Positive electrode active material for sodium secondary battery, method for preparing same, and sodium secondary battery including same

Abstract

An embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, the positive electrode active material comprising: composite particles; and a coating layer covering at least a portion of the surface of the composite particles. The composite particles contain: at least one transition metal selected from nickel, iron, and manganese; sodium; and a doping metal, and the coating layer contains oxides of the doping metal and the coating transition metal.

Description

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

[0001] The present invention relates to a positive electrode active material for a sodium secondary battery, a method for manufacturing the same, and a sodium secondary battery comprising the same.

[0002] With the surge in demand for lithium-ion rechargeable batteries, which are widely used as energy storage devices in various electronic technology fields, sodium-ion rechargeable batteries are attracting attention as a replacement for lithium, an expensive metal.

[0003] Sodium-ion secondary batteries are one of the next-generation materials with high potential for application as secondary batteries because they have an insertion / extraction reaction operating principle similar to that of lithium-ion secondary batteries. However, they show lower performance in terms of capacity, lifespan, and rate characteristics compared to lithium-ion secondary batteries, making commercialization difficult. Therefore, the development of high-performance cathode active materials is required for commercialization.

[0004] Layered transition metal oxides are typically used as cathode active materials for sodium-ion secondary batteries because they possess a simple structure, excellent electrochemical performance, and are easy to synthesize. Layered transition metal oxides are generally classified into O3-type and P2-type based on their crystal structure, and cathode active materials based on the O3-type structure are Na x The cathode active material, which exhibits a composition such as (TM)O2 (2 / 3 < x < 1.2) and is based on a P2-type structure, is Na x It has a composition of (TM)O2(x≤ 2 / 3). Although O3-type layered oxides have a higher energy density than P2-type layered oxide particles, they have disadvantages such as reduced cycle stability due to larger structural changes during the charge-discharge process, making commercial application difficult. Specifically, when O3-type layered oxide cathode active materials are used at high voltage, capacity and lifespan characteristics may deteriorate due to phase changes, and when used at low voltage, there is a problem of low capacity.

[0005] To address the aforementioned problems, active research is being conducted on doping the cathode active material with inert metals or coating the surface of the active material particles with transition metals.

[0006] [Prior Art Literature]

[0007] [Patent Literature]

[0008] (Patent Document 0001) Republic of Korea Published Patent No. 10-2024-0011635

[0009] (Patent Document 0002) Chinese Published Patent No. 117727920

[0010] The objective of the present invention is to improve the problems of degraded capacity and lifespan characteristics due to phase transitions when using layered oxide cathode active materials for sodium secondary batteries at high voltages, and low capacity when using them at low voltages, by simultaneously using a specific doping metal and a coated transition metal. Specifically, the invention aims to improve the phase transition problem of the cathode active material during charging and discharging at high voltages by coating a specific coated transition metal onto composite transition metal particles substituted with a specific doping metal, and to improve electrical conductivity by forming an oxide containing both the doping metal and the coated transition metal in the coating layer.

[0011] In addition, the present invention aims to improve the capacitance characteristics of the coating transition metal oxide within the crystal structure of the coating layer composition by uniformly distributing the doping metal and the coating transition metal within the coating layer, while simultaneously improving the lifespan characteristics through buffering the volume change during charging and discharging by the doping metal oxide.

[0012] Additionally, the present invention aims to improve the problem where, when forming a coating layer on the surface of a positive electrode active material particle substituted with a specific doping metal, the doping metal does not move from within the crystal structure to the surface of the particle or, conversely, moves excessively.

[0013] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising: a composite particle; and a coating layer covering at least a portion of the surface of the composite particle; wherein the composite particle comprises at least one transition metal selected from nickel, iron, and manganese, sodium, and a doping metal, and the coating layer comprises an oxide of the doping metal and the coating transition metal.

[0014] The oxides of the doping metal and the coating transition metal may be compounds represented by the following chemical formula 1.

[0015] [Chemical Formula 1]

[0016] (Ca 3-x ·A x )(Co 4-y ·B y )O9

[0017] In the above chemical formula 1,

[0018] A is Na, and

[0019] B is at least one transition metal element selected from Fe, Ni, and Mn, and

[0020] 0≤x≤1, 0≤y≤1.

[0021] The coating layer may further comprise an oxide (C2) of a coated transition metal and a carbonate (C3) of a doping metal, wherein the oxide (C1) of the doping metal and the oxide of the coated transition metal has a monoclinic crystal structure of the P2 space group, the oxide (C2) of the coated transition metal has a cubic crystal structure of the Fd-3m space group, and the carbonate (C3) of the doping metal has a trigonal crystal structure of the 32 / m space group.

[0022] The above coating layer further comprises an oxide of the coated transition metal and a carbonate of the doping metal, wherein the peak area ratio B / A according to X-ray diffraction analysis may be 0.7 to 1.3.

[0023] (A above is the peak area of ​​the oxide of the doping metal and coating transition metal located at 2θ=37.34±0.5° in X-ray diffraction analysis, and B above is the peak area of ​​the oxide of the coating transition metal located at 2θ=36.82±0.5° in X-ray diffraction analysis)

[0024] The above coating layer further comprises an oxide of the coated transition metal and a carbonate of the doping metal, wherein the peak area ratio C / A according to X-ray diffraction analysis may be 0.5 to 1.0.

[0025] (A above is the peak area of ​​the oxide of the doping metal and coated transition metal located at 2θ=37.34±0.5° in X-ray diffraction analysis, and C above is the peak area of ​​the carbonate of the doping metal located at 2θ=29.40±0.5° in X-ray diffraction analysis)

[0026] The doping metal is calcium and is substituted into the sodium layer of the composite particle, and the coating transition metal may be cobalt and may not substantially diffuse into the interior of the composite particle.

