Positive electrode active material for sodium secondary battery, manufacturing method thereof, positive electrode including same, and sodium secondary battery including positive electrode

Abstract

The present invention provides: a positive electrode active material for a sodium secondary battery; a manufacturing method thereof; a positive electrode for a sodium secondary battery, the positive electrode including the positive electrode active material; and a sodium secondary battery including the positive electrode.

Description

A positive electrode active material for a sodium secondary battery, a method for manufacturing the same, a positive electrode including the same, and a sodium secondary battery including the positive electrode

[0001] Cross-citation with related applications

[0002] This application claims the benefit of priority based on Korean Patent Application 10-2025-0037944 filed March 25, 2025, and all contents disclosed in the literature of said Korean patent applications are incorporated herein as part of this specification.

[0003]

[0004] Technology field

[0005] The present invention relates to a positive electrode active material for a sodium secondary battery, a method for manufacturing the same, a positive electrode for a sodium secondary battery comprising the same, and a sodium secondary battery comprising the said positive electrode.

[0006]

[0007] Rechargeable batteries are batteries that can be used repeatedly through the discharge process, in which chemical energy is converted into electrical energy, and the reverse process, charging. With the recent commercialization of portable electronic devices and electric vehicles, the demand for rechargeable batteries is surging. Currently, lithium-ion batteries are primarily used in commercial applications; however, due to the limited reserves of lithium, the primary raw material, they are expensive and insufficient to meet the demand for rechargeable batteries.

[0008] Accordingly, there is a demand for the development of new rechargeable batteries that can replace lithium-ion batteries, and recently, active research and development is being conducted on sodium rechargeable batteries using sodium instead of lithium.

[0009] A sodium secondary battery (sodium-ion secondary battery) consists of a positive electrode containing a positive active material capable of inserting and extracting sodium ions, a negative electrode containing a negative active material capable of inserting and extracting sodium ions, and a non-aqueous electrolyte containing sodium ions. Since sodium secondary batteries utilize sodium, which is abundant and inexpensive, as a material, it is expected that commercializing this technology will enable the mass supply of large-scale secondary batteries. Similar to lithium ions in lithium secondary batteries, charging and discharging of the sodium secondary battery occur as sodium ions travel back and forth between the negative and positive electrodes through the electrolyte.

[0010] Layered transition metal oxides, which are representative forms of cathode active materials, have the advantages of having a simple structure, excellent electrochemical performance, and ease of synthesis. However, sodium ions have disadvantages in terms of practical synthesis and electrochemical reaction kinetics because their ionic radius is approximately 70% larger than that of lithium ions. Since the insertion and extraction of sodium ions with large ionic radii into the layered structure alters the lattice structure, the cathode active material undergoes continuous phase transitions to maintain the bond between the transition metal and oxygen ions within the cathode active material structure during the charge-discharge process, resulting in an unstable structure. When charged to a high potential to obtain greater capacity, irreversible phase transitions occur, leading to a problem where the reversible capacity decreases with repeated cycles, and consequently, the cycle characteristics and high-rate characteristics of the sodium secondary battery deteriorate.

[0011] Meanwhile, although various materials have been reported for cathode active materials for high-capacity / high-voltage sodium secondary batteries, research on high-nickel (high-Ni) cathode active materials is insufficient.

[0012] Therefore, it is necessary to develop a cathode active material for sodium secondary batteries in which the aforementioned problems are improved through structural improvement and modification of the cathode active material particles.

[0013]

[0014] The present invention was devised to solve the problems of the prior art described above, and aims to provide a cathode active material for a sodium secondary battery in which oxygen generation during the charging process is suppressed and reversibility at high voltage is improved by doping Fe into a high-nickel (high-Ni) cathode active material to enhance structural stability.

[0015] In addition, the present invention aims to provide a method for manufacturing a positive electrode active material for a sodium secondary battery by mixing a sodium compound and an iron compound with a high-nickel transition metal precursor using a dry method, while adjusting the ratio to a specific ratio between the metals.

[0016] In addition, the present invention aims to provide a positive electrode comprising the positive electrode active material for the sodium secondary battery and a sodium secondary battery comprising the positive electrode.

[0017]

[0018] To solve the above problem, the present invention provides a positive electrode active material for a sodium secondary battery, a method for manufacturing the same, a positive electrode for a sodium secondary battery including the same, and a sodium secondary battery including the positive electrode.

[0019] (1) The present invention provides a positive electrode active material for a sodium secondary battery represented by the following chemical formula 1:

[0020] [Chemical Formula 1]

[0021] Na x (Ni a Mn b ) (1-c-d) Fe c M d O 2+α

[0022] In the above chemical formula 1,

[0023] 0.7≤x≤1, 0.8≤a≤1, 0≤b≤0.2, 0 <c≤0.5, 0≤d≤0.1, 0≤a≤0.05, a+b=1이고,

[0024] M is one or more selected from T, Cu, Mg, Al, and Sn.

[0025] (2) In the above (1), the present invention relates to Ni K-edge XANES spectrum at 8343 eV or higher and 8353 eV or lower in the X-ray absorption near-edge structure analysis. 3+ The peak, and Mn in the Mn K-edge XANES spectrum between 6560 eV and 6570 eV. 4+ A positive electrode active material for a sodium secondary battery having a peak is provided.

[0026] (3) The present invention provides a positive electrode active material for a sodium secondary battery having a hexagonal crystal structure in accordance with (1) or (2).

[0027] (4) The present invention provides a positive electrode active material for a sodium secondary battery, wherein, in any one of (1) to (3), a peak (104) which is the main peak in the range of 2θ to 40° to 45° is present in the XRD pattern measured using unmonochromatic CuKα rays.

[0028] (5) The present invention, in any one of (1) to (4), has an average particle size (D 50 The present invention provides a positive electrode active material for a sodium secondary battery having a thickness of 2 μm to 7 μm.

[0029] (6) In any one of (1) to (5) above, the present invention has a loading amount of 3.5 mg / cm² 2 to 6.0 mg / cm² 2 3.8 V (Na / Na) measured using the galvanostatic intermittent titration technique (GITT) on a coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm + ) Above, a positive electrode active material for a sodium secondary battery having a discharge capacity of 60 mAh / g or more is provided.

