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

Abstract

An embodiment of the present invention provides a cathode active material for a sodium secondary battery, comprising a sodium composite transition metal oxide comprising: a transition metal containing nickel (Ni), copper (Cu), M1, and M2; and sodium (Na), wherein M1 and M2 in the composite transition metal oxide are different metals from each other and are manganese (Mn), iron (Fe), or cobalt (Co), and the sodium composite transition metal oxide comprises single crystals and satisfies relational expression 1.

Description

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

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

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

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

[0004] Layered transition metal oxides are 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 are Na x (TM)O2(2 / 3 <x≤1)와 같은 조성을 보이며, P2-type 구조를 기반으로 하는 양극 활물질은 Na x (TM)O2(x≤2 / 3) has a composition.

[0005] Generally, P2-type layered oxides possess relatively excellent cycle stability, but their commercial application is difficult due to drawbacks such as relatively low energy density resulting from low sodium content. As an alternative, 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. The cause of these major problems is analyzed to be the high amount of gas generated from side reactions with the electrolyte during repeated charge-discharge at high temperatures, as well as the significant volume changes of the particles caused by an unstable crystal structure.

[0006] Conventionally, to solve the problems of the aforementioned O3-type layered oxides, techniques involving doping with various metal elements or coating the particle surfaces have been commonly applied. However, the polycrystalline particles constituting the layered oxides provide a large surface area at grain boundaries or internal pores that reacts with the electrolyte, resulting in degraded electrochemical performance.

[0007] In prior art document 1 (Chinese published patent CN 117457895 A), the cathode material is formed by introducing a Cu doping element and growing a single crystal shape into primary particles, but the particle size of the single crystal is not large enough to sufficiently improve the Press density (PD), and there is a problem that the capacitance characteristics deteriorate due to an excessive amount of Cu doping.

[0008] Prior art document 2 (Korean Published Patent KR 10-2024-0011099 A) provides a sodium ion cathode material having a single-crystal form by doping it with an M element, having a specific chemical composition. Although M doping elements such as Zn, Sr, Zr, Ti, K, Cu, W, Al, Nb, Mo, etc. are used in an amount of 0.1 to 6 mol%, the particle size D50 of the single-crystal structure is 3 to 6 μm, so it is a small particle size as in Prior art document 1, and the effect of improving press density is insufficient.

[0009] The objective of the present invention is to suppress gas generation caused by side reactions in the electrolyte during high-temperature operation by single-crystallizing the positive electrode active material of a sodium-ion secondary battery. Specifically, by using a positive electrode active material with a large particle size in a single crystal, internal pores in conventional polycrystalline particles can be reduced, and an improved PD effect can be obtained.

[0010] In addition, the objective of the present invention is to i) successfully achieve single crystallization by applying an optimal transition metal composition, ii) achieve high-grade hardening of the single crystal size to the level of 9 to 13 μm, and iii) minimize capacity loss due to dopant introduction by using a relatively low content of Cu.

[0011] In addition, when single crystallization is applied through Cu doping, the stability of the crystal structure is improved even during repeated charge and discharge processes.

[0012] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery comprising a sodium composite transition metal oxide comprising nickel (Ni), copper (Cu), a transition metal containing M1 and M2; and sodium (Na), wherein in the composite transition metal oxide, M1 and M2 are different metals and are manganese (Mn), iron (Fe), or cobalt (Co), and the sodium composite transition metal oxide comprises a single crystal and satisfies the following equation 1.

[0013] [Relationship 1]

[0014] 0.8 ≤ D / C (㎛ / at mol%) ≤ 1.6

[0015] In the above relationship 1,

[0016] D is the average grain size (D50) of the single crystal, and C is the copper content (atomic mol%) relative to the total metal excluding sodium in the composite transition metal oxide.

[0017] The sodium complex transition metal oxide has an O3-type crystal structure, and the total amount (M) of nickel (Ni) and copper (Cu) with respect to the total metals excluding sodium is 30 to 35 at mol%, and the ratio of the content of M1 (M1 / M) and the ratio of the content of M2 (M2 / M) to the total amount (M) of nickel and copper may each be 0.85 to 1.20.

[0018] The sodium composite transition metal oxide has a total amount of nickel (Ni) and copper (Cu) of 30 to 35 at mol% relative to the total metal excluding the sodium, and the copper is substituted in the transition metal layer, wherein the content of the copper relative to the total amount of nickel and copper (100 at mol%) is 10 to 35%.

[0019] The above sodium complex transition metal oxide may include a compound represented by the following chemical formula 1.

