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

Abstract

One implementation of the present invention provides a cathode active material for a sodium secondary battery, the cathode active material comprising a sodium composite transition metal oxide that includes at least sodium, nickel, manganese, M1 and a doping metal, wherein M1 is iron or cobalt, the doping metal is composed of calcium and copper, and the cathode active material satisfies relation 1A.

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 The cathode active material, which exhibits a composition such as (TM)O2 (2 / 3 < x ≤ 1) and is based on a P2-type structure, is Na x It has a composition of (TM)O2(x≤ 2 / 3).

[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 substituting a single element, taking these variables into account. Specifically, complex doping has the advantage of selectively and combinedly providing various effects that are 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] In this invention, a composite doping technology capable of optimizing the composition and content of doping elements is developed to improve the structural stability of O3-type cathode active materials and to realize high capacity and excellent lifespan characteristics.

[0010] In this invention, the structural stability of the O3-type cathode active material is improved through double doping to achieve high capacity and excellent lifespan characteristics.

[0011] Furthermore, the objective of the present invention is to address the problem of physical aggregation or the formation of impurity phases such as NiO due to chemical reactions when copper is used as a doping metal, by applying double doping of copper and calcium and a specific manufacturing method to uniformly distribute the doping metal to the surface and interior of the cathode active material particles.

[0012] In addition, the present invention aims to provide a positive electrode active material in which side reactions in the electrolyte are reduced by increasing the average particle size of the primary and secondary particles of the positive electrode active material and decreasing the specific surface area through double doping of calcium and copper.

[0013] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery comprising at least sodium, nickel, manganese, M1, and a doping metal, wherein M1 is iron or cobalt, and the doping metal is composed of calcium and copper, satisfying the following equation 1A.

[0014] [Relation 1A]

[0015] 80 ≤ A1(Cu) / At (%) ≤ 99

[0016] In the above relationship 1A, At is the total cross-sectional area of ​​the sodium composite transition metal oxide particles, and A1(Cu) is the area of ​​the region in which copper is within ±10% of the average content (Cu, average at mol%) for 100 at mol% of all metals excluding sodium and calcium among the total cross-sectional area.

[0017] The above positive active material may satisfy the following relationship 1B.

[0018] [Relationship 1B]

[0019] 80 ≤ B1(Cu) / Bt (%) ≤ 99

[0020] In the above relationship 1B, Bt is the total surface area of ​​the sodium composite transition metal oxide particles, and B1(Cu) is the area of ​​the region in which copper is within ±10% of the average content (Cu, average at mol%) for 100 at mol% of all metals excluding sodium and calcium among the total surface area.

[0021] The above positive active material may satisfy the following relationship 2A.

[0022] [Relationship 2A]

[0023] 80 ≤ A2(Ca) / At (%) ≤ 99

[0024] In the above relationship 2A, At is the total cross-sectional area of ​​the sodium composite transition metal oxide particles, and A2(Ca) is the area of ​​the region within the total cross-sectional area where the calcium content is within ±10% of the average content (Ca, average at mol%) for a total of 100 at mol% of sodium and calcium.

[0025] The above positive active material may satisfy the following relationship 2B.

[0026] [Relationship 2B]

[0027] 80 ≤ B2(Ca) / Bt (%) ≤ 99

[0028] In the above relationship 2B, Bt is the total surface area of ​​the sodium composite transition metal oxide particles, and B2(Ca) is the area of ​​the region within the total surface area where the calcium content is within ±10% of the average content (Ca, average at mol%) for a total of 100 at mol% of sodium and calcium.

[0029] The sodium complex transition metal oxide may contain the doping metal such that the molar ratio of copper to calcium (Cu / Ca) is 1.5 to 4.5.

[0030] The above sodium complex transition metal oxide may contain copper in an amount greater than 3 at mol% and less than 10 at mol% for 100 at mol% of all metals excluding sodium and calcium, and calcium in an amount greater than 0.5 at mol% and less than 3 at mol% for 100 at mol% of the total of sodium and calcium.

[0031] The above sodium complex transition metal oxide may include a compound represented by Chemical Formula 1-1.

[0032] [Chemical Formula 1-1]

[0033] Na a-2v Ca v [(Ni x Mn y M1 z Cu w )]O2

[0034] In the above chemical formula 1-1, M1 is Co or Fe, and 0.8 <a<1.2, 0.005<v<0.03, 0.1≤x≤0.9, 0.1≤y≤0.9, 0.1≤z≤0.9, 0.03<w<0.1, x+y+z+w=1이다.

[0035] The above sodium complex transition metal oxide may include a compound represented by Chemical Formula 1-2.

[0036] [Chemical Formula 1-2]

[0037] Na a-2v Ca v [(Ni x Mn y M1 z Cu w )]O2

[0038] In the above chemical formula 1-2, M1 is Fe, and 0.9 <a<1.1, 0.01<v<0.02, 0.27≤x≤0.37, 0.27≤y≤0.37, 0.27≤z≤0.37, 0.04<w<0.06, x+y+z+w=1이다.

[0039] The above sodium complex transition metal oxide may be such that the calcium is substituted in the sodium layer of the complex transition metal oxide, and the copper is substituted in the transition metal layer of the complex transition metal oxide.

[0040] The above sodium complex transition metal oxide may have an O3 crystal structure in X-ray diffraction analysis, and the full width at half maximum (FWHM) of the (003) peak located at 2θ=15 to 17.5° is 0.1699 to 0.2599, and the NiO peak may not appear substantially.

[0041] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, comprising: a coating process in which a composite transition metal hydroxide precursor comprising at least nickel, manganese, and M1 (Co and / or Fe) is mixed with a copper compound and subjected to a first heat treatment to coat at least a portion of the surface of the composite transition metal hydroxide precursor particles with copper; and a doping process in which the copper-coated transition metal hydroxide precursor, a calcium compound, and a sodium compound are mixed and subjected to a second heat treatment to substitute the surface and interior of the sodium composite transition metal oxide particles with copper and calcium.

[0042] In the above coating process (1) and doping process (2), the ratio of the heat treatment temperature (°C) (T2 / T1) may be 1.2 to 2, and the ratio of the heat treatment time (h) (H2 / H1) may be 1.5 to 2.5.