[0030] (7) In any one of (1) to (6) above, the present invention has a loading amount of 3.5 mg / cm² 2 to 6.0 mg / cm² 2 The sodium ion diffusion coefficient measured using the galvanostatic intermittent titration technique (GITT) for a coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm is 6.4 × 10⁻⁶. -12 cm 2 / s to 1.3×10 -10 cm 2 Provides a positive electrode active material for a sodium secondary battery that is / s.

[0031] (8) The present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery according to any one of (1) to (7), comprising the steps of: solid-state mixing of a sodium compound, an iron compound, and a transition metal precursor comprising nickel and manganese in the absence of a solvent (S1); and calcining (S2), wherein in step (S1), the sodium compound, the iron compound, and the transition metal precursor are each mixed in a molar ratio of 1:0.45 to 0.90:0 or greater than 0.50 based on sodium in the sodium compound, nickel in the transition metal precursor, and iron in the iron compound, and wherein the transition metal precursor is a nickel manganese hydroxide represented by the following chemical formula 2:

[0032] [Chemical Formula 2]

[0033] [Ni a1 Mn b1 ](OH)2

[0034] In the above chemical formula 2,

[0035] 0.8≤a1≤1, 0≤b1≤0.2, a1+b1=1.

[0036] (9) The present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the sodium compound in (8) is one or more selected from the group consisting of Na2O2, Na2CO3, NaOH, NaNO3, CH3COONa and Na2(COO)2.

[0037] (10) The present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, wherein, in (8) or (9), the iron compound is one or more selected from the group consisting of Fe2O3, Fe3O4, Fe(OH)2 and FeSO4.

[0038] (11) The present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, wherein in any one of (8) to (10), the solid-state mixing in step (S1) is performed by stirring for 30 minutes to 1 hour at room temperature under an inert atmosphere.

[0039] (12) The present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, wherein in any one of (8) to (11), the calcination in step (S2) is performed by heat treatment in a temperature range of 700 ℃ to 850 ℃.

[0040] (13) The present invention provides a positive electrode for a sodium secondary battery comprising a positive electrode active material for a sodium secondary battery according to any one of (1) to (7) above.

[0041] (14) The present invention provides a sodium secondary battery comprising a positive electrode for a sodium secondary battery according to (13) above.

[0042]

[0043] The positive electrode active material for a sodium secondary battery according to the present invention is a high-nickel transition metal oxide doped with Fe, and since structural stability is improved, oxygen generation during charging is suppressed, so excellent reversibility at high voltage can be achieved.

[0044] The sodium secondary battery according to the present invention may have excellent lifespan characteristics and high rate characteristics by including a positive electrode comprising the positive electrode active material for the sodium secondary battery described above.

[0045]

[0046] The following drawings attached to this specification illustrate specific embodiments of the present invention and serve to further enhance understanding of the technical concept of the present invention together with the aforementioned description; therefore, the present invention should not be interpreted as being limited only to the matters described in such drawings.

[0047] FIG. 1 is a spectrum of the oxidation number analysis of transition metals by X-ray absorption near-edge structure (XANES) analysis of the positive electrode active materials of Example 1 and Comparative Example, where (a) is the Mn K-edge XANES spectrum and (b) is the Ni K-edge XANES spectrum.

[0048] Figure 2 shows the XRD patterns of the positive active materials of Examples 1 to 5 and the Comparative Example.

[0049] FIG. 3 is a graph showing the charge-discharge characteristics of coin half-cells of Examples 1 to 5 and Comparative Example, where (a) is Example 1, (b) is Example 2, (c) is Example 3, (d) is Example 4, (e) is Example 5, and (f) is a graph of Comparative Example.

[0050] FIG. 4 is a graph showing the results of the thermodynamic irreversibility analysis of the coin half-cells of Example 1 and the Comparative Example through GITT, where (a) is the result graph of the Comparative Example and (b) is the result graph of Example 1.

[0051] Figure 5 is a graph showing the results of the analysis of the lifespan characteristics of the coin half-cells of Example 1 and the Comparative Example.

[0052] FIG. 6 is a graph showing the results of a comparative analysis of the gas generation amount of the cathode active materials of Example 1 and the Comparative Example, where (a) is the result graph of the Comparative Example and (b) is the result graph of Example 1.

[0053] FIG. 7 is a cross-sectional FE-SEM analysis result image of the positive electrode active material of Example 1 and Comparative Example after charging, where (a) is the result image of Comparative Example and (b) is the result image of Example 1.

[0054] FIG. 8 is a cross-sectional STEM analysis result image of the positive electrode active material after charging of Example 1 and Comparative Example, where (a) is the result image of Comparative Example and (b) is the result image of Example 1.

[0055]

[0056] Hereinafter, the present invention will be described in more detail to aid in understanding the invention.

[0057] Terms and words used in the description and claims of the present invention should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the present invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0058]

[0059] measurement method

[0060] In this specification, the Ni K-edge XANES spectrum and Mn K-edge XANES spectrum obtained by X-ray absorption near-edge structure (XANES) analysis were each measured under step size = 0.4 eV using a light source drawn from a bending magnet (1.4557 Tesla) located at the PLS-II (Pohang Light Source-II) storage ring 7D port.

[0061] In this specification, XRD patterns were measured using a D2 powder X-ray diffractometer (Bruker) and CuKα rays were used, and no separate monochromator was used. Measurements were taken with a voltage of 40 kV, a current of 40 mA, 2θ (Bragg angle) = 10°–80°, a step size = 0.02°, and a time / step = 1 sec.

[0062] In this specification, the average particle size (D 50 The particle size was measured using the laser diffraction method. After dispersing the particle in a dispersion medium, it was introduced into a laser diffraction measuring device (Microtrac S3500), and after irradiating it with 28 kHz ultrasound at an output of 60 W, the particle size corresponding to 50% of the volume accumulation in the measuring device was calculated.

[0063] In this specification, the galvanostatic intermittent titration technique (GITT) refers to a method of measuring potential change by applying a constant current at a rate of 0.1C to a cell for 10 minutes (i.e., applying current for 10 minutes) and stopping the 50-minute constant current experiment (i.e., cutting off current for 50 minutes) at room temperature (23±5 ℃) and a voltage range of 1.5 V to 4.3 V. Here, the cell is an anode (anode active material loading amount: 3.5 mg / cm²) formed by rolling an anode slurry containing an anode active material, a conductive material, and a binder onto an aluminum current collector. 2 to 6.0 mg / cm² 2 It may be a coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm, prepared by obtaining ) and using sodium metal as the counter electrode, a glass fiber separator as the separator, and a solution in which 2 wt% of fluoroethylene carbonate (FEC) and 1.0 M NaClO4 are added to a solvent mixed in a 1:1 volume ratio of ethylene carbonate:propylene carbonate as the electrolyte.