[0020] [Chemical Formula 1]

[0021] Na a (Ni x-b Cu b M1 y M2 z M3 v )O2

[0022] In the above chemical formula 1,

[0023] M1, M2, and M3 are different elements, and

[0024] M1 and M2 are Fe, Mn, or Co, respectively, and

[0025] M3 is at least one selected from Fe, Mn, Co, P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and

[0026] 0.80 <a<1.20, 0.30≤x≤0.35, 0.05≤b≤0.15, 0.20<x-b<0.30, 0.30≤y≤0.35, 0.30≤z≤0.35, 0≤v≤0.10, x+y+z+v=1이다.

[0027] The above sodium complex transition metal oxide may include a compound represented by the following chemical formula 2.

[0028] [Chemical Formula 2]

[0029] Na a (Ni x-b Cu b Fe y Mn z M3 v )O2

[0030] In the above chemical formula 2,

[0031] M3 is at least one selected from Co, P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and

[0032] 0.80 <a<1.20, 0.30≤x≤0.35, 0.05≤b≤0.15, 0.20<x-b<0.30, 0.30≤y≤0.35, 0.30≤z≤0.35, 0≤v≤0.10, x+y+z+v=1이다.

[0033] The above sodium composite transition metal oxide may include a single crystal having an average particle size (D50) of 9 to 15 μm.

[0034] The above-mentioned positive active material has a PD (press density) of 2.85 to 3.05 g / cm³ 3 And, after 300 charge / discharge cycles, the volume change rate may be less than 3.5%.

[0035] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, comprising: a process of mixing a transition metal hydroxide precursor containing nickel (Ni), copper (Cu), M1 and M2 with a sodium compound; and a heat treatment process of heat-treating the mixture to produce a sodium composite transition metal oxide single crystal satisfying the following equation 1.

[0036] [Relationship 1]

[0037] 0.8 ≤ D / C (㎛ / at mol%) ≤ 1.6

[0038] In the above relationship 1,

[0039] D is the average grain size (D50) of the single crystal, and C is the copper content (atomic mol%) relative to the total metal excluding sodium in the composite transition metal oxide.

[0040] The above transition metal hydroxide precursor has an average particle size (D50) of 4 to 7 μm, and the heat treatment process may be performed to satisfy the following equation 2.

[0041] [Relationship 2]

[0042] 2 ≤ P2 / P1 ≤ 4

[0043] In the above relationship 2,

[0044] P1 is the average particle size (D50) of the transition metal hydroxide precursor, and P2 is the average particle size (D50) of the sodium complex transition metal oxide.

[0045] The above heat treatment process may involve heat treating the mixture at 900 to 1,000°C for 5 to 20 hours.

[0046] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery comprising the positive electrode active material.

[0047] Another embodiment of the present invention provides a sodium secondary battery using the anode.

[0048] In the present invention, a hydroxide precursor with a transition metal composition of Ni, Fe, Mn, and Cu is mixed with Na2CO3 and calcined at a high temperature to obtain a large single-crystal cathode active material, thereby achieving excellent effects such as reducing gas generation and improving PD characteristics when evaluating cycle life under high-temperature conditions.

[0049] According to the present invention, there is a reduction in gas generation and an improvement in PD resulting from single crystallization and the formation of large particles. Furthermore, capacity loss is minimized and the volume change rate of the positive electrode active material is reduced during the charge-discharge process by using an optimal transition metal composition and a relatively low content of Cu dopant, and a reduction in gas generation can be achieved by manufacturing large-diameter single-crystal particles.

[0050] Figure 1 shows NFM111 (NaNi 0.33 Fe 0.33 Mn 0.33 O2) and NFM424 (NaNi 0.4 Fe 0.2 Mn 0.4 O2) This is the result of a comparative analysis of the initial discharge capacity of sodium secondary batteries using positive electrode active material.

[0051] Figures 2 to 4 are Scanning Electron Microscopy (SEM) images of the positive electrode active materials according to Example 1, Comparative Examples 1 and 2.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] In this specification, the singular form includes the plural form unless specifically stated otherwise in the text.

[0056] 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.

[0057] 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.

[0058]

[0059] 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 composite transition metal oxide comprising a transition metal containing nickel (Ni), copper (Cu), M1, and M2; and sodium (Na), wherein M1 and M2 are different metals and are manganese (Mn), iron (Fe), or cobalt (Co).

[0060] Specifically, according to the present invention, by mixing a transition metal hydroxide precursor of a specific composition with a sodium compound and heat-treating it at a high temperature to produce an anode active material in the shape of a large-particle single crystal, the PD is improved, side reactions in the electrolyte are suppressed, the amount of gas generated is reduced, and the lifespan characteristics are improved.

