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

Abstract

One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, the positive electrode active material comprising a sodium composite transition metal oxide containing at least sodium, a transition metal, a first doping metal, and a second doping metal, wherein the ratio of an ionic radius (RM1) value of the first doping metal to an ionic radius (R) value of the sodium (RM1 / R) is in the range of 0.8 to 1.2, and the ratio of an ionic radius (RM2) value of the second doping metal to the ionic radius (R) value of the sodium (RM2 / R) is in the range of 0.5 to 0.7.

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. In this case, 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] The objective of the present invention is to improve the structural stability of an O3-type cathode active material to achieve high capacity and excellent lifespan characteristics, specifically to improve air / moisture stability and enhance cycling life characteristics by applying a composite doping metal to the cathode active material.

[0011] In addition, the objective of the present invention is to improve the structural stability of the sodium layer by strengthening the Na-O bonding strength, prevent the deterioration of the structural stability of the transition metal layer during charging and discharging by strengthening the TM-O bonding strength, and improve capacity loss due to the doping metal.

[0012] One embodiment of the present invention comprises a sodium complex transition metal oxide containing at least sodium, a transition metal, a first doping metal, and a second doping metal, wherein the ionic radius (R) of the first doping metal relative to the ionic radius (R) of the sodium is M1 Ratio of ) values ​​(R M1 / R) is 0.8 to 1.2, and the ionic radius (R) of the second doping metal relative to the ionic radius (R) value of the sodium is M2 Ratio of ) values ​​(R M2A positive electrode active material for a sodium secondary battery is provided, wherein / R) is 0.5 to 0.7.

[0013] The above positive active material is trivalent nickel (Ni 3+ ionic radius (R) Ni3+ Ionic radius (R) of the second doping metal for the value M2 Ratio of ) values ​​(R M2 / R Ni3+ ) can be 1.0 to 1.2.

[0014] The first doping metal may be a divalent cation, and the second doping metal may be a quaternary to hexavalent cation.

[0015] The first doping metal is Ca, and the second doping metal may be at least one selected from the group consisting of V, Mo, W, and Nb.

[0016] The first doping metal may be included in an amount of 0.5 to 2 at mol% with respect to the total amount of sodium (Na) and the first doping metal of the sodium complex transition metal oxide, and the second doping metal may be included in an amount of 0.01 to 0.2 at mol% with respect to the total amount of metals excluding sodium (Na) and the first doping metal of the sodium complex transition metal oxide.

[0017] The ratio of the second doping metal-oxygen bonding force (M2-O) to the first doping metal-oxygen bonding force (M1-O) (M2-O / M1-O) may be 1.1 to 2, and the ratio of the content of the second doping metal (M2) (at mol) to the content of the first doping metal (M1) (M2 / M1) may be 0.03 to 15%.

[0018] The above sodium complex transition metal oxide may be represented by the following chemical formula 1.

[0019] [Chemical Formula 1]

[0020] Na a M1 b [TM x M2 1-x ]O 2+w

[0021] In the above chemical formula 1,

[0022] TM is at least one selected from Ni and Fe, Mn and Co, M1 is Ca, M2 is at least one selected from W, V, Mo and Nb, 0.8≤a≤1.1, 0.005≤b≤0.02, 0.8≤x≤0.9998, 0.0001≤1-x≤0.002, -0.1≤w≤0.1.

[0023] The sodium composite transition metal oxide comprises secondary particles formed by the aggregation of a plurality of primary particles, and the first doping metal and the second doping metal may be located on the surface of the secondary particles, on the surface of the primary particles and / or inside the primary particles.

[0024] The above sodium complex transition metal oxide may have an O3-type crystal structure.

[0025] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, comprising the steps of dry mixing (a) a transition metal hydroxide precursor, a first doping metal compound, a second doping metal compound, and a sodium compound, and calcining (b).

[0026] The above dry mixing may involve mixing an equal amount of sodium compound in Na / M (total metal excluding Na) = greater than 0.8 and less than 1, mixing the first doping metal in an amount of 0.5 to 2 at mol% with respect to the total amount of Na and the first doping metal, and mixing the second doping metal in an amount of 0.01 to 0.2 at mol% with respect to the total amount of metal excluding Na and the first doping metal.

[0027] The first doping metal compound is an acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof of Ca, and the second doping metal compound may be at least one acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof selected from W, V, Mo, and Nb.

[0028] The above firing may be carried out at a temperature of 700°C to 1,100°C for 5 to 40 hours.

[0029] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery comprising the positive electrode active material.

[0030] Another embodiment of the present invention provides a sodium secondary battery comprising the anode; the cathode; and the electrolyte.