[0027] The doping metal may be provided in the entire body including the center of the composite particle and in the coating layer, having a maximum concentration in the coating layer and substantially the same concentration in the entire body including the center of the composite particle.

[0028] The doping metal may be included in the coating layer at 10 to 30 weight% and in the composite particle at 70 to 90 weight% with respect to the total weight of the doping metal.

[0029] The coating layer may have a thickness of 0.05 to 1.5 μm.

[0030] The above composite particle may be characterized by being represented by the following chemical formula 2.

[0031] [Chemical Formula 2]

[0032] Na a-2x Ca x [(M y TM1-y )]O2

[0033] In the above chemical formula 2,

[0034] TM is at least one selected from Ni, Mn, and Fe, and

[0035] M is at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, and

[0036] 0.80 <a<1.20, 0.001≤x≤0.1, 0≤y≤0.1, 0.9≤1-y≤1이다.

[0037] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, comprising the steps of: mixing a precursor of a composite particle, a doping metal compound, and a sodium compound, and then heat-treating to produce a composite particle; and mixing the produced composite particle with a coating transition metal compound and then heat-treating to form a coating layer, wherein the coating layer comprises oxides of the doping metal and the coating transition metal.

[0038] In the above coating layer formation step, the heat treatment may be performed at 300 to 500°C for 2 to 10 hours.

[0039] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery comprising the positive electrode active material.

[0040] Another embodiment of the present invention provides a sodium secondary battery using the anode.

[0041] According to the present invention, even if doping and coating are applied simultaneously to the positive electrode active material, lifespan characteristics and rate characteristics can be improved simultaneously without capacity degradation, and electrical conductivity can be improved by forming a doping metal-coated transition metal oxide within the coating layer.

[0042] In addition, in the present invention, by controlling the concentration of the coating transition metal to a specific design range, i) the doping metal moves sufficiently from within the crystal structure of the positive active material particle to the surface of the particle to form an oxide coating layer of the doping metal and the coating transition metal, and ii) the doping metal can be substituted to have a maximum concentration in the coating layer of the positive active material particle (composite particle) and substantially the same concentration throughout the entire particle, including the center. Accordingly, the uniform substitution (doping) of the doping metal within the positive active material particle results in an excellent suppression of phase transition during charging and discharging and improved lifespan characteristics, while simultaneously preventing the improvement of electrical conductivity, capacity characteristics, and rate characteristics caused by the coating layer.

[0043] Figure 1a shows the particle surface and cross-sectional SEM-EDS (Scanning electron microscope-energy dispersive x-ray spectroscopy) analysis results for the sodium composite transition metal oxide prepared in Example 1.

[0044] Figure 1b is the result of X-ray diffraction (XRD) analysis of the sodium composite transition metal oxide prepared in Example 1, showing the peak intensity analysis originating from the (003) plane.

[0045] Figures 2 and 3 are the results of SEM-EDS analysis of the surface of the cathode active material prepared in Examples 1 and 3.

[0046] Figure 4 is the cross-sectional EDS Line scan Co analysis result of the positive electrode active material prepared in Example 1.

[0047] The advantages and features of the present invention and the methods for achieving them will become clear by referring to the embodiments described below in detail together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below but may be implemented in various different forms. These embodiments are provided merely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined only by the scope of the claims.

[0048] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) may be used in a meaning that is commonly understood by those skilled in the art to which the present invention pertains. Throughout the specification, when a part is described as "comprising" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components. Furthermore, the singular form includes the plural form unless specifically stated otherwise in the text.

[0049]

[0050] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery. The positive electrode active material comprises: a composite particle; and a coating layer covering at least a portion of the surface of the composite particle; wherein the composite particle comprises at least one transition metal selected from nickel, iron, and manganese, sodium, and a doping metal, and the coating layer comprises oxides of the doping metal and the coating transition metal.

[0051] The above-mentioned coating transition metal may be a metal different from the transition metal of the above-mentioned composite particle.

[0052] In the present invention, different coated transition metals are coated onto composite particles substituted with doping metals to improve the phase transition problem of the positive electrode active material when using high voltage, and electrical conductivity can be improved by forming an oxide containing both the doping metal and the coated transition metal in the coating layer. Specifically, the oxides of the doping metal and the coated transition metal can have high electrical conductivity due to a layered structure in which a coated transition metal oxide layer and a doping metal-coated transition metal oxide layer are alternately stacked within the crystal structure.

[0053] Specifically, among the crystal structures of the coating layer, i) the coated transition metal oxide layer can achieve additional capacity because the transition metal is bonded with oxygen and has sodium activity, and at the same time, ii) the doping metal-coated transition metal oxide layer can also increase cycle stability by buffering the volume change of the sodium active region within the coating layer during charging and discharging, even though the doping metal exhibits sodium inactivity due to the strong bonding force of oxygen.

[0054] As in one embodiment, the oxide containing the doping metal and the coating transition metal may be a compound represented by the following chemical formula 1.

[0055] [Chemical Formula 1]

[0056] (Ca 3-x ·A 2x )(Co 4-y ·B y )O9

[0057] In the above chemical formula 1, A is Na, B is at least one transition metal element selected from Fe, Ni, and Mn, and 0≤x≤1, 0≤y≤1.