[0061] The above sodium complex transition metal oxide comprises a single crystal and is characterized by satisfying the following relationship 1.

[0062] [Relationship 1]

[0063] 0.8 ≤ D / C (㎛ / at mol%) ≤ 1.6

[0064] In the above equation 1, D is the average grain size (D50) of the sodium composite transition metal oxide single crystal, and C is the copper content (atomic mol%) relative to the total metal excluding sodium in the composite transition metal oxide.

[0065] Specifically, the positive electrode active material may have a D / C value of 1 to 2 μm / at mol%, 1 to 1.5 μm / at mol%, 1 to 1.3 μm / at mol%, or 1 to 1.2 μm / at mol%.

[0066] In the present invention, as the amount of Cu doping increases within a certain range in a specific transition metal composition, a critical value (maximum value) of the average particle size of the single crystal may appear. Accordingly, by using a relatively small amount of Cu, a large particle size single crystal can be obtained while minimizing capacity loss due to the introduction of a dopant. On the other hand, if the D / C value is below the lower limit, the amount of Cu doping is excessive, which may degrade the capacity characteristics, and if the D / C value exceeds the upper limit, a small particle size single crystal anode active material or polycrystalline particles is obtained, which increases side reactions in the electrolyte and may degrade the press density.

[0067] Meanwhile, cathode active materials are classified into single-crystalline cathode active materials and polycrystalline cathode active materials according to the structure of the crystal. Here, single-crystalline cathode active materials are materials composed of a single particle of various metals such as nickel, iron, and manganese, and polycrystalline cathode active materials have a multi-particle (secondary particle) structure in which a single particle (primary particle) is aggregated. In addition, the cathode active material of the present invention may include a large-particle single-crystalline cathode active material among small particle diameters of 5 μm or less and large particle diameters of more than 5 μm and 10 to 20 μm, depending on the particle size. Furthermore, with respect to the total number of particles of the sodium composite transition metal oxide, the large-particle single-crystalline particles may be included in an amount of more than 50%, more than 60%, 70 to 100%, preferably 80 to 100%.

[0068] Meanwhile, the analysis of large-diameter single crystals or small-diameter polycrystalline particles can be performed using a transmission electron microscope (TEM) or a scanning electron microscope (SEM), but is not necessarily limited to these methods.

[0069] As in one embodiment, the sodium composite transition metal oxide of the present invention may include a single crystal having an average particle size (D50) of 9 to 15 μm, and for example, D50 may be 10 to 15 μm, 11 to 15 μm, or 12 to 14 μm.

[0070] Specifically, with respect to the total number of particles of the sodium composite transition metal oxide, the positive active material may contain more than 50%, more than 60%, 70 to 100%, preferably 80 to 100%, of single crystal particles having the D50 value.

[0071] Meanwhile, the particle size distribution of the above sodium composite transition metal oxide can be measured using the laser diffraction method. For example, after dispersing single-crystal particles in a dispersion medium, they can be introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000) and irradiated with ultrasound of about 28 kHz at an output of 60 W, and then a volume-cumulative particle size distribution graph can be obtained and the particle size corresponding to 50% of the volume-cumulative amount can be calculated.

[0072] Here, the particle size corresponding to 50% of the volume accumulation is defined as the average particle size (D50). Additionally, the particle size that is 50% of the volume accumulation is defined as D50 from the volume accumulation particle size distribution graph obtained by measuring the laser diffraction particle size distribution.

[0073] In addition, the above-mentioned positive electrode active material has a PD (press density) of 2.85 to 3.05 g / cm³ 3 It can be, for example, 2.9–3.05 g / cm³ 3 2.9~3.0 g / cm 3 or 2.95~3.0 g / cm³ 3 It may be, and the volume change rate after 300 charge / discharge cycles may be less than 3.5% or 3.0% or less, for example, 1 to 3.5% or 1 to 3.0%.

[0074] Meanwhile, the PD (Press density) measurement method involves feeding a sample of positive electrode active material into a press machine and applying a force of 2.5 tons to produce a positive electrode active material in the form of a pellet, and then measuring the density; however, the present invention is not necessarily limited to this.

[0075] In addition, the volume change rate after 300 charge-discharge cycles is calculated as a percentage of the volume change rate of the cell volume (V300) measured after 300 charge-discharge cycles (25℃, 1.5V ~ 4.0V, 0.5C / 0.5C) with respect to the initial volume (V1) of the cell measured after at least 3 formation operations (25℃, 1.5V ~ 4.0V, 0.1C / 0.1C) and 1 degassing, after manufacturing a sodium secondary battery manufactured using a positive electrode active material into a pouch-type cell, but the present invention is not necessarily limited thereto.

