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

Abstract

An embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, the positive electrode active material comprising: secondary particles which contain a composite transition metal oxide including at least one transition metal selected from nickel, iron, manganese, and cobalt, sodium, and a doping metal, and in which a plurality of primary particles are aggregated; and a coating layer containing an oxide of the doping metal and covering at least a portion of the surface of the primary particles and at least a portion of pores formed between the plurality of primary particles, wherein the doping metal is distributed throughout the secondary particles including the center, and the concentration of the doping metal is higher in the central portion than in the surface portion of the secondary particles.

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, making commercialization difficult. Therefore, the development of high-performance cathode active materials is required for commercialization.

[0004] Layered transition metal oxides are typically used as cathode active materials for sodium-ion secondary batteries because they possess a simple structure, excellent electrochemical performance, and are easy to synthesize. Layered transition metal oxides are generally 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 The cathode active material, which exhibits a composition such as (TM)O2 (2 / 3 < x < 1.2) and is based on a P2-type structure, is Na x It has a composition of (TM)O2(x≤ 2 / 3). Although O3-type layered oxides have a higher energy density than P2-type layered oxide particles, they have disadvantages such as reduced cycle stability due to larger structural changes during the charge-discharge process, making commercial application difficult.

[0005] Research on methods for doping and coating metals onto cathode active materials is actively underway for the purpose of improving the aforementioned problems.

[0006] Prior art document 1 (Registered Patent KR 2466222 B1) presents a calcium-doped cathode active material to achieve improved cycle characteristics by improving the cathode active material used in sodium secondary batteries. However, the prior art document has the disadvantage that when manufacturing the cathode active material, it is calcined at a relatively low temperature (600~850℃ or lower), so the calcium remains on the surface of the secondary particles of the cathode active material or is only doped into the interior of some primary particles, resulting in insufficient improvement in structural stability and difficulty in suppressing side reactions of the electrolyte at grain boundaries and improving particle compressive strength.

[0007] Prior art document 2 (Published Patent CN 118039868 A) presents an anode active material in which a coating layer is formed on the surface of secondary particles and on the grain boundaries of primary particles. However, in the prior art document, since the coating (doping) concentration on the surface is higher than the coating (doping) concentration on the center of the secondary particles, it is difficult to improve particle density and particle compressive strength. Furthermore, there are problems such as increased crack occurrence and deterioration of cycle life due to an increased reaction surface area with the electrolyte during the course of cycle life. Additionally, in the prior art document, an anode active material is manufactured by synthesizing a transition metal (NFM) precursor, selectively mixing NaCO3 and a doping element, performing a first sintering, and then mixing a coating source and performing a second sintering. However, since both the first sintering (doping process) and the second sintering (coating process) are carried out at high temperatures, there is a problem in that the doping element migrates excessively to the surface of the secondary particles, increasing the coating / doping concentration on the surface.

[0008] Therefore, it is necessary to develop Ca coating technology to manufacture stable and high-performance O3-type layered cathode active materials.

[0009] The present invention aims to improve the performance and stability of a positive electrode active material through calcium (Ca) coating / doping, wherein Ca is doped into the interior of the secondary particles of the positive electrode active material and some is coated on the grain boundaries of the primary particles.

[0010] In addition, the objective of the present invention is to control the composition, content, and thickness of the Ca doping / coating formed inside the primary grain boundaries by proceeding with calcination in a specific temperature range when manufacturing the positive electrode active material.

[0011] One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery comprising: a composite transition metal oxide comprising at least one transition metal selected from nickel, iron, manganese, and cobalt, sodium, and a doping metal, wherein a plurality of primary particles are aggregated into secondary particles; and a coating layer comprising an oxide of the doping metal covering at least a portion of the surface of the primary particles and at least a portion of the pores formed between the plurality of primary particles, wherein the doping metal is provided throughout including the center of the secondary particles, and exhibits a higher concentration in the center than in the surface portion of the secondary particles.

[0012] The ratio (C2 / C1) of the doping metal concentration (C2, atomic mol%) in the center of the secondary particle to the doping metal concentration (C1, atomic mol%) in the surface portion of the secondary particle may be 1.1 to 3.5.

[0013] The doping metal may be provided to the entire area including the center of the primary particle and to the coating layer, and may exhibit a maximum concentration in the coating layer.

[0014] The thickness of the coating layer formed on at least a portion of the surface of the primary particle may be 2 to 30 nm.