[0058] Specifically, compounds represented by the above chemical formula 1 include, for example, Ca3CO4O9 and Ca3(Co 3.7 Mn 0.3 )O9, Ca3(Co 3.7 Fe 0.3 )O9, Ca3(Co 3.7 Ni0.3 )O9, (Ca 2.7 Na 0.6 )Co4O9, (Ca 2.5 Na)Co4O9, (Ca 2.7 Na 0.6 )(Co 3.7 Mn 0.3 )O9, (Ca 2.7 Na 0.6 )(Co 3.7 Fe 0.3 )O9 and (Ca 2.7 Na 0.6 )(Co 3.7 Ni 0.3 It may be at least one selected from )O9, preferably calcium cobalt oxide or Ca3CO4O9.

[0059] The calcium cobalt oxide-based compound represented by Chemical Formula 1 above has activity with sodium, so there is no capacity degradation in conventional sodium composite particles, and when incorporated into a coating layer, it can exhibit stable cycle life characteristics. In addition, it has excellent electronic conductivity and sodium ion conductivity, which improves high-rate performance through enhanced electrical conductivity. Furthermore, during the manufacturing process, exposure to the atmosphere or the doping metal (Ca) that partially replaced Na sites after doping can move to the surface of the cathode and form carbonates (CaCO3) of the doping metal. Since carbonates (CaCO3) of the doping metal generate CO2 gas during battery operation and cause a decrease in battery performance, it is important to suppress the carbonation reaction of the doping metal by inducing a reaction between the coating transition metal (Co) and the doping metal (Ca) on the surface of the cathode active material.

[0060] Specifically, the oxides of the doping metal and the coating transition metal may have a monoclinic crystal structure of the P2 space group. In particular, the crystal structure of Ca3Co4O9 has a structure in which [CoO2] layers and [CoCa2O3] layers are alternately stacked, and due to this layered structure, it can exhibit high electrical conductivity. Transition metal oxides (Co-O system) will achieve high charge / discharge capacity because they have lithium activity, while Ca-O system oxides have lithium inactivity due to strong bonding forces, but can increase cycle stability by buffering volume changes of the active material during charging and discharging.

[0061] As in one embodiment, the coating layer may further include an oxide (C2) of the coated transition metal and a carbonate (C3) of the doping metal.

[0062] Specifically, the oxide of the coating transition metal may have a cubic crystal structure of the Fd-3m space group, and may be, for example, cobalt oxide (Co3O4). The crystal structure is similar to a spinel crystal structure, so that sodium ions can move three-dimensionally as in a spinel crystal structure. Accordingly, compared to a layered structure in which sodium ions can move two-dimensionally in a composite particle, the movement of sodium ions in the surface coating layer is smoother and faster, thereby improving capacity degradation and enhancing high-rate characteristics.

[0063] In addition, the carbonate of the doping metal may have a trigonal crystal structure of the 32 / m space group, and may be, for example, calcium carbonate (CaCO3). As described above, the carbonate of the doping metal (CaCO3) generates CO2 gas during battery operation, which causes a decrease in battery performance; therefore, it is important to suppress the carbonation reaction of the doping metal by inducing a reaction between the coated transition metal (Co) and the doping metal (Ca) on the surface of the positive electrode active material.

[0064] As in one embodiment, the positive electrode active material may have a peak area ratio B / A of 0.7 to 1.3 according to X-ray diffraction analysis. A is the peak area of ​​the oxide of the doping metal and the coated transition metal (Ca3CO4O9) located at 2θ=37.34±0.5° in X-ray diffraction analysis, and B is the peak area of ​​the oxide of the coated transition metal (Co3O4) located at 2θ=36.82±0.5° in X-ray diffraction analysis.

[0065] Specifically, the peak area ratio B / A may be 0.7 to 1.3, 0.8 to 1.2, 0.8 to 1.1, or 0.8 to 1.0. If the peak area ratio B / A deviates from the preferred design range, the presence of an excessive cobalt coating layer may act as a resistance during the insertion and extraction of Na ions.

[0066] As in one embodiment, the positive electrode active material may have a peak area ratio C / A of 0.5 to 1.0 according to X-ray diffraction analysis. A is the peak area of ​​the oxide of the doping metal and the coated transition metal (Ca3CO4O9) located at 2θ=37.34±0.5° in X-ray diffraction analysis, and C is the peak area of ​​the carbonate of the doping metal (CaCO3) located at 2θ=29.40±0.5° in X-ray diffraction analysis.

[0067] Specifically, the peak area ratio C / A may be 0.5 to 1.0, 0.6 to 1.0, 0.7 to 1.0, or 0.8 to 1.0. If the peak area ratio C / A exceeds the upper limit, the carbonate (CaCO3) of the doping metal may generate CO2 gas during battery operation, which may degrade battery performance. Conversely, if the peak area ratio C / A is below the lower limit, the presence of an excessive cobalt coating layer may act as a resistance during the insertion and extraction of Na ions, and a large amount of doping metal (Ca) may migrate to the anode surface, which is undesirable.

[0068] As in one embodiment, the doping metal may be calcium and may be substituted into the sodium layer of the composite particle.

[0069] Specifically, the calcium can be substituted into the sodium layer of the composite particle, and the calcium element can be doped into the sodium layer of the composite particle having an O3 crystal structure. Accordingly, due to the strong bonding force of the Ca-O-based oxide, the O3 crystal structure of the composite particle is stably maintained, and cycle characteristics and rate characteristics can be improved simultaneously.

[0070] As in one embodiment, the coating transition metal may be cobalt and may not substantially diffuse into the interior of the composite particle.

[0071] Specifically, the cobalt may be included in the coating layer and may not substantially diffuse into the composite particle. For example, the content of the coating transition metal (cobalt) within the composite particle may be less than 1 at mol%, 0.8 at mol% or less, 0.5 at mol% or less, or 0.3 at mol% or less among all metals excluding sodium, and may be 0 at mol% or more or 0.1 at mol% or more. Accordingly, it is possible to reduce the proportion of carbonates of the doping metal (calcium) migrating to the surface and to improve electrical conductivity and electrochemical properties through the calcium cobalt oxide coating.