[0043] In the above coating and doping processes, the copper compound and the calcium compound may be mixed such that the molar ratio of copper to calcium (Cu / Ca) is 1.5 to 4.

[0044] The coating process (1) and doping process (2) may be performed such that the composite transition metal hydroxide precursor and the sodium composite transition metal oxide are each secondary particles formed by aggregating at least one primary particle, and the ratio (D / C) of the average particle size (D50) of the sodium composite transition metal oxide secondary particle (D) to the composite transition metal hydroxide precursor secondary particle (C) is 1.2 to 2.

[0045] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery comprising the positive electrode active material.

[0046] Another embodiment of the present invention provides a sodium secondary battery comprising a positive electrode and a negative electrode.

[0047] In the case of the calcium and copper double-doped O3-type cathode active material of the present invention, structural stability is improved, which has the effect of realizing high capacity and excellent lifespan characteristics.

[0048] In addition, when compared to single doping after double doping, an improvement in lifespan is expected due to an increase in the size of primary particles of the cathode active material, a decrease in the specific surface area of ​​secondary particles of the cathode active material, a decrease in the surface area reacting with the electrolyte, and a decrease in side reactions with the electrolyte.

[0049] In addition, by selecting copper and calcium as double-doping metals and applying a specific manufacturing method to uniformly distribute the double-doping metals on the surface and inside the positive active material particles, the structural stability and moisture stability of the positive active material can be improved, and in particular, it is analyzed that copper is more advantageous in terms of lifespan characteristics as it is uniformly distributed on the surface and inside the particles.

[0050] Figure 1 is the result of surface SEM-EDS (Scanning electron microscopy and energy dispersive X-ray spectroscopy) mapping analysis of the surface copper-coated precursor particles prepared in Example 1.

[0051] Figures 2 and 3 are the results of SEM-EDS (Scanning electron microscopy and energy dispersive X-ray spectroscopy) mapping analysis of the cross-section (Figure 2) and surface (Figure 3) of the positive electrode active material particles prepared in Example 1, Comparative Examples 2 and 4.

[0052] Figures 4 and 5 are SEM images of primary particles of the positive active material prepared in Example 1 and Comparative Examples 1 to 3 (Figure 4) and SEM images of secondary particles of the positive active material (Figure 5).

[0053] Figure 6 shows the XRD analysis results of the cathode active materials prepared in Example 1 and Comparative Examples 1 to 4.

[0054] Figure 7 shows the SEAD (Selected area electron diffraction) analysis results of the cathode active materials prepared in Example 1 and Comparative Example 2.

[0055] 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.

[0056] 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 belongs.

[0057] When a part of a specification is described as "including" a certain component, unless specifically stated otherwise, this means that it does not exclude other components but may include additional components.

[0058] In this specification, the singular form includes the plural form unless specifically stated otherwise in the text.

[0059] 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.

[0060] 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.

[0061]

[0062] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, characterized in that it comprises a sodium complex transition metal oxide comprising at least sodium, nickel, manganese, M1, and a doping metal, wherein M1 is iron or cobalt, and the doping metal is composed of calcium and copper.

[0063] In the present invention, high capacity and excellent lifespan characteristics can be achieved by improving the structural stability of the O3-type cathode active material through calcium and copper double doping.

[0064] The above positive active material is characterized by satisfying the following relationship 1A.

[0065] [Relation 1A]

[0066] 80 ≤ A1(Cu) / At (%) ≤ 99

[0067] In the above relationship 1A, At is the total cross-sectional area of ​​the sodium composite transition metal oxide particles, and A1(Cu) is the area of ​​the region in which copper is within ±10% of the average content (Cu, average at mol%) for 100 at mol% of all metals excluding sodium and calcium among the total cross-sectional area.

[0068] Specifically, At can be calculated as the sum of five non-overlapping circular regions selected from the particle cross-section, each having a diameter equal to 1 / 8 of the particle diameter. A1(Cu) refers to a region within the total selected cross-section area (At) where the copper content is within ±10% of the average content for 100 at mol% of the total metals excluding sodium and calcium. For example, when the average copper content of the cathode active material is 5 at mol% (among the total metals excluding sodium and calcium), it refers to the region where the copper content in the At is 4.5 to 5.5 at mol%. Copper content analysis can be performed experimentally using Energy Dispersive X-ray Spectroscopy (EDS), such as that equipped in a transmission electron microscope or a scanning electron microscope.

[0069] In addition, the above A1(Cu) / At(%) may be 80% or more, 85% or more, 90% or more, or 95% or more and 99% or less, and if A1(Cu) / At(%) is within the design range of Equation 1A, it may mean that Cu diffuses into the cathode active material particles and is uniformly distributed.

[0070] As in one embodiment, the positive active material may satisfy the following relationship 1B.

[0071] [Relationship 1B]

[0072] 80 ≤ B1(Cu) / Bt (%) ≤ 99

[0073] In the above relationship 1B, Bt is the total surface area of ​​the sodium composite transition metal oxide particles, and B1(Cu) is the area of ​​the region in which copper is within ±10% of the average content (Cu, average at mol%) for 100 at mol% of all metals excluding sodium and calcium among the total surface area.

[0074] Specifically, Bt can be calculated as the sum of five randomly selected cross-sectional areas having a diameter equal to 1 / 8 of the particle diameter on the particle surface, without overlapping. B1(Cu) refers to an area within the total selected cross-sectional area (Bt) where the copper content is within ±10% of the average content relative to 100 at mol% of the total metals excluding sodium and calcium. For example, when the average copper content of the cathode active material is 5 at mol% (among the total metals excluding sodium and calcium), it refers to the area of ​​the region within Bt where the copper content is 4.5 to 5.5 at mol%.

[0075] The above B1(Cu) / At(%) may be 80% or more, 85% or more, 90% or more, or 95% or more and 99% or less, and if B1(Cu) / At(%) is within the design range of Equation 1B, it may mean that Cu is uniformly distributed on the surface of the positive active material particles. If the above B1 / Bt ratio is below the lower limit value, there is a problem in that the copper doped on the surface of the positive active material particles physically aggregates in some areas or forms an impurity phase by chemical reaction.