[0015] The molar ratio (M1:M2) of the doping metal (M1) included in the composite transition metal oxide and the doping metal (M2) included in the doping metal oxide may be 70:30 to 95:5.

[0016] The above doping metal may include calcium and be substituted into the sodium layer of the above complex transition metal oxide.

[0017] In the coating layer above, the oxide of the doping metal may comprise at least one selected from sodium calcium oxide (Na2CaO2) and calcium oxide (CaO).

[0018] The above complex transition metal oxide may include a compound represented by the following chemical formula 1.

[0019] [Chemical Formula 1]

[0020] Na a-2x Ca x [(M y TM 1-y )]O2

[0021] In the above chemical formula 1,

[0022] TM is at least one selected from Ni, Fe, Mn and Co, and

[0023] M is at least one selected from P, Sr, Ba, Ti, Zr, 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

[0024] 0.80 <a<1.20, 0.001≤x≤0.1, 0≤y≤0.1, 0.9≤1-y≤1일 수 있다.

[0025] The above secondary particles have an O3 crystal structure in X-ray diffraction analysis, and the full width at half maximum (FWHM) of the peak corresponding to the NiO (003) plane may be 0.15 to 0.2°.

[0026] The density of the center of the secondary particle may be higher than the density of the surface of the secondary particle.

[0027] The above secondary particles have a particle strength of 14 to 16 kgf / mm 2 It could be.

[0028] Another embodiment of the present invention provides a method for manufacturing a positive active material for a sodium secondary battery, wherein the positive active material comprises a positive active material precursor, a doping metal compound, and a sodium compound, and the mixture is calcined. The positive active material comprises: a secondary particle formed by the aggregation of a plurality of primary particles; and a coating layer comprising an oxide of the doping metal, which covers at least a portion of the surface of the primary particles and at least a portion of the pores formed between the plurality of primary particles. The calcination is performed such that the doping metal is provided throughout the secondary particle, including the center, and exhibits a higher concentration in the center than in the surface portion of the secondary particle.

[0029] The above calcination may be carried out on the mixture at 875 to 975°C for 5 to 40 hours.

[0030] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery comprising the positive electrode active material.

[0031] Another embodiment of the present invention provides a sodium secondary battery using the anode.

[0032] According to the present invention, structural stabilization of the positive electrode active material can be improved through Ca doping, and electrochemical performance can be improved by suppressing side reactions of the electrolyte at grain boundaries through Ca coating.

[0033] In addition, when manufacturing the cathode active material, increasing the calcination temperature within a specific range allows the Ca doped inside the primary particles to move to the surface of the primary particles, thereby controlling the thickness of the Ca coating at the grain boundaries. Furthermore, the compressive strength of the particles can be improved through Ca doping, and the reaction area with the electrolyte can be reduced, which can decrease the rate of degradation of the cycle life.

[0034] Figures 1a to 1d are the results of cross-sectional TEM-EDS (Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy) analysis of the cathode active materials according to Comparative Examples 1 and 2 and Examples 1 and 2.

[0035] Figure 2 shows the results of analyzing the thickness of the Ca coating layer through cross-sectional TEM-EDS (Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy) of the cathode active materials according to Comparative Examples 1 and 2 and Examples 1 and 2.

[0036] Figures 3a and 3b are X-ray Diffraction (XRD) analysis results of the cathode active materials according to Comparative Examples 1 and 2 and Examples 1 and 2, and a graph of the FWHM (full width half mean) analysis of the NiO (003) plane.

[0037] 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.

[0038] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) may be used in a meaning that is commonly understood by those skilled in the art to which the present invention pertains. Throughout the specification, when a part is described as "comprising" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components. Furthermore, the singular form includes the plural form unless specifically stated otherwise in the text.

[0039]

[0040] FIGS. 1c and 1d are cross-sectional TEM-EDS results of the positive electrode active material according to Examples 1 and 2. Referring to FIGS. 1c and 1d, the positive electrode active material for a sodium secondary battery according to the present invention comprises a composite transition metal oxide comprising at least one transition metal selected from nickel, iron, manganese, and cobalt, sodium, and a doping metal, wherein a plurality of primary particles are aggregated into secondary particles; and a coating layer comprising an oxide of the doping metal, which covers at least a portion of the surface of the primary particles and at least a portion of the pores formed between the plurality of primary particles.

