Positive electrode active material for sodium secondary batteries, method for producing the same, and sodium secondary battery containing the same

Abstract

The present invention provides a positive electrode active material for a sodium secondary battery comprising a sodium-manganese oxide containing at least sodium (Na), nickel (Ni), and manganese (Mn), with manganese accounting for 55 mol% or more of the total metals other than sodium, wherein the sodium-manganese oxide is a secondary particle formed by the aggregation of at least one primary particle, and the surface and interior of the secondary particle are doped with cobalt (Co), a method for producing the positive electrode active material, a positive electrode containing the positive electrode active material, and a sodium secondary battery.

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material for a sodium secondary battery, a method for producing the same, and a sodium secondary battery containing the same. [Background technology]

[0002] Lithium-ion rechargeable batteries have been widely used as energy storage devices in various fields of electronics technology. Recently, with the surge in demand for lithium-ion rechargeable batteries, sodium-ion rechargeable batteries are attracting attention as a substitute for lithium, an expensive metal. Sodium-ion rechargeable batteries have an insertion / deinsertion reaction operating principle similar to that of lithium-ion rechargeable batteries, making them one of the next-generation materials with high potential for application in rechargeable batteries.

[0003] Transition metal oxides with a layered structure, a typical form of positive electrode active material, have the advantages of a simple structure, excellent electrochemical performance, and ease of synthesis. They can be classified into O3 type and P2 type depending on the position of the sodium ions. Of these, P2 type positive electrode active materials have the advantages of relatively high atmospheric and moisture stability, and are not very sensitive to synthesis conditions such as temperature and atmosphere. However, when sodium ions with a large ionic radius are inserted / deinserted within the layered structure, the lattice structure changes, causing the positive electrode active material to undergo continuous phase transitions. The irreversible phase generated during this process reduces the cycle life and high-rate characteristics of sodium-ion secondary batteries.

[0004] Among layered transition metal oxides, high-manganese (high-Mn) sodium nickel manganese oxide (NNMO) has advantages over other transition metal oxides, such as high capacity, price competitiveness (advantageous reserves), and environmental friendliness. However, it has the disadvantage of relatively low electrical conductivity due to the excess manganese contained in the oxide, resulting in a low capacity rate for sodium secondary batteries. When the capacity rate is low, there is a problem of reduced charge / discharge capacity and capacity retention during the cycling of sodium secondary batteries.

[0005] Furthermore, sodium by-products (such as Na2CO3 and NaOH) that remain in large quantities on the particle surface during the firing of the positive electrode active material degrade the electrochemical performance of the battery, such as generating gas due to side reactions in the electrolyte during battery operation, and reducing the capacity and output of the positive electrode material. Conventionally, methods have been applied to remove by-products from the surface of the positive electrode active material by washing with water using distilled water or ethanol, but there are limitations to sufficiently removing residual sodium, and sodium ions present in the positive electrode active material may be desorbed during the washing process, potentially damaging the particle surface.

[0006] To address the aforementioned problems, attempts have been made to improve the high-manganese oxide issue by modifying the particle structure and surface, such as adjusting the particle size of the positive electrode active material or coating the particle surface. However, these attempts have not yet reached a level suitable for commercialization. [Overview of the project] [Problems that the invention aims to solve]

[0007] In order to improve the decrease in electrical conductivity caused by the excess manganese in high-manganese oxides, it is preferable to produce ternary transition metal oxides that further contain transition metals other than manganese. However, when coprecipitation reactions are carried out to produce ternary transition metal hydroxide precursors, there is a problem in that a large amount of manganese oxide (such as MnO2) is synthesized due to the high reactivity of manganese.

[0008] In order to solve the above-mentioned problems, the present invention provides a technique for doping a binary transition metal oxide containing a high manganese content with a different type of transition metal cobalt. Specifically, by applying a cobalt coating during the water washing treatment of a sodium-high manganese oxide, residual sodium by-products are removed, and cobalt is doped into the surface and interior of the oxide particles, thereby providing a ternary transition metal oxide.

[0009] The present invention provides a sodium-high manganese NCM (Ni-Co-Mn) oxide in which three or more transition metals are uniformly distributed on the surface and inside the oxide particles.

[0010] Furthermore, the present invention aims to provide a sodium secondary battery in which the low rate characteristics and life characteristics of conventional high-manganese oxides are improved by using a positive electrode containing the aforementioned sodium high-manganese oxide positive electrode active material. [Means for solving the problem]

[0011] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising a sodium-manganese oxide containing at least sodium (Na), nickel (Ni), and manganese (Mn), and containing 55 mol% or more of manganese among the total metals other than sodium, wherein the sodium-manganese oxide is a secondary particle formed by the aggregation of at least one primary particle, and the surface and interior of the secondary particle are doped with cobalt (Co).

[0012] The aforementioned sodium manganese oxide can satisfy the following relational equation 1. [Relationship 1] (Dc-Ds) / Dt ≤ ±10% In the relational expression 1, based on the cross-section of the secondary particles, among the lengths (R) from the center to the surface, the region corresponding to 0 to 50% is defined as the first region (R1), and the region corresponding to 50 to 100% is defined as the second region (R2). Ds, Dc, and Dt are respectively measured in the first region (R1), the second region (R2), and the entire cross-section of the particle (R1 + R2), and are respectively the molar concentrations of cobalt (Co) with respect to the total metal (M) other than sodium.

[0013] Based on the cross-section of the secondary particles, the cobalt concentration in the first region (R1) may be 50 mol% or more with respect to 100 mol% of the total cobalt concentration.

[0014] The sodium manganese-based oxide can satisfy the following relational expression 2. [Relational expression 2] (Mc - Ms) / Mt ≦ ±10% In the relational expression 2, based on the cross-section of the secondary particles, among the lengths (R) from the center to the surface, the region corresponding to 0 to 50% is defined as the first region (R1), and the region corresponding to 50 to 100% is defined as the second region (R2). Ms, Mc, and Mt are respectively measured in the first region (R1), the second region (R2), and the entire cross-section of the particle (R1 + R2), and are respectively the molar concentrations of nickel (Ni) or manganese (Mn) with respect to the total metal (M) other than sodium.

[0015] The sodium manganese-based oxide may have an aspect ratio of the primary particles of 1:2.5 to 1:5.5.

[0016] The sodium manganese-based oxide can be represented by the following chemical formula 1. [Chemical formula 1] Na a Ni x Co y M1 z Mn 1-x-y-z O2 In the chemical formula 1, M1 is P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf It is at least one selected from Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu, and 0.50 ≦ a ≦ 0.80, 0.05 ≦ x ≦ 0.45, 0.01 ≦ y ≦ 0.15, 0 ≦ z ≦ 0.1, 0.55 ≦ 1 - x - y - z ≦ 0.85.

[0017] The sodium manganese oxide can show at least one peak selected from the group consisting of (004), (100), (101), (102), (103), (104), and (002) peaks by X-ray diffraction analysis (XRD).

[0018] Based on X-ray diffraction analysis, the ratio of NiO measured by the Rietveld refinement method in the sodium manganese oxide may be 5% or less.

[0019] The sodium manganese oxide can include a P2-type layered structure.

[0020] The positive electrode active material can contain residual sodium of 5,000 ppm or less.

[0021] Another embodiment of the present invention provides a method for manufacturing a positive electrode active material for a sodium secondary battery, including: a) preparing a sodium manganese oxide containing at least sodium (Na), nickel (Ni), and manganese (Mn), and containing 55 mol% or more of manganese among the total metals other than sodium; b) charging the sodium manganese oxide, a solvent, and a cobalt precursor into a reactor to form a cobalt compound on the surface of the particles of the sodium manganese oxide; and c) heat-treating the product of step b) to dope cobalt on the surface and inside of the sodium manganese oxide particles.