[0064]

[0065] Cathode active material for sodium secondary batteries

[0066] The present invention provides a positive electrode active material for a high-nickel (high-Ni) sodium secondary battery, wherein structural stability is improved and irreversible phase transitions during the charge-discharge process are suppressed, thereby improving reversible capacity.

[0067] A positive electrode active material for a sodium secondary battery according to one embodiment of the present invention comprises sodium (Na), nickel (Ni), manganese (Mn), and (Fe), and specifically may be represented by the following chemical formula 1.

[0068] [Chemical Formula 1]

[0069] Na x (Ni a Mn b ) (1-c-d) Fe c M d O 2+α

[0070] In the above chemical formula 1,

[0071] 0.7≤x≤1, 0.8≤a≤1, 0≤b≤0.2, 0 <c≤0.5, 0≤d≤0.1, 0≤a≤0.05, a+b=1이고,

[0072] M is one or more selected from T, Cu, Mg, Al, and Sn.

[0073]

[0074] Sodium secondary batteries consist of a positive electrode containing a positive active material capable of inserting / extracting sodium ions, a negative electrode containing a negative active material capable of inserting / extracting sodium ions, and a non-aqueous electrolyte containing sodium ions. Similar to lithium ions in lithium secondary batteries, charging and discharging of the battery occur as sodium ions travel back and forth between the negative and positive electrodes through the electrolyte, making them a promising alternative to lithium secondary batteries.

[0075] However, since sodium ions are approximately 70% larger than lithium ions in terms of ionic radius ratio, the lattice structure changes when sodium ions with large ionic radii are inserted / extracted from the layered structure of the positive electrode active material. Consequently, the positive electrode active material undergoes continuous phase transitions to maintain the bond between the transition metal and oxygen ions within the structure during the charge / discharge process, resulting in an unstable structure. Irreversible phase transitions occur, leading to a problem where the reversible capacity decreases with each cycle, which in turn causes a deterioration in the cycle characteristics and high-rate characteristics of the sodium secondary battery.

[0076] Meanwhile, the above-mentioned irreversible characteristics at high voltages during charging and discharging are known to be due to oxygen evolution during charging caused by unstable oxygen oxidation / reduction reactions (Oxygen Redox).

[0077] However, the positive electrode active material for a sodium secondary battery according to the present invention has Fe doped in a specific ratio in the transition metal layer to improve structural stability, thereby suppressing oxygen generation during charging and improving reversible capacity, and thus can provide a sodium secondary battery with excellent lifespan characteristics and high rate characteristics even at high voltages.

[0078] In addition, the positive electrode active material for a sodium secondary battery according to the present invention is based on a high-nickel (high-Ni) transition metal composite oxide, and compared to a high-manganese (high-Mn) active material which has a relatively low initial capacity, it has a wide operating voltage range and high charge capacity, thereby providing excellent initial performance and high energy density.

[0079]

[0080] Hereinafter, a positive electrode active material for a sodium secondary battery according to one embodiment of the present invention will be described in more detail.

[0081] The positive electrode active material for a sodium secondary battery according to one embodiment of the present invention may include sodium (Na), nickel (Ni), manganese (Mn), and (Fe), and may include manganese (Mn) in an amount of 20 mol% or less among the total metals excluding sodium (Na), and specifically may be represented by the following chemical formula 1.

[0082] [Chemical Formula 1]

[0083] Na x (Ni a Mn b ) (1-c-d) Fe c M d O 2+α

[0084] In the above chemical formula 1,

[0085] 0.7≤x≤1, 0.8≤a≤1, 0≤b≤0.2, 0 <c≤0.5, 0≤d≤0.1, 0≤a≤0.05, a+b=1이고,

[0086] M is one or more selected from T, Cu, Mg, Al, and Sn.

[0087] In the above chemical formula 1, Na may be included in an amount corresponding to x, and if x is within the above range, the positive active material may be in the form of an O3-type octahedral structure. In this case, it provides a high initial ion storage capacity and may have excellent structural stability during electrochemical cycling.

[0088] In addition, in the above chemical formula 1, Ni may be included in an amount of a × (1-cd), and if it is within this range, the capacity development and energy density at high voltage can be improved as a high-nickel cathode active material.

[0089] In addition, in the above chemical formula 1, Mn may be included in an amount of b×(1-cd), and if it is within this range, the structural stability of the positive active material may be improved.

[0090] In addition, in the above chemical formula 1, Fe may be included in the content of c, and if it is within this range, the capacity development of the positive active material at high voltage may occur reversibly and oxygen generation may be suppressed.

[0091] In addition, in the above chemical formula 1, M may be a doping element included in the crystal structure within the positive active material and may be included in the content of d.

[0092] As another example, in the above Chemical Formula 1, 0.9≤x≤1.0, 0.8≤a≤1.0, 0≤b≤0.2, 0 <c≤0.3, 0=d, 0=α, a+b=1일 수 있다.

[0093] In addition, the above-mentioned cathode active material for the sodium secondary battery is Ni at 8343 eV to 8353 eV in the Ni K-edge XANES spectrum obtained by X-ray absorption near-edge structure (XANES) analysis. 3+ The peak, and Mn in the Mn K-edge XANES spectrum between 6560 eV and 6570 eV. 4+ It has a peak. Here, the Ni K-edge XANES spectrum and Mn K-edge XANES spectrum obtained by X-ray absorption near-edge structure analysis are an analysis of the oxidation number of a transition metal in a cathode active material for a sodium secondary battery according to one embodiment of the present invention. As shown in FIG. 1, due to Fe doping, the peak in the Mn K-edge XANES spectrum shifted to high energy (increase in oxidation number) and the peak in the Ni K-edge XANES spectrum shifted to low energy (decrease in oxidation number), and from this, it can be confirmed that Fe is doped into the transition metal layer in the cathode active material.