[0076] In the present invention, two heat treatments are performed when manufacturing the cathode active material, wherein i) first, copper is uniformly coated on the surface of the transition metal hydroxide precursor through an initial heat treatment, and ii) subsequently, sodium is inserted (sodiation) and copper can be uniformly diffused into the particle surface and interior through a second heat treatment. In addition, iii) the primary particles can be grown by double doping of copper and calcium, thereby reducing the surface and pores of the primary particles. Due to these particle characteristics, physical aggregation of copper diffusing into the interior / surface can be suppressed and distribution uniformity can be improved.

[0077] According to the present invention, structural stability can be improved through double doping of calcium and copper, and the size of the positive electrode active material particles (primary particles and secondary particles) is increased and the specific surface area in contact with the electrolyte is reduced, thereby suppressing side reactions of the electrolyte.

[0078] As in one embodiment, the positive active material may be characterized by further satisfying the following relationship 2A and / or 2B.

[0079] [Relationship 2A]

[0080] 80 ≤ A2(Ca) / At (%) ≤ 99

[0081] In the above relationship 2A, At is the total cross-sectional area of ​​the sodium composite transition metal oxide particles, and A2 is the area of ​​the region within the total cross-sectional area where calcium is within ±10% of the average molar concentration Ca(x) for a total sum of sodium and calcium of 100 at mol%.

[0082] [Relationship 2B]

[0083] 80 ≤ B2(Ca) / Bt (%) ≤ 99

[0084] In the above relationship 2B, Bt is the total surface area of ​​the sodium composite transition metal oxide particles, and B2 is the area of ​​the region within the total surface area where calcium is within ±10% of the average molar concentration Ca(x) for a total of 100 at mol% of sodium and calcium.

[0085] In the above relationships 2A and 2B, 85 ≤ A2 / At (%) ≤ 99, 90 ≤ A2 / At (%) ≤ 99 or 95 ≤ A2 / At (%) ≤ 99, and 85 ≤ B2 / Bt (%) ≤ 99, 90 ≤ B2 / Bt (%) ≤ 99 or 95 ≤ B2 / Bt (%) ≤ 99.

[0086] When the above A2 / At, B2 / Bt ratio is below the lower limit, there is a problem in that calcium doped throughout the inside and surface of the positive active material particles physically aggregates or forms an impurity phase through chemical reaction.

[0087] Meanwhile, as described above, the area ratios (A1 / At, A2 / At, B1 / Bt, B2 / Bt) of the above relationships 1A, 1B, 2A, and 2B may represent the distribution uniformity (dispersion) of copper and calcium doped inside and on the surface of sodium composite transition metal oxide particles. When copper is substituted alone, it aggregates on the surface and inside the particle at a doping amount exceeding a certain level, exhibiting a non-uniform distribution and forming side reaction phases. When calcium is substituted alone, it tends to be doped relatively uniformly. When copper and calcium are double-doped, there is an effect of improving the distribution uniformity of the two types of dopants.

[0088] As in one embodiment, the sodium composite transition metal oxide may contain the doping metal such that the molar ratio of copper to calcium (Cu / Ca) is 1.5 to 4.5, for example, 1.5 to 4.5, 2 to 4.5, 2.5 to 4, 3 to 4, or 3 to 3.5. If the Cu / Ca molar ratio is excessively low below the lower limit, the effects of improving structural stability and lifespan characteristics due to copper doping may not be sufficiently manifested, or the capacity characteristics may deteriorate due to excessive calcium doping. Conversely, if the Cu / Ca molar ratio exceeds the upper limit, the effect of improving the uniformity of copper distribution by calcium becomes insufficient, making it easy for the doped copper to physically aggregate or form an impurity phase through chemical reactions.

[0089] Specifically, the sodium composite transition metal oxide may contain copper dopants in an amount greater than 3 at mol% and less than 10 at mol% relative to 100 at mol% of the total metals excluding sodium and calcium, for example, 3 to 8 at mol%, 3 to 7 at mol%, 3.5 to 6.5 at mol%, or 4 to 6 at mol%. Below the lower limit of the copper doping amount, there is insufficient improvement in structural stability and lifespan characteristics due to copper doping, while conversely, above the upper limit, problems such as physical aggregation of doped copper and the formation of chemical impurity phases may occur.

[0090] Specifically, the sodium composite transition metal oxide may contain calcium dopants in an amount greater than 0.5 at mol% and less than 3 at mol% relative to the total of 100 at mol% of sodium and calcium, or in an amount of 0.5 to 2.5 at mol%, 0.5 to 2 at mol%, or 1 to 2 at mol%. Below the lower limit of the calcium doping amount, the effect of improving the uniformity of copper distribution by calcium is insufficient, and the effect of improving structural stability by calcium doping may be negligible. Conversely, above the upper limit, excessive calcium doping may lead to a shortage of sodium ions, which may degrade capacity characteristics and rate characteristics.

[0091] As in one example, the sodium complex transition metal oxide may include a compound represented by Chemical Formula 1-1.

[0092] [Chemical Formula 1-1]

[0093] Na a-2v Ca v [(Ni x Mn y M1 z Cu w )]O2

[0094] In the above chemical formula 1-1, M1 is Co or Fe, and 0.8 <a<1.2, 0.005<v<0.03, 0.1≤x≤0.9, 0.1≤y≤0.9, 0.1≤z≤0.9, 0.03<w<0.1, x+y+z+w=1이다.

[0095] 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.

[0096] The transition metals within the composite particles may each contain 0.1 to 0.9 moles of Ni, Fe, Co, or Mn per 1 mole of the 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 may become unstable due to oxidation, but the crystal structure can be stabilized by substituting Ca in the sodium layer and Cu in the transition metal layer as in the present invention.

[0097] The above-mentioned doping metals, calcium and copper, are as described above.

[0098] Specifically, the sodium complex transition metal oxide may include a compound represented by Chemical Formula 1-2.

[0099] [Chemical Formula 1-2]

[0100] Na a-2v Ca v [(Ni x Mn y M1 z Cu w )]O2

[0101] In the above chemical formula 1-2, M1 is Fe, and 0.9 <a<1.1, 0.01<v<0.02, 0.27≤x≤0.37, 0.27≤y≤0.37, 0.27≤z≤0.37, 0.04<w<0.06, x+y+z+w=1이다.