[0022] The sodium manganese oxide in step a) above is a secondary particle formed by the aggregation of at least one primary particle, and the aspect ratio of the primary particle may be 1:2.5 to 1:5.5.

[0023] Step b) may include b1) adding the sodium manganese oxide and solvent to a reactor and stirring, and b2) adding a cobalt precursor to the reactor and stirring to form cobalt hydroxide on the surface of the sodium manganese oxide.

[0024] The cobalt precursor in step b) above can be introduced into the reactor such that the molar ratio of cobalt (Co / M) to the total metal (M) other than sodium in the sodium-manganese oxide is 1 to 15.

[0025] The cobalt precursor in step b) above may include Co(OH)2, CoOOH, Co(OCOCH3)2, Co(NO3)2, CoSO4, Co(SO4)2, or a combination thereof.

[0026] The heat treatment in step c) above can be carried out at 600-900°C for 6-18 hours.

[0027] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery containing the positive electrode active material.

[0028] Another embodiment of the present invention provides a sodium secondary battery using the positive electrode. [Effects of the Invention]

[0029] In this invention, by uniformly doping the surface and interior of sodium manganese oxide particles with cobalt, surface degradation problems can be effectively mitigated even with a relatively small amount of Co doping, thereby improving the low electrical conductivity and rate characteristics of high manganese oxides.

[0030] Furthermore, the present invention offers the advantage of being able to uniformly form the Co compound on the surface of sodium manganese oxide particles by simultaneously performing the residual sodium washing step and the Co coating step under specific conditions, enabling uniform Co doping in subsequent steps, and simplifying the process by integrating multiple overlapping steps.

[0031] The sodium secondary battery of the present invention, by introducing the above-described positive electrode active material for sodium secondary batteries, can help realize a long-life sodium secondary battery with improved rate characteristics and lifespan characteristics compared to conventional high-manganese oxides. Furthermore, by using Na instead of the expensive metal Li, the proportion of Mn, which is relatively cheaper than Ni and Co, can be increased, thus making it price-competitive. [Brief explanation of the drawing]

[0032] [Figure 1a] This is an SEM image of the surface of the particles produced in step a) of Example 1 (before coating). [Figure 1b] This is an SEM image of the surface of the particles produced in step b) (after coating) of Example 1. [Figure 1c] This is an SEM image of the surface of the particles produced in step c) of Example 1 (after heat treatment). [Figure 1d] This is an SEM image of the surface of the positive electrode active material particles manufactured in Comparative Example 4. [Figure 2a] This is a SEM-EDS image of the surface of the Co-doped cathode active material particles manufactured in Example 1. [Figure 2b] This is a SEM-EDS image of the surface of the Co-doped cathode active material particles manufactured in Comparative Example 2. [Figure 2c] This is a SEM-EDS image of the surface of the Co-doped cathode active material particles manufactured in Comparative Example 3. [Figure 2d] This is a SEM-EDS image of the surface of the Co-doped cathode active material particles manufactured in Comparative Example 4. [Figure 3a] This is a SEM-EDS image of a cross-section of the Co-doped cathode active material particles manufactured in Example 1. [Figure 3b] This graph (EDS line scanning) shows the change in the content of metal elements (Ni, Co, Mn) present in sodium manganese oxide secondary particles, as determined by EDS analysis in the direction shown on the SEM image of the cross-section of the Co-doped cathode active material particles manufactured in Example 1. [Figure 4] These are the XRD analysis results of the positive electrode active material (bulk) produced in Example 1 and Comparative Examples 2-3. [Modes for carrying out the invention]

[0033] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and can be realized in a variety of different forms, provided that these embodiments complete the disclosure of the present invention and fully inform those who are ordinary skill in the art to which the invention pertains, and the present invention is defined only by the scope of the claims.

[0034] Unless otherwise defined, all terms used herein (including technical and scientific terms) should be used in a sense that is commonly understood by a person of ordinary skill in the art to which the invention pertains. Throughout the specification, when a part of the specification “includes” a component, this means, unless otherwise stated, that it may include other components rather than excluding them. Also, singular forms include plural forms unless otherwise specified in the text.

[0035] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising a sodium-manganese oxide containing at least sodium (Na), nickel (Ni), and manganese (Mn), and containing 55 mol% or more of manganese among the total metals other than sodium, wherein the sodium-manganese oxide is a secondary particle formed by the aggregation of at least one primary particle, and the surface and interior of the secondary particle are doped with cobalt (Co).

[0036] The sodium-manganese oxide is an NNMO-based sodium-manganese oxide containing at least sodium, nickel, and manganese. The sodium-manganese oxide is a high-manganese (high-Mn) oxide containing 55 mol% or more of manganese among the total metals other than sodium, and the manganese content among the metals other than sodium may be 55 mol% or more, 60 mol% or more, or 65 mol% or more. The upper limit is not particularly limited, but for example, it may be 85 mol% or less, 80 mol% or less, or 75 mol% or less. The higher the manganese content among the metals other than sodium, the more advantageous it is to be able to exhibit high capacity under high-voltage operating environments, and the concentration of both nickel and cobalt can be reduced to enhance price competitiveness.

[0037] The sodium manganese oxide of the present invention is characterized in that it is a secondary particle formed by the aggregation of at least one primary particle, and the surface and interior of the secondary particle are doped with cobalt (Co).

[0038] Most degradation of positive electrode active materials begins on the surface of oxide particles and diffuses into the interior of the material. In particular, surface degradation due to side reactions with the electrolyte can occur in the high-voltage region where high-content manganese oxides are in operation. The present invention uniformly dops the surface and interior of the sodium-manganese oxide particles with cobalt, thereby effectively mitigating the problem of surface degradation even with a relatively small amount of Co doping. This improves the low electrical conductivity and rate characteristics of high-manganese oxides and extends the battery cycle life.

[0039] On the other hand, as described above, when sodium oxide is produced using a binary transition metal hydroxide precursor and then doped with Co coating, the generation of manganese oxide (such as MnO2), a by-product that is produced in large quantities, which is a problem when producing high-manganese ternary hydroxide precursors, can be suppressed. Furthermore, compared to a technique in which Co is concentrated and distributed only on the surface of the oxide particles to form a coating layer, the present invention can further improve the above-mentioned effects by uniformly doping the inside and surface of the oxide particles with Co. Moreover, when Co, an electrochemically active metal, is doped, it not only prevents surface degradation of the oxide particles but also participates in oxidation-reduction reactions during charging / discharging of sodium secondary batteries, thereby increasing the discharge capacity compared to coating / doping with other metals.

[0040] The sodium manganese oxide does not necessarily have a substantially cobalt (Co) molar concentration gradient in the radial direction of the cross-section of the secondary particle. This can further improve the above-mentioned effects. Here, "substantially having no cobalt molar concentration gradient" may include a concept in which the cobalt concentration changes slightly in a certain region of the cross-section of the secondary particle, for example, a predetermined cobalt concentration that gradually decreases or increases in the radial direction of the particle. Cases where a trend in concentration changes is observed can be excluded.

[0041] The aforementioned sodium manganese oxide can satisfy the following relational equation 1.

