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

Abstract

The present invention provides a positive electrode active material for a sodium secondary battery, a method for preparing the same, and a sodium secondary battery comprising the positive electrode active material, the positive electrode active material for a sodium secondary battery being characterized by containing an O3-type sodium composite transition metal oxide containing at least sodium, a transition metal, and a doping metal, the sodium composite transition metal oxide is a secondary particle formed by aggregating a plurality of primary particles, and the aspect ratio of the primary particles is 1: 1 to 1: 2.5.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a sodium secondary battery, a method for preparing the same, and a sodium secondary battery including the positive electrode active material. Background Art

[0002] Lithium ion secondary batteries are widely used as energy storage devices in various electronic technology fields. Recently, with the sharp increase in the demand for lithium ion secondary batteries, sodium ion secondary batteries have attracted much attention in order to replace expensive metal lithium.

[0003] Sodium ion secondary batteries have a working principle similar to the insertion / extraction reaction of lithium ion secondary batteries. Therefore, they are one of the new generation materials with high potential for application in secondary batteries. However, compared with lithium ion secondary batteries, they show low performance in terms of capacity, life characteristics, rate characteristics, etc., and it is difficult to commercialize. In order to commercialize sodium ion secondary batteries, it is necessary to develop positive electrode active materials with high performance.

[0004] As the positive electrode active material of a sodium ion secondary battery, layered structure transition metal oxides with a simple structure, excellent electrochemical performance, and easy synthesis are typically used. Layered structure transition metal oxides are typically classified into O3 type and P2 type according to the crystal structure. The positive electrode active material based on the O3 type structure shows a composition such as Na x (TM)O2 (2 / 3 < x ≤ 1), and the positive electrode active material based on the P2 type structure has a composition of Na x (TM)O2 (x ≤ 2 / 3).

[0005] Generally, compared with P2 type layered oxide particles, O3 type layered oxides have a high energy density, but have the disadvantage of causing greater structural changes during charge and discharge, resulting in a decrease in cycle stability. P2 type layered oxides have relatively excellent cycle stability, but due to the low sodium content and relatively low energy density and other disadvantages, it is difficult to be applied commercially.

[0006] However, in O3 type oxide particles, due to sodium by-products existing in the form of Na2CO3 and NaOH on the particle surface, during the operation of the battery, there are problems such as gas generation caused by side reactions of the electrolyte, reduction of the capacity of the positive electrode active material, and reduction of the output, which reduce the battery life and stability. In O3 type oxide particles, when washing with water to remove residual Na, the internal Na is all lost, resulting in the problem of unable to maintain the structure.

[0007] In the present invention, high capacity and excellent life characteristics are to be achieved by improving the structural stability of the O3 type positive electrode active material. Summary of the Invention

[0008] Technical Problem to be Solved by the Invention

[0009] In the present invention, high capacity and excellent life characteristics are achieved by improving the structural stability of the O3 type positive electrode active material.

[0010] In addition, the object of the present invention is to provide a method for preparing a positive electrode active material, which performs a calcination process and a Cu coating process when preparing the positive electrode active material to improve battery performance such as structural stability, capacity characteristics, and life characteristics.

[0011] In addition, the purpose of the present invention is to increase the average particle size of the primary particles in the O3 type layered oxide secondary particles in which multiple primary particles are agglomerated, and to synthesize a transition metal oxide coated with Cu to improve the air stability and water stability, and to improve the problems of secondary particle fragmentation and structural collapse after water washing.

[0012] Means for solving technical problems

[0013] One example of the present invention provides a positive electrode active material for a sodium secondary battery, characterized in that it comprises an O3-type sodium composite transition metal oxide containing at least sodium, a transition metal and a doped metal, wherein the sodium composite transition metal oxide is a secondary particle formed by agglomeration of multiple primary particles, and the aspect ratio of the primary particles is 1:1 to 1:2.5.