[0094] In addition, the positive electrode active material for a sodium secondary battery according to one embodiment of the present invention may have a hexagonal crystal structure, specifically having a hexagonal crystal structure and a space group of R3m, and may be a layered compound in which a sodium layer and a transition metal oxide layer are alternately stacked. In addition, the positive electrode active material according to one embodiment of the present invention is doped with Fe and transitions from a monoclinic crystal structure to a hexagonal crystal structure, and transitions to a more stable hexagonal crystal structure as the amount of doped Fe increases, thereby suppressing the generation of impurities such as NiO and having excellent crystallinity. As another example, the positive electrode active material for a sodium secondary battery according to one embodiment of the present invention may have a (104) peak, which is the main peak in the range of 2θ to 40° to 45° in the XRD pattern measured using non-monochromatic CuKα rays. Through this, it can be seen that the positive electrode active material for the sodium secondary battery has the above crystal structure.

[0095]

[0096] As another example, a positive electrode active material for a sodium secondary battery according to one embodiment of the present invention has an average particle size (D 50 The average particle size may be 2 μm to 7 μm, specifically 3 μm to 5 μm. If the above average particle size is satisfied, the diffusion distance of the ions is shortened, the diffusion speed is increased, and the surface area is large, which may be advantageous for inducing a uniform reaction.

[0097] As another example, a positive electrode active material for a sodium secondary battery according to one embodiment of the present invention has a loading amount of 3.5 mg / cm² 2 to 6.0 mg / cm² 2 3.8 V (Na / Na) measured using the galvanostatic intermittent titration technique (GITT) on a coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm +The discharge capacity may be 60 mAh / g or more.

[0098] In addition, the above-mentioned positive electrode active material for a sodium secondary battery has a loading amount of 3.5 mg / cm² 2 to 6.0 mg / cm² 2 The sodium ion diffusion coefficient measured using the galvanostatic intermittent titration technique (GITT) for a coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm is 6.4 × 10⁻⁶. -12 cm 2 / s to 1.3×10 -10 cm 2 It may be / s. If this is satisfied, it alleviates the contraction of the sodium ion path during the charge / discharge process, thereby stabilizing the layered structure, and as a result, the lifespan characteristics and high-rate characteristics may be excellent.

[0099] In the present invention, the sodium ion diffusion coefficient of the positive electrode active material for the sodium secondary battery is measured through a coin half-cell, but since the distance between the negative electrode and the positive electrode in the coin half-cell of the above specifications (diameter 20 mm, thickness 3.2 mm) is very close, the electrolyte resistance is at a level where there is no resistance, and even in the case of the negative electrode, there is almost no voltage change, so the potential of the positive electrode can be considered absolute in voltage behavior, and thus the sodium ion diffusion coefficient is dependent on the characteristics of the positive electrode active material.

[0100] Furthermore, the above-described positive electrode active material for a sodium secondary battery possesses high crystallinity and high uniformity of microstructure, and since the particle size and distribution are uniform, it can possess the aforementioned sodium ion diffusion coefficient characteristics. These characteristics may be influenced by synthesis through solid-state mixing described later, and additionally, may be influenced by a combination of various factors such as the composition of the positive electrode active material, particle size, and calcination conditions after solid-state mixing. The present invention proposes a positive electrode active material that satisfies the above-described sodium ion diffusion coefficient by appropriately controlling various factors.

[0101] Meanwhile, in the present invention, the sodium ion diffusion coefficient may refer to the result of analyzing data measured using a galvanostatic intermittent titration technique (GITT) using the following Equation 1.

[0102] [Equation 1]

[0103]

[0104] In the above Equation 1,

[0105] D is the sodium ion diffusion coefficient, L is the sodium ion diffusion distance (cm), which is generally the radius of the particle, τ is the time (s) for the potential to change due to diffusion after current interruption, and dE s ε is the change in voltage during current application, dE τ is the amount of voltage change in the equilibrium state after current interruption.

[0106]

[0107] Method for manufacturing a positive electrode active material for a sodium secondary battery

[0108] The present invention provides a method for manufacturing the above-mentioned positive electrode active material for a sodium secondary battery.

[0109] A method for manufacturing a positive electrode active material for a sodium secondary battery according to one embodiment of the present invention is a dry method comprising the steps of: solid-state mixing of a sodium compound, an iron compound, and a transition metal precursor including nickel and manganese in the absence of a solvent (S1); and calcining (S2). The transition metal precursor contains manganese (Mn) in an amount of 20 mol% or less among the total metals in the precursor. In the step (S1), the sodium compound, the iron compound, and the transition metal precursor may be mixed in a molar ratio of 1:0.45 to 0.90:0 or greater than 0.50 based on sodium in the sodium compound, nickel in the transition metal precursor, and iron in the iron compound, respectively. The transition metal precursor may be a nickel manganese hydroxide represented by the following chemical formula 2.

[0110] [Chemical Formula 2]

[0111] [Ni a1 Mn b1 ](OH)2

[0112] In the above chemical formula 2,

[0113] 0.8≤a1≤1, 0≤b1≤0.2, a1+b1=1.

[0114]

[0115] A method for manufacturing a positive electrode active material for a sodium secondary battery according to one embodiment of the present invention can form a uniform particle size and distribution by mixing a sodium compound and an iron compound in a transition metal precursor using a dry method and doping iron, thereby having high crystallinity and increasing the uniformity of the microstructure.

[0116] Specifically, the method for manufacturing the positive electrode active material for a sodium secondary battery according to one embodiment of the present invention involves solid-state mixing of a sodium compound and an iron compound in a transition metal precursor by a dry method and doping with iron, thereby [addressing] Fe in the solution that occurs when co-precipitating occurs in the solution. 2+ It easily reacts with oxygen in the air to form Fe 3+ It is oxidized to, and thus Fe 2+ Unable to precipitate in the state of Fe 3+ Since Fe precipitates first in this form, non-uniform synthesis occurs, making it easy to control composition, structure, and shape uniformity without the occurrence of problems regarding composition and structural non-uniformity.

[0117] In addition, the method for manufacturing a positive electrode active material for a sodium secondary battery according to one embodiment of the present invention can produce a positive electrode active material having excellent crystallinity and a structurally stable hexagonal crystal structure by mixing a sodium compound, a transition metal precursor, and an iron compound in the aforementioned molar ratio range.