[0031] According to the present invention, as a method to improve weakened bonding between TM and O, the introduction of a specific composite doping metal effectively controls the charge transfer between Na and O, thereby enabling the establishment of a stronger Na2O bond energy. Accordingly, structurally stable and excellent air / water stability can be achieved.

[0032] In addition, in the present invention, the first doping metal and Na + Ionic radius of and and second doping metal and Na + The difference in ionic radius can facilitate doping into the sodium layer and the transition metal layer. Subsequently, due to the relatively high oxidation states of the first and second doping metals, the bonding strength with oxygen is increased, which can improve the structural stability of the sodium layer and the transition metal layer and improve ionic conductivity.

[0033] In addition, in the present invention, Ni 3+ and By doping with cations that have a similar ionic radius and a high oxidation state, Ni is balanced by charge 3+ By reducing the ratio of Ni2+ / Ni 3+ It is possible to increase the ratio, improve the structural stability of the transition metal layer, and prevent capacity loss caused by the doping metal.

[0034] In addition, in the present invention, by controlling the ratio of the bonding strength of the first and second doping metals with oxygen to a specific range and simultaneously satisfying the ratio of the doping amounts of the first and second doping metals to a specific range, the distortion of the crystal structure caused by the difference in bonding strength between the sodium layer and the transition metal layer and oxygen during composite doping metal substitution can be suppressed.

[0035] Figure 1 is a surface / cross-sectional SEM-EDS mapping analysis image of the positive electrode active material particles prepared in Examples 1 to 4 and Comparative Example 1.

[0036] Figures 2a, 2b, and 2c are a scanning TEM image, a TEM-EDS analysis image, and a TEM image showing the interplanar distance of the sodium layer of the cathode active material prepared in Comparative Example 1.

[0037] Figures 3a, 3b, and 3c are a scanning TEM image, a TEM-EDS analysis image, and a TEM image showing the interplanar distance of the sodium layer of the cathode active material prepared in Example 1.

[0038] Figures 4a, 4b, and 4c are a scanning TEM image, a TEM-EDS analysis image, and a TEM image showing the interplanar distance of the sodium layer of the cathode active material prepared in Example 3.

[0039] 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.

[0040] 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.

[0041]

[0042] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery. The positive electrode active material comprises a sodium complex transition metal oxide containing at least sodium, a transition metal, a first doping metal, and a second doping metal.

[0043] Generally, O3-type oxide cathode active materials have poor air / water stability, so when exposed to air or in contact with water, they react with H2O and CO2, undergoing oxidation reactions and H + / Na + Exchange occurs, generating residual Na, such as NaOH and Na2CO3, on the surface of the cathode active material. The generated CO3 is embedded in the transition metal layer to form CO4 tetrahedra, and as a result, Na +Diffusion slows down and electrochemical properties deteriorate. In the present invention, as a method to improve the weakened bonding between TM and O, the introduction of a specific composite doping metal effectively controls the charge transfer between Na and O, thereby enabling the establishment of a stronger Na2O bond energy. Accordingly, structurally stable and excellent air / water stability can be achieved.

[0044] The first and second doping metals are such that the ionic radius (R) of the first doping metal relative to the ionic radius (R) value of the sodium. M1 Ratio of ) values ​​(R M1 / R) is 0.8 to 1.2, and the ionic radius (R) of the second doping metal is M2 Ratio of ) values ​​(R M2 / R) is 0.5 to 0.7.

[0045] For example, the ratio of the above ionic radius values ​​(R M1 / R) may be 0.9~1.2, 1.0~1.2, 1.0~1.15, or 1.0~1.1, and the ratio of the ionic radius values ​​(R M2 / R) may be 0.5~0.65, 0.55~0.7, 0.55~0.65, or 0.59~0.64. The first doping metal may have an ionic radius of 0.9~1.2Å, 1.0~1.2Å, or 1.0~1.15Å, and the second doping metal may have an ionic radius of 0.55~0.7Å, 0.5~0.65Å, 0.55~0.7Å, 0.55~0.65Å, or 0.59~0.64Å.

[0046] The first doping metal (M1) is Na + Because it has an ionic radius range similar to (ionic radius: 1.02 Å), it can be easily doped with a sodium layer, and M1 2+ The oxidation number of Na 1+ Due to the high contrast, the binding affinity M1-O with oxygen increases, which can improve the structural stability of the sodium layer. As a result, the empty space within the lattice structure increases, leading to an increase in the c-axis within the lattice, which can improve ionic conductivity.