[0042] [Relationship 1] (Dc-Ds) / Dt ≤ ±10%

[0043] In relational equation 1, if we define the region corresponding to 0-50% of the length (R) from the center to the surface, based on the cross-section of the secondary particle, as the first region (R1) and the region corresponding to 50-100% as the second region (R2), then Ds, Dc, and Dt are measured in the first region (R1), the second region (R2), and the entire cross-section of the particle (R1+R2), respectively, and may each be the molar concentration of cobalt (Co) relative to the total metal (M) other than sodium.

[0044] The sodium manganese oxide may be, for example, (Dc-Ds) / Dt≦±9%, (Dc-Ds) / Dt≦±8%, (Dc-Ds) / Dt≦±7%, (Dc-Ds) / Dt≦±6%, (Dc-Ds) / Dt≦±5%, (Dc-Ds) / Dt≦±4%, (Dc-Ds) / Dt≦±3%, (Dc-Ds) / Dt≦±2%, or (Dc-Ds) / Dt≦±1%, and may also be 0≦(Dc-Ds) / Dt≦9%, 0≦(Dc-Ds) / Dt≦8%, 0≦(Dc-Ds) / Dt≦7%, 0≦(Dc-Ds) / Dt≦6%, or 0≦(Dc-Ds) / D t≦5%, 0≦(Dc-Ds) / Dt≦4%, 0≦(Dc-Ds) / Dt≦3%, 0≦(Dc-Ds) / Dt≦2%, or 0≦(Dc-Ds) / Dt≦1%, and 0≧(Dc-Ds) / Dt≧-9%, 0≧(Dc-Ds) / Dt≧-8%, 0≧(Dc-Ds) / Dt≧-7%, 0≧(Dc-Ds) / Dt≧-6%, 0≧(Dc-Ds) / Dt≧-5%, 0≧(Dc-Ds) / Dt≧-4%, 0≧(Dc-Ds) / Dt≧-3%, 0≧(Dc-Ds) / Dt≧-2%, or 0≧(Dc-Ds) / Dt≧-1%. In this invention, Co doping is uniformly performed on the surface and interior of secondary particles of sodium manganese oxide, suppressing the synthesis of by-products such as manganese oxide (MnO2) and cobalt oxide (CoO2), and enabling the production of high-manganese transition metal ternary oxides with a uniform transition metal composition on the surface and interior of the secondary particles.

[0045] The sodium manganese oxide may have a cobalt concentration of 50 mol% or more, or 60 mol% or more, and 80 mol% or less, or 70 mol% or less, relative to a total cobalt concentration of 100 mol%, based on the cross-section of the secondary particles. For example, it may be 50-80 mol%, 50-75 mol%, 50-70 mol%, or 50-60 mol%. This can further improve the effects described above.

[0046] The aforementioned sodium manganese oxide can satisfy the following relational equation 2.

[0047] [Relationship 2] (Mc-Ms) / Mt ≤ ±10%

[0048] In relational equation 2, if we define the region corresponding to 0-50% of the length (R) from the center to the surface, based on the cross-section of the secondary particle, as the first region (R1) and the region corresponding to 50-100% as the second region (R2), then Ms, Mc, and Mt are measured in the first region (R1), the second region (R2), and the entire cross-section of the particle (R1+R2), respectively, and may be the molar concentrations of nickel (Ni) or manganese (Mn) relative to the total metal (M) other than sodium.

[0049] The sodium manganese oxides mentioned above include, for example, (Mc-Ms) / Mt≦±9%, (Mc-Ms) / Mt≦±8%, (Mc-Ms) / Mt≦±7%, and (Mc-Ms) / Mt≦±6%. %, (Mc-Ms) / Mt≦±5%, (Mc-Ms) / Mt≦±4%, (Mc-Ms) / Mt≦±3%, (Mc-Ms) / Mt≦±2%, or (Mc-Ms) / Mt≦±1%, and 0≦(Mc-Ms) / Mt≦9%, 0≦(Mc-Ms) / Mt≦8%, 0≦(Mc-Ms) / Mt≦7%, 0≦(Mc-Ms) / Mt≦6%, 0≦(Mc-Ms) / Mt≦5%, 0≦(Mc-Ms) / Mt≦4%, 0≦(Mc-Ms) / Mt≦3%, 0≦( The ratio of Mc-Ms) / Mt to Mt may be ≤2%, or 0≤(Mc-Ms) / Mt≤1%, and may also be 0≥(Mc-Ms) / Mt≥-9%, 0≥(Mc-Ms) / Mt≥-8%, 0≥(Mc-Ms) / Mt≥-7%, 0≥(Mc-Ms) / Mt≥-6%, 0≥(Mc-Ms) / Mt≥-5%, 0≥(Mc-Ms) / Mt≥-4%, 0≥(Mc-Ms) / Mt≥-3%, 0≥(Mc-Ms) / Mt≥-2%, or 0≥(Mc-Ms) / Mt≥-1%. In this invention, the synthesis of by-products such as nickel oxide (NiO) and manganese oxide (MnO2) is suppressed, and a high-manganese transition metal ternary oxide with a uniform transition metal composition on the surface and inside of secondary particles can be produced.

[0050] On the other hand, the molar concentrations of Co, Ni, and Mn distributed radially in the secondary particles may be obtained by performing EDX mapping on a cross-sectional SEM or TEM image of the sodium manganese oxide particles, followed by line scanning analysis, but the present invention is not limited thereto.

[0051] The primary particles of the sodium manganese oxide may have an aspect ratio of 1:2.5 to 1:5.5, 1:2.5 to 1:5, or 1:2.7 to 1:4.5, and the shape of the primary particles may be plate-type or sheet-type.

[0052] If the aspect ratio of the primary particles is less than 1:2.5, the resistance may increase during charging / discharging, and if it is greater than 1:5.5, it is undesirable because Na and Air in the layered structure may react to form byproducts.

[0053] The term "aspect ratio" as used in this application refers to the ratio (Length / Width ratio) of the major axis (Length; a-axis) and minor axis (Width; b-axis) of the primary particle. When the major axis indicates the direction of the relatively long region of the primary particle, the minor axis is located on the same plane as the major axis and indicates the length of the relatively short region. Here, the primary particle may have a sheet shape, meaning that the length in the thickness direction of the primary particle is significantly smaller than the length in the plane direction (major axis and minor axis) of the primary particle. On the other hand, the minor axis may be in a direction perpendicular to the major axis, and the "aspect ratio" of the primary particle can be calculated as the ratio of the major axis to the minor axis of the primary particle measured from the surface of the primary particle. This allows the overall shape of the primary particle to be determined according to the lengths of the major axis (a-axis) and the minor axis (b-axis). For example, if the aspect ratio, which is the ratio of the major axis to the minor axis of the primary particle, exceeds 5, the shape of the primary particle may be relatively closer to a rod shape than a sheet shape. On the other hand, the closer the aspect ratio of the primary particle is to 1, the closer the shape of the primary particle is to a sheet shape. In other words, in the present invention, the aspect ratio of the primary particle can be used as an indicator of the sheet shape of the primary particle, and the closer the aspect ratio of the primary particle is to 1, the more it can be understood to match the sheet shape of the primary particle.

