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

Abstract

An embodiment of the present invention provides a cathode active material for a sodium secondary battery, the cathode active material comprising a single-crystal sodium composite transition metal oxide comprising: at least sodium; two or more main elements selected from transition metals; and two or more doping elements selected from transition metals, and the cathode active material satisfying relational expression 1 indicating the uniformity of the doping elements.

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] Rechargeable batteries have been widely used as energy storage devices in various fields of electronic technology. Recently, with the surge in demand for lithium-ion rechargeable batteries, 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 essential for the commercialization of sodium-ion secondary batteries.

[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 typically classified into O3-type and P2-type based on their crystal structure, and cathode active materials based on the O3-type structure include Na x (TM)O2(2 / 3 <x≤1)와 같은 조성을 보이며, P2-type 구조를 기반으로 하는 양극 활물질은 Na x (TM)O2(x≤2 / 3) has a composition.

[0005] P2-type layered oxides possess relatively excellent cycle stability, but their commercial application is difficult due to drawbacks such as relatively degraded capacity characteristics resulting from their low sodium content. O3-type layered oxides have a higher energy density compared to P2-type layered oxide particles, but they suffer from reduced cycle stability due to greater structural changes during the charge-discharge process. Specifically, they exhibit poor air and water stability; this leads to problems such as structural degradation caused by reacting with surrounding H2O and CO2 during storage and processing to form sodium byproducts in the form of Na2CO3 and NaOH on the particle surface.

[0006] As one of the various methods for doping cathode active materials to enhance properties, the introduction of mono- and multi-valence cations as doping elements is being studied. Since the doping elements are positioned within the lattice of the cathode active material, they can provide an effect that improves the physical and electrochemical properties of the cathode active material according to the unique characteristics of each doping element, such as binding energy and oxidation state.

[0007] These doping elements can be selected from a variety of elements and adjusted to optimal concentrations depending on the desired effect. However, since the doping effect can vary depending on various internal and external factors, such as size, diffusivity, and the manufacturing environment of the cathode active material, complex doping combining multiple elements may be more advantageous for improving the characteristics of the cathode active material than single doping involving the substitution of a single element, taking these variables into account. Specifically, complex doping has the advantage of selectively and combinedly providing various effects available for each doping element, such as structural stability, thermal stability, changes in cation mixing, and capacity changes of the cathode active material.

[0008] However, even in the case of such complex doping, the potential effects of the doping elements must be considered, along with the characteristics of the sodium and transition metal elements in the cathode active material into which the doping elements are introduced, and the correlations between the doping elements. If complex doping elements are formulated without such consideration, problems may arise where doping efficiency decreases or, conversely, the characteristics of the cathode active material are degraded. Therefore, there is a high need for technology capable of optimizing the composition and content of doping elements to more efficiently improve the characteristics of the cathode active material.

[0009] [Prior Art Literature]

[0010] [Patent Literature]

[0011] (Prior Art 1: Chinese Registered Patent CN 117457895 B)

[0012] (Prior Art 2: Chinese Published Patent CN 117012948 A)

[0013] The objective of the present invention is to provide a single-crystal positive electrode active material having improved lifespan characteristics and preventing particle breakage during electrode rolling through the uniform arrangement of double-doping elements within the positive electrode active material particles and efficient single crystallization using the doping elements.

[0014] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery comprising a single-crystal sodium composite transition metal oxide comprising at least sodium; two or more major elements selected from transition metals; and two or more doping elements selected from transition metals, satisfying the following relationship 1 which indicates the compositional uniformity of the first doping element.

[0015] [Relation 1] B / A ≤ 0.15

[0016] (In Equation 1, A and B are measured at at least 30 arbitrary points along a cross-section passing through the center of the positive electrode active material, and represent the average value (A) and standard deviation (B) of the content (wt%) of the first doping element for all metals excluding sodium.)

[0017] The above positive active material may satisfy the following relationship 2, which indicates the uniformity of the distribution of the first doping element among a plurality of sodium complex transition metal oxide single crystal particles.

[0018] [Equation 2] D / C ≤ 0.10

[0019] (In Equation 2, C and D are measured among at least 10 randomly selected single-crystal grains and represent the average value (C) and standard deviation (D) of the content (wt%) of the first doping element in the total metal excluding sodium.)

[0020] The above positive active material may contain less than 3 weight percent of the oxide of the first doping element.

[0021] The above sodium composite transition metal oxide single crystal has an O3-type crystal structure, and the transition metal comprises at least one of Ni; Mn; and Fe and Co; and among two or more doping elements selected from the transition metals, the first doping metal may be Cu and the second doping metal may be Zn or Zr.

[0022] The above sodium complex transition metal oxide may include a compound represented by the following chemical formula 1.

[0023] [Chemical Formula 1] Na a [(Ni x Mn y Fe z Cu b M1 c )]O2

[0024] (In Chemical Formula 1, M1 is Zn or Zr, and 0.8 <a<1.2, 0.1≤x≤0.9, 0.1≤y≤0.9, 0.1≤z≤0.9, 0.05≤b≤0.13, 0.005≤c≤0.02, x+y+z+b+c=1이다)

[0025] The above sodium complex transition metal oxide may include a compound represented by Chemical Formula 2.