[0047] The second doping metal (M2) is Na + By having an ionic radius range of 0.5 to 0.7 times (ionic radius: 1.02 Å), it is easy to dope the transition metal layer rather than the sodium layer, and due to the high oxidation state, the binding force M2-O with oxygen is increased, which can improve the structural stability of the transition metal layer during charging and discharging.

[0048] Furthermore, when the first or second doping metal is doped alone, there is a problem in that an unstable structure is formed due to distortion caused by the difference in bonding strength between the sodium layer and the transition metal layer and oxygen. On the other hand, in the present invention, as the first doping metal is composite doped into the sodium layer and the second doping metal is composite doped into the transition metal layer, the bonding strength with oxygen is increased in both the sodium layer and the transition metal layer, thereby reducing the relative difference in bonding strength between the sodium layer-O and the transition metal layer-O caused by single doping. As a result, distortion is suppressed throughout the crystal structure, structural stability is improved, and electrochemical lifespan degradation can be prevented.

[0049] The above second doping metal is trivalent nickel (Ni 3+ ionic radius (R) Ni Ionic radius (R) of the second doping metal for the value M2 Ratio of ) values ​​(R M2 / R Ni ) can be 0.9~1.2, 1.0~1.2, or 1.0~1.15.

[0050] Ni in sodium transition metal oxides 3+ The oxidation state can change from 2+ to 4+ during the calcination step or charge / discharge; since Ni is unstable in terms of electron configuration when existing with an oxidation state of 3+ and has a stable structure when existing with an oxidation state of 2+, Ni 3+ and By doping with cations that have a similar ionic radius and a high oxidation state, Ni is balanced by charge 3+ By reducing the ratio of Ni 2+ / Ni 3+The ratio can be increased.

[0051] In addition, in conventional lithium transition metal oxides (LIBs), Li + Ni with an ionic radius similar to (0.76 Å). 2+ (0.69Å) is Li + While it is important to suppress cation mixing, where sites and locations change, in the case of sodium transition metal oxides, Na + Since the ionic radius of is 1.02 Å, Ni with an ionic radius of 0.69 Å 2+ Even if the ratio increases, it is difficult for cation mixing problems to occur.

[0052] The first doping metal may be Ca as a divalent cation, and the second doping metal may be at least one selected from the group consisting of V, Mo, W, and Nb as a tetravalent to hexagonal cation. The first doping metal Ca 2+ (1.10Å) is Na + Compared to (1.02 Å), the ionic radius is within a similar range of ±10% and the oxidation number is higher, so the aforementioned effect can be further improved. The above-mentioned second doping metal V 5+ (0.59Å), Mo 6+ (0.59Å), W 6+ (0.60Å) and Nb 5+ In the case of (0.64Å), Ni 3+ (0.56Å), Fe 3+ (0.63Å) and Mn 4+ Since the ionic radius is within a similar range of ±20% compared to (0.53 Å), it can be easily doped into a transition metal layer; and during charge / discharge, among transition metals, it exhibits oxidation state ranges of Ni 2+ / 3+ / 4+ and Mn 3+ / 4+, and since the second doping metal with an oxidation state of +5 or +6 has a higher oxidation state, when doping into a transition metal layer Ni 2+ The ratio increases, which can prevent capacity loss caused by doping metals.

[0053] The first doping metal may have a binding force (M1-O) with oxygen of 350 to 550 kJ / mol, for example, 400 to 500 kJ / mol or 450 to 480 kJ / mol, and the second doping metal may have a binding force (M2-O) with oxygen of 550 to 800 kJ / mol, for example, 600 to 770 kJ / mol or 650 to 770 kJ / mol. Accordingly, the binding force M1-O with oxygen in the sodium layer within the crystal structure of the positive electrode active material is increased, thereby improving the structural stability of the sodium layer. Additionally, the empty space within the lattice structure increases, causing the c-axis within the lattice to increase, which can thereby improve ionic conductivity. Furthermore, the binding force M2-O with oxygen in the transition metal layer within the crystal structure of the positive electrode active material is increased, thereby improving the structural stability of the transition metal layer during charging and discharging.

[0054] The content of the first doping metal (M1) may be 0.5 to 2 at mol% with respect to the total amount of sodium (Na) and the first doping metal (M1) of the sodium complex transition metal oxide, and for example, 1.0 to 2 at mol%, 1.2 to 1.8 at mol%, or 1.3 to 7 at mol%.

[0055] The content of the second doping metal (M2) may be 0.02 to 0.2 at mol% with respect to the total amount of metal excluding sodium (Na) and the first doping metal (M1) of the sodium complex transition metal oxide, and for example, may be 0.02 to 0.1 at mol%, 0.02 to 0.08 at mol%, 0.02 to 0.07 at mol%, or 0.02 to 0.06 at mol%.