[0102] According to the above sodium composite transition metal oxide composition, by adopting an optimal composition in a multi-component system in which Cu and Ca are substituted based on transition metals Ni, Fe, and Mn, it is possible to synthesize an anode material that is structurally stable and has high capacity and excellent lifespan characteristics.

[0103] As in one embodiment, the sodium complex transition metal oxide may have calcium substituted in the sodium layer of the complex transition metal oxide, and copper substituted in the transition metal layer of the complex transition metal oxide.

[0104] Specifically, the calcium may be substantially entirely substituted into the sodium layer of the composite transition metal oxide, for example, 90 to 100 mol%, 90 to 99 mol%, or 95 to 99 mol% of the total molar amount of calcium may be substituted into the sodium layer. Additionally, the copper may be substantially entirely substituted into the transition metal layer of the composite transition metal oxide, for example, 90 to 100 mol%, 90 to 99 mol%, or 95 to 99 mol% of the total molar amount of copper may be substituted into the transition metal layer.

[0105] In the present invention, i) a double-doping metal is selected such that the ionic radii of calcium (0.99 Å) and sodium (1.02 Å) are similar, and the ionic radii of copper (0.73 Å) and transition metal (Ni 0.78 Å) are similar; ii) copper is first coated on the surface and interior of a transition metal hydroxide precursor at low temperature heat treatment, and then calcium is mixed in during the sodium insertion step to increase the reactivity of copper-transition metal and allow copper to be uniformly diffused into the surface and interior of the precursor particles; and iii) by mixing the calcium compound with the sodium compound during sodium insertion and performing high-temperature heat treatment, calcium can be substituted into the sodium layer competitively with sodium, while copper can be uniformly diffused into the interior.

[0106] As in one embodiment, the positive active material 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 2θ = 15 to 17.5° may be 0.1699 to 0.2599. A lower FWHM (003) indicates higher O3 crystallinity, and electrochemical performance can be improved by increasing structural stability after double doping with Cu and Ca.

[0107] In addition, the NiO peak may not substantially appear in the XRD analysis. The proportion of NiO measured by the Rietveld refinement method based on X-ray diffraction analysis may be 3% or less, 2% or less, or 1% or less, but is not limited thereto. At the same time, the c-lattice length of the cathode active material may be greater than 16.000, greater than 16.001, greater than 16.002, greater than 16.003, or greater than 16.004, and may be 16.010 or less, or 16.07 or less. According to the present invention, by using a Cu-coated NFM precursor to double-dope Cu and Ca, the c-lattice length extension effect due to the introduction of Cu can be achieved, and the problem of side reaction phases such as the NiO phase being synthesized upon Na insertion when Cu is introduced alone can be improved through the introduction of Ca. In addition, as Ca and Cu are doped relatively uniformly across the entire surface / cross-section of the positive active material, the aforementioned effects can be achieved throughout the entire positive active material particle.

[0108]

[0109] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery.

[0110] The above manufacturing method comprises: a coating process in which a complex transition metal hydroxide precursor comprising at least nickel, manganese, and M1 (Co and / or Fe) is mixed with a copper compound and subjected to a first heat treatment to coat at least a portion of the surface of the complex transition metal hydroxide precursor particles with copper; and a doping process in which the copper-coated transition metal hydroxide precursor, a calcium compound, and a sodium compound are mixed and subjected to a second heat treatment to substitute the surface and interior of the sodium complex transition metal oxide particles with copper and calcium.

[0111] In the present invention, structural stability can be improved through a coating process and a doping process involving double doping of calcium and copper, thereby providing an O3-type cathode active material with high capacity and excellent lifespan characteristics. Furthermore, by coating and doping copper and calcium separately in separate stages, copper can be uniformly diffused into the sodium composite transition metal oxide particles. On the other hand, if the copper coating process is omitted and the copper compound and calcium compound are mixed and heat-treated simultaneously in the doping process, or if copper or calcium is doped alone, or conversely, if the calcium compound is mixed and heat-treated in the coating process and the copper compound is mixed and heat-treated in the doping process, doping uniformity may be reduced.

[0112] The above coating process is intended to uniformly coat copper on at least a portion, preferably the entire surface, of the particle surface of the composite transition metal hydroxide precursor, increase the size of the precursor primary particle, and increase the reactivity between the transition metal and copper, by mixing the composite transition metal hydroxide precursor and a copper compound and performing a first heat treatment.

[0113] As in one embodiment, the mixture may be composed of the composite transition metal hydroxide and the copper compound in an amount greater than 3 at mol% and less than 10 at mol% relative to 100 at mol% of the total metal, for example, 3 to 8 at mol%, 3 to 7 at mol%, 3.5 to 6.5 at mol%, or 4 to 6 at mol%. If the amount of copper compound mixed is less than the lower limit, the improvement in structural stability and lifespan characteristics due to copper doping is insufficient, and conversely, if it exceeds the upper limit, physical aggregation of the doped copper may occur, making it difficult to coat uniformly and potentially synthesizing impurity phases.

[0114] As in one embodiment, the above mixture can uniformly dope copper on at least a portion of the surface of the precursor particles, preferably the entire surface, by applying a dry method, whereas doping by a wet method may not be desirable because the selection of doping compounds is limited and there is a problem of increased costs due to process complexity.

[0115] In addition, the copper compound may be a copper acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof.

[0116] In addition, the above-mentioned 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 relationship 2A.

[0117] [Chemical Formula 2-1]

[0118] [(Ni x Mn y M1 z )](OH)2

[0119] In the above chemical formula 2-1, 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.

[0120] Specifically, the complex transition metal hydroxide precursor may include a compound represented by the following relationship 2B.

[0121] [Chemical Formula 2-2]

[0122] [(Ni x Mn y M1 z )](OH)2

[0123] In the above chemical formula 2-2, M1 is Fe, and 0.27≤x≤0.37, 0.27≤y≤0.37, 0.27≤z≤0.37, x+y+z=1.

[0124] As in one embodiment, the first 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, copper can be uniformly coated over the entire surface of the precursor particles, and the reactivity between the transition metal and copper 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.

[0125] Specifically, the first 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.