[0076] As in one embodiment, the sodium composite transition metal oxide may have an O3-type crystal structure, and the total amount (M) of nickel (Ni) and copper (Cu) may be 30 to 35 at mol% with respect to the total metal (100 at mol%) excluding sodium, and the ratio of the content of M1 (M1 / M) and the ratio of the content of M2 (M2 / M) to the total amount (M) of nickel and copper may each be 0.85 to 1.20.

[0077] If the above M1 / M and M2 / M are below the design range, Cu may replace M1 and M2 (Mn, Co, or Fe), and grow into a pseudo-single crystal or polycrystalline form rather than a single crystal form even during high-temperature heat treatment, which may be disadvantageous in terms of grain growth. Additionally, since Cu is used in excess and synthesized at high temperatures, single crystallization occurs easily and large grain sizes can be grown, but capacity and life characteristics are degraded. Furthermore, as Cu is a material with a flux effect, it can coarsen primary grains even in small amounts, so using an excess amount of Cu may not be desirable.

[0078] The copper is substituted into the transition metal layer, and the content of the copper relative to the total amount of nickel and copper (100 at mol%) may be 10 to 35%, for example, 15 to 35%, 20 to 35%, 25 to 35%, or 30 to 35%. If the Cu content replacing Ni is less than 10%, it is difficult to obtain a one-body single crystal form at the same temperature, and conversely, if the Cu content replacing Ni exceeds 35%, there is a problem of deterioration in capacity and life characteristics due to the decrease in Ni content.

[0079] As in one embodiment, the copper may be included in an amount of more than 3 at mol%, 5 to 15 at mol%, 7 to 15 at mol%, 7 to 13 at mol%, or 7 to 11 at mol% with respect to the total metal excluding sodium. By using a relatively low amount of Cu, large-diameter single crystals can be obtained while minimizing the reduction in capacity and lifespan due to the introduction of doping metals.

[0080] As in one embodiment, the sodium complex transition metal oxide may include a compound represented by the following chemical formula 1.

[0081] [Chemical Formula 1]

[0082] Na a (Ni x-b Cu b M1 y M2 z M3 v )O2

[0083] In the above chemical formula 1, M1, M2, and M3 may be different elements, M1 and M2 may each be Fe, Mn, or Co, and M3 may be at least one selected from Fe, Mn, Co, P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and 0.80 <a<1.20, 0.30≤x≤0.35, 0.05≤b≤0.15, 0.20<x-b<0.30, 0.30≤y≤0.35, 0.30≤z≤0.35, 0≤v≤0.10, x+y+z+v=1일 수 있다.

[0084] Specifically, the sodium complex transition metal oxide may include a compound represented by the following chemical formula 2.

[0085] [Chemical Formula 2]

[0086] Na a (Ni x-b Cu b Fe y Mn z M3 v )O2

[0087] In the above chemical formula 2,

[0088] M3 may be at least one selected from Fe, Mn, Co, P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and 0.80 <a<1.20, 0.30≤x≤0.35, 0.05≤b≤0.15, 0.20<x-b<0.30, 0.30≤y≤0.35, 0.30≤z≤0.35, 0≤v≤0.10, x+y+z+v=1일 수 있다.

[0089] The sodium (Na) is included in an amount of 0.8 to 1.2 moles per mole of sodium composite transition metal oxide, so that the composite particles are formed with an O3 crystal structure. Although the composite transition metal oxide exhibits high reversible capacity, there is a problem in that the limited sodium ions of the cathode material are consumed due to irreversible phase transitions, such as the formation of a solid electrolyte interface (SEI, CEI) film on the electrode surface during the initial charging process, thereby degrading the energy density and cycle stability of the secondary battery. Accordingly, according to the present invention, yttrium-doped sodium zirconium oxide coated on the cathode active material functions as a sacrificial cathode material, thereby improving the problem of sodium ion consumption during initial charging and enabling the expression of additional capacity.

[0090]

[0091] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery.

[0092] The above manufacturing method includes a process of mixing a transition metal hydroxide precursor containing nickel (Ni), copper (Cu), M1 and M2 with a sodium compound; and a process of heat treating to produce a sodium composite transition metal oxide single crystal satisfying the following equation 1.

[0093] As in one embodiment, in the mixing process, the transition metal hydroxide precursor and the sodium compound can be mixed in an equivalent amount of Na / M (total metal excluding Na) = greater than 0.8 and less than 1.2, and the mixing can be performed using a dry mixing method. In the case of the dry method, the transition metal hydroxide precursor particles and the sodium compound can be mixed uniformly, and the problem of increased costs due to process complexity in the wet method can be improved.