[0056] In addition, the ratio (M2-O / M1-O) of the second doping metal-oxygen bonding force (M2-O) to the first doping metal-oxygen bonding force (M1-O) may be 1.1 to 2, and the ratio (M2 / M1) of the content (doping amount, at mol) of the second doping metal (M2) to the content (doping amount, at mol) of the first doping metal (M1) may be 0.03 to 15. Specifically, the ratio of the first / second doping metal-oxygen bonding force (M2-O / M1-O) may be 1.1 to 1.7 or 1.4 to 1.7 and the ratio of the doping amount (M2 / M1) may be 0.1 to 10, 0.3 to 10, 0.5 to 10, 0.7 to 10, 1 to 10, 3 to 10, 3 to 7, or 3 to 5.

[0057] When the above-mentioned numerical range is satisfied, the difference in relative bonding strength between the sodium layer-O and the transition metal layer-O can be minimized, and accordingly, when substituting a composite doping metal, the distortion of the crystal structure caused by the difference in bonding strength between the sodium layer-oxygen and the transition metal layer-oxygen can be suppressed.

[0058] The above sodium complex transition metal oxide is a positive electrode active material for a sodium secondary battery represented by the following chemical formula 1:

[0059] [Chemical Formula 1]

[0060] Na a M1 b [TM x M2 1-x ]O 2+w

[0061] In the above chemical formula 1, TM may be at least one selected from Fe, Mn, Ni, and Co, M1 may be Ca, M2 may be at least one selected from W, V, Mo, and Nb, and 0.8≤a≤1.1, 0.005≤b≤0.02, 0.8≤x≤0.9998, 0.0001≤1-x≤0.002, -0.1≤w≤0.1.

[0062] In the above chemical formula 1, TM (transition metal) may comprise at least one selected from Ni and Fe, Mn and Co. Preferably, TM may be Ni, Co and Mn, or Ni, Fe and Mn.

[0063] The above M1 (first doping metal) and M2 (second doping metal) are the same as those previously described.

[0064] Meanwhile, the sodium composite transition metal oxide may further include a metal (M3) different from TM, M1, and M2, and M3 may be at least one selected from P, Sr, Ba, Zr, Co, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nb, B, Nb, Gd, and Cu.

[0065] The sodium composite transition metal oxide may include a secondary particle formed by the aggregation of at least one primary particle, and the first doping metal and the second doping metal may be located on the surface of the secondary particle, on the surface of the primary particle, and / or inside the primary particle. For example, the first and second doping metals may each be present partially or wholly at the interface between the primary particles located inside the secondary particle and / or on the surface of the secondary particle. When the first and second doping metals are partially present on the surface of the primary particle and / or the secondary particle, the shell may exist in the form of an island, and when they are entirely present, the shell may exist in the form of a uniform film.

[0066] Meanwhile, the above "secondary particles formed by the aggregation of at least one primary particle" should be interpreted to include both "particles formed by the aggregation of multiple primary particles" and "non-aggregated single particles composed of a single crystallite."

[0067] In addition, the primary particle and the secondary particle may each independently have a rod shape, an elliptical shape, and / or an irregular shape.

[0068] When the average long axis length is used as an indicator of the size of the primary particles and the secondary particles, the average long axis length of the primary particles constituting the sodium composite transition metal oxide may be 0.1 μm to 5 μm, and the average long axis length of the secondary particles may be 1 μm to 30 μm. The average long axis length of the secondary particles may vary depending on the number of primary particles constituting the secondary particles, and the cathode active material may contain particles having various average long axis lengths.

[0069] The sodium complex transition metal oxide has an O3-type crystal structure, and the sodium complex transition metal oxide may be physically and / or chemically bonded to the primary particle. Additionally, the sodium complex transition metal oxide may exist in a state in which it forms a solid solution with the primary particle.

[0070] With respect to the secondary particle, the sodium complex transition metal oxide may be present partially or wholly at the interface between the primary particle located inside the secondary particle and / or on the surface of the secondary particle. When the sodium complex transition metal oxide is partially present on the surface of the primary particle and / or the secondary particle, the shell may exist in the form of an island.

[0071]

[0072] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery.

[0073] The above manufacturing method includes a process of dry mixing a transition metal hydroxide precursor, a first doping metal compound, a second doping metal compound, and a sodium (Na) compound, and calcining them.