[0026] [Chemical Formula 2] Na a [(Ni x Mn yFe z Cu b M1 c )]O2

[0027] (In Chemical Formula 2, M1 is Zn, and 0.9 <a<1.1, 0.16≤x≤0.26, 0.27≤y≤0.37, 0.27≤z≤0.37, 0.05≤b≤0.13, 0.005≤c≤0.02, 0.27≤x+b≤0.37, x+y+z+b+c=1이다)

[0028] The above sodium complex transition metal oxide may be characterized in that the ratio of the content of the first doping element to the content of the second doping element is 10 to 25 molar ratios.

[0029] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, comprising: a step of manufacturing a precursor comprising a first doping element and at least two transition metals; and a step of manufacturing a positive electrode active material by mixing the precursor and a sodium compound and heat-treating to uniformly dope the surface and interior of the positive electrode active material particles with the first doping element, wherein the step of manufacturing the precursor or the step of manufacturing the positive electrode active material further involves mixing a compound of a second doping element.

[0030] The above precursor manufacturing step may involve mixing at least two transition metal compounds and a compound of the first doping element, and then manufacturing a transition metal hydroxide precursor through a co-precipitation reaction.

[0031] The above precursor manufacturing step may involve mixing a transition metal hydroxide precursor with a compound of a first doping element and heat-treating it to coat at least a portion of the surface of the transition metal hydroxide precursor with the first doping element.

[0032] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery comprising the positive electrode active material.

[0033] Another embodiment of the present invention provides a sodium secondary battery comprising the anode.

[0034] The positive electrode active material for a sodium secondary battery according to the present invention comprises a sodium composite transition metal oxide in the form of a single crystal, comprising at least sodium; two or more major elements selected from transition metals; and two or more doping elements selected from transition metals. By doing so, the compositional uniformity of the first doping element is excellent, the single crystallization efficiency by the second doping element is improved, and the particle size distribution characteristics of the single crystal can be uniformly displayed, and particle breakage during electrode rolling is prevented and lifespan characteristics are improved.

[0035] Figures 1 and 2 are cross-sectional SEM-EDS (Scanning electron microscope-energy dispersive x-ray spectroscopy) Cu analysis images of the transition metal hydroxide precursors prepared in Comparative Example 1 and Examples 1 and 2 (Figure 1), and cross-sectional SEM-EDS Cu, Zn analysis images of the prepared cathode active material particles (Figure 2).

[0036] Figure 3 is an analysis result showing the average value (A) and standard deviation (B) of the total metal content (atomic mol%) excluding sodium by SEM-EDS line mapping at at least 30 arbitrary points on a cross-section passing through the center of the cathode active material prepared in Example 1.

[0037] Figure 4 shows the results of SEM-EDS cross-sectional analysis of 10 randomly selected single-crystal particles of the cathode active material prepared in Example 1.

[0038] 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.

[0039] 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.

[0040] In this specification, the singular form includes the plural form unless specifically stated otherwise in the text.

[0041] Additionally, when a part such as a layer, film, region, plate, etc. is described in this specification as being “on” or “on” another part, this includes not only cases where it is “immediately on” another part, but also cases where there is another part in between.

[0042] In this specification, "A to B" means "A or more and B or less" unless specifically defined otherwise. Additionally, "A and / or B" means at least one selected from the group consisting of A and B, unless specifically defined otherwise.

[0043]

[0044] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery comprising a sodium composite transition metal oxide in a single crystal form, comprising at least sodium; two or more major elements selected from transition metals; and two or more doping elements selected from transition metals.

[0045] The sodium composite transition metal oxide of the present invention has the effect of improving the stability of the crystal structure by uniformly distributing the first doping element into the crystal lattice structure, thereby improving the problem of active material particle breakage and structural collapse during electrode rolling. In addition, when manufacturing the sodium composite transition metal oxide by sintering, a compound of the second doping element is used as a flux, which is a type of substance that lowers the melting point, to grow the positive active material into a uniform single-crystal shape, while simultaneously providing a double doping effect by substituting the second doping element into the crystal lattice structure.

[0046] The present invention provides a positive electrode active material for a sodium secondary battery that satisfies the following relationship 1, which indicates the compositional uniformity of a first doping element among two or more doping elements.

[0047] [Relationship 1]

[0048] B / A ≤ 0.15

[0049] In the above equation 1, A and B are measured at at least 30 arbitrary points on a cross-section passing through the center of the positive electrode active material, and are the average value (A) and standard deviation (B) of the content (wt%) of the first doping element for all metals excluding sodium.

[0050] Specifically, A and B of Equation 1 may be the average value (A) and standard deviation (B) of the content (wt%) of the first doping element for the entire metal excluding sodium at each point, after selecting at least 30 points in the linear profile of the first doping element for a cross-section passing through the center of the positive electrode active material.

[0051] In addition, the above B / A is a parameter representing the uniformity of distribution of the first doping metal within the lattice structure of a single-crystal sodium composite transition metal oxide crystal, for example, B / A may be 0.15 or less, 0.145 or less, 0.14 or less, 0.135 or less, or 0.13 or less, and although not particularly limited, may be 0.05 or more or 0.10 or more. In the present invention, when the first doping metal is co-precipitated with the main element and the second doping metal, it exists in a partially aggregated form within the lattice structure, which can improve the problem of reduced stability of the crystal structure and air / water stability and the occurrence of particle breakage of the single-crystal active material during electrode rolling.