[0041] The plurality of primary particles may extend in a direction radiating toward the surface of the secondary particle from a region inside the secondary particle. The region inside the secondary particle may be the center of the secondary particle. In other words, the primary particle may be rod-shaped, plate-shaped, or needle-shaped, extending toward the surface of the secondary particle from the region inside the secondary particle. A path for the movement of sodium ions and electrolyte may be provided between the plurality of primary particles. Accordingly, the present invention covers at least a portion of the surface of the primary particle and at least a portion of the pores formed between the plurality of primary particles, and through a coating layer containing an oxide of the doping metal, the side reaction of the electrolyte at the grain boundary is suppressed and the particle compressive strength is improved, thereby enhancing electrochemical performance.

[0042] The doping metal is provided throughout the entire secondary particle, including the center, and is characterized by having a higher concentration in the center than in the surface portion of the secondary particle.

[0043] Unlike conventional technology, which simply coats the surface of primary particles and the exterior of secondary particles of a cathode active material with an oxide of sodium inactive metal or an oxide of sodium active metal, the present invention may include a doping metal at a high concentration within the internal primary particles and grain boundaries located in the center of the secondary particles. As a result, the size of the primary particles increases intensively in the center of the secondary particles due to the doping metal, thereby reducing the voids in the center of the secondary particles and improving particle density and particle compressive strength. Furthermore, crack formation is suppressed during the course of cycle life, and the reaction surface area with the electrolyte is reduced, thereby improving lifespan degradation.

[0044] In other words, the doping metal may exhibit a higher concentration in the doping and coating layer of the primary particle aggregated in the center of the secondary particle than in the doping and coating layer of the primary particle aggregated in the surface of the secondary particle.

[0045] As in one embodiment, the ratio (C2 / C1) of the doping metal concentration (C2, atomic mol%) in the center of the secondary particle to the doping metal concentration (C1, atomic mol%) in the surface portion of the secondary particle may be 1.1 to 3.5, for example, 1.3 to 3.5, 1.4 to 3.5, 1.4 to 3.0, 1.4 to 2.5, 1.4 to 2.0, or 1.4 to 1.7. Accordingly, the size of the primary particle in the center of the secondary particle is increased, and as a result, the voids in the center of the secondary particle are reduced, the particle density is increased, and the particle compressive strength is increased, thereby suppressing crack occurrence during battery charging and discharging and reducing electrolyte reaction, thereby improving lifespan degradation.

[0046] Meanwhile, the surface portion of the secondary particle may refer to a depth region of 200 nm from the outermost surface in the direction of the center (0 to 200 nm), and the center may refer to an internal region excluding the surface portion (200 nm to the center), and the concentration of the doping metal in the surface portion of the secondary particle and the concentration of the doping metal in the center of the secondary particle can be measured by FE-SEM EDS mapping analysis.

[0047] Additionally, the doping metal may be provided to the entire area including the center of the primary particle and to the coating layer, and may exhibit a maximum concentration in the coating layer. The doping metal is doped into the interior of the primary particle, and a portion of the doping metal moves sufficiently into the surface of the primary particle and the internal pores of the (secondary particle) to form a doping metal oxide coating layer, wherein the maximum concentration in the coating layer and the concentration in the entire area including the center of the primary particle may be substituted to have substantially the same concentration.

[0048] In the present invention, during the process of mixing and calcining a positive active material precursor and a doping metal compound, the calcination temperature can be controlled to induce a coating of the primary particle located at the center of the positive active material secondary particle with the doping metal. As the calcination temperature is increased within a specific numerical range, the doping metal that was uniformly substituted within the secondary particle moves to the surface and internal pores of the primary particle to form an oxide coating layer of the doping metal. Specifically, a coating layer with a thickness of 2 to 30 nm, for example, 3 to 20 nm, 3 to 15 nm, or 3 to 10 nm, can be formed on at least a portion of the surface of the primary particle.

[0049] As in one embodiment, the molar ratio (M1:M2) of the doping metal (M1) included in the composite transition metal oxide and the doping metal (M2) included in the doping metal oxide may be 70:30 to 95:5, 75:25 to 95:5, 80:20 to 95:5, or 85:30 to 95:5. If the doping metal molar ratio (M1:M2) exceeds an upper limit value, the coating layer may not be formed as a uniform film with sufficient thickness, making it difficult to improve side reactions in the electrolyte. On the other hand, if the doping metal molar ratio (M1:M2) is below a lower limit value, the proportion of doping metal migrating to the coating layer increases, making it difficult to improve structural stability within the primary particle.