[0074] In the above dry mixing process, the first and second doping metals can be uniformly doped on the surface and inside the transition metal hydroxide precursor particles by applying the dry method, whereas when doping transition metals by the wet method, the selection of doping compounds is limited and there is a problem of increased costs due to process complexity.

[0075] The above dry mixture can be mixed in an equivalent amount of Na / M (total metal excluding Na) = greater than 0.8 and less than 1.

[0076] When the mixing amount of the sodium compound is within the above range, the manufactured cathode active material may have an O3-type layered crystal structure, thereby possessing a higher energy density, high atmospheric and moisture safety, and being less sensitive to synthesis conditions (temperature and atmosphere, etc.). Additionally, within the above sodium content range, the battery discharge capacity can be improved, and unreacted residual Na can be minimized.

[0077] The first doping metal may be mixed in an amount of 0.5 to 2 at mol% with respect to the total amount of Na and the first doping metal, and the second doping metal may be mixed in an amount of 0.02 to 0.2 at mol% with respect to the total amount of metals excluding Na and the first doping metal.

[0078] The above transition metal hydroxide precursor may be represented by the following chemical formula 2.

[0079] [Chemical Formula 2]

[0080] TM(OH)2

[0081] Here, TM may be at least one selected from Co, Ni, Mn and Fe.

[0082] The first doping metal compound may be an acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof of Ca, and the second doping metal compound may be at least one acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof selected from W, V, Mo and Nb.

[0083] The above 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.

[0084] The above calcination process can be performed at a temperature of 700 to 1,100°C. When the calcination temperature is within the above range, sufficient reaction between raw materials can occur and particles can grow uniformly. More preferably, the calcination may be performed at a temperature of 750 to 1,050°C, 850 to 1,050°C, or 900 to 1,000°C. The calcination may be performed for 5 to 40 hours. When the calcination time is within the above range, a highly crystalline cathode active material can be obtained, the particle size is appropriate, and production efficiency can be improved. More preferably, the calcination may be performed for 5 to 20 hours, 5 to 18 hours, 8 to 15 hours, or 10 to 14 hours.

[0085]

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097]

[0098] 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.

[0099] Examples

[0100] (Examples 1–9 and Comparative Examples 1–2)

[0101] NFM111 precursor (Ni 0.33 Fe 0.33 Mn 0.33 (OH)2) and the first doping metal (M1) precursor and the second doping metal (M2) precursor of Table 1 below were dry mixed using a Hand Mixer.

[0102] Na2CO3 was mixed into the mixed precursor at a ratio of Na / (Ni+Fe+Mn+M2)=1.0 equivalents.

[0103] The prepared mixture was placed into an alumina crucible and calcined at 900°C for 10 hours in an air atmosphere, and then cooled to room temperature to produce an O3-type cathode active material.

[0104] The manufactured positive electrode active material was put into a mixer and crushed for 1 minute and 30 seconds.

[0105] A cathode slurry was prepared by dispersing 2g of 96wt% of the manufactured cathode active material, 2wt% of carbon black, and 2wt% of PVdF binder in N-methyl-2-pyrrolidone (NMP). The cathode slurry was uniformly coated onto an aluminum film with a thickness of 15㎛ and vacuum dried at 135℃ to produce a cathode for a sodium secondary battery.

[0106] 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.

[0107] First doping metal (M1) Second doping metal (M2) M1 precursor M1 content (at mol%) M1 oxidation state / (M1 / Na + ) Ionic radius ratio M2 Precursor M2 content (at mol%) M2 Oxidation number / (M1 / Na + ) Ion radius ratio (M1 / Ni 3+ ) Ionic Radius Example 1Ca(OH)2 1.5Ca 2+ / 1.1Nb2O50.05Nb 5+ / 0.62 1.14 Example 2Ca(OH)2 1.5Ca 2+ / 1.1V2O50.05V 5+ / 0.57 1.05 Example 3Ca(OH)2 1.5Ca 2+ / 1.1MoO30.05Mo 6+ / 0.57 1.05 Example 4Ca(OH)2 1.5Ca 2+ / 1.1WO30.05W 6+ / 0.58 1.07 Example 5Ca(OH)2 1.5Ca 2+ / 1.1Nb2O50.1Nb 5+ / 0.62 1.14 Example 6Ca(OH)2 1.5Ca 2+ / 1.1Nb2O50.2Nb 5+ / 0.62 1.14 Example 7 Cal(OH)2 1.5 Cal 2+ / 1.1WO30.1W 6+ / 0.58 1.07 Example 8Ca(OH)2 1.5Ca 2+ / 1.1WO30.2W 6+ / 0.58 1.07 Example 9Ca(OH)2 1.5Ca 2+ / 1.1Nb2O50.5Nb 5+ / 0.62 1.14 Comparative Example 1Ca(OH)2 1.5Ca 2+ / 1.1----Comparative Example 2-------* Oxidation number (ionic radius): Na + (1.02Å), Ca 2+ (1.10Å), Nb 5+ (0.64Å), Ni 3+ (0.56Å), V 5+ (0.59Å), Mo 6+ (0.59Å), W 6+ (0.60 Å)* Binding energy (kJ / mol): Ca-O 464.8 kJ / mol, Nb-O 753 kJ / mol, WO 720 kJ / mol, VO 637 kJ / mol, Mo-O 607 kJ / mol