[0014] The ratio (D2 / D1) of the average particle size (D2) of the secondary particles to the average particle size (D1) of the primary particles may be 2.5 to 10.

[0015] The average particle size (D1) of the primary particles may be 0.8 μm to 2.5 μm, and the average particle size (D2) of the secondary particles may be 6 μm to 12 μm.

[0016] The doping metal may be copper (Cu).

[0017] The sodium composite transition metal oxide can be represented by the following Chemical Formula 1.

[0018] [Chemical formula 1]

[0019] Na a [Cu x M y TM 1-x-y ]O2

[0020] In the above chemical formula 1, TM is at least one selected from Co, Ni, Mn and Fe, M is at least one selected from P, Sr, Ba, Ti, Zr, W, Co, Mg, Al, Zn, Ce, Hf, Ta, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb and Gd, 0.8 <a<1.0,0.01≤x≤0.1,0≤y≤0.1,0.8≤1-x-y≤0.99。

[0021] For the positive electrode active material, in XRD analysis, the half maximum width (FWHM (003)) at 2θ of 15° to 17.5° may be 0.1599 to 0.3399.

[0022] In the positive electrode active material, the residual Na content (total sodium (TTS)) may be 100 ppm to 3000 ppm.

[0023] Another example of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery, characterized in that it includes: a process of calcining a transition metal hydroxide precursor; a process of dry mixing the calcined precursor prepared in the above calcination process and a doped metal compound; and a process of calcining after mixing the metal-doped calcined precursor prepared in the above dry mixing process and a sodium compound at an equivalent ratio of Na / M (all metals except Na) = greater than 0.8 and less than 1.

[0024] The calcination process is performed at a temperature of 700° C. to 1100° C. in an oxidative atmosphere.

[0025] In the dry mixing process, the doping metal compound may be an acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide or a combination thereof of copper (Cu).

[0026] The sintering process may be performed at a temperature of 800°C to 1100°C.

[0027] The method may further include washing the sodium transition metal oxide prepared in the calcining process with water.

[0028] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery including the positive electrode active material, and a sodium secondary battery including the positive electrode and a negative electrode.

[0029] Effects of the Invention

[0030] In the present invention, after water washing, the fragmentation of the secondary particles can be minimized. In this regard, the half-peak width (FWHM (003)) of the main peak (003) of the O3 type is maintained at the same level as before water washing, and the residual Na on the particle surface can be removed at a low level while maintaining the O3 type crystal structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1a The SEM-EDS mapping analysis results of the surface of the positive electrode active material (secondary) particles prepared in Example 1 are as follows: Figure 1b The cross-sectional SEM-EDS mapping analysis results of the positive electrode active material (secondary) particles prepared in Example 1 and Comparative Example 1 are shown.

[0032] Figure 2a and Figure 2b The SEM comparative analysis results of the positive electrode active material particles prepared in Example 1 and Comparative Examples 1 to 3 before and after washing with water are shown.

[0033] Figure 3 The XRD comparative analysis results of the positive electrode active material particles prepared in Example 1 and Comparative Examples 1 to 3 before and after water washing are shown.

[0034] Figure 4 It is a graph showing the residual Na content (total sodium (TTS)) on the surface of the positive electrode active material after water washing in Example 1 and Comparative Examples 1 to 3. DETAILED DESCRIPTION

[0035] The advantages, features and methods of achieving the advantages and features of the present invention can be clearly seen by referring to the accompanying drawings and the embodiments described in detail below. However, the present invention is not limited to the embodiments disclosed below, but can be embodied in a variety of different forms. The embodiments are only used to make the disclosure of the present invention more complete and to fully inform the ordinary technicians in the technical field to which the present invention belongs of the scope of the invention. The present invention is only defined by the scope of the claims.

[0036] Unless otherwise defined, all terms (including technical terms and scientific terms) used in this specification can be used with the meanings commonly understood by ordinary technicians in the technical field to which the present invention belongs. Throughout the specification, when a part is mentioned to "include" a component, unless there is a special record to the contrary, it means that other components are also included, rather than excluding other components. In addition, unless otherwise mentioned, the singular form can also include the plural form in a sentence.