[0094] In addition, M1 and M2 in the above transition metal hydroxide precursor are the same as described above, and the transition metal hydroxide precursor may include a compound represented by the following chemical formula 3 and / or 4.

[0095] [Chemical Formula 3]

[0096] Ni x-b Cu b M1 y M2 z M3 v (OH)2

[0097] In the above chemical formula 3, M1, M2, and M3 may be different elements, M1 and M2 may each be Fe, Mn, or Co, and M3 may be at least one selected from Fe, Mn, Co, P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and 0.30≤x≤0.35, 0.05≤b≤0.15, 0.20 <x-b<0.30, 0.30≤y≤0.35, 0.30≤z≤0.35, 0≤v≤0.10, x+y+z+v=1일 수 있다.

[0098] [Chemical Formula 4]

[0099] Ni x-b Cu b Fe y Mn z M3 v (OH)2

[0100] In the above chemical formula 4, M3 may be at least one selected from Fe, Mn, Co, P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and 0.80 <a<1.20, 0.30≤x≤0.35, 0.05≤b≤0.15, 0.20<x-b<0.30, 0.30≤y≤0.35, 0.30≤z≤0.35, 0≤v≤0.10, x+y+z+v=1일 수 있다.

[0101] As in one embodiment, 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.

[0102] As in one embodiment, the transition metal hydroxide precursor may have an average particle size (D50) of 4 to 10 μm, for example, 4 to 7 μm. Since using a precursor with an average particle size (D50) exceeding 10 μm may result in multiple large-sized particles being aggregated to form a semi-single crystal, it may be preferable to use a precursor with an average particle size (D50) of 4 to 7 μm, although this is not necessarily limited.

[0103] As in one embodiment, the heat treatment process may be performed to satisfy the following relationship 2.

[0104] [Relationship 2]

[0105] 2 ≤ P2 / P1 ≤ 4

[0106] In the above relationship 2, P1 may be the average particle size (D50) of the transition metal hydroxide precursor, and P2 may be the average particle size (D50) of the sodium complex transition metal oxide.

[0107] Specifically, the heat treatment process may involve heat treating the mixture at 900 to 1,000°C for 5 to 20 hours.

[0108] Meanwhile, the particle size distribution of the above transition metal hydroxide precursor and sodium composite transition metal oxide can be measured using the laser diffraction method. For example, after dispersing the precursor particles and oxide particles in a dispersion medium, they can be introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000) and irradiated with ultrasound of about 28 kHz at an output of 60 W, and then a volume cumulative particle size distribution graph can be obtained and the particle size corresponding to 50% of the volume cumulative amount can be calculated.

[0109] Here, the particle size corresponding to 50% of the volume accumulation is defined as the average particle size (D50). Additionally, the particle size that is 50% of the volume accumulation is defined as D50 from the volume accumulation particle size distribution graph obtained by measuring the laser diffraction particle size distribution.

[0110] In the present invention, when the mixing ratio of the transition metal composition of the transition metal hydroxide precursor and the sodium compound is applied within a preferred range, there is an aspect in that large-diameter single crystallization becomes easier as the heat treatment temperature increases within the heat treatment temperature range.

[0111] The manufactured cathode active material may subsequently undergo additional washing, filtration, and drying processes.

[0112] The cathode active material produced by the method for producing a cathode active material for a sodium secondary battery according to the present invention suppresses gas generation due to side reactions in the electrolyte during high-temperature operation through single crystallization. Specifically, by producing a cathode active material with a large single-crystal particle size, internal pores in conventional polycrystalline particles can be reduced, and an improved PD effect can be obtained.

[0113]

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] 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.

[0125]

[0126] 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.

[0127] Examples

[0128] (Example 1)

[0129] Ni 0.22 Cu 0.11 Fe 0.33 Mn 0.33 Sodium compound Na2CO3 was added to an (OH)2 precursor (D50=4㎛) in an amount of Na / M (M=Ni+Cu+Fe+Mn) = 1.0 equivalent, and heat-treated at 950℃ (heating rate 4℃ / min) in an air atmosphere for 10 hours to produce a complex transition metal oxide powder (Na[Ni 0.22 Cu 0.11 Fe 0.33 Mn 0.33 [O2] powder) was manufactured.

[0130] A cathode slurry was prepared by dispersing 85 wt% of the manufactured coated 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.

[0131] 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.

[0132] (Comparative Example 1)

[0133] 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 heat treatment was performed at 850℃.

[0134] (Examples 2 and Comparative Examples 2 to 5)

[0135] 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 transition metal composition of Table 1 below was used.