[0118]

[0119] Hereinafter, a method for manufacturing a positive electrode active material for a sodium secondary battery according to one embodiment of the present invention will be described in more detail by dividing it into steps.

[0120] (S1) Step

[0121] The above step (S1) is a step of mixing a sodium compound, a transition metal precursor, and an iron compound, and specifically, can be performed by solid-state mixing of the sodium compound, the transition metal precursor, and the iron compound in the absence of a solvent.

[0122] In the above solid-state mixing step, the sodium compound, the transition metal precursor, and the iron compound may be mixed in a molar ratio of 1:0.45 to 0.90:0 or greater than 0.50, specifically 1:0.63 to 0.90:0 or greater than 0.30, or 1:0.63 to 0.72:0.2 to 0.3, based on sodium in the sodium compound, nickel in the transition metal precursor, and iron in the iron compound, respectively.

[0123] In addition, the sodium compound may be one or more selected from the group consisting of Na2O2, Na2CO3, NaOH, NaNO3, CH3COONa, and Na2(COO)2, and the iron compound may be one or more selected from the group consisting of Fe2O3, Fe3O4, Fe(OH)2, and FeSO4.

[0124] In addition, the above solid-state mixing is not particularly limited and can be performed using mixing devices commonly used in the field, such as ball mills, V-mixers, stirrers, mortars, etc. Specifically, it can be performed by stirring for 30 minutes to 1 hour under an inert gas atmosphere at room temperature (25±5 ℃).

[0125]

[0126] (S2) Step

[0127] The above step (S2) is a step for manufacturing a positive electrode active material, and can be performed by calcining the mixture after solid-state mixing.

[0128] The above calcination may be performed by heat treatment in a temperature range of 700 ℃ to 850 ℃ under an oxidizing atmosphere, and specifically, may be performed by heat treatment in a temperature range of 800 ℃ to 850 ℃ under an oxidizing atmosphere.

[0129] In addition, the above calcination is not particularly limited but may be performed for 5 to 40 hours, 5 to 20 hours, 5 to 15 hours, or 8 to 12 hours, in which case a highly crystalline positive active material can be obtained.

[0130] In the above, the oxidation differentiation site may be an air atmosphere or an oxygen (O2) atmosphere.

[0131]

[0132] anode

[0133] The present invention provides a positive electrode comprising the aforementioned positive electrode active material for a sodium secondary battery.

[0134] An anode according to one embodiment of the present invention comprises an anode current collector; and an anode active material layer located on at least one surface of the anode current collector and comprising the anode active material.

[0135] The above positive active material may be included in an amount of 80% to 99% by weight, specifically 85% to 98% by weight, based on the total weight of the positive active material layer, and in this case, excellent capacity characteristics may be exhibited.

[0136] The above positive current collector is not specifically limited as long as it is conductive without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the above positive current collector may have a thickness of 3 μm to 500 μm.

[0137] In addition, the above positive current collector can form fine irregularities on its surface to increase the adhesion of the positive active material, and can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

[0138] In addition, the above-mentioned positive active material layer may include a conductive material and a binder together with the aforementioned positive active material.

[0139] The above conductive material is used to impart conductivity to the electrode and can be used without special limitations as long as it has electronic conductivity without causing chemical changes in the battery being constructed. Specifically, the above conductive material may include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, or silver; conductive tubes such as carbon nanotubes; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used.

[0140] In addition, the binder serves to improve adhesion between positive active material particles and adhesion between the positive active material and the current collector, and examples include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0141] The above-described anode can be manufactured according to a conventional anode manufacturing method, except for using the aforementioned anode active material, and, for example, can be manufactured by applying a composition (slurry) for forming an anode active material layer, prepared by dissolving or dispersing the above-described anode active material and optionally a binder and a conductive material in a solvent, onto an anode current collector, and then drying and rolling.

[0142] The above solvent may be a general solvent used in the field, and examples include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more may be used. The amount of the above solvent used is sufficient if it has a viscosity that dissolves or disperses the anode active material, conductive material, and binder, taking into account the coating thickness of the slurry and the manufacturing yield, and subsequently provides excellent thickness uniformity when coated for anode manufacturing.

[0143] Alternatively, the anode may be manufactured by casting the composition for forming the anode active material layer onto a separate support, and then laminating the film obtained by peeling off from the support onto an anode current collector.

[0144]

[0145] Sodium secondary battery

[0146] The present invention provides a sodium secondary battery comprising the above-mentioned positive electrode.

[0147] According to one embodiment of the present invention, the sodium secondary battery may comprise the positive electrode, the negative electrode positioned opposite the positive electrode, the separator interposed between the positive electrode and the negative electrode, and the electrolyte. The positive electrode is as described above, and only the remaining components will be described below.

[0148] In the above sodium secondary battery, the negative electrode comprises a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0149] The above-mentioned negative current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used. In addition, the above-mentioned negative current collector may typically have a thickness of 3 μm to 500 μm, and, similar to the positive current collector, fine irregularities may be formed on the surface of the current collector to strengthen the bonding strength of the negative active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0150] The above-mentioned cathode active material layer optionally includes a binder and a conductive material together with the cathode active material.

[0151] As the above-mentioned negative electrode active material, a compound capable of reversible insertion and extraction of sodium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides capable of doping and dedoping lithium, such as SiOβ (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites comprising the above-mentioned metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or more of these may be used. Additionally, a metallic sodium thin film may be used as the above-mentioned negative electrode active material.

[0152] Meanwhile, the binder and conductive material mentioned above may be the same as those described above for the anode.

[0153] The above cathode may be manufactured by applying a composition (slurry) for forming a cathode active material layer, prepared by dissolving or dispersing a cathode active material and optionally a binder and a conductive material in a solvent, onto a cathode current collector and drying it, or by casting the slurry for forming a cathode active material layer onto a separate support and then laminating the film obtained by peeling off from the support onto a cathode current collector.

[0154] According to one embodiment of the present invention, the separator separates the negative electrode and the positive electrode and provides a pathway for the movement of sodium ions. It can be used without special limitations as long as it is a separator commonly used in sodium secondary batteries, and it is particularly desirable that it has low resistance to the movement of electrolyte ions and excellent electrolyte moisture retention capacity. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer like an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, a coated separator containing a ceramic component or a polymer material may be used to ensure heat resistance or mechanical strength, and it may optionally be used in a single-layer or multi-layer structure.