[0126] The above doping process is intended to prepare a sodium complex transition metal oxide by diffusing copper coated on the surface of precursor particles into the interior of the particles, uniformly doping the double-doping metal onto the surface and interior of the oxide particles, substituting it into the crystal structure of the sodium complex transition metal oxide, and inserting sodium. This is achieved by mixing the copper-coated transition metal hydroxide precursor, the calcium compound, and the sodium compound, and performing a second heat treatment.

[0127] As in one embodiment, the mixture may be composed of the copper-coated transition metal hydroxide precursor and the calcium compound in an amount greater than 0.5 at mol% and less than 3 at mol% relative to the total of 100 at mol% of sodium and calcium, for example, 0.5 to 2.5 at mol%, 0.5 to 2 at mol%, or 1 to 2 at mol%.

[0128] If the calcium mixing amount is below the lower limit, the effect of improving the uniformity of copper distribution by calcium is insufficient, and the effect of improving structural stability by calcium doping may be negligible. Conversely, if it exceeds the upper limit, the sodium ions may be deficient due to excessive calcium doping, which may degrade capacity and rate characteristics.

[0129] 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.

[0130] As in one embodiment, the above mixing can uniformly mix calcium, sodium, and copper on at least a portion, preferably the entire surface, of the precursor particle surface by applying a dry method, whereas mixing by a wet method may not be desirable because the selection of calcium compounds and sodium compounds is limited and there is a problem of increased costs due to process complexity.

[0131] As in one embodiment, the calcium compound may be an acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof.

[0132] 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.

[0133] As in one embodiment, the second heat treatment can be performed at a temperature of 800°C to 1,000°C for 8 to 12 hours. When the temperature and time of the second heat treatment are within the above range, copper can be diffused into the oxide particles, the double-doping metal can be uniformly doped onto the surface and interior of the oxide particles, the sodium can be substituted into the crystal structure of the sodium complex transition metal oxide, and sodium can be inserted to produce a sodium complex transition metal oxide. In addition, sufficient reaction between the raw materials can occur, and the particles can grow uniformly.

[0134] Specifically, the second 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.

[0135] As in one embodiment, the coating process (1) and doping process (2) can be performed such that the ratio of the heat treatment temperature (°C) (T2 / T1) is 1.2 to 2 and the ratio of the heat treatment time (h) (H2 / H1) is 1.5 to 2.5. Specifically, the ratio of the heat treatment temperature (°C) (T2 / T1) can be 1.2 to 2, 1.2 to 1.8, 1.2 to 1.6, or 1.4 to 1.6, and the ratio of the heat treatment time (h) (H2 / H1) can be 1.5 to 2.5, 1.5 to 2.3, 1.5 to 2, or 1.5 to 1.8. Accordingly, by applying different heat treatment temperatures in the coating process (1) and doping process (2), copper and calcium can be evenly diffused into the cathode active material. If the heat treatment temperature of the above coating process (1) is high, there is a possibility that Cu will diffuse into the interior before sodium insertion, making it difficult for Ca and Na to insert sodium into the structure, so a higher temperature heat treatment is required in the doping process (2).

[0136] In addition, in the coating and doping processes described above, the copper compound and the calcium compound may be mixed such that the molar ratio of copper to calcium (Cu / Ca) is 1.5 to 4.5. Specifically, the mixture may be mixed such that the (Cu / Ca) molar ratio is 1.5 to 4.5, 2 to 4.5, 2.5 to 4, 3 to 4, or 3 to 3.5. If the Cu / Ca molar ratio is mixed below a lower limit, the effects of improving structural stability and lifespan characteristics due to copper doping may not be sufficiently manifested, or the capacitance characteristics may deteriorate due to excessive calcium doping. Conversely, if the mixture exceeds an upper limit, the effect of improving the uniformity of copper distribution by calcium becomes insufficient, making it easy for the doped copper to physically aggregate or form impurity phases through chemical reactions.

[0137] The coating process (1) and doping process (2) described above can be performed such that the composite transition metal hydroxide precursor and the sodium composite transition metal oxide are each secondary particles formed by aggregating at least one primary particle, and the ratio (D / C) of the average particle size (D50) of the sodium composite transition metal oxide secondary particle (D) to the composite transition metal hydroxide precursor secondary particle (C) is 1.2 to 2. Specifically, the ratio (D / C) of the average particle size (D50) may be 1.2 to 2, 1.2 to 1.8, or 1.3 to 1.5. Accordingly, the size of the precursor primary particle can be increased, and copper can be uniformly coated over the entire surface of the precursor particle. As a result of increasing the reactivity between the transition metal and copper, copper can be uniformly diffused on the surface and inside the oxide particle. In addition, by reducing the specific surface area in contact with the electrolyte, the side reaction of the electrolyte is suppressed.

[0138] 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, and 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.

[0139] In addition, a drying process may be performed to remove moisture from the positive active material containing moisture after 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.

[0140]

[0141] 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.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] 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.

[0146] 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.

[0147] 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.

[0148] 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.

[0149] 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.

[0150] 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.

[0151] 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.

[0152]

[0153] 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.

[0154] Examples

[0155] (Example 1)

[0156] 1) Coating process

[0157] Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 precursor and copper compound Cu(OH)2 were mixed using a hand mixer in an amount of 0.05 equivalents of Cu / M (M=Ni+Fe+Mn+Cu), and the copper coating was applied to the surface of the precursor by heat treatment at 600°C in an air atmosphere for 6 hours.

[0158] 2) Doping process

[0159] Cu-coated Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 precursor and sodium compound Na2CO3 were added in an amount of Na / M (M=Ni+Fe+Mn) = 1.0 equivalent, and calcium compound Ca(OH)2 was added in an amount of Ca / M (M=Ni+Fe+Mn) = 0.015 equivalent, and heat-treated at 900°C in an air atmosphere for 10 hours to obtain a cathode active material powder (Na 0.97 Ca 0.015 [(Ni 0.29 Fe 0.33 Mn 0.33 Cu 0.05 )]O2 powder) was manufactured.

[0160] 3) Anode manufacturing process

[0161] 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.

[0162] 4) Sodium secondary battery manufacturing process

[0163] 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.

[0164] (Comparative Example 1)

[0165] 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 1) the coating process was not performed and 2) a calcium compound was not added during the doping process.