[0136]

[0137] Experimental Example

[0138] Experimental Example 1: Evaluation of Capacity Characteristics According to Increase in Transition Metal Ni Content

[0139] Figure 1 is the result of a comparative analysis of the initial discharge capacity of sodium secondary batteries using NFM111 positive active material and NFM424 positive active material.

[0140] Specifically, NFM111 (NaNi 0.33 Fe 0.33 Mn 0.33 O2) Cathode active material and NFM424 (NaNi 0.4 Fe0.2 Mn 0.4 O2) A positive electrode and a sodium secondary battery were manufactured using the positive electrode active material, respectively.

[0141] The initial discharge capacity of the manufactured sodium secondary battery was measured through charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 2.2–4.2V, and a discharge rate of 0.1C.

[0142] Research is being conducted to improve capacity characteristics in conventional lithium-ion secondary batteries by increasing the Ni content in the transition metal composition. However, in sodium-ion secondary batteries, as shown in Figure 1, even when the Ni content was increased to 40 mol% (NFM424), no significant difference in capacity was observed compared to the case where a cathode active material with a composition of 33 mol% Ni was used; instead, a phenomenon of deterioration in lifespan characteristics was confirmed. Conversely, when the Ni content was reduced to the 20 mol% level, precursor synthesis was not easy due to the high degree of Fe oxidation caused by the relatively high Fe composition.

[0143]

[0144] Experimental Example 2: Evaluation of Physical Properties of Anode Active Material

[0145] Figures 2 to 4 show the Scanning Electron Microscopy (SEM) analysis results of the cathode active materials according to Example 1, Comparative Examples 1 and 2.

[0146] As shown in FIGS. 2 to 4, in Example 1, the positive active material was synthesized as a single crystal, but in Comparative Examples 1 and 2, it was confirmed that the positive active material was prepared in the form of secondary particles in which primary particles were aggregated due to the low heat treatment temperature and low Cu content.

[0147] Additionally, for the cathode active materials prepared in Examples 1 to 2 and Comparative Examples 1 to 5, PD, volume change rate (300 cycles), and average particle size (D50) were measured by the following method, and the ratio D / C of the average particle size (D50) of the single crystal to the copper content in the transition metal composition was calculated and summarized in Table 1 below.

[0148] Analysis of particle size distribution of positive electrode active material

[0149] The particle size distribution of the single crystals among the positive active materials was analyzed using a known laser diffraction method. Specifically, after dispersing each positive active material in a dispersion medium, a laser diffraction particle size measuring device (Microtrac MT 3000) was used to irradiate ultrasound of approximately 28 kHz with an output of 60 W, and then a volumetric cumulative particle size distribution graph was obtained.

[0150] Next, the particle size corresponding to 50% of the volume accumulation amount was calculated from the volume accumulation particle size distribution graph above.

[0151] PD (Press density) measurement method

[0152] A sample of positive active material (3.00 g) was weighed in a PD measuring device and placed into a mold, then placed in a press machine and 2.5 tons of force was applied to manufacture it into a pellet shape and the density was measured.

[0153] 300-cycle volume change rate measurement method

[0154] The sodium secondary batteries prepared in Examples 1 to 2 and Comparative Examples 1 to 5 were manufactured into small pouch forms (single-plate cells) and then subjected to a formation process (25℃, 1.5V to 4.0V, 0.1C / 0.1C charge / discharge 3 times).

[0155] After the phosphating process was completed, the initial volume of the single-plate cell was measured after degassing. Subsequently, the volume of the single-plate cell after 300 cycles of charge-discharge were performed under charge-discharge conditions (25℃, 1.5V ~ 4.0V, 0.5C / 0.5C).

[0156] Specific gravity and volume were measured using an electronic hydrometer (Alpha Mirage), and the measurement principle was based on Archimedes' principle; specifically, the principle applied was to automatically calculate the specific gravity and volume by measuring the weight in the air and then measuring the weight in water.

[0157] Manufacturing Method, Evaluation of Anode Active Material Properties, Heat Treatment Temperature (°C), Transition Metal Composition (Ni / Cu / Fe / Mn (Molar Ratio), PD (g / cm³) 3 ) Volume change rate (300cy)(%) D50(㎛) Relationship 1(D / C) Comparative Example 29 500 0.30 / 0.03 / 0.33 / 0.33 2.79 7.36.9 2.30 Example 29 500 0.26 / 0.07 / 0.33 / 0.33 2.93 3.11 1.4 1.63 Example 19 500 0.22 / 0.11 / 0.33 / 0.33 2.96 2.91 2.6 1.15 Comparative Example 5 9 500 0.11 / 0.22 / 0.33 / 0.33 2.95 2.81 2.20.55 Comparative Example 18 500 0.22 / 0.11 / 0.33 / 0.33 2.78 8.28.20.75 Comparative Example 39500.33 / 0.11 / 0.22 / 0.332.805.78.50.77 Comparative Example 49500.33 / 0.11 / 0.33 / 0.222.796.17.80.71