[0037] An embodiment of the present invention provides a positive electrode active material for a sodium secondary battery. The positive electrode active material comprises an O3-type sodium composite transition metal oxide containing at least sodium, a transition metal, and a doped metal, wherein the sodium composite transition metal oxide is a secondary particle formed by agglomeration of a plurality of primary particles, and the aspect ratio of the primary particles is 1:1 to 1:2.5.

[0038] Generally, it is well known that O3 oxide positive electrode materials have poor air stability. Specifically, when exposed to air or in contact with water, water oxidation reaction and H + / Na + Exchange, thereby generating residual Na such as NaOH and Na2CO3 on the surface. The generated CO3 is buried in the transition metal layer to form CO4 tetrahedron, thereby slowing down the diffusion of Na+ and deteriorating the electrochemical properties. Therefore, for the weakened bonding between TM-O, by introducing doping metals, the charge transfer between Na and O is effectively adjusted, thereby constructing a stronger Na2O binding energy. It has structural stability and excellent atmospheric stability.

[0039] Furthermore, in the present invention, a calcined precursor is prepared by calcining at a high temperature to achieve the above-mentioned aspect ratio of primary particles, and then a doping metal is prepared by dry mixing so as to be uniformly dispersed on the surface of the calcined precursor particles.

[0040] In the present invention, a calcined precursor that is oxidatively calcined at high temperature is used to increase the size of the primary particles, reduce the specific surface area, and increase the density of the primary particles formed by the agglomeration of the secondary particles. At this time, ① the binding force between the primary particles is increased, thereby improving the particle fragmentation after water washing, and increasing the cohesion between the primary particles to prevent the Na from being separated from the crystal structure due to water washing damage. In addition, ② due to the characteristics of the doped metal used in a small amount, the following effects are achieved: the doping dispersion is relatively increased on the surface of the primary particles with reduced specific surface area and the surface of the secondary particles. Therefore, compared to the previous goal or effect of improving the surface characteristics mainly by doping the particle surface, in the present invention, the doped metal is uniformly arranged inside the secondary particles and at the crystal boundary of the primary particles, and the structural stability of the O3 type sodium composite transition metal oxide can be further improved.

[0041] In the present invention, specifically, the aspect ratio of the primary particles is 1:1 to 1:2.5, for example, 1:1 to 1:2.4, 1:1 to 1:2.3, 1:1 to 1:2.2, 1:1 to 1:2.1, 1:1 to 1:2, 1:1 to 1:1.9, 1:1 to 1:1.8, 1:1 to 1:1.7, 1:1 to 1:1.6, preferably 1:1 to 1:1.5. In the present invention, in the oxidation roasting process of the positive active material precursor, oxidation roasting can be performed at a high temperature to achieve the aspect ratio range of the primary particles. In addition, a positive active material can be provided, which is roasted and doped in steps under specific conditions to increase the size of the primary particles constituting the secondary particles in the positive active material, reduce the aspect ratio, and thus improve the energy density, high voltage stability, life characteristics and high rate characteristics.

[0042] The ratio (D2 / D1) of the average particle size (D2) of the secondary particles to the average particle size (D1) of the primary particles can be 2.5 to 10, for example, 2.5 to 8, 2.5 to 6 or 2.5 to 5. When the primary particle size ratio (D2 / D1) is greater than 10, the size of the primary particles is too small, so the primary particles may not be formed in shape, thus having an O3 structure, and a large amount of by-products (impurity) may be generated. These results are due to the fact that the roasting reaction does not proceed smoothly, or does not crystallize into an O3 structure, for example, roasting at a low temperature, roasting for a short time and / or roasting unevenly. In addition, the size of the primary particles can be the length of the major axis.