[0052] Meanwhile, the content of the first doping element or the linear profile analysis can be performed experimentally using energy dispersive X-ray spectroscopy (EDS) equipped in a transmission electron microscope or a scanning electron microscope.

[0053] As in one embodiment, the positive active material may satisfy the following relationship 2, which indicates the uniformity of the distribution of the first doping element among a plurality of sodium complex transition metal oxide single crystal particles.

[0054] [Relationship 2]

[0055] D / C≤ 0.1

[0056] In Equation 2, C and D are measured among at least 10 randomly selected single-crystal grains and represent the average value (C) and standard deviation (D) of the content (wt%) of the first doping element among all metals excluding sodium.

[0057] Specifically, the plurality of single crystal particles may be at least 10, for example, 10 to 300, 10 to 200, or 10 to 100 single crystal particles. The above D / C represents the uniformity of the distribution of the first doping element among the single crystal particles in the entire cathode active material, and calculates the ratio of the standard deviation (D) to the average value (C) of the content (wt%) of the first doping element among the single crystal particles. For example, the above D / C may be 0.1 or less, 0.09 or less, 0.08 or less, or 0.07 or less, and may be 0.01 or more or 0.05 or more, without limitation.

[0058] When the D / C of the above positive active material is within the above range, the distribution of the first doping element among the plurality of single-crystal particles is uniform, and there is an advantage that the mechanical and electrochemical properties of the positive active material containing the plurality of single-crystal particles become more uniform.

[0059] As in one embodiment, the positive electrode active material may contain less than 3 weight% of the oxide of the first doping element. Specifically, if the first doping element is unevenly distributed in the positive electrode active material and partially aggregates, it is easy to synthesize as an oxide side reaction phase. Consequently, there is a disadvantage in that the single crystallization effect of large particle size is poor, structural stability is reduced, and lifespan characteristics deteriorate. For example, the positive electrode active material may contain 2.9 weight% or less, 2.7 weight% or less, or 2.5 weight% or less of the oxide of the first doping element, or 0.5 weight% or more, or 1 weight% or more.

[0060] As in one embodiment, the sodium composite transition metal oxide has a single-crystal shape, where the term "single-crystal shape" refers to a form in which a single single-crystal particle or 1 to 10, 1 to 5, 1 to 3, or 1 to 2 single crystals are aggregated, and it is obvious that some polycrystalline particles may be included in small amounts.

[0061] By having a single crystal form, it has superior crystal structure stability compared to a positive active material having a secondary particle form, which is an aggregate form of primary particles, and prevents the breaking of positive active material particles during the rolling process when manufacturing electrodes, and enables high electrode density.

[0062] As in one embodiment, the sodium composite transition metal oxide single crystal may have an O3-type crystal structure, and the specific compound composition may include at least one of Ni; Mn; and Fe and Co as the main element transition metal, and among two or more doping elements selected from the transition metals, the first doping metal may be Cu, and the second doping metal may be Zn or Zr.

[0063] According to the above sodium composite transition metal oxide composition, it can be seen that an optimal composition has been adopted in a multi-component transition metal composition by substituting the double doping elements Cu and Zn (or Zr) based on the major elements, the transition metals Ni, Fe (or Co), and Mn. Through this multi-component transition metal composition, it is possible to synthesize a cathode active material that is structurally stable, has high capacity, and possesses excellent lifespan characteristics.

[0064] Specifically, the sodium complex transition metal oxide may include a compound represented by the following chemical formula 1.

[0065] [Chemical Formula 1]

[0066] Na a [(Ni x Mn y Fe z Cu b M1 c )]O2

[0067] In Chemical Formula 1, M1 is Zn or Zr, and 0.8 <a<1.2, 0.1≤x≤0.9, 0.1≤y≤0.9, 0.1≤z≤0.9, 0.05≤b≤0.13, 0.001≤c≤0.005, x+y+z+b+c=1이다.

[0068] More specifically, the sodium complex transition metal oxide may include a compound represented by the following chemical formula 2.

[0069] [Chemical Formula 2]

[0070] Na a [(Ni x Mn y Fe z Cu b M1 c )]O2

[0071] In Chemical Formula 2, M1 is Zn, and 0.9 <a<1.1, 0.16≤x≤0.26, 0.27≤y≤0.37, 0.27≤z≤0.37, 0.08≤b≤0.13, 0.005≤c≤0.02, 0.27≤x+b≤0.37, x+y+z+b+c=1이다.

[0072] The molar ratio (Na / M) of sodium (Na) to the total metal (M) excluding sodium is greater than 0.8 and less than 1.2, so that the composite particle can be formed with an O3 crystal structure. In the above chemical formula 1, if the content of Na corresponding to a is 0.8 or less, the capacity may decrease, and if it is 1.2 or more, it becomes more sensitive to atmospheric and moisture stability and synthesis conditions (temperature and atmosphere, etc.), and structural stability and moisture stability may deteriorate.

[0073] Specifically, the sodium composite transition metal oxide has an O3 crystal structure in X-ray diffraction (XRD) analysis, and the full width at half maximum (FWHM) of the (003) peak located at a diffraction angle 2θ = 15 to 17.5° may be 0.1699 to 0.2599°. A lower FWHM (003) indicates higher O3 crystallinity, and has the advantage of improved electrochemical performance due to increased structural stability after double doping with Cu, Zn (or Zr).