[0166] (Comparative Example 2)

[0167] 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 1) the coating process was not performed and 2) in the doping process, a copper compound Cu(OH)2 was added in an amount of 5.0 equivalents of Cu / M (M=Ni+Fe+Mn+Cu) instead of a calcium compound.

[0168] (Comparative Example 3)

[0169] 1) 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 coating process was not performed.

[0170] (Comparative Example 4)

[0171] 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 1) the coating process was not performed, and 2) in addition to the addition of calcium and sodium compounds in the doping process, a copper compound Cu(OH)2 was added in an amount of 5.0 equivalents of Cu / M (M=Ni+Fe+Mn+Cu).

[0172]

[0173] Experimental Example

[0174] Experimental Example 1: SEM-EDS (Scanning electron microscopy and energy dispersive X-ray spectroscopy) mapping analysis

[0175] Surface SEM-EDS (Scanning electron microscopy and energy dispersive X-ray spectroscopy) mapping analysis was performed on the surface of the copper-coated precursor particles prepared in Example 1, and the results are shown in Figure 1.

[0176] Referring to Figure 1, it was confirmed that Cu was uniformly distributed on the surface of the precursor.

[0177] Cross-sectional / surface SEM-EDS (Scanning electron microscopy and energy dispersive X-ray spectroscopy) mapping analysis was performed on the cathode active material particles prepared in Example 1, Comparative Examples 2 and 4, and the results are shown in Figures 2 and 3.

[0178] In addition, five non-overlapping circular regions with a diameter approximately 1 / 8 of the particle diameter were randomly selected on the particle surface and cross-section, respectively (refer to the yellow circles in the SEM image of Fig. 2), and the compositions of Cu and Ca were quantified by SEM-EDS analysis to calculate the area ratios of equations 1A, 1B, 2A, and 2B (A1 / At, A2 / At, B1 / Bt, B2 / Bt, %).

[0179] Next, among the total cross-sectional area (At) selected above, the area (A1) of the region where copper is within ±10% of the average content (5±0.5 at mol%) for 100 at mol% of all metals excluding sodium and calcium, and the area (A2) of the region where calcium is within ±10% of the average content (1.5±0.15 at mol%) for 100 at mol% of the total of sodium and calcium were calculated as a percentage (%) according to equations 1A and 2A. The calculation results are shown in Table 1 below.

[0180] In addition, among the total surface area (Bt) selected above, the area (B1) of the region where copper is within ±10% of the average content (5±0.5 at mol%) for 100 at mol% of all metals excluding sodium and calcium, and the area (B2) of the region where calcium is within ±10% of the average content (1.5±0.15 at mol%) for 100 at mol% of the total of sodium and calcium were calculated as a percentage (%) as shown in equations 1B and 2B. The calculation results are shown in Table 1 below.

[0181] Cross-section of positive active material particle surface of positive active material particle Relationship Formula 1 AA1(Cu) / At (%) Relationship Formula 2 AA2(Ca) / At (%) Relationship Formula 1 BB1(Cu) / Bt (%) Relationship Formula 2 BB2(Ca) / Bt (%) Example 1 99999998 Comparative Example 2 84-87- Comparative Example 4 79788575

[0182] Referring to Table 1 and Figures 2 to 3 above, in Example 1, when using an NFM precursor with Cu uniformly coated on the surface, Ca doping and Na insertion, it was confirmed that Ca and Cu diffused into the surface and interior of the positive electrode active material particles and were uniformly distributed.

[0183] In addition, by referring to the surface SEM image and the cross-sectional SEM image, it was confirmed that the specific surface area and internal pores decreased as the size of the primary particles increased.

[0184] On the other hand, in the case of Comparative Examples 2 and 4, it was confirmed that the doping uniformity of Cu on the surface and inside the positive active material particles was poor, and the size of the primary particles was also smaller compared to Example 1.

[0185] SEM images of the primary particles of the positive active material and the secondary particles of the positive active material prepared in Example 1 and Comparative Examples 1 to 3 are shown in Figures 4 and 5, respectively.

[0186] As shown in Fig. 4, compared to Comparative Example 1 without metal doping, the size of the primary particles in Comparative Examples 2 and 3, which were doped with Ca alone or Cu alone, increased slightly, but the size of the primary particles in Example 1 increased the most significantly. These results are analyzed to be due to the combined effect of Ca and Cu.

[0187] In addition, as shown in Fig. 5, it was confirmed that precursor secondary particles with an average particle size (D50) of 9.0 to 16.7 μm were manufactured into cathode active material secondary particles with an average particle size (D50) of 12.0 to 18.5 μm. These results are analyzed to be due to double doping of Ca and Cu, similar to the increase in primary particle size as shown in Fig. 4.

[0188] In summary, it is expected that the average particle size of primary and secondary particles will increase and the specific surface area will decrease, thereby reducing the surface area reacting with the electrolyte, increasing energy density, and improving lifespan characteristics.

[0189]

[0190] Experimental Example 2: XRD (X-ray diffraction) and SAED (Selected Area Electron Diffraction) Analysis

[0191] The XRD analysis results of the cathode active materials prepared in Example 1 and Comparative Examples 1 to 4 are shown in FIG. 6, and the (003) peak full width at half maximum (FWHM), c-lattice length, and NiO ratio were summarized through XRD analysis and are shown in Table 2 below.

[0192] Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Doping Elements Ca, Cu-CuCaCa, Cuc Axis Length (Å) 16.004 16.000 16.007 15.995 16.005 NiO Ratio (%)--0.43-0.07

[0193] As shown in Figure 6 and Table 2, the c-axis length increased in Example 1, Comparative Example 2, and Comparative Example 4, and this result is analyzed to be due to the expansion of the interlayer spacing through Cu doping. However, referring to Comparative Example 3, it was found that Ca doping did not contribute to the expansion of the interlayer spacing.

[0194] Considering that Example 1 has an accurate peak position and the highest peak intensity, it was confirmed that the O3 crystallinity is improved compared to Comparative Examples 2 and 4. In addition, no other peaks, such as impurity phases or second phases, were observed. This suggests that using a Cu-coated NFM precursor suppresses the generation of NiO peaks during Ca doping and Na insertion, which is advantageous for improving electrochemical performance.