[0043] The average particle size (D1) of the primary particles may be 0.8 μm to 2.5 μm, for example, 0.8 μm to 2.3 μm, 0.8 μm to 2 μm, 0.8 μm to 1.7 μm or 1 μm to 1.5 μm. In addition, the average particle size (D2) of the secondary particles may be 6 μm to 12 μm, for example, 6 μm to 10 μm or 6 μm to 8 μm. The primary particles and the secondary particles contained in the positive electrode active material at least meet the above conditions, thereby increasing the particle density in the positive electrode active material. Thus, the electrochemical properties of the positive electrode active material may be improved.

[0044] In addition, the term "aspect ratio" used in the present application is the ratio (Length / Width ratio) of the major axis (Length) and the minor axis (Width) of the above-mentioned primary particles. In the case where the above-mentioned major axis shows the direction of the relatively long region of the above-mentioned primary particles, the above-mentioned minor axis shows the length of the relatively short region located on the same surface as the above-mentioned major axis. At this time, the above-mentioned primary particles may have a plate shape, which means that the length in the thickness direction of the primary particles is significantly smaller than the length in the surface direction (major axis and minor axis) of the primary particles. In addition, the above-mentioned minor axis may be a direction perpendicular to the above-mentioned major axis, and the "aspect ratio" of the above-mentioned primary particles can be calculated by the ratio of the major axis and the minor axis of the above-mentioned primary particles measured from the surface of the above-mentioned primary particles.

[0045] In addition, in the present invention, more than 50% of the total number of primary particles constituting the above-mentioned secondary particles, for example, more than 60% or 70%, have an aspect ratio, primary particle size, and a ratio of the secondary particle size to the primary particle size (D2 / D1) within the above-mentioned ranges, or at least 10 or at least 20 primary particles among the primary particles constituting the above-mentioned secondary particles may have an aspect ratio, primary particle size, and a ratio of the secondary particle size to the primary particle size (D2 / D1) within the above-mentioned ranges.

[0046] The doping metal may be copper (Cu). The Cu doping may replace part of Fe and Ni in the transition metal and reduce the amount of Fe and Ni migration. In addition, Cu doping improves the average atomic valence state of Mn by charge compensation, thereby reducing the Mn 3+ Jahn-Teller effect. Moreover, compared with the case where Co is uniformly coated, Cu shows a tendency to be relatively unevenly coated due to its own characteristics, but in the present invention, the coating dispersion is relatively increased on the surface of the primary particles with reduced specific surface area and the surface of the secondary particles. Therefore, the coating unevenness of Cu can be greatly improved. As a result, when the O3 type oxide particles are washed with water to remove residual Na, the internal Na is lost and the structure cannot be maintained, but the atmospheric stability (air stability) and water stability (water stability) can be improved. After washing with water, the secondary particle fragmentation and structural collapse problems can be improved.

[0047] Specifically, the sodium composite transition metal oxide may be represented by the following Chemical Formula 1.

[0048] [Chemical formula 1]

[0049] Na a [Cu x M y TM 1-x-y ]O2

[0050] In the above Chemical Formula 1,

[0051] TM is at least one selected from Co, Ni, Mn, and Fe,

[0052] M is at least one selected from P, Sr, Ba, Ti, Zr, W, Co, Mg, Al, Zn, Ce, Hf, Ta, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd; 0.8 ≤ a ≤ 1, 0.01 ≤ x ≤ 0.1, 0 ≤ y ≤ 0.1, 0.8 ≤ 1 - x - y ≤ 0.99.

[0053] When the Na equivalent of the above O3-type layered oxide is less than 0.80 (a < 0.80), the oxide has a P3-type structure, and the electrochemical characteristics may deteriorate according to the arrangement of the lattice unit. On the contrary, when the Na equivalent is greater than 1.0 (0.1 < a), there are disadvantages such as reduced atmospheric and moisture stability and sensitivity to synthesis conditions such as temperature and atmosphere brightness.