[0050] The doping metal is uniformly substituted into the crystal structure of the primary particles to improve structural stability, and at the same time, electrochemical performance can be improved by suppressing side reactions of the electrolyte at the grain boundaries and improving the compressive strength of the particles through the primary particle coating.

[0051] The above secondary particles comprise a complex transition metal oxide comprising at least one transition metal selected from nickel, iron, manganese, and cobalt, sodium, and a doping metal, wherein the doping metal may include calcium and may be substituted into the sodium layer of the complex transition metal oxide in the secondary particles. Since Ca has an ionic radius similar to that of Na, it is easy to substitute into the Na layer, and because Ca has a stronger bonding force with the metals (Ni, Mn, Fe, Co, etc.) constituting the transition metal layer compared to Na, the stability of the crystal structure can be improved. Specifically, during charging, Na ions move to the cathode, causing a decrease in Na ions in the Na layer and potentially causing the crystal structure to collapse; however, at this time, Ca can act as a pillar (pillar effect) to improve structural stability.

[0052] The complex transition metal oxide included in the above secondary particle may include a compound represented by the following chemical formula 1, and preferably may be a compound represented by the following chemical formula 1.

[0053] [Chemical Formula 1]

[0054] Na a-2x Ca x [(M y TM 1-y )]O2

[0055] In the above Chemical Formula 1, TM may be at least one selected from Ni, Fe, Mn, and Co, for example, Ni, Fe, and Mn, and specifically Ni 0.33 Fe 0.33 Mn 0.33 It may be. In addition, the above M may be at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu.

[0056] In the above Chemical Formula 1, 0.80 <a<1.20, 0.001≤x≤0.1, 0≤y≤0.1, 0.9≤1-y≤1일 수 있고, 예를 들면, 상기 화학식 1로 표시되는 화합물은 Na 0.94 Ca 0.03 Ni 0.33 Fe 0.33 Mn 0.33 It could be O2.

[0057] The sodium (Na) is included in an amount of 0.8 to 1.2 moles per mole of the composite transition metal oxide, so that the composite particles are formed with an O3 crystal structure. When the composite particles are used at low voltage, their capacity is low, requiring improvement of electrochemical characteristics; when used at high voltage, there are problems such as low stability and reduced capacity and lifespan due to phase transition. Accordingly, in the present invention, as described above, a coating layer can be formed by moving some calcium from the calcium-substituted composite transition metal oxide particles to the surface of the primary particles. As a result, degradation caused by the electrolyte can be minimized. Consequently, a cathode active material with a long lifespan, high capacity, and high stability can be provided.

[0058] The TM in the above composite particle may contain Ni, Fe, and Mn in amounts of 0.1 to 0.9 moles each per 1 mole of composite particle oxide, for example, Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 It can be included in the composition. If iron is included within the composite particles, Fe at high voltage 3+ Ga Fe 4+ The crystal structure can become unstable due to oxidation, but the crystal structure can be stabilized by substituting Ca into the sodium layer as in the present invention.

[0059] The above doping metal may be included in an amount of 0.1 to 10 at mol%, 0.1 to 5 at mol%, 1 to 4 at mol%, or 1.5 to 3.5 at mol% with respect to the total metal excluding sodium contained in the composite particle and the coating layer. Accordingly, the aforementioned effect can be further improved.

[0060] The secondary particles of the above-mentioned positive active material have an O3 crystal structure in X-ray diffraction analysis, and the full width at half maximum (FWHM) of the peak corresponding to the NiO (003) plane may be 0.15 to 0.2°, and the content of residual sodium (TTS, Total Sodium) may be 3,500 ppm or less, for example, 2,000 to 3,200 ppm. Accordingly, in the positive active material, the O3 crystal structure of the composite particles is stably maintained due to the strong bonding force of the Ca-O-based oxide as the calcium element is doped into the sodium layer in the positive active material having an O3 crystal structure, thereby improving cycle life characteristics. In addition, side reactions in the electrolyte caused by residual sodium can be improved, and the formation of a Na deficient layer and NiO heterogeneous phase can be suppressed. As a result, the crystal structure of the positive active material is stabilized, so the collapse of the crystal structure of the positive active material during the charge-discharge process is minimized, and deterioration caused by the electrolyte can be prevented.