[0155] According to one embodiment of the present invention, the electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc., which are usable when manufacturing a sodium secondary battery, but are not limited thereto. As a specific example, the electrolyte may include an organic solvent and a sodium salt.

[0156] According to one embodiment of the present invention, the organic solvent may be used without special limitations as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvent may be an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether-based solvent such as dibutyl ether or tetrahydrofuran; a ketone-based solvent such as cyclohexanone; or an aromatic hydrocarbon-based solvent such as benzene or fluorobenzene. Carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, a carbonate-based solvent is preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred.

[0157] According to one embodiment of the present invention, the sodium salt can be used without special limitations as long as it is a compound capable of providing sodium ions used in a sodium secondary battery. Specifically, the sodium salt may be NaPF6, NaClO4, NaAsF6, NaBF4, NaB(C6H5)4, NaCF3SO3, NaC4F9SO3, NaN(C2F5SO3)2, NaN(C2F5SO2)2, or NaN(CF3SO2)2. It is preferable to use the sodium salt within the range of 0.1 M to 2.0 M. When the concentration of the sodium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and sodium ions can move effectively.

[0158] According to one embodiment of the present invention, in addition to the electrolyte components, for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, the electrolyte may include, for example, vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), propane sulfone (PS), 1,3-propane sulfone (PRS), ethylene sulfate (ESA), succinonitrile (SN), adiponitrile (AN), hexane tricarbonitrile (HTCN), γ-butyrolactone, biphenyl (BP), cyclohexyl benzene (CHB), t-amyl One or more additives selected from the group consisting of benzene (tert-amyl benzene, TAB), or haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride may be further included. In this case, the said additives may be included in an amount of 0.1% to 5% by weight based on the total weight of the electrolyte.

[0159]

[0160] Since the sodium secondary battery containing the positive electrode active material according to the present invention stably exhibits excellent capacity characteristics, output characteristics, and lifespan characteristics, it is useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs) and electric vehicles (EVs).

[0161] The external shape of the sodium secondary battery of the present invention is not particularly limited, but can be a cylindrical shape using a can, a prismatic shape, a pouch shape, or a coin shape.

[0162] The sodium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but can also preferably be used as a unit cell in a medium-to-large battery module comprising a plurality of battery cells.

[0163] Accordingly, according to one embodiment of the present invention, a battery module comprising the sodium secondary battery as a unit cell and a battery pack comprising the same are provided.

[0164] According to one embodiment of the present invention, the battery module or battery pack may be used as a power source for one or more medium-to-large devices, including a power tool; an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0165]

[0166] Examples

[0167] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0168] Example 1

[0169] Na2O2, Ni 0.9 Mn 0.1 (OH)2 and Fe2O3 powders were hand-mixed using a mortar and pestle for 60 minutes at room temperature (25±5 ℃) under an inert gas atmosphere to achieve a molar ratio of Na:Ni:Fe of 1:0.63:0.3, thereby ensuring uniform mixing. Subsequently, the mixture was placed in an alumina crucible and heat-treated in a kiln under an oxygen atmosphere at 800 ℃ for 10 hours to produce Na[Ni 0.9 Mn 0.1 ] 0.7 Fe 0.3 A positive electrode active material for a sodium secondary battery with an O2 composition was prepared.

[0170]

[0171] Example 2

[0172] In the above Example 1, Na2O2, Ni 0.9 Mn 0.1 Except for mixing (OH)2 and Fe2O3 powders in amounts such that the atomic ratio of Na:Ni:Fe is 1:0.855:0.05 molar ratio, the procedure was carried out in the same manner as Example 1 above, so that Na[Ni 0.9 Mn 0.1 ] 0.95 Fe 0.05 A positive electrode active material for a sodium secondary battery with an O2 composition was prepared.

[0173]

[0174] Example 3

[0175] In the above Example 1, Na2O2, Ni 0.9 Mn 0.1 Except for mixing (OH)2 and Fe2O3 powders in amounts such that the atomic ratio of Na:Ni:Fe is 1:0.8:0.1 molar ratio, the procedure was carried out in the same manner as Example 1 above, so that Na[Ni 0.9 Mn 0.1 ] 0.9 Fe 0.1A positive electrode active material for a sodium secondary battery with an O2 composition was prepared.

[0176]

[0177] Example 4

[0178] In the above Example 1, Na2O2, Ni 0.9 Mn 0.1 Except for mixing (OH)2 and Fe2O3 powders in amounts such that the atomic ratio of Na:Ni:Fe is 1:0.72:0.20 molar ratio, the procedure was carried out in the same manner as Example 1 above, so that Na[Ni 0.9 Mn 0.1 ] 0.8 Fe 0.2 A positive electrode active material for a sodium secondary battery with an O2 composition was prepared.

[0179]

[0180] Example 5

[0181] In the above Example 1, Na2O2, Ni 0.9 Mn 0.1 Except for mixing (OH)2 and Fe2O3 powders in amounts such that the atomic ratio of Na:Ni:Fe is 1:0.45:0.50 molar ratio, the procedure was carried out in the same manner as Example 1 above, so that Na[Ni 0.9 Mn 0.1 ] 0.5 Fe 0.5 A positive electrode active material for a sodium secondary battery with an O2 composition was prepared.

[0182]

[0183] Comparative example

[0184] Na2O2 and Ni 0.9 Mn 0.1 (OH)2 powder was hand-mixed using a mortar and pestle for 60 minutes at room temperature (25±5 ℃) under an inert gas atmosphere to achieve a Na:Ni atomic ratio of 1:0.9 molar ratio, ensuring uniform mixing without particle damage. Subsequently, the mixture was placed in an alumina crucible and heat-treated in a kiln under an oxygen atmosphere at 800 ℃ for 10 hours to produce NaNi 0.9 Mn 0.1A positive electrode active material for a sodium secondary battery with an O2 composition was prepared.

[0185]

[0186] Experimental Example 1

[0187] X-ray absorption near-edge structure (XANES) analysis of each sodium secondary battery positive electrode active material prepared in the examples and comparative examples was performed using photomolecular spectroscopy (XPS) to confirm that Fe was doped in the transition metal layer, and the results are shown in Figure 1.