[0074] The transition metals in the above composite particles may include Ni, Fe, and Mn in amounts of 0.1 to 0.9 moles each for every mole of sodium composite transition metal oxide, for example, Ni may be included in a molar ratio of 0.16≤x≤0.26, 0.18≤x≤0.24, or 0.20≤x≤0.24, and Fe and Mn may be included in molar ratios of 0.27≤y, z≤0.37, 0.30≤y, z≤0.35, and 0.31≤y, z≤0.35, respectively.

[0075] The first and second doping elements in the above composite particles may include Cu in a molar ratio of 0.05≤b≤0.13, 0.07≤b≤0.13, or 0.07≤b≤0.11 per mole of sodium composite transition metal oxide, and also include Zn (or Zr) in a molar ratio of 0.005≤c≤0.02, 0.005≤c≤0.015, or 0.005≤c≤0.01.

[0076] As the doping amount of the first doping element (Cu) increases, the effect of large-diameter single crystallization tends to increase, but the uniformity of Cu distribution decreases, leading to the synthesis of side phases such as CuO, which is undesirable in terms of capacity reduction due to Ni substitution; therefore, it is desirable to apply the Cu doping amount within the numerical range mentioned above. In addition, since the same level of single crystallization appears regardless of the doping amount in the range of 0.5 to 2 mol% for the second doping element (Zn), it is desirable to apply the Zn doping amount as 0.5 to 1 mol% in terms of capacity.

[0077] In addition, the ratio (Cu / Zn) of the doping amount (content) of the first doping element to the doping amount (content) of the second doping element may be 10 to 25, 12 to 25, or 14 to 22. If the Cu / Zn doping amount ratio (content ratio) exceeds the above design range, the uniformity of Cu distribution decreases, which may increase the synthesis of side reaction phases such as CuO, and conversely, if it is below the above design range, the efficiency of large particle size single crystallization may decrease.

[0078]

[0079] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery.

[0080] The above manufacturing method comprises: a precursor manufacturing step comprising a first doping element and at least two transition metals; and a positive electrode active material manufacturing step comprising mixing the precursor and a sodium compound and heat-treating to uniformly dope the surface and interior of positive electrode active material particles with the first doping element; wherein the precursor manufacturing step or the positive electrode active material manufacturing step is characterized by further mixing a compound of a second doping element.

[0081] In the present invention, by performing a specific process for double doping with a first doping element (Cu) and a second doping element (Zn or Zr), the structural stability of the manufactured single-crystal cathode active material is improved, thereby providing an O3-type cathode active material with improved particle strength resulting from enhanced mechanical properties and excellent lifespan characteristics. Furthermore, by simultaneously co-precipitating the first doping element at the center and surface of the precursor or coating the precursor surface during the sodium insertion and separation stages, the first doping element, which exhibits an aggregation and non-uniform distribution during double doping, can be uniformly diffused into the crystal structure of the sodium composite transition metal oxide particles. On the other hand, if the compounds of the first and second doping elements are mixed and heat-treated all at once during the doping process or the multi-component transition metal hydroxide precursor co-precipitation without first proceeding with the first doping element co-precipitation or coating process, or if the first or second doping element is single-substituted, or conversely, if the second doping element compound is mixed and heat-treated during the coating process and the second doping element compound is mixed and heat-treated during the doping process, it may be undesirable as doping uniformity is reduced or single crystallization does not proceed.

[0082] As in one embodiment, the precursor preparation step may be to prepare a transition metal hydroxide precursor by mixing at least two transition metal compounds and a compound of the first doping element and then performing a co-precipitation reaction.

[0083] Specifically, the precursor manufacturing step may involve mixing at least two transition metal compounds and a compound of the first doping element, and then manufacturing a transition metal hydroxide precursor through a co-precipitation reaction.

[0084] Alternatively, the precursor manufacturing step may involve mixing a transition metal hydroxide precursor with a compound of a first doping element and heat-treating it to coat at least a portion of the surface of the transition metal hydroxide precursor with the first doping element.

[0085] The above mixture may be a mixture of at least two transition metal compounds or a complex transition metal hydroxide and a first doping element (copper) compound, wherein the first doping element is mixed in an amount of 5 to 13 mol% of the total metal 100 mol%, for example, 7 to 13 mol% or 7 to 11 mol%. If the amount of the first doping element compound mixed is less than the lower limit, the improvement in structural stability and lifespan characteristics due to the introduction of the first doping element is insufficient, and conversely, if it exceeds the upper limit, physical aggregation of the first doping element may occur, making it difficult to coat uniformly and allowing impurity phases to be synthesized.

[0086] The above heat treatment can be performed at a temperature of 500 to 700°C for 4 to 8 hours. When the heat treatment temperature and time are within the above range, the first doping element can be uniformly coated over the entire surface of the precursor particles, and the reactivity between the transition metal and the first doping element can be increased. In addition, the size of the primary precursor particles can be increased, the pore ratio within the secondary particles and the specific surface area of ​​the primary particles can be reduced, and a highly crystalline precursor can be synthesized.

[0087] Specifically, the heat treatment can be performed at a temperature of 500 to 700°C, 550 to 650°C, or 550 to 600°C for 4 to 8 hours, 4.5 to 7.5 hours, or 5 to 7 hours, and the above-described effect can be further improved.