[0072] The doping metal may be provided to the entire composite particle, including the center, and to the coating layer, having a maximum concentration in the coating layer and substantially the same concentration throughout the entire composite particle, including the center. In the present invention, during the process of mixing the composite particle and the coating material to coat the surface of the composite particle, the cobalt concentration of the coating material can be controlled to induce the formation of a coating layer of the doping metal and the coating metal oxide. Specifically, by providing a certain concentration of cobalt, the doping metal, which is uniformly provided within the composite particle precursor, may migrate to the surface via the cobalt provided to the surface of the composite particle (anode active material) precursor to form an oxide of the coating layer.

[0073] Specifically, the doping metal may be included in the coating layer in an amount of 10 to 30 wt%, 10 to 25 wt%, or 15 to 25 wt% with respect to the total weight, and may be included in the composite particle in an amount of 70 to 90 wt%, 75 to 90 wt%, or 75 to 85 wt%. If the weight ratio of the doping metal exceeds the upper limit within the coating layer or is below the lower limit within the composite particle, the substitution amount of the doping metal within the composite particle is low, so phase transition is not suppressed and lifespan characteristics may be degraded. Conversely, if the weight ratio of the doping metal is below the lower limit within the coating layer or exceeds the upper limit within the composite particle, the electrical conductivity of the surface coating layer is low, the initial efficiency (ICE) is reduced, and high-rate characteristics may deteriorate.

[0074] As in one embodiment, the coating layer may have a thickness of 0.05 to 1.5 μm, and for example, the coating layer thickness may be 1.5 μm or less, 1.2 μm or less, or 1 μm or less, and 0.05 μm or more, or 0.1 μm or more. Accordingly, the above-described effect can be further improved.

[0075] The above composite particle may be represented by the following chemical formula 2.

[0076] [Chemical Formula 2]

[0077] Na a-2x Ca x [(M y TM 1-y )]O2

[0078] In the above chemical formula 2, TM may be at least one selected from Ni, Mn, and Fe, and M may be at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu, and 0.80 <a<1.20, 0.001≤x≤0.1, 0≤y≤0.1, 0.9≤1-y≤1일 수 있다.

[0079] The above Na is included in an amount of 0.8 to 1.2 moles per 1 mole of composite particle oxide, so that the composite particle is formed with an O3 crystal structure. When the composite particle is used at low voltage, the capacity is low, requiring improvement of electrochemical characteristics, and when used at high voltage, there is a problem of reduced capacity and lifespan due to phase change. Accordingly, in the present invention, as described above, by coating a calcium-substituted composite particle with a coating transition metal, the lifespan and rate characteristics can be improved without a decrease in capacity.

[0080] The TM in the above composite particle may contain Ni, Fe, and Mn in amounts of 0.1 to 0.9 moles each per 1 mole of composite particle oxide, for example, Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 It can be included in the composition. If iron is included within the composite particles, Fe at high voltage 3+ Ga Fe 4+ The crystal structure can become unstable due to oxidation, but the crystal structure can be stabilized by substituting Ca in the sodium layer as in the present invention.

[0081] Meanwhile, the composite particle may be composed of a plurality of primary particles aggregated into secondary particles. Here, the coating layer may be formed in a manner that covers at least a portion of the primary particles and / or secondary particles.

[0082]

[0083] Another embodiment of the present invention provides a method for manufacturing the positive electrode active material.

[0084] The above manufacturing method comprises the steps of: preparing composite particles by mixing a precursor of composite particles, a doping metal compound, and a sodium compound, and then heat-treating the mixture; and forming a coating layer by mixing the prepared composite particles with a coating transition metal compound and then heat-treating the mixture, wherein the coating layer comprises oxides of the doping metal and the coating transition metal.

[0085] First, in the step of manufacturing the composite particles, the precursor of the composite particles may be a transition metal complex hydroxide precursor and may be manufactured through a co-precipitation process commonly used in the relevant technical field. The transition metal complex hydroxide precursor may include a compound represented by the following chemical formula 3.

[0086] [Chemical Formula 3]

[0087] [M y TM 1-y ](OH)2

[0088] In the above chemical formula 3, TM may be at least one selected from Ni, Mn, and Fe, M may be at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu, and 0≤y≤0.1, 0.9≤1-y≤1.

[0089] The TM in the above transition metal complex hydroxide precursor may contain Ni, Fe, and Mn in amounts of 0.1 to 0.9 moles each per 1 mole of the precursor, for example, Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 It can be included in the composition.

[0090] The above doping metal compound may be a calcium salt, and at least one selected from, for example, calcium chloride, calcium oxide, calcium carbonate, calcium phosphate, calcium carbide, and calcium hydroxide may be used, and preferably, it may be calcium carbonate or calcium hydroxide.

[0091] The above sodium compound may be at least one selected from the group consisting of sodium carbonate (Na2CO3), sodium hydroxide (NaOH), sodium nitrate (NaNO3), sodium acetate (CH3COONa), and sodium oxalate (Na2(COO)2), and preferably may be sodium carbonate (Na2CO3), sodium hydroxide (NaOH), or a combination thereof.

[0092] The above mixture may involve mixing the precursor of the composite particle and the sodium compound in an amount of Na / M (total metal excluding Na) = 0.8 to 1.2 equivalents, 0.8 to 1.1 equivalents, or 0.95 to 1.05 equivalents. When the amount of sodium compound mixed is within the above range, the cathode active material produced may have an O3-type layered crystal structure, thereby having a higher energy density, high atmospheric and moisture safety, and being less sensitive to synthesis conditions (temperature and atmosphere, etc.). Additionally, the battery discharge capacity may be improved within the above sodium content range, and unreacted residual Na may be minimized.