[0188] Specifically, in the analysis of the X-ray absorption near-edge structure (XANES), the Ni K-edge XANES spectrum and Mn K-edge XANES spectrum were analyzed by photomolecular spectroscopy (XPS) to confirm whether Fe was doped by analyzing the oxidation states of Ni and Mn. Each of the above spectra was measured under step size = 0.4 eV using a light source extracted from a bending magnet (1.4557 Tesla) located at the PLS-II storage ring 7D port.

[0189] As confirmed through Fig. 1, the cathode active material of the example is Ni at 8343 eV to 8353 eV in the Ni K-edge XANES spectrum. 3+ A peak appears, and along with this, in the Mn K-edge XANES spectrum, Mn appears between 6560 eV and 6570 eV. 4+ Peaks were observed, and when compared with the results of the cathode active material of the comparative example not doped with Fe, it was confirmed that in the Ni K-edge XANES spectrum, the peak shifted to a low energy, indicating a decrease in the oxidation number of Ni, and in the Mn K-edge XANES spectrum, the peak shifted to a high energy, indicating an increase in the oxidation number of Mn.

[0190]

[0191] Experimental Example 2

[0192] XRD analysis was performed on each cathode active material prepared in the above examples and comparative examples to confirm the crystal structure, and the results are shown in Figure 2.

[0193] XRD analysis was performed using a Brucker D2 powder x-ray diffractiometer (Brucker) with a current of 40 kV, a voltage of 40 mA CuKα, 2θ (Bragg angle) = 10°–80°, and a scan speed of 0.02° / 1 sec.

[0194] As confirmed in Fig. 2, the positive electrode active material for the sodium secondary battery of the example has a main peak (104) in the XRD pattern in the range of 2θ to 40° to 45° and has a hexagonal O3 type crystal structure.

[0195]

[0196] Experimental Example 3

[0197] The sodium ion diffusion coefficient of each positive electrode active material for a sodium secondary battery prepared in the examples was confirmed.

[0198] The sodium ion diffusion coefficient was analyzed using Equation 1 below from data measured by a galvanostatic intermittent titration technique (GITT) in which a coin half-cell was prepared using each cathode active material, and the coin half-cell was tested for 10 minutes at a constant current of 0.1 C rate (20 mA / g) in a voltage range of 1.5 V to 4.3 V relative to sodium metal, and the constant current test was stopped and repeated. The results are shown in Table 1 below.

[0199] [Equation 1]

[0200]

[0201] In the above Equation 1,

[0202] D is the sodium ion diffusion coefficient, L is the sodium ion diffusion distance (cm), which is generally the radius of the particle, τ is the time (s) for the potential to change due to diffusion after current interruption, and dE s ε is the change in voltage during current application, dE τ is the amount of voltage change in the equilibrium state after current interruption.

[0203] Each of the above coin half-cells was manufactured as follows.

[0204] Each positive active material, carbon black (Super P) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were weighed in a weight ratio of 80:10:10. Then, the positive active material and the conductive material were first mixed using a mortar and pestle, and the mixed powder was mixed with the binder and N-methylpyrrolidone (NMP) and uniformly mixed using a stirrer to prepare a positive slurry.

[0205] The above anode slurry was applied to an aluminum current collector to a thickness of 200 μm using an applicator, dried to remove NMP, and then rolled to manufacture an electrode.

[0206] After punching the manufactured electrode to a diameter of 20 mm, it was vacuum-rolled at 120 ℃ for 8 hours to remove moisture, and the anode (anode active material loading amount: 5 mg / cm² 2 Manufactured ).

[0207] A coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm was prepared using the above anode, sodium metal as the counter electrode, a glass fiber separator as the separator, and a solution in which 2 wt% of fluoroethylene carbonate (FEC) and 1.0 M NaClO4 were added to a solvent in which ethylene carbonate:propylene carbonate solvents were mixed in a 1:1 volume ratio as the electrolyte.

[0208] Classification Example 1 2 3 4 5 Sodium ion diffusion coefficient (cm 2 / s)4.037×10 -10 8.740×10 -11 1.682×10 -10 3.059×10-10 5.451×10 -10

[0209] As shown in Table 1 above, the cathode active material for the sodium secondary battery of the example has a loading amount of 3.5 mg / cm² 2 to 6.0 mg / cm² 2 The sodium ion diffusion coefficient measured using the galvanostatic intermittent titration technique (GITT) for a coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm is 6.4 × 10⁻⁶. -12 cm 2 / s to 1.3×10 -10 cm 2 I confirmed that it is / s.

[0210]

[0211] Experimental Example 4

[0212] Coin half-cells were manufactured using each sodium secondary battery positive active material prepared in the examples and comparative examples, battery characteristics were measured, and electrode cross-sectional analysis was performed.

[0213] 1) Manufacture of coin half-cells

[0214] Each positive active material, carbon black (Super P) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were weighed in a weight ratio of 80:10:10. Then, the positive active material and the conductive material were first mixed using a mortar and pestle, and the mixed powder was mixed with the binder and N-methylpyrrolidone (NMP) and uniformly mixed using a stirrer to prepare a positive slurry.

[0215] The above anode slurry was applied to an aluminum current collector to a thickness of 200 μm using an applicator, dried to remove NMP, and then rolled to manufacture an electrode.

[0216] After punching the manufactured electrode to a diameter of 20 mm, it was vacuum-rolled at 120 ℃ for 8 hours to remove moisture, and the anode (anode active material loading amount: 5 mg / cm² 2Manufactured ).

[0217] A coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm was prepared using the above anode, sodium metal as the counter electrode, a glass fiber separator as the separator, and a solution in which 2 wt% of fluoroethylene carbonate (FEC) and 1.0 M NaClO4 were added to a solvent in which ethylene carbonate:propylene carbonate solvents were mixed in a 1:1 volume ratio as the electrolyte.

[0218]

[0219] 2) Battery Charge / Discharge Profile Analysis

[0220] The charge-discharge characteristics of the above coin half-cell were analyzed by charging and discharging it at a current density of 0.1C in a voltage range of 1.5 V to 4.3 V relative to sodium metal, and the results of the first, second, and tenth cycles are shown in Figure 3.