[0195] On the other hand, in Comparative Example 2, impurity peaks and NiO peaks were observed, and in Comparative Example 4, some NiO peaks were also observed. Referring to Table 1 and Figures 1 and 2, it was confirmed that because the uniformity of the distribution of the Cu composition is poor, the effect of Cu and Ca double doping is not sufficiently expressed in Comparative Example 4, along with Comparative Example 2 which was doped with Cu alone, as in Example 1.

[0196] Figure 7 shows the SEAD analysis results of the cathode active materials prepared in Example 1 and Comparative Example 2, where the transition metal layer spacing is 3.96 nm in Example 1. -1 3.87 nm of Comparative Example 1 -1 It was confirmed that it increased significantly compared to.

[0197] In summary, in the case of Example 1, the expansion of the interlayer spacing due to Cu doping resulted in an increase in both the c-lattice length and the transition metal layer spacing. As the doping uniformity of Cu and Ca double doping improved, impurity phases such as NiO did not occur, and O3 crystallinity increased, leading to an improvement in the Na ion diffusion rate and initial cycle capacity.

[0198]

[0199] Experimental Example 3: Battery Performance Evaluation

[0200] For the sodium secondary batteries prepared in Example 1 and Comparative Examples 1 to 4, 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, voltage range 2.5V to 4.3V, and 0.1C / 0.1C.

[0201] In addition, for the same sodium secondary battery, 50 charge / discharge cycles were performed under 1C / 1C conditions within a driving voltage range of 2.5V to 4.3V 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 3 below.

[0202] Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 1st CH (mAh / g) 157.5 156.6 157.1 158.8 157.11 st DCH (mAh / g) 143.6 144.5 143.3 144.6 143.3 ICE (%) 91.2 92.3 91.2 91.1 91.21 Cycle DCH (mAh / g) 135.9 133.6 137.6 134.0 134.8 50 Cycle DCH (mAh / g) 123.4 111.8 122.0 116.5 120.0 50 Cycle retention (%) 90.9 83.2 88.7 87.0 89.0

[0203] Referring to Table 3, Example 1 showed the best results with a 50-cycle life retention rate of over 90%.

[0204] Comparative Examples 1 to 4 are confirmed to have deteriorated electrochemical performance, such as a decrease in both the 50-cycle life retention rate and discharge capacity compared to Example 1.

[0205]

[0206] Experimental Example 4: Measurement of Residual Na Content (ppm) and Moisture Stability Test

[0207] The cathode active materials prepared in Example 1 and Comparative Examples 1 to 4 were exposed to a wet room with 60% humidity for 1 to 14 days, and residual Na content measurement and moisture stability tests were performed.

[0208] The residual sodium content was determined as the value (TTS, Total Sodium) obtained by measuring the residual Na-containing compound (e.g., NaOH or Na2CO3) separately using potentiometric titration and then calculating the total amount of Na alone. The calculation method is as shown in Formula 1 below.

[0209] [Formula 1]

[0210] TTS(Total Na) = NaOH Analysis Value(%)×1(Na / NaOH) + Na2CO3 Analysis Value(%)×2(Na / Na2CO3)

[0211] The moisture content was measured by placing the sample in a vial dedicated to a KF Oven (Karl Fischer Oven), heating it to vaporize the moisture within the sample, and transferring the vaporized moisture to a measuring vessel using flow gas (air or N2).

[0212] Residual Na (ppm) Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 14 Comparative Example 14,512 9,381 12,662 14,327 17,017 18,438 28,312 42,333 Comparative Example 24,807 8,303 11,181 13,596 15,299 16,391 22,421 36,391 Comparative Example 32,495 3,905 5,007 5,989 6,791 7,550 8,500 11,046 Example 13,608 4,097 4,906 5,631 6,003 6,837 7,258 8,628

[0213] Moisture (ppm) Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 14 Day Comparative Example 1 10 2.3 47 1.6 59 7.1 67 5.3 73 5.6 75 4.7 80 6.8 34 73.6 Comparative Example 2 5 9.6 41 1.2 48 7.9 59 0.9 65 5.0 69 3.4 70 0.1 22 39.6 Comparative Example 3 5 130 5.5 39 0.6 45 1.1 49 1.9 55 1.0 58 1.6 60 7.6 Example 1 7 3.5 34 8.8 39 0.5 44 0.3 48 0.3 50 0.2 52 8.8 57 5.3

[0214] As shown in Tables 4 and 5, moisture stability was found to be excellent in the order of Example 1, Comparative Examples 3, 2, and 1. In the case of Example 1, moisture stability was improved due to the effect of reducing residual Na, and this effect was maintained even after prolonged exposure in a wet chamber. This is analyzed to be due to the improvement in structural stability and moisture stability resulting from the double doping of Ca and Cu and the improvement of doping uniformity.

[0215]

[0216] Experimental Example 5: Review of Optimization for Ca and Cu Double Doping Contents

[0217] (Examples 1-1 to 1-5)

[0218] 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 doping metal precursor was used with the composition of Table 6 below.

[0219] [Equation 1A] A1(Cu) / At (%) was calculated in the same way as in Experimental Example 1, and the results are shown in Table 6 below.

[0220] In addition, cycle life characteristics were calculated using the same method as in Experimental Example 3, and the results are shown in Table 6 below.

[0221] Type of doping metal, content [M1 / M2 = (mol% / mol%)] Cu distribution in particle cross-section [Equation 1A] A1(Cu) / At(%) Cycle life (50cy, %) Example 1 Cu / Ca = 5.0 / 1.59 90.9 Example 1-1 Cu / Ca = 3.0 / 1.59 187.8 Example 1-2 Cu / Ca = 1.0 / 1.58 287.0 Example 1-3 Cu / Ca = 5.0 / 1.09 589.2 Example 1-4 Cu / Ca = 5.0 / 0.59 3.588.9 Example 1-5 Cu / Ca = 1.5 / 0.58 587.1

[0222] Referring to Table 6, it is analyzed that in Examples 1, 1-1, and 1-2, as the amount of Cu doping decreases, the effect of improving structural stability becomes negligible, and thus the cycle life characteristics deteriorate.