[0054] The sodium manganese-based oxide may be located on the particles and further include a cobalt-containing coating layer or may not include a cobalt-containing coating layer. Here, it is preferable that the sodium manganese-based oxide does not include the cobalt-containing coating layer. The cobalt-containing coating layer can be present on the surface of the primary particles and / or secondary particles of the sodium manganese-based oxide, and can be formed into a continuous or non-continuous coating layer with a thickness of 30 nm or less, 20 nm or less, 0.1 to 10 nm, 0.1 to 5 nm, or 0.5 to 3 nm. Thereby, there is an effect of preventing surface deterioration that occurs during charge and discharge in a high voltage region. When the cobalt coating layer exceeds the above thickness range, a cobalt aggregation phenomenon (non-uniform Co layer) may occur, and the synthesis of by-products such as cobalt oxide (CoO2) on the surface of the sodium manganese-based oxide particles increases, making it difficult to uniformly dope cobalt into the surface and inside of the particles. During the production of the high-manganese NCM ternary transition metal oxide, the amount of cobalt doping is not sufficient, and it may be difficult to improve the low rate characteristics and life characteristics, which are the demerits of conventional high-manganese oxides. Therefore, in the present invention, by optimizing the Co content and mixing Co simultaneously with the water washing process of residual sodium, Co is uniformly dispersed on the surface of the oxide particles, and by adjusting the heat treatment temperature and time in the subsequent process, the doping metal is uniformly doped into the surface and inside of the oxide particles, so that a Co coating layer can be substantially not formed. This can prevent surface deterioration that occurs during charge and discharge in a high voltage region. When the cobalt coating layer exceeds the above thickness range, a cobalt aggregation phenomenon (non-uniform Co layer) may occur, and the synthesis of by-products such as cobalt oxide (CoO2) on the surface of the sodium manganese-based oxide particles increases, making it difficult to uniformly dope cobalt into the surface and inside of the particles. During the production of the high-manganese NCM ternary transition metal oxide, the amount of cobalt doping is not sufficient, and it may be difficult to improve the low rate characteristics and life characteristics, which are the demerits of conventional high-manganese oxides. Therefore, in the present invention, by optimizing the Co content and mixing Co simultaneously with the water washing process of residual sodium, Co is uniformly dispersed on the surface of the oxide particles, and by adjusting the heat treatment temperature and time in the subsequent process, the doping metal is uniformly doped into the surface and inside of the oxide particles, so that a Co coating layer can be substantially not formed.

[0055] The sodium manganese-based oxide can be represented by the following Chemical Formula 1.

[0056] [Chemical Formula 1] Na a Ni x Co y M1 z Mn 1-x-y-z O2

[0057] In the above chemical formula 1, M1 may be at least one selected from P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu, and may be 0.50≦a≦0.80, 0.05≦x≦0.45, 0.01≦y≦0.15, 0≦z≦0.1, and 0.55≦1-xyz≦0.85.

[0058] The sodium manganese oxide of chemical formula 1 may have a molar ratio (Na / M) of sodium (Na) to total metal (M) other than sodium of 0.5 to 0.8. In chemical formula 1, if the content corresponding to a is less than 0.5, the capacity may decrease, and if it exceeds 0.8, the position of the sodium ions may change, and an O3 type crystal structure may be observed. The O3 type positive electrode active material may have lower atmospheric and moisture stability and be more sensitive to synthesis conditions (temperature and atmosphere, etc.) compared to the P2 type positive electrode active material. The Na may more preferably be 0.60 ≤ a ≤ 0.80, 0.60 ≤ a ≤ 0.75, or 0.65 ≤ a ≤ 0.75.

[0059] The sodium-manganese oxide may have a molar ratio (Mn / M) of manganese (Mn) to the total metal (M) other than sodium of 0.55 to 0.85. When the manganese content is within the specified range, high capacity can be achieved under high-voltage operating conditions, ensuring price competitiveness. The Mn content may more preferably be 0.60 ≤ 1-xyz ≤ 0.85, 0.60 ≤ 1-xyz ≤ 0.80, or 0.60 ≤ 1-xyz ≤ 0.70 in chemical formula 1.

[0060] The sodium manganese oxide may have a molar ratio of nickel (Ni) to the total metal (M) other than sodium (Ni / M) of 0.05 to 0.45. This can suppress the problem that arises as the nickel content increases within the range of nickel content, namely, the problem of decreased structural and chemical stability of the active material due to changes in the oxidation state of nickel. The Ni content may more preferably be 0.1 ≤ x ≤ 0.45, 0.2 ≤ x ≤ 0.45, 0.25 ≤ x ≤ 0.45, or 0.3 ≤ x ≤ 0.4 in chemical formula 1.

[0061] Furthermore, the sodium manganese oxide may have a molar ratio of nickel to manganese (Ni / Mn) of 0.05-0.75, 0.1-0.7, 0.2-0.7, 0.3-0.6, or 0.4-0.6. If the molar ratio is below the upper limit of the designed range, In addition, during the production of oxides by high-temperature calcination, the synthesis of by-products such as nickel oxide (Ni-O) due to nickel leaching can be suppressed. Conversely, if the molar ratio is above the lower limit of the designed range, the synthesis of by-products such as manganese oxide (MnO2) due to the high reactivity of manganese can be suppressed, enabling the effective production of ternary transition metal oxides and improving electrochemical performance, such as the high discharge capacity of sodium secondary batteries.

[0062] The sodium-manganese oxide may have a molar ratio of cobalt (Co) to the total metal (M) other than sodium (Co / M) of 0.01 to 0.15, preferably 0.01 to 0.12, 0.01 to 0.1, 0.01 to 0.09, or 0.02 to 0.09.

[0063] The sodium manganese oxide can exhibit at least one peak selected from the group consisting of (004), (100), (101), (102), (103), (104), and (002) peaks by X-ray diffraction analysis (XRD), preferably all of the (004), (100), (101), (102), (103), (104), and (002) peaks. These peaks form a P2-type layered structure, and a technical feature of the present invention is that the conventional P2-type layered structure is stably maintained even after Co coating and doping of sodium manganese oxide particles with a P2-type layered structure under high-temperature conditions. As described above, P2-type layered cathode active materials have the advantage of relatively high atmospheric and moisture stability, and are not very sensitive to synthesis conditions such as temperature and atmosphere.

[0064] The sodium manganese oxide may have a NiO ratio of 5% or less, as measured by the Rietveld refinement method based on X-ray diffraction analysis. For example, it may be 0.5-5%, 0.5-4.5%, 0.5-4.0%, or 0.5-3.5%. More specifically, it may be 1% or more, 1.5% or more, 2% or more, 2.5% or more, 3% or more, or 3.1% or more and 5% or less, 4.5% or less, 4% or less, or 3.5% or less. More specifically, it may be 3.1%-3.5%, 3.2%-3.5%, 3.3%-3.5%, or 3.4%-3.5%. If the NiO ratio of the sodium manganese oxide is below the above range, Co doping to the surface and interior of the secondary particles may not be performed uniformly with sufficient content. If it exceeds the above range, the amount of heterogeneous phase impurities such as NiO may increase.

[0065] The positive electrode active material may contain 5,000 ppm or less of residual sodium, for example, 0.1-0.5% by weight, 0.2-0.5% by weight, 0.1-0.49% by weight, 0.1-0.48% by weight, or 0.1-0.47% by weight of residual sodium relative to the total positive electrode active material. In the present invention, by simultaneously performing a water washing step and a Co mixing step on a positive electrode active material of high-content manganese-based sodium manganese oxide (NNMO), the sodium by-products remaining on the surface of the particles can be adjusted to a low content, and a cobalt compound can be uniformly formed on the surface of the particles.

[0066] Furthermore, the positive electrode active material may contain 15% or less by weight of sodium hydroxide (NaOH) or 10% or less by weight of sodium hydroxide (NaOH) per 100% by weight of residual sodium. Of the residual sodium, NaOH is uniformly coated onto the surface of the oxide particles by performing a residual sodium washing step and a Co coating step simultaneously. During subsequent heat treatment, it is Co-doped, and the by-product sodium salt (such as Na2SO4) can be easily removed with water.