[0061] The above may include a density at the center of the secondary particle of the positive active material that is higher than the density on the surface of the secondary particle, and accordingly, the particle strength of the secondary particle of the positive active material is 14 to 16 kgf / mm 2 As a result of the primary particle size increasing due to the doping metal, the particle density and particle compressive strength at the center of the secondary particle are improved, and crack formation is suppressed during the course of the cycle life and the reaction area with the electrolyte is reduced, thereby reducing life degradation.

[0062]

[0063] Another embodiment of the present invention provides a method for manufacturing the positive electrode active material.

[0064] The above manufacturing method involves mixing a positive electrode active material precursor, a doping metal compound, and a sodium compound, and calcining the mixture to produce a positive electrode active material.

[0065] First, the precursor of the positive electrode active material may be a complex transition metal hydroxide precursor and may be manufactured through a co-precipitation process commonly used in the relevant technical field. The complex transition metal hydroxide precursor may include a compound represented by the following chemical formula 2.

[0066] [Chemical Formula 2]

[0067] [M y TM 1-y ](OH)2

[0068] In the above chemical formula 2, TM may be at least one selected from Ni, Fe, Mn and Co, M may be at least one selected from P, Sr, Ba, Ti, Zr, 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 0≤y≤0.1, 0.9≤1-y≤1.

[0069] The TM in the above-mentioned complex transition metal hydroxide precursor may contain Ni, Fe, Mn, and Co in amounts of 0.1 to 0.9 moles each per 1 mole of the precursor, for example, Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 It can be included in the composition.

[0070] The above doping metal compound may be a calcium salt, for example, at least one selected from calcium chloride, calcium oxide, calcium phosphate, calcium carbide, and calcium hydroxide, and preferably calcium hydroxide may be used.

[0071] The above sodium compound may be at least one selected from the group consisting of sodium carbonate (Na2CO3), sodium hydroxide (NaOH), sodium nitrate (NaNO3), sodium acetate (CH3COONa), and sodium oxalate (Na2(COO)2), and preferably, sodium carbonate (Na2CO3), sodium hydroxide (NaOH), or a combination thereof may be used.

[0072] The above mixture may be a mixture of the cathode active material precursor and the doping metal compound in an amount of Ca / M (all metals excluding Ca) = 0.01 to 0.1 equivalents, 0.01 to 0.05 equivalents, 0.01 to 0.04 equivalents, or 0.015 to 0.035 equivalents. Additionally, the above mixture may be a mixture of the composite particle precursor and the sodium compound in an amount of Na / M (all metals excluding Na) = 0.8 to 1.2 equivalents, 0.8 to 1.1 equivalents, or 0.95 to 1.05 equivalents. When the amount of the sodium compound and the doping metal compound mixed is within the above ranges, the cathode active material produced may have an O3-type layered crystal structure, which may have a higher energy density, high atmospheric and moisture safety, and be less sensitive to synthesis conditions (temperature and atmosphere, etc.). In addition, the battery discharge capacity can be improved within the above sodium content range, and unreacted residual Na and Ca can be minimized.

[0073] Meanwhile, the above mixing may be a dry mixing method in which raw materials are mixed dry, and is not particularly limited as long as it is a dry mixing method generally used in the manufacture of cathode active materials. Accordingly, the degradation of electrolyte side reactions caused by the interior of secondary particles, the surface / interior of primary particles, and pores can be improved through uniform doping metal substitution even into the interior of the cathode active material secondary particles.

[0074] The above calcination can be performed at a temperature of 875 to 975°C, 875 to 950°C, or 875 to 925°C for 1 to 40 hours, 1 to 30 hours, 5 to 20 hours, 5 to 15 hours, or 7.5 to 12.5 hours. When the calcination temperature and calcination time are within the above ranges, sufficient reaction between raw materials can occur, and the doping metal can be uniformly substituted throughout the entire area including the center of the cathode active material secondary particle, a highly crystalline cathode active material can be obtained, and the particle size is appropriate, thereby improving production efficiency.

[0075] Specifically, as the firing temperature is increased within the designed range, some of the doping metal uniformly substituted within the secondary particles may migrate to the surface of the primary particles and internal pores to form an oxide coating layer of the doping metal. The thickness of the coating layer formed on at least a portion of the surface of the primary particles may be increased to 2 to 30 nm, for example, 3 to 20 nm, 3 to 15 nm, or 3 to 10 nm, and the doping metal included as a doping metal oxide in the coating layer may be included in a higher proportion than the doping metal substituted in the complex transition metal oxide of the primary particles.