[0108] Experimental Example

[0109] Experimental Example 1: SEM-EDS Analysis of Anode Active Material Particles

[0110] Surface / cross-sectional SEM-EDS mapping analysis was performed on the cathode active material particles prepared in Examples 1 to 4 and Comparative Example 1, and the results are shown in Figure 1.

[0111] As shown in Figure 1, it was confirmed that Nb, W, V, and Mo are uniformly distributed across the surface, but some exist in a partial island form. Additionally, all doping metals were diffused into the interior, and in the case of Nb and Mo, some were found to exist partially.

[0112] In addition, in the case of Example 1, a change in the shape of the cathode active material to a plate-like form due to Nb doping was confirmed. Accordingly, it is analyzed that side reactions with the electrolyte will be suppressed due to the reduction in surface area.

[0113]

[0114] Experimental Example 2: STEM (Scanning TEM) and TEM-EDS Analysis of Anode Active Material Particles

[0115] Referring to Figures 2a, 2b, and 2c, in the case of the cathode active material prepared in Comparative Example 1, a Na deficient layer was found in a region 10 nm thick from the surface in a direction perpendicular to the TM layer on the surface of the primary particle.

[0116] In addition, compared to before Ca doping, the interplanar distance (3.95–4.00 Å) of the sodium layer increased, and it was confirmed that Ca was present at the grain boundaries and inside the primary grains.

[0117] Referring to Figures 3a, 3b, and 3c, it was confirmed that the interplanar distance (4.15–4.25 Å) of the sodium layer due to simultaneous doping of Ca and Nb in the cathode active material prepared in Example 1 increased compared to Comparative Example 1. This is attributed to the effect of expanding the interplanar distance of Na layers and is analyzed to be advantageous for Na ion diffusion.

[0118] In addition, Ca and Nb exist in an island form at the grain boundaries between primary grains, and it is analyzed that Nb is coated as Nb oxide (NbO2) on a part of the surface of the primary grains when doped.

[0119] Referring to Figures 4a, 4b, and 4c, it was confirmed that the cathode active material prepared in Example 3 showed an increase in interplanar distance (4.10–4.20 Å) due to the simultaneous doping of Ca and W compared to Comparative Example 1. This is analyzed to be advantageous for Na ion diffusion due to the effect of expanding the interplanar distance of the sodium layer.

[0120] In addition, it is confirmed that Ca is present inside and on the surface of the primary particles and that W is uniformly distributed.

[0121]

[0122] Experimental Example 3: Evaluation of Charge Transfer Resistance

[0123] Impedance (EIS; Electrochemical Impedance Spectroscopy) characteristics

[0124] For the same lithium secondary battery, after charging under 1C conditions, the resistance was measured within the frequency range (10kHz~0.01Hz, 10mV) using electrochemical impedance spectroscopy to calculate the Rct (charge transfer resistance) value.

[0125] M1 Precursor M1 Content (at mol%) M2 Precursor M2 Content (at mol%) Doping Amount Ratio (M2 / M1) (%) EIS (Ω) Example 1 Ca(OH)2 1.5 Nb2O 50.0 53.3 9.78 Example 5 Ca(OH)2 1.5 Nb2O 50.1 6.7 10.58 Example 6 Ca(OH)2 1.5 Nb2O 50.2 13.3 12.47 Example 7 Ca(OH)2 1.5 WO 30.1 6.7 10.39 Example 8 Ca(OH)2 1.5 WO 30.2 13.3 11.98 Example 9 Ca(OH)2 1.5 Nb2O 50.5 33.3 20.14 Comparative Example 1 Ca(OH)2 1.5 -0 11.17 Comparative Example 2-----12.03

[0126] Referring to Table 2, Examples 1, 5, and 7 have lower resistance values ​​compared to Comparative Examples 1 and 2, which is interpreted as a result of structural stabilization through doping.