[0088] In addition, the first doping element compound may be an acetate compound of the first doping element, a sulfur oxide, a nitrogen oxide, a phosphoric acid, an oxide, an oxyhydroxide, a hydroxide, or a combination thereof. The first doping element may be copper (Cu).

[0089] In addition, the complex transition metal hydroxide precursor may include at least nickel, manganese and M1 (Co and / or Fe), and may include a compound represented by the following chemical formula 3.

[0090] [Chemical Formula 3]

[0091] [(Ni x Mn y M1 z )](OH)2

[0092] In the above chemical formula 3, M1 is Co or Fe, and 0.1≤x≤0.9, 0.1≤y≤0.9, 0.1≤z≤0.9, x+y+z=1.

[0093] Specifically, the complex transition metal hydroxide precursor may include a compound represented by the following chemical formula 4.

[0094] [Chemical Formula 4]

[0095] [(Ni x Mn y M1 z )](OH)2

[0096] In the above chemical formula 4, M1 is Fe, and 0.27≤x≤0.37, 0.27≤y≤0.37, 0.27≤z≤0.37, x+y+z=1.

[0097] The above step of manufacturing the positive electrode active material is intended to prepare a sodium complex transition metal oxide by diffusing a first doping element, which is co-precipitated or coated on the surface and interior of the precursor particles, into the interior of the particles, uniformly doping a second doping element on the surface and interior of the oxide particles, substituting the first and second doping elements into the crystal structure of the sodium complex transition metal oxide, and inserting sodium. This is achieved by mixing the transition metal hydroxide precursor coated or co-precipitated with the first doping element (copper), the second doping element compound, and the sodium compound, and then proceeding with heat treatment.

[0098] As in one embodiment, the mixture may be a transition metal hydroxide precursor coated with the first doping element and a second doping element compound such that the second doping element is 0.5 to 2 mol% of the total metal 100 mol%, for example, 0.5 to 1.5 mol% or 0.5 to 1 mol%.

[0099] In addition, the sodium compound can be mixed in an equivalent amount of Na / M (all metals excluding Na and Ca) = greater than 0.8 and less than 1.2, for example, in an equivalent amount of 0.9 to 1.1.

[0100] As in one embodiment, the above mixing can have a uniform composition of first and second doping elements and sodium on at least a portion of the surface of the precursor particles, preferably the entire surface, by applying a dry method. On the other hand, when mixing by a wet method, the selection of the second doping element compound and the sodium compound is limited, and it may not be desirable due to the problem of increased costs caused by process complexity, but the present invention is not limited thereto.

[0101] As in one embodiment, the second doping element compound may be an acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof of the second doping element. The second doping element may be zinc (Zn) or zirconium (Zr), and preferably zinc (Zn).

[0102] In addition, the sodium compound may be at least one selected from the group consisting of Na2CO3, NaOH, NaNO3, CH3COONa, and Na2(COO)2, and preferably may be Na2CO3, NaOH, or a combination thereof.

[0103] As in one embodiment, the heat treatment can be performed at a temperature of 800 to 1,000°C for 8 to 12 hours. When the heat treatment temperature and time are within the above range, the first doping element is uniformly diffused into the oxide particles, the first and second doping elements are uniformly distributed among a plurality of single crystal particles, sufficient reaction between raw materials can occur, and the single crystal particles can grow uniformly.

[0104] Specifically, the heat treatment can be performed at a temperature of 800 to 1,000°C, 850 to 950°C, or 900 to 950°C for 8 to 12 hours, 8.5 to 11.5 hours, or 9 to 11 hours, and the above-described effect can be further improved.

[0105] Subsequently, the process may further include a water washing process for the cathode active material produced in the above doping process. The water washing process is a process for removing unreacted materials, impurities, and residual sodium. The cathode active material produced in the above calcination process is introduced into a reactor into which one or more selected from deionized water, distilled water, and ethanol are introduced, and the material may be washed for 0.5 to 5 hours, 0.5 to 4 hours, 0.5 to 3 hours, or 0.5 to 1.5 hours at a temperature of 1 to 80°C or 5 to 50°C at a stirring speed of 200 to 500 rpm, 200 to 400 rpm, or 300 to 400 rpm, but the present invention is not limited thereto.

[0106] In addition, a drying process may be performed to remove moisture from the positive electrode active material containing moisture through a washing process, and the drying process may be performed under vacuum conditions at a temperature of 100 to 300°C for 12 hours or more, but the present invention is not limited thereto.

[0107]

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119]

[0120] 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.

[0121] Examples

[0122] (Example 1)

[0123] Ni 0.33 Fe 0.33 Mn 0.33 An aqueous solution of the first doping element compound (CuSO4) corresponding to 11 mol% (Cu / M (M=Ni+Fe+Mn)) and an aqueous solution of the second doping element compound (ZnSO4) corresponding to 0.5 mol% (Zn / M) are added to a slurry of (OH)2 precursor and DIW in a mass ratio of 1:1 over a period of 1 hour. At this time, a 25 wt% aqueous solution of NaOH is added together to maintain the slurry pH at 10, thereby Ni 0.22 Fe 0.33 Mn 0.33 Cu 0.11 Zn 0.005 (OH)2 was produced.