[0093] In addition, the above mixture may be a mixture of the precursor of the composite particle and the doping metal compound at a ratio of Ca / M (total metal excluding Ca) = 0.5 to 5 mol% or 0.5 to 2.5 mol%. If the amount of Ca mixed exceeds the upper limit, the ratio of Ca(OH)2 or CaCO3 compounds increases compared to the ratio of compounds reacting with Co in the coating layer, which may cause an increase in resistance, a decrease in capacity, etc. Conversely, if too little is added, the formation of Co-Ca-O compounds may be insufficient, making it difficult to improve electrochemical performance.

[0094] The above heat treatment can be performed at a temperature of 700°C to 1,100°C. When the calcination temperature is within the above range, sufficient reaction between raw materials can occur, and the doping metal can be uniformly substituted throughout the transition metal composite particle, including the center. The above heat treatment may preferably be performed at a temperature of 750 to 1,050°C, 800 to 1,000°C, or 850 to 950°C. The calcination may be performed for 5 to 40 hours, 5 to 20 hours, 5 to 18 hours, 8 to 15 hours, or 10 to 14 hours. When the calcination time is within the above range, a highly crystalline cathode active material can be obtained, the particle size is suitable, and production efficiency can be improved.

[0095] The manufactured transition metal composite particles may subsequently undergo additional washing, filtration, and drying processes.

[0096] Next, the step of forming a coating layer is performed by mixing the above-mentioned composite particles with a coating transition metal compound and then heat-treating them.

[0097] The above-mentioned coating transition metal compound may include a cobalt precursor, and the cobalt precursor may be Co3O4, Co(OH)2, CoOOH, Co(OCOCH3)2ㆍ4H2O, Co(NO3)2ㆍ6H2O, CoSO4, CoSO4ㆍ7H2O, or a combination thereof, and preferably Co3O4 may be used.

[0098] The above mixing allows for the formation of a coating layer on the surface of the composite particles by controlling the concentration of the coating transition metal compound during the process of mixing the composite particles and the coating transition metal compound and heat treating. Specifically, the coating transition metal compound may be mixed to a ratio of Co / M (where M is the total transition metal excluding sodium of the composite particles) of 0.1 to 5 mol% relative to the total molar amount of the composite particles and the coating transition metal compound, followed by heat treatment, or preferably, the coating transition metal compound may be mixed to a ratio of 0.1 to 4 mol%, 0.3 to 3 mol%, 0.4 to 3 mol%, 0.5 to 3 mol%, or 0.5 to 1 mol%, followed by heat treatment.

[0099] As described above, by providing a coating transition metal compound, a coating transition metal can be provided on the surface of the composite particle, and the doping metal, which is uniformly provided throughout the entire composite particle including the center, can move to the surface by the coating transition metal on the surface, thereby forming the coating layer. On the other hand, if the amount of the coating transition metal compound mixed is less than the design range, the coating layer may not be easily formed, and conversely, if the amount of the mixture exceeds the design range, the doping metal is excessively provided to the surface, resulting in insufficient doping effect, and the doping metal and coating transition metal oxide are not easily formed within the coating layer, and the unreacted coating transition metal compound increases, which may simply mix with the composite particle or appear as an uneven coating on the surface of the composite particle.

[0100] The heat treatment can be performed at 300 to 500°C for 2 to 10 hours, for example, at 350 to 500°C or 350 to 450°C for 2 to 8 hours or 2 to 6 hours. By performing the heat treatment at a temperature within the range, a coating layer comprising oxides of a doping metal and a coating transition metal can be formed on the surface of the composite particle.

[0101] The coating layer covering the surface of the composite particle is characterized by comprising oxides of the doping metal and the coating transition metal, and the oxides of the doping metal and the coating transition metal are the same as those described above.

[0102] The composite particles with the manufactured coating layer formed thereon may subsequently undergo additional washing, filtration, and drying processes.

[0103]

[0104] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery and a sodium secondary battery comprising the positive electrode active material.

[0105] The above-mentioned positive electrode comprises a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, and a positive electrode active material according to one aspect of the present invention is present in the positive electrode active material layer.

[0106] The above positive current collector is not particularly 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 positive current collector may typically have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive active material. Such positive current collectors may be provided in various forms such as films, sheets, foils, nets, porous bodies, foams, nonwoven bodies, etc.

[0107] In addition, the positive active material layer may be a layer comprising a conductive material and a binder together with the positive active material described above.

[0108] Here, the conductive material is used to impart conductivity to the electrode, and can be used without special restrictions as long as it is conductive without causing chemical changes to the positive electrode active material. Non-limiting examples of conductive materials 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, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskies such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. The conductive material may typically be included in an amount of 1% to 30% by weight based on the total weight of the positive electrode active material layer.

[0109] In addition, the binder is a material that serves to improve adhesion between positive active material particles and adhesion between the positive active material and the current collector. Non-limiting examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof. The binder may typically be included in an amount of 1% to 30% by weight based on the total weight of the positive active material layer.

[0110] A positive electrode according to one embodiment of the present invention may be manufactured according to a conventional method for manufacturing a positive electrode for a sodium secondary battery, except for using the positive electrode active material described above. For example, a positive electrode may be manufactured by applying a slurry for forming a positive electrode active material layer, comprising a positive electrode active material and optionally a binder and a conductive material, onto a positive electrode current collector, and then drying and rolling. According to another example, a positive electrode may be manufactured by casting a slurry for forming a positive electrode active material layer onto a separate support, and then laminating a film obtained by peeling off the positive electrode active material layer from the support onto a positive electrode current collector.