[0127] In addition, it was confirmed that the resistance value of Example 1 is the lowest, and compared to this, it was confirmed that the resistance increases as the content of the doping element increases or as the doping ratio (M2 / M1) increases, as in Examples 5, 6, and 9. Similarly, in the case of Examples 7 and 8, it was confirmed that the resistance increases as the content of the doping element or the doping ratio (M2 / M1) increases.

[0128] While performance may improve due to structural stabilization effects as the content of doping elements increases, performance degradation occurs when doped beyond the optimal level. Therefore, the above results are interpreted as being caused by the doping elements acting as a resistance due to doping at a level exceeding the optimal conditions.

[0129]

[0130] Experimental Example 4: Evaluation of Electrochemical Characteristics of Sodium Secondary Battery

[0131] * Charging capacity, discharging capacity, and charge / discharge efficiency

[0132] For the lithium secondary batteries (coin cells) prepared in Examples 1 to 4, 9 and Comparative Examples 1 to 2, the initial charge capacity, initial discharge capacity, initial reversibility efficiency, and rate capability (discharge capacity ratio; rate capability (C-rate)) were measured through charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, with a voltage range of 2.2V to 4.1V and a discharge rate of 0.1C to 2.0C.

[0133] * Life characteristics (cycle capacity retention rate)

[0134] For the same sodium secondary battery, 50 charge / discharge cycles were performed under conditions of 0.5C / 0.5C within a driving voltage range of 2.2V to 4.1V at 25℃, and the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention rate) was measured.

[0135] * Output efficiency (C-rate)

[0136] For the same sodium secondary battery, the output efficiency (C-rate) of 0.1C / 0.1C, 0.2C / 0.2C, 0.33C / 0.33C, 1.0C / 1.0C, and 2.0C / 2.0C was measured through charge-discharge experiments applied at 25℃, a voltage range of 2.2V to 4.1V, and a discharge rate of 0.1C to 5.0C using an electrochemical analyzer (Toyo, Toscat-3100).

[0137] The above measurement results are shown in Tables 3 and 4 below.

[0138] Example 1 Example 2 Example 3 Example 4 Example 9 Comparative Example 1 Comparative Example 2 1st CH (mAh / g) 159.6 162.5 173.8 161.8 153.3 162.4 161.6 1st DCH (mAh / g) 146.5 146.8 157.8 151.7 143.2 147.3 149.0 ICE (%) 91.8 90.3 90.8 93.7 93.4 90.7 92.2 1 Cycle DCH (mAh / g) 137.8 136.1 146.9 136.2 134.6 136.8 139.6 50 Cycle DCH (mAh / g) 124.7 119.3 128.0 117.2 88.5 118.0 116.7 50 Cycle retention (%)90.587.687.286.065.786.283.6

[0139] Referring to Table 3, the sodium secondary battery prepared in Comparative Example 1 showed a lifespan retention rate of 86.2% at 50 cycles, while Example 1 showed the best lifespan characteristics at 90.5% at 50 cycles. These results are analyzed to be due to the fact that the surface area increased as the size of the primary particles increased, resulting in a reduced contact area with the electrolyte, and consequently, side reactions with the electrolyte are suppressed.

[0140] In the case of Example 2, the volume change of the positive active material during charging and discharging is suppressed by the strong binding of VO (637 kJ / mol), which is analyzed as a result of improved structural stability.

[0141] In the case of Example 3, an increase in capacity was confirmed, which is due to charge balance occurring through the specific multivalent cation doping of the present invention, resulting in Ni 2+ / Ni 3+ It is analyzed as a result of the increase in the ratio.

[0142] In the case of Example 4, it is analyzed that the migration of transition metals (TM) is reduced because there are fewer oxygen vacancies due to the strong bonding of Mo-O (607 kJ / mol).

[0143] In the case of Example 9, a low initial charge / discharge capacity was exhibited and a decrease in lifespan was observed. This result is analyzed to be due to the excess doping element acting as a resistor, and is consistent with the aforementioned EIS results.

[0144] (2.2-4.1V) Example 1 Example 2 Example 3 Example 4 Comparative Example 10.1C (%) 100 100 100 100 100 0.2C (%) 96.7 95.4 95.8 96.2 95.4 0.33C (%) 91.0 88.2 89.0 89.5 88.01C (%) 73.7 72.8 74.0 74.6 72.12C (%) 54.7 53.1 56.7 56.2 52.5

[0145] Referring to Table 4, performance improvement at high rates was confirmed in Examples 1 to 4 compared to Comparative Example 1. This is analyzed to be because the sodium layer spacing is expanded due to multivalent cation doping, which is favorable for Na diffusion.