[0124] Ni 0.22 Fe 0.33 Mn 0.33 Cu 0.11 Zn 0.005 (OH)2 precursor and Na2CO3 were added in an amount of Na / M (M=Ni+Fe+Mn) = 1.0 equivalent, and heat-treated at 900°C in an air atmosphere for 10 hours to obtain the cathode active material powder Na[(Ni 0.22 Fe 0.33 Mn 0.33 Cu 0.11 Zn 0.005 )]O2 was manufactured.

[0125] 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.

[0126] 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.

[0127] (Example 2)

[0128] Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and CuSO4 were mixed using a hand mixer in an amount of 0.11 equivalents of Cu / M (M=Ni+Fe+Mn), and heat-treated at 600°C in an air atmosphere for 6 hours to coat the surface of the precursor with Cu.

[0129] Cu-coated Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 precursor and Na2CO3 were added in an amount of Na / M (M = Ni + Fe + Mn) = 1.0 equivalent, Zn(OH)2 was added in an amount of Zn / M (M = Ni + Fe + Mn) = 0.005 equivalent, and heat-treated at 900°C in an air atmosphere for 10 hours to obtain the cathode active material powder Na[(Ni 0.22 Fe 0.33 Mn 0.33 Cu 0.11 Zn 0.005 )]O2 was manufactured.

[0130] Next, a positive electrode and a sodium secondary battery were manufactured using the manufactured positive electrode active material and proceeded in the same manner as in Example 1.

[0131] (Examples 3 to 6)

[0132] 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 composition of the first and second doping element compounds was changed as shown in Table 2 below.

[0133] (Comparative Example 1)

[0134] Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and CuSO4 were mixed using a hand mixer in an amount of 0.11 equivalents of Cu / M (M=Ni+Fe+Mn), 1.0 equivalent of Na2CO3 (Na / M (M=Ni+Fe+Mn)), and 0.005 equivalents of Zn(OH)2 (Zn / M (M=Ni+Fe+Mn)). The mixture was then heat-treated at 900°C in an air atmosphere for 10 hours to produce the cathode active material powder Na[(Ni 0.22 Fe 0.33 Mn 0.33 Cu 0.11 Zn 0.005 )]O2 was manufactured.

[0135] Next, a positive electrode and a sodium secondary battery were manufactured using the manufactured positive electrode active material and proceeded in the same manner as in Example 1.

[0136] (Comparative Example 2)

[0137] Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and Zn(OH)2 were mixed using a hand mixer in an amount of Zn / M (M=Ni+Fe+Mn) = 0.005 equivalents, and heat-treated at 600°C in an air atmosphere for 6 hours to coat the surface of the precursor with Zn, and Zn-coated Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 precursor and Na2CO3 were added in an amount of Na / M (M=Ni+Fe+Mn) = 1.0 equivalent, and CuSO4 was added in an amount of Cu / M (M=Ni+Fe+Mn) 0.11 equivalent, and heat-treated at 900°C in an air atmosphere for 10 hours to obtain the cathode active material powder Na[(Ni 0.22 Fe 0.33 Mn 0.33 Cu 0.11 Zn 0.005 Except for manufacturing O2, the process was carried out in the same manner as in Example 1 to manufacture a positive active material, a positive electrode, and a sodium secondary battery.

[0138] (Comparative Example 3)

[0139] The procedure was carried out in the same manner as Example 1, except that the coating process was not performed and CuSO4 and Zn(OH)2 were not used, and the cathode active material powder Na[(Ni 0.33 Fe 0.33 Mn 0.33 )]O2, anode, and sodium secondary batteries were manufactured.

[0140]

[0141] Experimental Example

[0142] Experimental Example 1: Confirmation of uniformity of Cu distribution in anode active material by transition metal precursor surface Cu coating (immersion) method

[0143] Figures 1 and 2 are cross-sectional SEM-EDS Cu analysis images of transition metal precursors prepared in the hydroxide precursor preparation steps of Comparative Example 1 and Examples 1 to 2 (Figure 1), and cross-sectional SEM-EDS Cu, Zn analysis images of prepared cathode active material particles (Figure 2).

[0144] Referring to Fig. 1, when a precursor is prepared such that the distribution of Cu in the transition metal precursor is simultaneous presence at the center / surface and presence at the surface as in Examples 1 and 2, it was confirmed that Cu diffuses into the interior and is uniformly distributed during the preparation of the cathode active material.

[0145] On the other hand, it is analyzed that if simultaneous doping of Cu and Zn is performed when manufacturing a positive electrode active material as in Comparative Example 1, Cu will aggregate, resulting in reduced structural stability and deterioration of lifespan characteristics.

[0146]

[0147] Experimental Example 2: Analysis of the uniformity of distribution of the doping element Cu within the crystal structure of the positive active material

[0148] The average value (A) and standard deviation (B) of the total metal content (atomic mol%) excluding sodium were calculated by SEM-EDS line-mapping at at least 30 arbitrary points on a cross-section passing through the center of the cathode active material prepared in Example 1, and are shown in Fig. 3.

[0149] As shown in Figure 3, the average value of Cu concentration in the cross-sectional line passing through the center of the particle was calculated to be 10.8 atomic mol%, the standard deviation was 1.2, and the ratio of the standard deviation (B) to the average value (A) (B / A) was calculated to be 0.110.

[0150] After performing the same SEM-EDS line-mapping analysis as above on 10 randomly selected cathode active material particles, the average of the 10 average values ​​and the average of the 10 standard deviations were calculated and are shown in Table 1 below.