[0054] For the positive electrode active material of the present invention, in XRD analysis, the full width at half maximum (FWHM(003)) of (003) at 2θ from 15° to 17.5° can be from 0.1599 to 0.3399. The XRD analysis result can be the result of analysis after water washing for removing residual Na on the surface of the prepared positive electrode active material. Thus, after removing residual Na on the surface by water washing, the above positive electrode active material can well maintain the O3-type crystal structure without particle fragmentation or structural collapse.

[0055] In the positive electrode active material of the present invention, the content of residual Na (Total Sodium, TTS) can be reduced to 100 ppm to 3000 ppm, specifically, it can be reduced to 500 ppm to 3000 ppm. Thus, gas generated due to residual Na can be inhibited, and the battery life characteristics can be improved.

[0056] In addition, the content of the above residual Na (Total Sodium, TTS) can be a value (Total Sodium, TTS) obtained by solely calculating the total amount of Na in a compound containing residual Na (for example, NaOH or Na2CO3).

[0057] Another example of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery.

[0058] The preparation method includes: a process of calcining a transition metal hydroxide precursor; a process of dry mixing the calcined precursor prepared in the calcination process and a doped metal compound; and a process of calcining after mixing the metal-doped calcined precursor prepared in the dry mixing process and a sodium compound at an equivalent ratio of Na / M (all metals except Na) = greater than 0.8 and less than 1.

[0059] The above-mentioned roasting process is a process for heat-treating a transition metal hydroxide precursor at a high temperature. In the absence of existing roasting, and in the case where the transition metal is coated on the hydroxide precursor by a dry method, in the subsequent process, after the heat treatment for inserting sodium, particle fragmentation may occur in the oxide secondary particles or primary particles. Upon analysis, this phenomenon originates from organic elements such as acetate, sulfide, nitride, phosphide or oxygen elements such as oxide, oxyhydroxide, hydroxide contained in the anion group in the coating compound. In addition, in the case of coating the transition metal by a wet method, there is a problem of increased cost caused by limited selection of coating compounds and complicated processes.

[0060] The above-mentioned calcination process is carried out at a temperature of 750°C to 1050°C under an oxidizing atmosphere, preferably, it can be carried out at a temperature of 800°C to 1050°C, 800°C to 950°C, 850°C to 1000°C or 850 to 950°C. In the case of calcination at a temperature higher than the above range, the size of the synthesized calcined precursor primary particles is excessively increased, so that the sodium ion diffusion (Na ion diffusion) in the particle surface is limited. On the contrary, in the case of calcination at a temperature lower than the above range, the primary particles are not fully grown, so that the target primary particle aspect ratio cannot be achieved. At this time, the calcination time is not particularly limited, and preferably, it can be carried out for 6 hours to 15 hours, 8 hours to 15 hours or 8 hours to 13 hours.

[0061] When the above calcination process is performed, the following phenomena occur: the specific surface area and porosity of the calcined precursor decrease, the particle size of the plurality of primary particles in the secondary particles increases, and the degree of agglomeration increases. As a result, the tap density of the calcined precursor increases, and the particle fragmentation problem that occurs when dry coating is performed without calcination can be prevented.

[0062] The transition metal hydroxide precursor may be represented by the following chemical formula 3, and the calcined precursor may be represented by the following chemical formula 4.

[0063] [Chemical formula 3]

[0064] TM(OH)2

[0065] (TM is at least one selected from Co, Ni, Mn and Fe)

[0066] [Chemical formula 4]

[0067] (TM)O4

[0068] (TM is at least one selected from Co, Ni, Mn and Fe)

[0069] The dry mixing process is a process of dry mixing the calcined precursor prepared in the calcination process and the doped metal compound. In the case of dry coating after the calcination process, the bonding force between the primary particles is increased by calcination to have a dense shape of primary particles, thereby improving the metal coating effect (cohesion between primary particles). In addition, when Cu coating is performed without calcination, although some structures are maintained after water washing, cracks are easily generated between the primary particles.