[0067] The positive electrode active material particles have an average particle size (D 50 The thickness may be 2 to 15 μm, for example, 4 to 10 μm. In this case, the electrode density increases, and the amount of electrode per unit volume is Energy density can be improved.

[0068] The following describes a method for producing a positive electrode active material for a sodium secondary battery according to the present invention.

[0069] The manufacturing method includes the steps of: a) preparing a sodium-manganese oxide containing at least sodium (Na), nickel (Ni), and manganese (Mn), and containing 55 mol% or more of manganese among the total metals other than sodium; b) introducing the sodium-manganese oxide, a solvent, and a cobalt precursor into a reactor to form a cobalt compound on the surface of the sodium-manganese oxide particles; and c) heat-treating the product of step b) to dope the surface and interior of the sodium-manganese oxide particles with cobalt.

[0070] In this invention, a Co-doped sodium nickel manganese oxide (NNMO) positive electrode active material is manufactured using a wet process in a positive electrode active material containing a high manganese oxide. This allows for a uniform distribution across the entire surface of the positive electrode active material particles, thereby suppressing surface defects. Furthermore, sufficient Co diffusion into the interior of the particles improves surface degradation caused by changes in lattice structure and contraction / expansion that occur during charging and discharging in the high-voltage region.

[0071] a) Step is to prepare a high-content manganese (NNMO) oxide (NNMO) containing 55 mol% or more of manganese among the total metals other than sodium.

[0072] For example, the oxide preparation step may include a1) a step of producing a nickel-manganese hydroxide precursor containing 55 mol% or more of manganese in the total metal, and a2) a step of mixing the nickel-manganese hydroxide precursor with a sodium compound and then heat-treating it to produce the sodium-manganese oxide.

[0073] The precursor manufacturing step a1) is not limited to any method for manufacturing a common cathode active material precursor for sodium secondary batteries.

[0074] For example, the method for producing the precursor involves adding a nickel salt, a manganese salt, and a complexing agent to a reactor, and then producing a complexed transition metal hydroxide by coprecipitation. The nickel salt and manganese salt may be nickel or manganese-containing acetates, nitrates, sulfates, halides, sulfides, hydroxides, oxides, or oxyhydroxides, respectively. Specifically, the nickel salt may be Ni(OH)2, NiOOH, Ni(OCOCH3)2·4H2O, Ni(NO3)2·6H2O, NiSO4, Ni(SO4)2·7H2O, or a combination thereof, and the manganese salt may be Mn(OH)2, MnOOH, Mn(OCOCH3)2·4H2O, Mn(NO3)2·6H2O, MnSO4, Mn(SO4)2·7H2O, or a combination thereof, but the present invention is not limited thereto. A basic solution can be added to the reactor to adjust the pH to 10-12, and a small amount of ammonium solution can be used as a complexing agent.

[0075] The aforementioned complex transition metal hydroxide precursor can be represented by the following chemical formula 2.

[0076] [Chemical formula 2] Nix2Mny2(OH)2

[0077] In the above chemical formula 2, 0.05 ≤ x² ≤ 0.45 and 0.55 ≤ y² ≤ 0.95 may also be applied.

[0078] The aforementioned x2 represents the ratio of the number of moles of Ni to the total number of moles of transition metals, where 0.1 ≤ x2 ≤ 0. It may also be 0.45, 0.2≦x2≦0.45, 0.25≦x2≦0.45, or 0.3≦x2≦0.4, where y2 represents the ratio of moles of Mn to the total number of moles of transition metal, and may be 0.60≦y2≦0.90, 0.60≦y2≦0.80, or 0.60≦y2≦0.70.

[0079] When the composite transition metal hydroxide precursor has the composition represented by the chemical formula 2, the positive electrode active material produced from the precursor can exhibit high capacity under high-voltage operating conditions, and can reduce the concentration of both nickel and cobalt, thereby increasing price competitiveness.

[0080] The step of producing the sodium manganese oxide described in a2) above is not limited to any method for producing an oxide of a common positive electrode active material for sodium secondary batteries.

[0081] The sodium-containing raw material may be at least one selected from the group consisting of Na2CO3, NaOH, NaNO3, CH3COONa, and Na2(COO)2, and preferably Na2CO3, NaOH, or a combination thereof.

[0082] During the production of the positive electrode active material, the positive electrode active material precursor and the sodium compound can be mixed in a transition metal:sodium ratio of 1:0.5 to 1:0.8, 1:0.6 to 1:0.8, 1:0.6 to 1:0.75, or 1:0.65 to 1:0.75. When the sodium compound is within the above range, the produced positive electrode active material may have a P2 crystal structure, thereby enabling high atmospheric and moisture stability and less sensitivity to synthesis conditions (temperature and atmosphere, etc.). Furthermore, within the range of sodium content, the battery discharge capacity can be improved and unreacted residual Na can be minimized.

[0083] The heat treatment can be carried out at a temperature of 700°C to 1,100°C. When the firing temperature is within this range, the reaction between the raw materials can proceed sufficiently, and the particles can grow uniformly. The heat treatment can more preferably be carried out at a temperature of 750 to 1,050°C, 850 to 1,050°C, or 900 to 1,000°C.

[0084] The heat treatment can be carried out for 5 to 40 hours. When the firing time is within the above range, a highly crystalline positive electrode active material can be obtained, the particle size is appropriate, and production efficiency can be improved. The heat treatment can more preferably be carried out for 5 to 20 hours, 5 to 18 hours, 8 to 15 hours, or 10 to 14 hours.

[0085] b) Step is a step of forming a cobalt compound on the surface of the sodium manganese oxide particles, and may further include b1) a step of introducing the sodium manganese oxide and solvent into a reactor and stirring, and b2) a step of introducing a cobalt precursor into the reactor and stirring to form cobalt hydroxide (Co(OH)2) on the surface of the sodium manganese oxide.

[0086] In this invention, by performing the water washing and coating processes simultaneously under specific conditions, a cobalt compound can be uniformly formed on the surface of the secondary particles, integrating multiple overlapping steps and simplifying the process. Furthermore, after coating, during high-temperature heat treatment, as described above, uniform cobalt doping to the surface and interior of the oxide particles is possible, enabling the production of high-manganese ternary transition metal oxides with a uniform transition metal composition.

[0087] Conventional dry coating process or coating performed after water washing of the surface of the positive electrode active material In the coating process, if heat treatment is performed at high temperatures after coating, it may be difficult to form a uniform coating layer overall, such as forming island-like coating layers on the surface of the particles, and furthermore, uniform and sufficient cobalt doping into the interior of the particles cannot be expected. Such coating layers cause surface defects, increase the contact area with the electrolyte (specific surface area), and have problems such as battery swelling due to side reactions of the electrolyte and a decrease in cycle life. In the present invention, a uniform Co distribution is formed overall on the surface of the positive electrode active material particles, so uniform Co doping can be performed in the subsequent process. As a result, surface defects of the positive electrode active material particles are minimized, surface stability is improved, capacity is increased, and cycle life is improved.

[0088] b1) Step may be the step of adding the sodium manganese oxide and solvent to the reactor and stirring.

[0089] The sodium manganese oxide and solvent can be mixed in a reactor in a weight ratio of 0.5:1.5 to 1.5:0.5, or 0.7:1.2 to 1.2:0.7, for example, a weight ratio of 1:1. When mixed within the above content range, residual sodium can be effectively removed, sodium elution from the oxide particles can be prevented, and surface defects of the particles can be prevented.