[0221] As shown in Fig. 3, it was confirmed that the positive electrode active material of the example exhibits reversible charge and discharge capacities at high voltage. On the other hand, the positive electrode active material of the comparative example showed an irreversible phenomenon in which no discharge capacity was exhibited at high voltage. In addition, it was confirmed that the positive electrode active material according to one embodiment of the present invention has superior charge and discharge capacity characteristics compared to the comparative example.

[0222]

[0223] 3) Analysis of Thermodynamic Irreversibility

[0224] The thermodynamic irreversibility of the above coin half-cell was analyzed using a galvanostatic intermittent titration technique (GITT) in which the experiment was conducted for 10 minutes with a constant current rate of 0.1C and the experiment was stopped for 50 minutes in a voltage range of 1.5 V to 4.3 V relative to sodium metal, and the results are shown in Figure 4.

[0225] As shown in Figure 4, it was confirmed that the battery using the positive active material of the example exhibits a reversible discharge capacity at high voltage, whereas the battery using the positive active material of the comparative example exhibits an irreversible phenomenon in which the discharge capacity is not exhibited at high voltage.

[0226]

[0227] 4) Analysis of battery life characteristics

[0228] The above coin half-cell was charged and discharged at a constant current of 0.1C rate in a voltage range of 1.5 V to 4.3 V relative to sodium metal, and the lifespan characteristics over 100 cycles were measured, and the results are shown in Fig. 5.

[0229] As shown in Figure 5, it was confirmed that the battery using the positive active material of the example had significantly improved lifespan characteristics compared to the battery using the positive active material of the comparative example.

[0230]

[0231] 5) Analysis of gas generation volume

[0232] The above coin half-cell was charged with a constant current of 0.1C rate in a voltage range of 1.5 V to 4.3 V relative to sodium metal, and the amount of gas generated by the positive electrode active material was analyzed by Differential Electrochemical Mass Spectrometry (DEMS), and the results are shown in Figure 6.

[0233] As shown in Fig. 6, it was confirmed that the positive electrode active material of the example suppressed the generation of O2 at high voltage and reduced the generation of CO2. Through this, it was confirmed that the positive electrode active material for a sodium secondary battery according to one embodiment of the present invention suppresses oxygen generation even when charging proceeds to a high voltage region due to structural stabilization, and thus has the effect of improving reversibility at high voltage.

[0234]

[0235] 6) Electrode cross-section analysis

[0236] After charging the above coin half-cell to 4.3 V, the cell was disassembled and the anode cross-section was analyzed using FE-SEM (SUPRA 55VP, Carl Zeiss Auriga) and STEM (JEM-ARM200F, JEOL Ltd), and the results are shown in Figures 7 and 8, respectively.

[0237] As shown in FIGS. 7 and 8, it was confirmed that the positive active material of the example significantly suppresses crack formation during the charging process compared to the comparative example, while stably maintaining a layered structure on the surface.

Claims

1. A positive electrode active material for a sodium secondary battery represented by the following chemical formula 1: [Chemical Formula 1] So x (Ni a Mr b ) (1-c-d) Feb c M d O 2+α In the above chemical formula 1, 0.7≤x≤1, 0.8≤a≤1, 0≤b≤0.2, 0 <c≤0.5, 0≤d≤0.1, 0≤a≤0.05, a+b=1이고, M is one or more selected from T, Cu, Mg, Al, and Sn.

2. In Paragraph 1, In the Ni K-edge XANES spectrum obtained by X-ray absorption near-edge structure (XANES) analysis, Ni between 8343 eV and 8353 eV 3+ The peak, and Mn in the Mn K-edge XANES spectrum between 6560 eV and 6570 eV. 4+ A positive electrode active material for a sodium secondary battery having a peak.

3. In Paragraph 1, A positive electrode active material for a sodium secondary battery having a hexagonal crystal structure.

4. In Paragraph 1, A positive electrode active material for a sodium secondary battery, wherein the main peak (104) is present in the XRD pattern measured using unmonochromatic CuKα rays in the range of 40° to 45° 2θ.

5. In Paragraph 1, Average particle size (D 50 A positive electrode active material for a sodium secondary battery having a thickness of 2 μm to 7 μm.

6. In Paragraph 1, Loading dose 3.5 mg / cm² 2 to 6.0 mg / cm² 2 3.8 V (Na / Na) measured using the galvanostatic intermittent titration technique (GITT) on a coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm + A positive electrode active material for a sodium secondary battery having a discharge capacity of 60 mAh / g or more.

7. In Paragraph 1, Loading dose 3.5 mg / cm² 2 to 6.0 mg / cm² 2 The sodium ion diffusion coefficient measured using the galvanostatic intermittent titration technique (GITT) for a coin half-cell with a diameter of 20 mm and a thickness of 3.2 mm is 6.4 × 10⁻⁶. -12 cm 2 / s to 1.3×10 -10 cm 2 A positive electrode active material for sodium secondary batteries that is / s.

8. A step (S1) of solid-state mixing of a sodium compound, an iron compound, and a transition metal precursor including nickel and manganese in the absence of a solvent; and It includes a firing step (S2), In the above (S1) step, the sodium compound, the iron compound, and the transition metal precursor are each mixed in a molar ratio of 1:0.45 to 0.90:0 or greater than 0.50 based on sodium in the sodium compound, nickel in the transition metal precursor, and iron in the iron compound, respectively. A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the above transition metal precursor is a nickel manganese hydroxide represented by the following chemical formula 2: [Chemical Formula 2] [Ni a1 Mn b1 ](OH)2 In the above chemical formula 2, 0.8≤a1≤1, 0≤b1≤0.2, a1+b1=1.

9. In Paragraph 8, A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the above sodium compound is one or more selected from the group consisting of Na2O2, Na2CO3, NaOH, NaNO3, CH3COONa, and Na2(COO)2.

10. In Paragraph 8, A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the above iron compound is one or more selected from the group consisting of Fe2O3, Fe3O4, Fe(OH)2 and FeSO4.

11. In Paragraph 8, A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein in the above step (S1), solid mixing is performed by stirring for 30 minutes to 1 hour at room temperature under an inert atmosphere.

12. In Paragraph 8, A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the calcination in the above (S2) step is performed by heat treatment in a temperature range of 700 ℃ to 850 ℃.

13. A cathode for a sodium secondary battery comprising a cathode active material for a sodium secondary battery according to claim 1.

14. A sodium secondary battery comprising a positive electrode for a sodium secondary battery according to paragraph 13.

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