[0076] The manufactured cathode active material may subsequently undergo additional washing, filtration, and drying processes.

[0077]

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089]

[0090] 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.

[0091] Examples

[0092] (Example 1)

[0093] Ni 0.33 Fe 0.33 Mn 0.33 Sodium compound Na2CO3 is added to the (OH)2 precursor in an amount of Na / M (M=Ni+Fe+Mn) = 1.0 equivalent, and doping metal compound Ca(OH)2 is added in an amount of Ca / M (M=Ni+Fe+Mn) = 0.03 equivalent, and calcined at 900°C in an air atmosphere for 10 hours to obtain a positive electrode active material powder (Na 0.98 Ca 0.03 Ni 0.33 Fe 0.33 Mn 0.33O2 powder was manufactured.

[0094] A cathode slurry was prepared by dispersing 85 wt% of the manufactured cathode active material, 10 wt% of carbon black, and 5 wt% of PVdF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The cathode slurry was uniformly coated onto an aluminum film with a thickness of 15 μm and vacuum dried at 135°C to produce a cathode for a sodium secondary battery.

[0095] 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.

[0096] (Example 2 and Comparative Examples 1-2)

[0097] 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 calcination was performed at temperatures of 950℃, 800℃, and 850℃.

[0098]

[0099] Experimental Example

[0100] Experimental Example 1: Evaluation of Ca coating layer according to calcination temperature via TEM-EDS analysis

[0101] FIGS. 1a to 1d and FIG. 2 are cross-sectional TEM-EDS results of the cathode active materials according to Comparative Examples 1 and 2 and Examples 1 and 2.

[0102] Referring to Figures 1a–1d and Figure 2, it was confirmed that as the firing temperature increased, i) the primary particles grew and the formation of pores in the center decreased compared to the formation of pores on the surface of the secondary particles (see Figures 1a–1d), ii) the concentration of Ca increased at the grain boundaries of the primary particles, such as pores between the surface and the primary particles (see Figures 1a–1d), and iii) the thickness of the Ca coating layer formed at a firing temperature of 900–950°C increased significantly compared to the thickness of the Ca coating layer formed at 800–850°C.

[0103]

[0104] Experimental Example 2: Evaluation of Ca mapping in the center / surface of cathode active material secondary particles via FE-SEM EDS mapping analysis

[0105] FE-SEM EDS mapping analysis was performed to calculate the metal molar ratio of the surface (surface ~ 200 nm) and center (200 nm ~ center) of the cathode active material secondary particles and the Ca molar ratio of the surface and center.

[0106] Manufacturing Method SEM-EDS Analysis Results (Na / Ca / Ni / Fe / Mn) (atomic mol%) Calcination Temperature (°C) Secondary Particle Surface (M1) (Outermost Surface ~ 200 nm) Secondary Particle Center (M2) (200 nm ~ Center) Center / Surface Molar Ratio (Ca2 / Ca1) Comparative Example 18 00 49 / 1.5 / 16.5 / 16.5 / 16.5 49 / 1.5 / 16.5 / 16.5 / 16.5 1 Comparative Example 28 50 49.1 / 1.5 / 16.5 / 16.5 / 16.5 48.9 / 1.6 / 16.5 / 16.5 / 16.5 1.1 Example 190049.2 / 1.3 / 16.5 / 16.5 / 16.548.6 / 1.9 / 16.5 / 16.5 / 16.51.5 Example 295049.6 / 0.9 / 16.5 / 16.5 / 16.547.8 / 2.7 / 16.5 / 16.5 / 16.53.0

[0107] Referring to Table 1, it was confirmed that as the calcination temperature increases during the manufacture of the positive active material, the Ca concentration in the center (Ca2) increases relative to the Ca concentration in the surface part (Ca1) of the secondary particles of the positive active material.

[0108] It is expected that the size of the primary particles will increase intensively in the center of the secondary particles due to the above Ca, and it is analyzed that the particle density and particle compressive strength can be improved by reducing the voids in the center.

[0109]

[0110] Experimental Example 3: Evaluation of Ca-doped / coated compound composition and ratio via TEM EDS mapping analysis

[0111] The composition of the Ca-doped compound and Ca-coated compound and the Ca molar ratio obtained through TEM-EDS mapping analysis of the cathode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2 are summarized in Table 1 below.