[0158] Referring to Table 1, it was confirmed that in Comparative Example 2, Example 2, Example 1, and Comparative Example 5, by adjusting the ratio of Cu to a preferred range of 7 to 11 atomic mol% while keeping the content of Ni and Cu fixed, it is possible to synthesize single crystals with optimal large particle size using a relatively low content of Cu and minimize capacity loss due to the introduction of Cu-doping metal. In the case of Comparative Example 2, large particle hardening of the single crystal is difficult due to insufficient Cu doping, whereas in Comparative Example 5, the large particle hardening effect of the single crystal is poor relative to the Cu doping amount, and deterioration of capacity characteristics due to Ni loss is expected.

[0159] In Comparative Example 1, a precursor with the same transition metal composition as in Example 1 was used, but the heat treatment temperature was excessively low, so large-diameter single crystallization did not proceed sufficiently.

[0160] In Comparative Examples 3 and 4, it was analyzed that when the content of Fe and Mn was reduced and Cu was doped, the single crystal did not grow to a large particle size.

[0161]

[0162] Additionally, the average particle size (D50) of the cathode active material (sodium composite transition metal oxide) was measured according to the change in the average particle size (D50) of the transition metal hydroxide precursor, and the results are listed in Table 2 below.

[0163] (Examples 3-1 to 3-3)

[0164] Ni 0.22 Cu 0.11 Fe 0.33 Mn 0.33 The average particle size (D50) of the (OH)2 precursor was applied as described in Table 2 below, and the positive electrode active material was prepared in the same manner as in Example 1.

[0165] Precursor average particle size (D50) P1 (㎛) Cathode active material single crystal average particle size (D50) P2 (㎛) Relationship 2 (P2 / P1) Example 14 12.6 3.15 Example 3-16 12.8 2.13 Example 3-28 12.3 1.54 Example 3-310 13.5 1.35

[0166] As shown in Table 2, even when using a small particle size precursor of 4 μm based on D50, the positive active material could be grown into a large particle size single crystal shape of 12 to 13 μm. Specifically, in Examples 3-1 to 3-3, it was confirmed that when using precursor particles of 4 to 7 μm, which is the preferred average particle size (D50) range, a positive active material in the form of a large particle size single crystal was produced regardless of the average particle size.

[0167] However, referring to Example 3-3, if the average particle size of the precursor increases excessively to 10 μm or more, multiple large-diameter particles may aggregate and a similar single-crystal form may appear; therefore, it is desirable to control the average particle size (D50) of the precursor to 4 to 7 μm to improve single crystallization and process efficiency.

[0168]

[0169] Experimental Example 3: Evaluation of Electrochemical Performance of Sodium Secondary Battery

[0170] For the sodium secondary batteries prepared in Examples 1 to 2 and Comparative Examples 1 to 5, the initial charge capacity and initial discharge capacity were measured through charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 2.2V to 4.0V, and a discharge rate of 0.1C.

[0171] In addition, for the same sodium secondary battery, 1 to 50 charge / discharge cycles were performed under conditions of 0.5C / 0.5C within a driving voltage range of 1.5V to 4.0V at 25℃, and the ratio of the 50-cycle discharge capacity to the 1-cycle discharge capacity (capacity retention) was measured.

[0172] Manufacturing Method Electrochemical Performance Evaluation (1.5-4.0V) Heat Treatment Temperature (°C) Transition Metal Composition Ni / Cu / Fe / Mn (Molar Ratio) Initial Capacity Characteristics (mAh / g) Lifetime Characteristics (50cy) (%) Charge Capacity Discharge Capacity Efficiency (%) Comparative Example 29 500 0.30 / 0.03 / 0.33 / 0.33 15 3.21 40.69 1.87 9.3 Example 29 500 0.26 / 0.07 / 0.33 / 0.33 148.11 35.49 1.48 1.2 Example 19 500 0.22 / 0.11 / 0.33 / 0.33 147.51 34.09 0.98 2.1 Comparative Example 59500.11 / 0.22 / 0.33 / 0.33141.0129.191.578.1 Comparative Example 18500.22 / 0.11 / 0.33 / 0.33145.2136.894.271.0 Comparative Example 39500.33 / 0.11 / 0.22 / 0.33151.0142.194.177.5 Comparative Example 49500.33 / 0.11 / 0.33 / 0.22150.3142.995.173.1

[0173] Referring to Table 3, it was confirmed that in Examples 1 and 2 corresponding to the present invention, the capacity characteristics and lifespan characteristics were improved compared to Comparative Examples 1 to 5.