[0090] The pH can be adjusted to 10-12 by adding a basic solution to the reactor, a small amount of ammonium solution can be used as a complexing agent, and the stirring speed may be 200-500 rpm, 200-400 rpm, or 300-400 rpm, but the present invention is not limited thereto. This allows the cobalt compound to be uniformly formed on the surface of the sodium manganese oxide particles in the subsequent step b2).

[0091] b2) Step may also be the step of adding a cobalt precursor to the reactor and stirring to form cobalt hydroxide (Co(OH)2) on the surface of the sodium manganese oxide.

[0092] The cobalt precursor can be fed into the reactor such that the total amount of the sodium manganese oxide, the solvent, and the cobalt compound are mixed in a weight ratio of 5:1 to 1.1:1, 4.5:1 to 1.5:1, preferably 4:1 to 1.5:1 or 3:1 to 1.5:1. Here, the cobalt precursor can be fed at a rate of 20 to 200 ml / hr, 20 to 180 ml / hr, or 20 to 160 ml / hr for 0.5 to 5 hours, 0.5 to 4 hours, 0.5 to 3 hours, or 0.5 to 1.5 hours. If the feeding rate exceeds the above rate, it is difficult to uniformly coat the surface of the sodium manganese oxide particles with cobalt hydroxide. Conversely, if the feeding rate is below the above rate, the cost increases as the process time lengthens, and Na elution occurs inside the positive electrode active material oxide particles, which is undesirable.

[0093] The cobalt precursor can be introduced into the reactor such that the cobalt (Co) content ratio (Co / M) to the total metal (M) other than sodium in the sodium-manganese oxide is 0.01 to 0.15, preferably 0.01 to 0.12, 0.01 to 0.1, 0.01 to 0.09, or 0.02 to 0.09.

[0094] Step b) above can be carried out in an inert atmosphere such as nitrogen or argon at a temperature of 40-70°C or 45-65°C, and the coating time may be 60-90 minutes, or for example, 75-105 minutes. This can further improve the effects described above.

[0095] The solvent may be a water washing solution, or one or more selected from deionized water, distilled water, and ethanol, and preferably deionized water. Here, the temperature of the water washing solution used during the water washing may be 1 to 80°C or 5 to 50°C, but the present invention is not limited thereto.

[0096] The basic solution may be an alkali metal or alkaline earth metal hydroxide such as NaOH, KOH, or Ca(OH)2, or a combination thereof, and may be added to adjust the pH of the solution in the reactor.

[0097] The ammonium solution may contain NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, NH4CO3, or a combination thereof, and as a complexing agent, it can effectively coprecipitate the cobalt-containing raw material in the reactor.

[0098] The cobalt precursor may be a cobalt-containing solution and can be produced by adding a cobalt-containing raw material to water or a mixed solvent of an organic solvent such as an alcohol that can be homogeneously mixed with water.

[0099] On the other hand, the cobalt-containing raw material may be Co(OH)2, CoOOH, Co(OCOCH3)2, Co(NO3)2, CoSO4, Co(SO4)2, or a combination thereof, preferably CoSO4 and / or Co(SO4)2. Here, the cobalt-containing raw material may include hydroxides and acid anhydrides of the above-mentioned substances.

[0100] The reactor can be any reactor used for coprecipitation reactions, which are generally used in the production of positive electrode active materials, without any limitations.

[0101] The manufacturing method of the present invention may further include a drying step after step b) above. The drying step is a step for removing moisture from the positive electrode active material, which has a water-containing Co compound formed after the washing and coating steps, and can be dried for 12 hours or more under vacuum conditions and at a temperature of 100 to 300°C.

[0102] Step c) involves heat-treating the product of step b) to dope the surface and interior of the sodium manganese oxide particles with cobalt. Uniform cobalt doping of the surface and interior of the sodium manganese oxide particles not only improves the surface stability of the positive electrode active material but also improves the capacitance characteristics. In this invention, Co doping of the positive electrode active material can suppress the phase transition (P2-O2) that occurs in the high voltage range during charging / discharging, which not only suppresses surface side reactions of the particles and improves the degradation of the initial lifespan but also prevents the P2-O2 phase transition that occurs in the high voltage range during charging / discharging, thus improving the lifespan characteristics.

[0103] The heat treatment can be carried out at 600-900°C for 6-18 hours, for example, at 700-900°C or 750-900°C for 8-16 hours, 10-14 hours, or 11-13 hours. If such heat treatment is carried out below the temperature and / or time range, the doping of Co to the surface and interior of the secondary particles may not be carried out uniformly with sufficient content, and conversely, if it exceeds the temperature and / or time range, the amount of heterogeneous phases such as NiO may increase.

[0104] The coating source may be further mixed during the heat treatment in step c) above. The coating source may be P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, M The compound may also contain at least one selected from o, Ge, Nd, B, Nb, Gd, and Cu.

[0105] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery and a sodium secondary battery, both containing the positive electrode active material.

[0106] The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, and the positive electrode active material according to one aspect of the present invention is present in the positive electrode active material layer.

[0107] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatments such as carbon, nickel, titanium, or silver can be used. The positive electrode current collector can also typically have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. Such positive electrode current collectors can be provided in various forms, such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0108] Furthermore, the positive electrode active material layer may also be a layer containing a conductive material and a binder, along with the positive electrode active material described above.

[0109] Here, the conductive material is used to impart conductivity to the electrode and can be used without particular limitations as long as it does not cause a chemical change in the positive electrode active material and is conductive. Non-restrictive examples of conductive materials include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. The conductive material can usually be included in an amount of 1% to 30% by weight relative to the total weight of the positive electrode active material layer.

[0110] Furthermore, the binder is a substance that improves the adhesion between positive electrode active material particles and the adhesion between the positive electrode 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 can typically be present in an amount of 1% to 30% by weight relative to the total weight of the positive electrode active material layer.

[0111] A positive electrode according to one embodiment of the present invention can be manufactured by a conventional method for manufacturing positive electrodes for sodium secondary batteries, except that the positive electrode active material described above is used. For example, a positive electrode can be manufactured by applying a slurry for forming a positive electrode active material layer, which includes the positive electrode active material and selectively a binder and a conductive material, onto a positive electrode current collector, followed by drying and rolling. In another example, a positive electrode can be manufactured by casting the slurry for forming a positive electrode active material layer onto another support, peeling the positive electrode active material layer from the support, and then laminating the resulting film onto a positive electrode current collector.

[0112] According to yet another aspect of the present invention, an electrochemical element including the above-described positive electrode is provided. Specifically, the electrochemical element may be a battery, a capacitor, or, more specifically, a sodium secondary battery.

[0113] A sodium-based secondary battery includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator interposed between the positive and negative electrodes, and an electrolyte. The sodium-based secondary battery may also include a battery container (case) for housing the electrode assembly, including the positive electrode, negative electrode, and separator, and a sealing member for sealing the battery container.

[0114] Here, depending on the shape of the battery container (case), sodium secondary batteries can be divided into can-type sodium secondary batteries, in which the electrode assembly is housed in a metal can, and pouch-type sodium secondary batteries, in which the electrode assembly is housed in a pouch made of a sheet such as aluminum laminate.

[0115] In particular, in the case of a pouch-type sodium secondary battery using a positive electrode containing a positive electrode active material according to various embodiments of the present invention, there is a low possibility of side reactions between the positive electrode active material and the electrolyte, which has the advantage of improving stability during storage and / or operation and reducing gas generation.