[0111] According to another aspect of the present invention, an electrochemical device comprising the anode described above is provided. Herein, the electrochemical device may specifically be a battery, a capacitor, etc., and more specifically, a sodium secondary battery.

[0112] A sodium secondary battery comprises a positive electrode, a negative electrode positioned opposite the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. Additionally, the sodium secondary battery may include a battery container (case) housing an electrode assembly comprising a positive electrode, a negative electrode, and a separator, and a sealing member for sealing the battery container.

[0113] At this time, depending on the shape of the battery container (case), sodium secondary batteries can be classified into can-type sodium secondary batteries in which the electrode assembly is embedded in a metal can and pouch-type sodium secondary batteries in which the electrode assembly is embedded in a pouch made of a sheet such as aluminum laminate.

[0114] In particular, in the case of a pouch-type sodium secondary battery using a cathode containing a cathode active material according to various embodiments of the present invention, there is an advantage in that stability during storage and / or operation is improved and gas generation can be reduced as the possibility of side reactions between the cathode active material and the electrolyte is low.

[0115]

[0116] The present invention will be described in detail below through examples, but these are intended to explain the invention in more detail and the scope of the present invention is not limited by the following examples.

[0117] Examples

[0118] (Example 1)

[0119] Ni 0.33 Fe 0.33 Mn 0.33 Sodium compound Na2CO3 is added to the (OH)2 precursor in an amount of Na / M (M=Ni+Fe+Mn) = 1.0 equivalent, and doping metal compound Ca(OH)2 is added in an amount of Ca / M (M=Ni+Fe+Mn) = 0.02 equivalent (2 mol%), and heat-treated at 900°C in an air atmosphere for 10 hours to produce sodium complex transition metal oxide particle powder (Na 0.98 Ca 0.02 [Ni 0.33 Fe0.33 Mn 0.33 I obtained O2]powder.

[0120] A coating transition metal compound Co3O4 was added to the prepared sodium composite transition metal oxide particles in an amount of Co / M (M=Ni+Fe+Mn) = 0.005 equivalents (0.5 mol%) and dry-mixed using a hand mixer. The prepared mixture was placed in an alumina crucible and calcined at 400°C for 4 hours in an air atmosphere, and then cooled to room temperature to prepare an anode active material with a coating layer formed thereon.

[0121] A cathode slurry was prepared by dispersing 85 wt% of the manufactured cathode active material, 10 wt% of carbon black, and 5 wt% of PVdF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The cathode slurry was uniformly coated onto an aluminum film with a thickness of 15 μm and vacuum dried at 135°C to produce a cathode for a sodium secondary battery.

[0122] A sodium secondary battery (coin cell) was manufactured using a sodium metal plate as the counter electrode for the above anode, a porous glass fiber (thickness: 200 μm) as the separator, and an electrolyte in which NaPF6 is present at a concentration of 1.0 M in a solvent mixed in a volume ratio of 98:2 of propylene carbonate and fluoroethylene carbonate.

[0123] (Examples 2 to 5)

[0124] A positive electrode active material, a positive electrode, and a sodium secondary battery were prepared in the same manner as in Example 1, except that the amount of coating transition metal compound added was as listed in Table 1 below.

[0125] (Comparative Example 1)

[0126] A positive electrode active material, a positive electrode, and a sodium secondary battery were prepared in the same manner as in Example 1, except that Co3O4 was used to replace Ca(OH)2 as the doping metal compound and Ca(OH)2 was used to replace Co3O4 as the coating transition metal compound. Here, the amounts of Ca(OH)2 and Co3O4 used are the same as the amounts of each compound added in Example 1.

[0127] (Comparative Example 2)

[0128] A positive electrode active material, a positive electrode, and a sodium secondary battery were prepared in the same manner as in Example 1, except that no doping metal compound was used and Ca(OH)2 and Co3O4 were simultaneously used as coating transition metal compounds. Here, the amounts of Ca(OH)2 and Co3O4 used were the same as the amounts of each compound added in Example 1.

[0129]

[0130] Experimental Example

[0131] Experimental Example 1: SEM-EDS Analysis and X-ray Diffraction Analysis of Ca-Doped Sodium Complex Transition Metal Oxide Particles

[0132] The particle surface and cross-sectional SEM-EDS analysis results for the sodium composite transition metal oxide prepared in Example 1 are shown in Figure 1a.

[0133] In addition, sodium composite transition metal oxides of Examples 1, 1-1, 1-2 and Ref were prepared by proceeding in the same manner as in Example 1, except that the doping metal compound Ca(OH)2 was used at Ca / M(M=Ni+Fe+Mn) = 2 mol%, 1.5 mol%, 1 mol%, and 0 mol%. The results of the peak intensity analysis originating from the (003) plane by XRD analysis of the prepared sodium composite transition metal oxides are shown in Fig. 1b.

[0134] As in Figures 1a and 1b, Na + and Ca 2+Since the ionic radius is similar and there is no change in the c-axis length of the (003) plane in XRD analysis after Ca doping, it is confirmed that Ca partially replaces Na sites within the crystal lattice inside the sodium complex transition metal oxide. In addition, it is analyzed that it forms a CaCO3 composition on the surface of the sodium complex transition metal oxide.

[0135]

[0136] Experimental Example 2: SEM-EDS Analysis of Anode Active Material

[0137] Figures 2 and 3 are the results of SEM-EDS analysis of the surface of the cathode active material prepared in Examples 1 and 3.

[0138] As shown in Figures 2 and 3, a coating layer is formed covering the surface of the manufactured composite particle cathode active material, and it was confirmed that the coating layer contains Co and Ca.