Claims

1. A sodium complex transition metal oxide comprising nickel (Ni), copper (Cu), M1 and M2; and sodium (Na); and In the above composite transition metal oxide, M1 and M2 are different metals, and are manganese (Mn), iron (Fe), or cobalt (Co), and A positive electrode active material for a sodium secondary battery, wherein the above sodium composite transition metal oxide comprises a single crystal and satisfies the following Equation 1: [Relationship 1] 0.8 ≤ D / C (㎛ / at mol%) ≤ 1.6 In the above equation 1, D is the average grain size (D50) of the single crystal, and C is the copper content (atomic mol%) relative to the total metal excluding sodium in the complex transition metal oxide.

2. In Paragraph 1, The above sodium complex transition metal oxide has an O3-type crystal structure, and The total amount (M) of nickel (Ni) and copper (Cu) with respect to all metals excluding the sodium is 30 to 35 at mol%, and A positive electrode active material for a sodium secondary battery, wherein the ratio of the content of M1 (M1 / M) and the ratio of the content of M2 (M2 / M) to the total amount of nickel and copper (M) are each 0.85 to 1.

20.

3. In Paragraph 1, The above sodium complex transition metal oxide has a total amount of nickel (Ni) and copper (Cu) of 30 to 35 at mol% with respect to the total metal excluding sodium, and A positive electrode active material for a sodium secondary battery, wherein the copper is substituted in a transition metal layer, and the content of the copper is 10 to 35% relative to the total amount of nickel and copper (100 at mol%).

4. In Paragraph 1, The above sodium complex transition metal oxide is a positive electrode active material for a sodium secondary battery comprising a compound represented by the following chemical formula 1: [Chemical Formula 1] So a (Ni x-b Cu b M1 y M2 z M3 v )O2 In the above chemical formula 1, M1, M2, and M3 are different elements, and M1 and M2 are Fe, Mn, or Co, respectively, and M3 is at least one selected from Fe, Mn, Co, P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and 0.80 <a<1.20, 0.30≤x≤0.35, 0.05≤b≤0.15, 0.20<x-b<0.30, 0.30≤y≤0.35, 0.30≤z≤0.35, 0≤v≤0.10, x+y+z+v=1이다.

5. In Paragraph 1, The above sodium complex transition metal oxide is a positive electrode active material for a sodium secondary battery comprising a compound represented by the following chemical formula 2: [Chemical Formula 2] So a (Ni x-b Cu b Feb y Mr z M3 v )O2 In the above chemical formula 2, M3 is at least one selected from Co, P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and 0.80 <a<1.20, 0.30≤x≤0.35, 0.05≤b≤0.15, 0.20<x-b<0.30, 0.30≤y≤0.35, 0.30≤z≤0.35, 0≤v≤0.10, x+y+z+v=1이다.

6. In Paragraph 1, The above sodium composite transition metal oxide comprises a single crystal having an average particle size (D50) of 9 to 15 μm, and is a positive electrode active material for a sodium secondary battery.

7. In Paragraph 1, The above-mentioned positive active material has a PD (press density) of 2.85 to 3.05 g / cm³ 3 And, A positive electrode active material for a sodium secondary battery having a volume change rate of less than 3.5% after 300 charge-discharge cycles.

8. A process of mixing a transition metal hydroxide precursor containing nickel (Ni), copper (Cu), M1 and M2 with a sodium compound; and A method for manufacturing a positive electrode active material for a sodium secondary battery, comprising: a heat treatment process for manufacturing a sodium composite transition metal oxide single crystal by heat treating the above mixture to satisfy the following relationship 1; [Relationship 1] 0.8 ≤ D / C (㎛ / at mol%) ≤ 1.6 In the above equation 1, D is the average grain size (D50) of the single crystal, and C is the copper content (atomic mol%) relative to the total metal excluding sodium in the complex transition metal oxide.

9. In Paragraph 8, The above transition metal hydroxide precursor has an average particle size (D50) of 4 to 7 μm, and A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the above heat treatment process is performed to satisfy the following relationship 2: [Relationship 2] 2 ≤ P2 / P1 ≤ 4 In the above relationship 2, P1 is the average particle size (D50) of the transition metal hydroxide precursor, and P2 is the average particle size (D50) of the sodium complex transition metal oxide.

10. In Paragraph 8, A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the heat treatment process described above involves heat-treating the mixture at 900 to 1,000°C for 5 to 20 hours.

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

12. A sodium secondary battery using a positive electrode according to Paragraph 11.

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