[0151] NiCuFeMn Average Value (A) (at mol%) 23.9 11.1 32.2 33.6 Standard Deviation (B) 1.5 1.4 0.4 1.6 B / A 0.0 6 30.1 26 0.0 1 20.0 48

[0152] Additionally, the average value (E) and standard deviation (F) of the Cu content (atomic mol%) for the cathode active materials prepared in Examples 3 to 6 and Comparative Examples 1 to 2 were summarized in Table 2 below through the same SEM-EDS line-mapping analysis as above.

[0153] Amount of 1st and 2nd doping elements (molar ratio) (Cu or Zn) / (M=Ni+Fe+Mn) Inside a single single crystal grain "Cu content (atomic mol%) Distribution uniformity 1st doping element Cu(OH)2 2nd doping element Zn(OH)2 Average value (A) Standard deviation (B) B / A Example 3 0.03 0.005 2.8 0.4 0.143 Example 4 0.07 0.005 7.2 0.9 0.125 Example 10.11 0.005 11.11 40.126 Example 50.11 0.01 10.9 1.5 0.138 Example 60.11 0.015 11.11 50.135 Comparative Example 10.11 0.005 8.64 90.569 Comparative Example 20.110.0057.54.10.547

[0154] Referring to Table 2 and Figure 3, Example 1 showed the best B / A of 0.126, confirming that Cu was uniformly distributed within the single-crystal particles of the single-crystal cathode active material. On the other hand, Comparative Examples 1 and 2 showed a non-uniform distribution with severe aggregation of Cu.

[0155] In Examples 1 and 3 to 6, the degree of large-diameter single crystallization increased as the Cu doping amount increased; however, it is analyzed that when the ratio of Cu doping amount to Zn doping amount decreases excessively (Examples 3, 5, and 6), B / A also increases, and the uniformity of Cu distribution deteriorates. In addition, since single crystallization proceeds at the same level regardless of the content in the range of 0.5 to 2.0 mol% for Zn doping amount, it is analyzed that a Zn doping amount of 0.5 mol% is desirable in terms of capacity.

[0156] In summary, the optimal doping composition was experimentally confirmed at 7–11 mol% Cu and 0.5 mol% Zn.

[0157] In addition, to confirm the uniformity of the distribution of the first doping element (Cu) among multiple single crystal particles, the average value (E) and standard deviation (F) of the Cu content (atomic mol%) among 10 randomly selected single crystal particles were calculated through cross-sectional EDS line-mapping analysis, and the results are summarized in Table 3 below.

[0158] The above analysis was performed by analyzing the Cu content corresponding to the area marked with a circle in the cross-sectional SEM image of each single crystal grain as shown in Figure 4 using EDS line-mapping.

[0159] Amount of 1st and 2nd doping elements (molar ratio) (Cu or Zn) / (M=Ni+Fe+Mn)” between 10 random single-crystal grains” Uniformity of Cu content (atomic mol%) distribution 1st doping element Cu(OH)2 2nd doping element Zn(OH)2 Average value (C) Standard deviation (D) D / C Example 3 0.0 30.0 53.2 0.2 10.0 66 Example 4 0.0 70.0 56.8 0.5 0.0 74 Example 10.1 10.0 511.5 0.8 0.0 70 Example 50.1 10.0 111.2 1.0 0.0 89 Example 60.1 10.0 1511.3 0.9 0.0 80 Comparative Example 10.110.00510.92.90.266Comparative Example 20.110.00511.13.60.324

[0160] Referring to Table 3, it was confirmed that in Examples 1, 3, and 4, the D / C ratio was excellent at 0.066–0.074 and Cu was uniformly distributed among multiple single-crystal grains. On the other hand, in Comparative Examples 1 and 2, Cu was severely agglomerated and showed a non-uniform distribution.

[0161] Meanwhile, as the amount of Zn doping increased, the doping uniformity of Cu tended to decrease slightly.

[0162]

[0163] Experimental Example 3: Confirmation of CuO impurity phase formation

[0164] The content of the CuO impurity phase of the cathode active materials prepared in Examples 1, 3 to 6 and Comparative Examples 1 to 2 was analyzed by X-ray diffraction (XRD) and Rietveld refinement (XRD = Rigaku D / MAX-2500 / PC, 40 kV, 15 mA, 4° / min, Cu-Kα radiation, λ = 0.1540 nm).

[0165] Specifically, the average value of the CuO content (wt%) of 10 randomly selected single crystal particles among the cathode active materials prepared in each example and comparative example was calculated and is shown in Table 4 below.

[0166] Amount of CuO phase synthesized (wt%) (average of 10 arbitrary single-crystal grains) Example 12.4 Example 32.2 Example 42.8 Example 52.3 Example 62.3 Comparative Example 15.2 Comparative Example 24.8

[0167] Referring to Table 4, it was confirmed that the synthesis of the CuO impurity phase was suppressed to less than 3% in the examples. On the other hand, in Comparative Examples 1 and 2, it was analyzed that the CuO phase was synthesized inside and on the surface of the aggregated Cu particles.

[0168]

[0169] Experimental Example 4: Analysis of Cathode Active Material Particle Breakage During Anode Rolling

[0170] Figure 5 is a cross-sectional SEM image of the anode prepared after rolling in Example 1 and Comparative Example 3.

[0171] Referring to Fig. 5, in Comparative Example 3, it was observed that in the case of a polycrystalline cathode active material that was not doped with Cu or Zn, grain breakage occurred during electrode rolling.