[0139] Specifically, it was confirmed that even when the amount of Co coating was increased from 0.5 mol% to 2 mol% in Example 3 compared to Example 1, Co was distributed only in the coating layer and did not diffuse into the interior of the cathode active material composite particle, while Ca was uniformly distributed inside and on the surface of the composite particle and at a high concentration in the coating layer.

[0140]

[0141] Experimental Example 3: EDS Line scan (Co, Ca) analysis

[0142] Figure 4 shows the cross-sectional EDS line scan Co analysis results of the cathode active material prepared in Example 1. Specifically, the Co content (wt%) was measured at 30 random points based on a cross-section passing through the center of the cathode active material particles.

[0143] As shown in Fig. 4, it was confirmed that the positive active material particles with the coating layer formed in Example 1 have a particle size of 12.9 μm and a coating layer of about 1 μm thickness is formed on the surface.

[0144] Additionally, cross-sectional EDS line scan Co, Ca analysis was performed on the cathode active materials prepared in Examples 1 to 5 and Comparative Examples 1 to 2, and the results are listed in Table 1 below.

[0145] Manufacturing Method Coating Layer Composition Cathode Active Material Internal Composition Doping Metal / Doping Amount (mol%) Coating Metal / Coating Amount (mol%) Co : Ca (Weight Ratio) Co (mol%) Ca (mol%) Example 1 Ca / 2Co / 0.563 : 3801.7 Example 2 Ca / 2Co / 170 : 3001.57 Example 3 Ca / 2Co / 280 : 2001.50 Example 4 Ca / 2Co / 0.154 : 4601.92 Example 5 Ca / 2Co / 589 : 1101.37 Comparative Example 1 Co / 0.5Ca / 20 : 1000.50 Comparative Example 2-Co / 0.5Ca / 220 : 8000

[0146] Referring to Table 1, it was confirmed that in the positive electrode active material of the example, Co is distributed only in the coating layer and does not diffuse into the interior of the positive electrode active material composite particle, while Ca is uniformly distributed inside and on the surface of the composite particle and at a high concentration in the coating layer.

[0147] As in Comparative Example 1, when Co is doped and Ca is coated, it is analyzed that the reactivity of Co and Ca is low during the second heat treatment step for forming the coating layer, so the Ca-Co-O compound is not synthesized.

[0148] In the case of Comparative Example 2, during the second heat treatment step (coating temperature of 400℃), Ca was not diffused into the cathode active material, and thus it is expected that no Ca doping effect will occur.

[0149] Meanwhile, in the case of Example 4, the amount of Co coating is low, so the Co-Ca coating layer is formed in small quantities, there is no improvement in ion conductivity, and it is predicted that the lifespan and rate characteristics will be degraded.

[0150] In addition, in the case of Example 5, a large amount of Co coating material was used, resulting in the formation of an excessive coating layer. The presence of the excessive coating layer acts as a resistance during the insertion and extraction of Na ions, and a large amount of doping metal (Ca) migrates to the anode surface, reducing the doping effect, so it is predicted that it will be difficult to improve the lifespan and rate characteristics.

[0151]

[0152] Experimental Example 4: X-ray diffraction analysis of the positive electrode active material

[0153] X-ray diffraction (XRD) analysis was performed on the cathode active materials prepared in Examples 1 to 5 and Comparative Examples 1 to 2. The peaks obtained from the XRD analysis were deconvolved to obtain the peak area A(Ca3Co4O9) corresponding to (2θ=37.34±0.5°) of Ca3Co4O9, the peak area A(Co3O4) corresponding to (2θ=36.82±0.5°) of Co3O4, and the peak area A(CaCO3) corresponding to (2θ=29.40±0.5°) of CaCO3. The peak area ratios A(Co3O4) / A(Ca3Co4O9) and A(CaCO3) / A(Ca3Co4O9) were calculated and are shown in Table 3 below.

[0154] Compound space group crystal structure XRD peak diffraction angles Ca3CO4O9P2Monoclinic 37.34 °Co3O4Fd-3mCubic 36.82 °CaCO332 / mTrigonal 29.40 °

[0155] Coating layer composition A(Co3O4) / A(Ca3Co4O9) peak area ratio A(CaCo3) / A(Ca3Co4O9) peak area ratio Example 1 0.9 10.90 Example 2 1.0 80.71 Example 3 0.9 60.52 Example 4 1.0 11.66 Example 5 0.6 60.40 Comparative Example 1--Comparative Example 2--

[0156] Referring to Table 3, in Examples 1 to 5, it was confirmed that as the amount of cobalt coating increased, the reaction between the coated transition metal (Co) and the doping metal (Ca) on the surface of the positive electrode active material was induced, thereby suppressing the synthesis of the CaCo3 phase.

[0157] However, in the case of Example 5, the presence of an excessive cobalt coating layer can act as a resistance during the insertion and extraction of Na ions, and a large amount of doping metal (Ca) may migrate to the anode surface, so it is predicted that it will be disadvantageous to improve lifespan characteristics and rate characteristics.

[0158] Referring to Examples 1 to 3, the A(CaCO3) / A(Ca3CO4O9) peak area ratio is 0.5 to 0.9, showing excellent results, so it is predicted that the lifespan and rate characteristics will be improved at a coating content of Co 0.5 to 5 mol%.

[0159] Meanwhile, in Comparative Examples 1 and 2, the Ca3CO4O9 peak did not occur.

[0160]

[0161] Experimental Example 5: Evaluation of Electrochemical Performance of Sodium Secondary Battery

[0162] For the sodium secondary batteries prepared in Examples 1 to 5 and Comparative Examples 1 to 2, the initial charge capacity, initial discharge capacity, initial reversible efficiency, and rate capability (discharge capacity ratio; rate capability (C-rate)) were measured through charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, with a voltage range of 2.2V to 4.0V and a discharge rate of 0.1C to 2.0C.