[0116] The present invention will be described in detail below with reference to examples, but these are for the purpose of explaining the present invention in more detail and the scope of the rights of the present invention is not limited by the following examples.

[0117] Examples Manufacturing Example 1: Manufacturing of Cathode Active Material Example 1 a) Production of sodium nickel manganese oxides Ni 0.35 Mn 0.65 (OH)2 precursor is treated with a sodium compound Na2CO3 in an amount of Na / M = 0.67 equivalents, and heat-treated at 950°C in an air atmosphere for 12 hours to produce sodium nickel manganese oxide cathode active material powder (Na 0.67 Ni 0.35 Mn 0.65 O2 We obtained powder.

[0118] b) Formation of cobalt compounds 400g of DIW was added to a 5L batch-type reactor, followed by 23.9g of 1M NaOH solution. The mixture was then maintained at 25°C with a stirring speed of 300 rpm for 15 minutes until the pH reached 12.8. Subsequently, Na 0.67 Ni 0.35 Mn 0.65 O2 was added and stirred at 300 rpm.

[0119] Next, a 1.7 M aqueous cobalt sulfate solution was added at a controlled rate of 51.18 ml / hr, and the mixture was reacted for 60 minutes while maintaining a pH of 11.5-12 to form cobalt hydroxide on the surface of the sodium manganese oxide particles. The obtained particles were separated and dried in a vacuum oven at 110°C for 24 hours.

[0120] c) Cobalt doping Next, the aforementioned particle powder was placed in an alumina crucible and then heat-treated in an air atmosphere at 800°C for 12 hours to produce a cobalt-doped cathode active material.

[0121] Comparative Example 1 b) and c) are omitted, and sodium nickel manganese oxide powder (Na) is used in step a). 0.67 Ni 0.35 Mn 0.65 O2 powder was used as the positive electrode active material.

[0122] Comparative Examples 2-3 c) Cobalt-doped cathode active materials were produced in the same manner as in Example 1, except that the heat treatment in step c) was carried out at temperatures of 600°C (Comparative Example 2) and 700°C (Comparative Example 3), respectively.

[0123] Comparative Example 4 Ni 0.35 Mn 0.65 Co acetate and sodium compound Na2CO3 were added to the (OH)2 precursor in amounts of Co / (Ni+Mn+Co)=0.1 and Na / (Ni+Mn+Co)=0.67 equivalents, and the mixture was heat-treated at 950°C in an air atmosphere for 12 hours to produce a sodium nickel manganese oxide cathode active material (Na 0.67 Ni 0.315 Co 0.1 Mn 0.585 O2powder was obtained as the positive electrode active material.

[0124] Manufacturing Example 2: Manufacturing of Sodium Rechargeable Batteries A cathode slurry was prepared by dispersing 30 g of N-methyl-2-pyrrolidone (NMP) with 90 wt% of the manufactured cathode active material, 5.5 wt% of carbon black, and 4.5 wt% of PVdF binder. The cathode slurry was uniformly coated onto a 15 μm thick aluminum thin film and vacuum-dried at 135°C to produce a cathode for a sodium secondary battery.

[0125] A sodium secondary battery (coin cell) was manufactured using a sodium metal plate as the counter electrode for the positive electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separator, and an electrolyte containing ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7 with NaPF6 present at a concentration of 1.15 M.

[0126] Experimental example Experimental Example 1: Analysis of SEM images of the surface of positive electrode active material particles Figures 1a to 1c are SEM images of the surface of positive electrode active material particles produced in step a) (before coating), step b) (after coating), and step c) (after heat treatment) of Example 1, and Figure 1d is an SEM image of the surface of positive electrode active material particles produced in Comparative Example 4.

[0127] Referring to Figures 1a to 1c, it was confirmed that the sodium nickel manganese oxide before coating exhibited a plate-like primary particle structure with an aspect ratio of 1:2.7 to 1:5, and that this primary particle shape was well maintained even after coating and heat treatment.

[0128] Experimental Example 2: Confirmation of Co-doping uniformity on the surface and interior of manganese oxide particles by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis. Figures 2a to 2d are SEM-EDS images of the surface of Co-doped cathode active material particles produced in Example 1 and Comparative Examples 2 to 4.

[0129] Referring to Figures 2a to 2c, it was confirmed that during heat treatment after Co coating, uniform Co doping occurred on the surface of the positive electrode active material secondary particles as the temperature increased. In Comparative Examples 2 and 3, when Co coating heat treatment was performed at a relatively low temperature, the doped Co concentrated and distributed only on the surface of the oxide particles, forming a coating layer with a non-uniform island shape. As a result, it was predicted that in Comparative Examples 2 and 3, there would be a doping Co concentration gradient decreasing from the surface to the center of the cross-section of the secondary particles. Referring to Figure 2d, in Comparative Example 4, it was confirmed that a non-uniform coating layer was formed, such as when Co aggregated on the surface of the positive electrode active material particles during dry coating with a Co acetate source.

[0130] Figures 3a and 3b are graphs (EDS line scanning) showing the change in the content of metal elements (Ni, Co, Mn) present in sodium manganese oxide secondary particles, obtained by cross-sectional SEM-EDS images of Co-doped cathode active material particles manufactured in Example 1 and EDS analysis in the direction represented by the cross-sectional SEM image. Table 1 below shows the molar concentrations of Ni, Co, and Mn relative to 100 mol% of the total metal (Ni+Co+Mn) other than sodium in the center, surface, and overall (bulk) of the secondary particles. The molar concentrations of Ni, Co, and Mn in the center and surface of the secondary particles were calculated from EDS line scanning, while the molar concentrations of Ni, Co, and Mn in the overall secondary particles (bulk) were calculated by ICP analysis.

[0131] [Table 1]

[0132] In the case of the Co-doped cathode active material produced in Example 1, uniform cobalt doping was confirmed on the surface and inside of the secondary particles, confirming that the coating / doping technology of the present invention can produce transition metal ternary oxides with a uniform composition.

[0133] Experimental Example 3: Confirmation of P2-type layered structure and heterogeneous phases by XRD analysis Figure 4 shows the XRD analysis results of the positive electrode active material (bulk) produced in Example 1 and Comparative Examples 2-3. Table 2 below shows the ratio of NiO (heterogeneous phase) formed within the positive electrode active material at different Co-doping heat treatment temperatures, measured using the TOPAS program.

[0134] [Table 2]

[0135] (In Table 2, "Reference" refers to the cobalt-coated positive electrode active material that did not undergo step c) cobalt doping (heat treatment) in Example 1.)

[0136] Referring to Figure 4 and Table 2, XRD analysis of the positive electrode active materials produced in Example 1 and Comparative Examples 2-3 revealed peaks at (004), (100), (101), (102), (103), (104), and (002), confirming the presence of a P2-type layered structure. Furthermore, while the proportion of NiO impurities (NiO peaks confirmed around 2theta = 37°, 43°, and 63°) increased with increasing doping heat treatment temperature, the magnitude of this increase (ΔNiO) gradually decreased around 800°C. In Example 1, compared to Comparative Examples 2 and 3, uniform Co doping was possible, and the proportion of the heterogeneous phase NiO was adjusted to within 5%. Also, referring to Figure 3a, it was confirmed that in the present invention, although a small amount of NiO was synthesized during Co coating and heat treatment doping, heterogeneous phases related to Co and Mn (CoO2, MnO2, etc.) were not generated.