[0112] Manufacturing Method TEM EDS Analysis Results Calcination Temperature (°C) Ca Compound Composition (Doping / Coating Compound) Doping Ca : Coating Ca (Molar Ratio) Comparative Example 1800 Na 0.98 Ca 0.03 TMO2 / CaO99.3:0.7 Comparative Example 2850Na 0.98 Ca 0.03 TMO2 / CaO99.1:0.9 Example 1900Na 0.98 Ca 0.03 TMO2 / CaO9 4.9:5.1 Example 2950Na 0.98 Ca 0.03 TMO2 / CaO84.2:15.8

[0113] (In Table 2 above, TM is Ni 0.33 Fe 0.33 Mn 0.33 lim)

[0114] Referring to Table 2, the Ca doping and Ca coating ratios of the primary particles of the cathode active material could be controlled to a desirable range within a specific calcination temperature range during the manufacture of the cathode active material. Accordingly, it is analyzed that in the case of the cathode active materials prepared in Examples 1 and 2, structural stability within the primary particles is improved, and at the same time, the coating layer is formed as a uniform film with sufficient thickness, thereby suppressing side reactions in the electrolyte.

[0115]

[0116] Experimental Example 4: Evaluation of Cathode Active Material Crystal Structure via XRD Analysis

[0117] The crystal structure of the cathode active material and the formation of impurity phases (NiO (003) planes) were confirmed through XRD analysis, and the results are shown in Figures 3a and 3b.

[0118] As shown in Figures 3a and 3b, it was confirmed that the cathode active material was synthesized in the O3-phase overall. However, when the cathode active material was prepared by calcining at a low temperature of 800–850°C, the NiO peak appeared high with a full width at half maximum of 0.2 or higher, whereas NiO formation could be suppressed by preparing the cathode active material by increasing the calcination temperature to 900–950°C.

[0119]

[0120] Experimental Example 5: Evaluation of Electrochemical Performance of Sodium Secondary Battery

[0121] * Electrochemical performance measurement

[0122] For the sodium secondary batteries prepared in Examples 1 and 2 and Comparative Examples 1 and 2, the initial charge capacity, initial discharge capacity, and initial reversible efficiency were measured through charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 2.2V to 4.1V, and a discharge rate of 0.1C.

[0123] 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 2.2V to 4.1V at 25℃, and the ratio of the discharge capacity at the 50th cycle to the discharge capacity at the 1st cycle (cycle capacity retention rate) was measured, and the measurement results are shown in Table 3 below.

[0124] * Measurement of residual Na content (TTS)

[0125] The residual sodium content was determined as the value (TTS, Total Sodium) obtained by measuring the residual Na-containing compound (e.g., NaOH or Na2CO3) separately using potentiometric titration and then calculating the total amount of Na alone. The calculation method is as shown in Formula 1 below.

[0126] [Formula 1]

[0127] TTS(Total Na) = NaOH Analysis Value (%) × Na / NaOH + Na2CO3 Analysis Value (%) × 2Na / Na2CO3

[0128] * Measurement of compressive strength of positive electrode active material particles

[0129] Compressive strength was determined using a Micro Compression Testing Machine (Manufacturer: Shimadzu, Model: MCT-510). The cathode active material particles were compressed in the MD direction with a test force of 10 gf at a loading speed of 0.079 gf / sec, and the load at which the specimen fractured was measured. The compressive strength was calculated as follows:

[0130] Compressive strength (kgf / mm²) 2 ) = Load (kgf) / Thickness (mm) x Width (mm)

[0131] (2.2-4.1V) Comparative Example 1 Comparative Example 2 Example 1 Example 2 Particle compressive strength (kgf / mm² 2)12.8 13.5 15.5 12.5 Residual Na (Total Na, ppm) 36,339 2,742 2,447 2,775 Residual Na NaOH ppm N / D 947 96 21,029 Na 2CO 3 ppm 85,581 5,066 4,367 5,034 Total Nappm36,3392,7422,4472,775 Initial Charge Capacity mAh / g 140.8 160.2 160.3 160.0 Discharge Capacity 129.2 145.7 146.4 146.8 Efficiency % 91.7 90.9 91.4 91.8 Lifespan 1 cy mAh / g 114.1 133.7 136.4 138.8 50 cy 95.8 115.5 120.7 122.8% 82.5 86.4 88.5 88.5

[0132] As shown in Table 3, it can be confirmed that the lithium secondary battery containing the positive active material according to the embodiment of the present invention has improved initial charge / discharge characteristics and lifespan characteristics compared to the comparative example.