[0163] In addition, for the same sodium secondary battery, 50 charge-discharge cycles were performed under conditions of 0.5C / 0.5C within a driving voltage range of 2.2V to 4.0V at 25℃, and the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention rate) was measured, and the measurement results are shown in Table 4 below.

[0164] Initial Capacity Characteristics Lifetime Characteristics (50cy) (%) Rate Characteristics (%) CH (mAh / g) DCH (mAh / g) ICE (%) Example 1 150.6 139.4 92.5 97.1 89.7 Example 2 148.4 138.4 93.3 95.6 87.4 Example 3 147.2 138.5 94.1 95.3 86.5 Example 4 150.2 137.9 91.8 89.1 75.1 Example 5 145.1 138.4 95.4 88.0 76.3 Comparative Example 1 148.3 126.7 85.5 83.2 76.4 Comparative Example 2 147.4 137.4 95.1 87.5 74.8

[0165] As shown in Table 4, it can be confirmed that the lifespan and rate characteristics of a lithium secondary battery containing a positive electrode active material according to an embodiment of the present invention are further improved compared to a comparative example. Accordingly, it was found that using a positive electrode active material substituted with a doping metal and including oxides of the doping metal and the coated transition metal in the coating layer is an effective method for improving lifespan and rate characteristics.

Claims

1. Composite particles; and a coating layer covering at least a portion of the surface of the composite particles; comprising, The above composite particles comprise at least one transition metal selected from nickel, iron, and manganese, sodium, and a doping metal, and The above coating layer comprises an oxide of the doping metal and the coating transition metal, a positive electrode active material for a sodium secondary battery.

2. In Paragraph 1, A positive electrode active material for a sodium secondary battery, wherein the oxides of the above-mentioned doping metal and coating transition metal are compounds represented by the following chemical formula 1: [Chemical Formula 1] (Ca 3-x A x )(Co 4-y ·B y )O9 In the above chemical formula 1, A is Na, and B is at least one transition metal element selected from Fe, Ni, and Mn, and 0≤x≤1, 0≤y≤1.

3. In Paragraph 1, The above coating layer further comprises an oxide (C2) of the coated transition metal and a carbonate (C3) of the doping metal, wherein The oxide (C1) of the above doping metal and coating transition metal has a monoclinic crystal structure of the P2 space group, and The oxide (C2) of the above-mentioned coating transition metal has a cubic crystal structure of the Fd-3m space group, and The carbonate (C3) of the above-mentioned doping metal is a positive electrode active material for a sodium secondary battery having a trigonal crystal structure of the 32 / m space group.

4. In Paragraph 1, The above coating layer further comprises an oxide of the coated transition metal and a carbonate of the doping metal, wherein A positive electrode active material for a sodium secondary battery having a peak area ratio B / A of 0.7 to 1.3 according to X-ray diffraction analysis. (A above is the peak area of ​​the oxide of the doping metal and coating transition metal located at 2θ=37.34±0.5° in X-ray diffraction analysis, and B above is the peak area of ​​the oxide of the coating transition metal located at 2θ=36.82±0.5° in X-ray diffraction analysis) 5. In Paragraph 1, The above coating layer further comprises an oxide of the coated transition metal and a carbonate of the doping metal, wherein A positive electrode active material for a sodium secondary battery having a peak area ratio C / A of 0.5 to 1.0 according to X-ray diffraction analysis. (A above is the peak area of ​​the oxide of the doping metal and coated transition metal located at 2θ=37.34±0.5° in X-ray diffraction analysis, and C above is the peak area of ​​the carbonate of the doping metal located at 2θ=29.40±0.5° in X-ray diffraction analysis) 6. In Paragraph 1, The above doping metal is calcium and is substituted into the sodium layer of the above composite particle, and A positive electrode active material for a sodium secondary battery, wherein the above-mentioned coating transition metal is cobalt and does not substantially diffuse into the interior of the above-mentioned composite particle.

7. In Paragraph 1, The above-mentioned doping metal is, Provided to the entire body including the center of the above composite particle and to the coating layer, having a maximum concentration in the coating layer, A positive electrode active material for a sodium secondary battery having substantially the same concentration throughout, including the center of the above-mentioned composite particle.

8. In Paragraph 1, A positive electrode active material for a sodium secondary battery, wherein the doping metal is included in the coating layer at 10 to 30 weight% and in the composite particle at 70 to 90 weight% based on the total weight of the doping metal.

9. In Paragraph 1, The above coating layer is a positive electrode active material for a sodium secondary battery, having a thickness of 0.05 to 1.5 μm.

10. In Paragraph 1, A positive electrode active material for a sodium secondary battery, characterized in that the above composite particles are represented by the following chemical formula 2: [Chemical Formula 2] And a-2x Ca x [(I y TM 1-y )]O2 In the above chemical formula 2, TM is at least one selected from Ni, Mn, and Fe, and M is at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, and 0.80 <a<1.20, 0.001≤x≤0.1, 0≤y≤0.1, 0.9≤1-y≤1이다.

11. A step of preparing composite particles by mixing a precursor of the composite particles, a doping metal compound, and a sodium compound, and then heat-treating the mixture; and The method includes the step of forming a coating layer by mixing the above-manufactured composite particles with a coating transition metal compound and then heat-treating the mixture. A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the coating layer comprises an oxide of the doping metal and the coating transition metal.

12. In Paragraph 11, A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein, in the step of forming the coating layer, the heat treatment is performed at 300 to 500°C for 2 to 10 hours.

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

14. A sodium secondary battery using a positive electrode according to Paragraph 13.

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