[0146] In addition, in the case of Example 3, the capacity retention rate was found to be the best at high rates.

[0147] As described above, although the present invention has been illustrated and described in relation to specific embodiments, it will be obvious to those skilled in the art that the present invention can be modified and changed in various ways without departing from the technical spirit of the invention as provided by the following claims.

Claims

1. A sodium complex transition metal oxide comprising at least sodium, a transition metal, a first doping metal, and a second doping metal, and The ionic radius (R) of the first doping metal relative to the ionic radius (R) value of the sodium above M1 Ratio of ) values ​​(R M1 / R) is 0.8 to 1.2, and The ionic radius (R) of the second doping metal relative to the ionic radius (R) value of the sodium above M2 Ratio of ) values ​​(R M2 A positive electrode active material for a sodium secondary battery having / R) of 0.5 to 0.

7.

2. In Paragraph 1, Trivalent nickel (Ni 3+ ionic radius (R) Ni3+ Ionic radius (R) of the second doping metal for the value M2 Ratio of ) values ​​(R M2 / R Ni3+ A positive electrode active material for a sodium secondary battery, wherein ) is 1.0 to 1.

2.

3. In Paragraph 1, The first doping metal is a divalent cation, and The above second doping metal is a positive electrode active material for a sodium secondary battery, wherein the second doping metal is a 4 to 6 valent cation.

4. In Paragraph 3, The first doping metal mentioned above is Ca, and The above second doping metal is at least one selected from the group consisting of V, Mo, W, and Nb, a positive active material for a sodium secondary battery.

5. In Paragraph 1, The first doping metal is included in an amount of 0.5 to 2 at mol% with respect to the total amount of sodium (Na) and the first doping metal of the sodium complex transition metal oxide, and A positive electrode active material for a sodium secondary battery, wherein the second doping metal is included in an amount of 0.01 to 0.2 at mol% with respect to the total amount of metals excluding sodium (Na) and the first doping metal of the sodium composite transition metal oxide.

6. In Paragraph 1, The ratio (M2-O / M1-O) of the second doping metal-oxygen bonding force (M2-O) to the first doping metal-oxygen bonding force (M1-O) is 1.1 to 2, and A positive electrode active material for a sodium secondary battery, wherein the ratio (M2 / M1) of the content (at mol) of the second doping metal (M2) to the content (at mol) of the first doping metal (M1) is 0.03 to 15%.

7. In Paragraph 1, The above sodium complex transition metal oxide is a positive electrode active material for a sodium secondary battery represented by the following chemical formula 1: [Chemical Formula 1] Na a M1 b [TM x M2 1-x ]O 2+w In the above chemical formula 1, TM is at least one selected from Ni and Fe, Mn and Co, and M1 is Ca, and M2 is at least one selected from W, V, Mo and Nb, and 0.8≤a≤1.1, 0.005≤b≤0.02, 0.8≤x≤0.9998, 0.0001≤1-x≤0.002, -0.1≤w≤0.

1.

8. In Paragraph 1, The above sodium complex transition metal oxide comprises secondary particles formed by the aggregation of a plurality of primary particles, and A positive electrode active material for a sodium secondary battery, wherein the first doping metal and the second doping metal are located on the surface of the secondary particle, on the surface of the primary particle and / or inside the primary particle.

9. In Paragraph 1, The above sodium composite transition metal oxide is a positive electrode active material for a sodium secondary battery having an O3-type crystal structure.

10. A method for manufacturing a positive electrode active material for a sodium secondary battery according to claim 1, comprising the steps of dry mixing a transition metal hydroxide precursor, a first doping metal compound, a second doping metal compound, and a sodium compound (a) and calcining (b).

11. In Paragraph 10, The above dry mixing involves mixing an equal amount of a sodium compound with a Na / M (total metal excluding Na) ratio greater than 0.8 and less than 1, and The above-mentioned first doping metal is mixed in an amount of 0.5 to 2 at mol% with respect to the total amount of Na and the first doping metal, and A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the second doping metal is mixed at a concentration of 0.01 to 0.2 at mol% relative to the total amount of metals excluding Na and the first doping metal.

12. In Paragraph 10, The first doping metal compound is an acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof of Ca, and A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the second doping metal compound is at least one acetate compound selected from W, V, Mo and Nb, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide, or a combination thereof.

13. In Paragraph 10, A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the above calcination is carried out at a temperature of 700 to 1,100℃ for 5 to 40 hours.

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

15. A sodium secondary battery comprising a positive electrode according to paragraph 14; a negative electrode; and an electrolyte.

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