[0137] Experimental Example 4: Evaluation of the electrochemical performance of a sodium secondary battery For the sodium secondary batteries manufactured in Example 1 and Comparative Examples 1-2, 50x charge / discharge cycles were performed under 1C / 1C conditions at 25°C and within a driving voltage range of 2.0V to 4.6V. The ratio of the discharged capacity at 50 cycles to the initial capacity (cycle capacity retention) was then measured.

[0138] [Table 3]

[0139] Referring to Table 3, it was confirmed that the cycle life characteristics of the sodium secondary battery using the positive electrode active material of Example 1 were improved compared to the sodium secondary battery using the positive electrode active material of Comparative Example 1.

[0140] Experimental Example 5: Analysis of Unreacted Residual Sodium To measure residual sodium, 1 g of coated positive electrode active material was immersed in 5 g of distilled water, stirred for 5 minutes, and the filtrate was taken. The filtrate was titrated with 0.1 M HCl, and the volume of HCl added until the pH of the filtrate reached 5 was measured to analyze the unreacted sodium remaining on the surface of the positive electrode active material.

[0141] [Table 4]

[0142] Referring to Table 4, it was confirmed that the residual sodium content of the positive electrode active material according to Example 1 was significantly reduced as the Co coating / doping process was carried out. In particular, NaOH of the residual sodium was used in the Co coating / doping reaction (coating reaction: NaOH + CoSO4 → Co(OH)2 + Na2SO4), the generated Co(OH)2 was uniformly coated on the surface of the oxide particles, and then Co doped the surface and interior of the particles during heat treatment, while the by-product Na2SO4 was easily removed with water. On the other hand, the positive electrode active material according to Comparative Example 4 was dry coated with a Co acetate source, and cobalt aggregation (non-uniform coating) was observed, resulting in uncontrolled residual Na.

[0143] As described above, the present invention has been illustrated and explained with respect to specific embodiments, but it will be obvious to those of ordinary skill in the art that the present invention can be improved and modified in various ways without departing from the technical spirit of the invention provided by the following claims.

Claims

1. It contains at least sodium (Na), nickel (Ni), and manganese (Mn), and includes a sodium-manganese oxide containing 55 mol% or more of manganese among the total metals other than sodium. The sodium manganese oxide is a secondary particle formed by the aggregation of at least one primary particle, and the surface and interior of the secondary particle are doped with cobalt (Co), wherein this is a positive electrode active material for a sodium secondary battery.

2. The sodium manganese oxide satisfies the following relational expression 1, wherein the sodium manganese oxide is a positive electrode active material for a sodium secondary battery according to claim 1. [Relationship 1] (Dc-Ds) / Dt≦±10% In the relational equation 1, the region corresponding to 0 to 50% of the length (R) from the center to the surface, with respect to the cross-section of the secondary particle, is defined as the first region (R １ ), the area corresponding to 50-100% is the second area (R ２ If we define it as follows, then Ds, Dc, and Dt are, respectively, the first region (R １ ), second region (R ２ ) and the entire cross-section of the particle (R １ +R ２ These values ​​were measured using the ) method and represent the molar concentration of cobalt (Co) relative to the total metal (M) other than sodium.

3. Based on the cross-section of the secondary particle, the first region (R) is defined as a percentage of the total cobalt concentration of 100 mol%. １ The positive electrode active material for a sodium secondary battery according to claim 2, wherein the cobalt concentration of ) is 50 mol% or more.

4. The sodium manganese oxide satisfies the following relational equation 2, wherein the sodium manganese oxide is a positive electrode active material for a sodium secondary battery according to claim 1. [Relationship Equation 2] (Mc-Ms) / Mt≦±10% In the relational expression 2, based on the cross-section of the secondary particles, among the lengths (R) from the center to the surface, the region corresponding to 0 to 50% is defined as the first region (R １ ), and the region corresponding to 50 to 100% is defined as the second region (R ２ ). Then, Ms, Mc, and Mt are measured in the first region (R １ ), the second region (R ２ ), and the entire cross-section of the particle (R １ + R ２ ), respectively, and are the molar concentrations of nickel (Ni) or manganese (Mn) with respect to the total metal (M) other than sodium.

5. The sodium manganese oxide has an aspect ratio of primary particles of 1:2.5 to 1:5.5, as described in claim 1, for a positive electrode active material for a sodium secondary battery.

6. The sodium manganese oxide is represented by the following chemical formula 1, and is the positive electrode active material for a sodium secondary battery according to claim 1. [Chemical formula 1] Na ａ Ni ｘ Co ｙ M1 ｚ Mn １－ｘ－ｙ－ｚ O ２ In the above chemical formula 1, M1 is at least one selected from P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu. 0.50 ≤ a ≤ 0.80, 0.05 ≤ x ≤ 0.45, 0.01 ≤ y ≤ 0.15, 0 ≤ z ≤ 0.1, and 0.55 ≤ 1 - x - y - z ≤ 0.

85.

7. The sodium manganese oxide was found to have peaks (004), (100), (101), (102), (103), (104), and (002) by X-ray diffraction analysis (XRD). The positive electrode active material for a sodium secondary battery according to claim 1, wherein at least one peak selected from the group is shown.

8. The sodium manganese oxide has a NiO ratio of 5% or less, as measured by the Rietveld refinement method based on X-ray diffraction analysis, as a positive electrode active material for a sodium secondary battery according to claim 1.

9. The sodium manganese oxide comprises a P2-type layered structure, as described in claim 1, for a positive electrode active material for a sodium secondary battery.

10. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the positive electrode active material contains 5,000 ppm or less of residual sodium.

11. a) A step of preparing a sodium-manganese oxide containing at least sodium (Na), nickel (Ni), and manganese (Mn), and containing 55 mol% or more of manganese among the total metals other than sodium, b) The steps of introducing the sodium manganese oxide, solvent and cobalt precursor into a reactor to form a cobalt compound on the surface of the sodium manganese oxide particles, c) A method for producing a positive electrode active material for a sodium secondary battery, comprising the step of heat-treating the product of step b) to dope the surface and interior of the sodium manganese oxide particles with cobalt.

12. The method for producing a positive electrode active material for a sodium secondary battery according to claim 11, wherein the sodium manganese oxide in step a) is a secondary particle formed by the aggregation of at least one primary particle, and the aspect ratio of the primary particle is 1:2.5 to 1:5.

5.

13. Step b) above is, b1) The steps of adding the sodium manganese oxide and solvent to the reactor and stirring, b2) A method for producing a positive electrode active material for a sodium secondary battery according to claim 11, comprising the step of introducing a cobalt precursor into the reactor and stirring to form a cobalt hydroxide on the surface of the sodium manganese oxide.

14. The cobalt precursor in step b) is A method for producing a positive electrode active material for a sodium secondary battery according to claim 11, wherein the sodium manganese oxide is introduced into a reactor such that the molar ratio (Co / M) of cobalt (Co) to the total metal (M) other than sodium is 1 to 15.

15. The cobalt precursor in step b) is Co(OH) ２ , CoOOH, Co(OCOCH ３ ) ２ Co(NO ３ ) ２ CoSO ４ Co(SO ４ ) ２ A method for producing a positive electrode active material for a sodium secondary battery according to claim 11, or a combination thereof.

16. The method for producing a positive electrode active material for a sodium secondary battery according to claim 11, wherein the heat treatment in step c) is performed at 600 to 900°C for 6 to 18 hours.

17. A positive electrode for a sodium secondary battery, comprising the positive electrode active material described in claim 1.

18. A sodium secondary battery using the positive electrode described in claim 17.

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