[0133] In addition, as the calcination temperature increased, the particle strength of the cathode active material improved. As a result, it was predicted that the occurrence of internal cracks during the rolling process in electrode manufacturing would decrease, and crystal structure variation and particle deformation during charging and discharging would be suppressed, thereby improving electrochemical performance. However, it was confirmed that surface degradation of primary particles may occur if calcination is performed at excessively high temperatures (see Example 2).

[0134] Accordingly, it was found that using a cathode active material substituted with a doping metal and including an oxide of the doping metal in the coating layer is an effective method to improve initial charge / discharge characteristics and lifespan characteristics.

Claims

1. A complex transition metal oxide comprising at least one transition metal selected from nickel, iron, manganese, and cobalt, sodium, and a doping metal, wherein a plurality of primary particles are aggregated into secondary particles; and A coating layer comprising an oxide of the doping metal, covering at least a portion of the surface of the primary particle and at least a portion of the pores formed between the plurality of primary particles; A positive electrode active material for a sodium secondary battery, wherein the doping metal is provided throughout the entire secondary particle including the center, and exhibits a higher concentration in the center than in the surface portion of the secondary particle.

2. In Paragraph 1, A positive electrode active material for a sodium secondary battery, wherein the ratio (C2 / C1) of the doping metal concentration (C1, atomic mol%) in the center of the secondary particle to the doping metal concentration (C2, atomic mol%) in the surface portion of the secondary particle is 1.1 to 3.

5.

3. In Paragraph 1, A positive electrode active material for a sodium secondary battery, wherein the doping metal is provided in the entire body including the center of the primary particle and in the coating layer, and exhibits a maximum concentration in the coating layer.

4. In Paragraph 1, The above coating layer is a positive electrode active material for a sodium secondary battery, wherein the thickness of the coating layer formed on at least a portion of the surface of the primary particle is 2 to 30 nm.

5. In Paragraph 1, A positive electrode active material for a sodium secondary battery, wherein the molar ratio (M1:M2) of the doping metal (M1) included in the composite transition metal oxide and the doping metal (M2) included in the doping metal oxide is 70:30 to 95:

5.

6. In Paragraph 1, A positive electrode active material for a sodium secondary battery, wherein the doping metal comprises calcium and is substituted into the sodium layer of the complex transition metal oxide.

7. In Paragraph 1, A positive electrode active material for a sodium secondary battery, wherein in the coating layer, the oxide of the doping metal comprises at least one selected from sodium calcium oxide (Na2CaO2) and calcium oxide (CaO).

8. In Paragraph 1, The above-mentioned 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] And a-2x Ca x [(I y TM 1-y )]O2 In the above chemical formula 1, TM is at least one selected from Ni, Fe, Mn and Co, and M is at least one selected from P, Sr, Ba, Ti, Zr, 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 0.80 <a<1.20, 0.001≤x≤0.1, 0≤y≤0.1, 0.9≤1-y≤1이다.

9. In Paragraph 1, The above secondary particle has an O3 crystal structure in X-ray diffraction analysis, and A positive electrode active material for a sodium secondary battery, having a full width at half maximum (FWHM) of the peak corresponding to the NiO (003) plane of 0.15 to 0.2°.

10. In Paragraph 1, A positive electrode active material for a sodium secondary battery, comprising a secondary particle center density that is higher than the secondary particle surface density.

11. In Paragraph 1, The above secondary particles have a particle strength of 14 to 16 kgf / mm 2 Phosphorus, sodium positive electrode active material for secondary batteries.

12. A method for manufacturing a positive electrode active material by mixing a positive electrode active material precursor, a doping metal compound and a sodium compound, and calcining the mixture, wherein The above positive active material comprises: secondary particles formed by the aggregation of a plurality of primary particles; and a coating layer covering at least a portion of the surface of the primary particles and at least a portion of the pores formed between the plurality of primary particles, the coating layer comprising an oxide of the doping metal. A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the above calcination is performed such that the doping metal is provided throughout the entire secondary particle including the center, and exhibits a higher concentration in the center than in the surface portion of the secondary particle.

13. In Paragraph 12, A method for manufacturing an anode active material, wherein the above calcination is performed by calcining the above mixture at 875 to 975°C for 5 to 40 hours.

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

15. A sodium secondary battery using a positive electrode according to Paragraph 14.

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