[0223] In addition, in Examples 1, 1-3, and 1-4, it is analyzed that as the amount of Ca doping decreases, the uniformity of the distribution of Cu inside the cathode active material particles deteriorates, and the cycle life characteristics are reduced.

[0224] In addition, as in Examples 1-5, even if the Cu / Ca molar ratio falls within the preferred range of the present invention, if the content of Cu and Ca is excessively low, the effect of introducing double-doped metals is negligible, and it is analyzed that structural stability and cycle life deteriorate.

Claims

1. A sodium complex transition metal oxide comprising at least sodium, nickel, manganese, M1, and a doping metal, and The above M1 is iron or cobalt, and The above doping metal is composed of calcium and copper, and is a positive electrode active material for a sodium secondary battery satisfying the following relationship 1A: [Relation 1A] 80 ≤ A1(Cu) / At (%) ≤ 99 In the above relationship 1A, At is the total cross-sectional area of ​​the above sodium composite transition metal oxide particles, and A1(Cu) is the area of ​​the region in which copper is within ±10% of the average content (Cu, average at mol%) for 100 at mol% of all metals excluding sodium and calcium among the total cross-sectional area above.

2. In Paragraph 1, A positive electrode active material for a sodium secondary battery satisfying the following relationship 1B: [Relationship 1B] 80 ≤ B1(Cu) / Bt (%) ≤ 99 In the above relationship 1B, Bt is the total surface area of ​​the sodium complex transition metal oxide particles, and B1(Cu) is the area of ​​the region in which copper is within ±10% of the average content (Cu, average at mol%) for 100 at mol% of all metals excluding sodium and calcium among the total surface area above.

3. In Paragraph 1, A positive electrode active material for a sodium secondary battery satisfying the following relationship 2A: [Relationship 2A] 80 ≤ A2(Ca) / At (%) ≤ 99 In the above relationship 2A, At is the total cross-sectional area of ​​the above sodium composite transition metal oxide particles, and A2(Ca) is the area of ​​the region in which, among the total cross-sectional area, the calcium content is within ±10% of the average content (Ca, average at mol%) for a total of 100 at mol% of sodium and calcium.

4. In Paragraph 1, A positive electrode active material for a sodium secondary battery satisfying the following relationship 2B: [Relationship 2B] 80 ≤ B2(Ca) / Bt (%) ≤ 99 In the above relationship 2B, Bt is the total surface area of ​​the sodium complex transition metal oxide particles, and B2(Ca) is the area of ​​the region in which, among the total surface area, calcium is within ±10% of the average content (Ca, average at mol%) for a total of 100 at mol% of sodium and calcium.

5. In Paragraph 1, The above sodium composite transition metal oxide is a positive electrode active material for a sodium secondary battery comprising the doping metal such that the molar ratio of copper to calcium (Cu / Ca) is 1.5 to 4.

5.

6. In Paragraph 1, The above sodium complex transition metal oxide contains copper in an amount greater than 3 at mol% and less than 10 at mol% with respect to 100 at mol% of total metals excluding sodium and calcium, and A positive electrode active material for a sodium secondary battery containing more than 0.5 at mol% and less than 3 at mol% of calcium, based on a total of 100 at mol% of sodium and calcium.

7. 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 1-1: [Chemical Formula 1-1] So a-2v Approx v [(Ni x Mr y M1 z Cu w )]O2 In the above chemical formula 1-1, M1 is Co or Fe, and 0.8 <a<1.2, 0.005<v<0.03, 0.1≤x≤0.9, 0.1≤y≤0.9, 0.1≤z≤0.9, 0.03<w<0.1, x+y+z+w=1이다.

8. 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 1-2: [Chemical Formula 1-2] So a-2v Approx v [(Ni x Mr y M1 z Cu w )]O2 In the above chemical formula 1-2, M1 is Fe, and 0.9 <a<1.1, 0.01<v<0.02, 0.27≤x≤0.37, 0.27≤y≤0.37, 0.27≤z≤0.37, 0.04<w<0.06, x+y+z+w=1이다.

9. In Paragraph 1, The above sodium composite transition metal oxide is a positive electrode active material for a sodium secondary battery in which the calcium is substituted in the sodium layer of the composite transition metal oxide and the copper is substituted in the transition metal layer of the composite transition metal oxide.

10. In Paragraph 1, The above sodium complex transition metal oxide has an O3 crystal structure in X-ray diffraction analysis, and the full width at half maximum (FWHM) of the (003) peak located at 2θ=15 to 17.5° is 0.1699 to 0.2599, and A positive electrode active material for sodium secondary batteries in which the NiO peak is substantially absent.

11. A complex transition metal hydroxide precursor comprising at least nickel, manganese and M1 (Co and / or Fe) is mixed with a copper compound and subjected to a first heat treatment, A coating process for coating at least a portion of the surface of the above-mentioned composite transition metal hydroxide precursor particles with copper; and The above copper-coated transition metal hydroxide precursor, calcium compound, and sodium compound are mixed and subjected to a second heat treatment, A method for manufacturing a positive electrode active material for a sodium secondary battery, comprising a doping process of substituting the surface and interior of sodium composite transition metal oxide particles with copper and calcium.

12. In Paragraph 11, A method for manufacturing a positive electrode active material for a secondary battery, wherein in the above coating process (1) and doping process (2), the ratio of the heat treatment temperature (°C) (T2 / T1) is 1.2 to 2 and the ratio of the heat treatment time (h) (H2 / H1) is 1.5 to 2.

5.

13. In Paragraph 11, A method for manufacturing a positive electrode active material for a secondary battery, wherein, in the above coating and doping processes, the copper compound and the calcium compound are mixed such that the molar ratio of copper to calcium (Cu / Ca) is 1.5 to 4.

14. In Paragraph 11, The above coating process (1) and doping process (2) are, The above-mentioned complex transition metal hydroxide precursor and sodium complex transition metal oxide are each secondary particles formed by the aggregation of at least one primary particle, wherein A method for manufacturing a positive electrode active material for a secondary battery, wherein the ratio (D / C) of the average particle size (D50) of the sodium composite transition metal oxide secondary particle (D) to the composite transition metal hydroxide precursor secondary particle (C) is 1.2 to 2.

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

16. A sodium secondary battery comprising a positive electrode according to paragraph 15; and a negative electrode.

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