[0172] It was confirmed that the strength of the single crystal particles of the positive active material was improved in Example 1, where no particle breakage occurred.

[0173]

[0174] Experimental Example 5: Performance Evaluation of Sodium Secondary Battery

[0175] For the sodium secondary batteries prepared in Examples 1, 3 to 6 and Comparative Examples 1 to 3, the 1st charge capacity, 1st discharge capacity, and 1st reversible efficiency were measured through charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) under conditions of 25°C, a voltage range of 2.2V to 4.0V, and 0.1C / 0.1C.

[0176] In addition, for the same sodium secondary battery, 50 charge-discharge cycles were performed under 1C / 1C conditions 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 discharge capacity (cycle capacity retention rate) was measured, and the results are shown in Table 5 below.

[0177] Initial Charge Capacity (mAh / g) Initial Discharge Capacity (mAh / g) Lifetime Characteristics (50cy, %) Example 1 142.1 130.586 Example 3 143.2 134.181 Example 4 144.01 33.884 Example 5 141.9 131.386 Example 6 141.1 130.985 Comparative Example 1 141.2 128.569 Comparative Example 2 140.8 130.172

[0178] Referring to Table 5, it was confirmed that in the case of Examples 1 and 3 to 6, Cu and Zn were uniformly doped on the surface and inside the cathode active material particles, and as a result of efficiently applying large-diameter single crystallization, the initial capacity characteristics and lifespan characteristics were improved compared to Comparative Examples 1 and 2.

Claims

1. A sodium complex transition metal oxide in the form of a single crystal, comprising at least sodium; two or more major elements selected from transition metals; and two or more doping elements selected from transition metals. A positive electrode active material for a sodium secondary battery satisfying the following relationship 1, which indicates the compositional uniformity of the first doping element. [Relationship 1] B / A ≤ 0.15 (In Equation 1, A and B are measured at at least 30 arbitrary points along a cross-section passing through the center of the positive electrode active material, and represent the average value (A) and standard deviation (B) of the content (wt%) of the first doping element for all metals excluding sodium.) 2. In Paragraph 1, The above positive active material is a positive active material for a sodium secondary battery satisfying the following relationship 2, which indicates the uniformity of distribution of a first doping element among a plurality of sodium composite transition metal oxide single crystal particles. [Relationship 2] D / C ≤ 0.10 (In Equation 2, C and D are measured among at least 10 randomly selected single-crystal grains and represent the average value (C) and standard deviation (D) of the content (wt%) of the first doping element in the total metal excluding sodium.) 3. In Paragraph 1, The above positive active material is a positive active material for a sodium secondary battery containing less than 3 weight% of an oxide of a first doping element.

4. In Paragraph 1, The above sodium complex transition metal oxide single crystal has an O3-type crystal structure, and The above transition metal comprises Ni; Mn; and at least one of Fe and Co; and A positive electrode active material for a sodium secondary battery, wherein among two or more doping elements selected from the above transition metals, the first doping metal is Cu and the second doping metal is Zn or Zr.

5. In Paragraph 1, The above sodium complex transition metal oxide is a positive electrode active material for a sodium secondary battery comprising a compound represented by the following chemical formula 1. [Chemical Formula 1] So a [(Ni x Mr y Feb z Cu b M1 c )]O2 (In Chemical Formula 1, M1 is Zn or Zr, and 0.8 <a<1.2, 0.1≤x≤0.9, 0.1≤y≤0.9, 0.1≤z≤0.9, 0.05≤b≤0.13, 0.005≤c≤0.02, x+y+z+b+c=1이다) 6. In Paragraph 1, The above sodium complex transition metal oxide is a positive electrode active material for a sodium secondary battery comprising a compound represented by Chemical Formula 2. [Chemical Formula 2] So a [(Ni x Mr y Feb z Cu b M1 c )]O2 (In Chemical Formula 2, M1 is Zn, and 0.9 <a<1.1, 0.16≤x≤0.26, 0.27≤y≤0.37, 0.27≤z≤0.37, 0.05≤b≤0.13, 0.005≤c≤0.02, 0.27≤x+b≤0.37, x+y+z+b+c=1이다) 7. In Paragraph 1, The above sodium composite transition metal oxide is characterized by a molar ratio of the content of the first doping element to the content of the second doping element of 10 to 25, and is a positive electrode active material for a sodium secondary battery.

8. A step for preparing a precursor comprising a first doping element and at least two transition metals; and A step of manufacturing an anode active material by mixing the above precursor and sodium compound and heat-treating to uniformly dope the surface and interior of the anode active material particles with a first doping element; A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the above-mentioned precursor manufacturing step or positive electrode active material manufacturing step involves further mixing a compound of a second doping element.

9. In Paragraph 8, The above precursor manufacturing step is, A method for manufacturing a positive electrode active material for a sodium secondary battery, comprising mixing at least two transition metal compounds and a compound of the first doping element, and then manufacturing a transition metal hydroxide precursor through a co-precipitation reaction.

10. In Paragraph 8, The above precursor manufacturing step is, A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein a compound of a transition metal hydroxide precursor and a first doping element is mixed and heat-treated to coat at least a portion of the surface of the transition metal hydroxide precursor with the first doping element.

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

12. A sodium secondary battery comprising a positive electrode according to paragraph 11.

Images