[0070] By applying the dry method, Cu can be uniformly coated on the surface and inside of the calcined precursor particles. On the contrary, when the transition metal is coated by the wet method, the selection of coating compounds is limited and there is a problem of increased cost due to complicated processes, which is not preferred.

[0071] The copper (Cu) compound may be an acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide or a combination thereof.

[0072] The above-mentioned sintering process is the following steps: in order to prepare O3 type sodium transition metal oxide, the above-mentioned copper (Cu) coated calcined precursor and sodium compound are mixed in an equivalent ratio of Na / M (all metals except Na) = greater than 0.8 and less than 1, and then sintered.

[0073] The mixture of the copper (Cu) coated calcined precursor and the sodium compound can be mixed in an equivalent of Na / M (all metals except Na) = greater than 0.8 and less than 1 or greater than 0.8 and less than 0.95. When the mixing amount of the sodium compound is within the above range, the crystalline structure of the prepared positive electrode active material can be an O3 type layered structure, thereby having a higher energy density, high atmospheric and moisture stability, and is not very sensitive to synthesis conditions (temperature and atmosphere, etc.). In addition, within the above sodium content range, the battery discharge capacity can be improved, and the unreacted and residual Na can be minimized.

[0074] The above-mentioned calcination can be performed at a temperature of 700°C to 1100°C. When the calcination temperature is within the above range, the raw materials can fully react with each other and the particles can grow uniformly. More preferably, the above-mentioned calcination can be performed at a temperature of 750°C to 1050°C, 850°C to 1050°C or 900°C to 1000°C. The above-mentioned calcination can be performed for 5 to 40 hours. When the calcination time is within the above range, a highly crystalline positive electrode active material can be obtained, the particle size is appropriate, and the production efficiency can be improved. Preferably, the above-mentioned calcination can be performed for 5 to 20 hours, 5 to 18 hours, 8 to 15 hours or 10 to 14 hours.

[0075] The sodium compound may be at least one selected from the group consisting of Na2CO3, NaOH, NaNO3, CH3COONa and Na2(COO)2, preferably Na2CO3, NaOH or a combination thereof.

[0076] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery including the positive electrode active material, and a sodium secondary battery.

[0077] The positive electrode comprises a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector. The positive electrode active material according to one aspect of the present invention is present in the positive electrode active material layer.

[0078] The positive electrode collector is not limited as long as it does not cause chemical changes in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the positive electrode collector can generally have a thickness of 3 μm to 500 μm, and fine concavoconvexities can be formed on the surface of the collector to improve the adhesion of the positive electrode active material. This positive electrode collector can be provided in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

[0079] In addition, the positive electrode active material layer may be a layer including the above-mentioned positive electrode active material, a conductive material, and a binder.

[0080] Among them, the conductive material is used to impart conductivity to the electrode, and can be used without restriction as long as it does not cause chemical changes in the positive electrode active material and has conductivity. As non-limiting examples of conductive materials, there are graphites such as natural graphite or artificial graphite; carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber and other carbon materials; 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. Usually, based on the total weight of the positive electrode active material layer, it can contain 1 weight percent to 30 weight percent of conductive material.

[0081] In addition, the binder is a substance that serves to improve the adhesion between the positive electrode active material particles and the bonding force between the positive electrode active material and the collector. As a non-limiting example of the binder, there are polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile (polyacrylonitrile), carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber or their various copolymers. Usually based on the total weight of the positive electrode active material, etc., 1 weight percent to 30 weight percent of the binder may be included.

[0082] In addition to using the above-mentioned positive electrode active material, the positive electrode of one example of the present invention can be prepared according to a conventional positive electrode preparation method for sodium secondary batteries. For example, the positive electrode active material and the slurry for forming the positive electrode active material layer containing the binder and the conductive material are selectively applied to the positive electrode collector, and then dried and rolled to prepare the positive electrode. According to another example, after the slurry for forming the positive electrode active material layer is cast on an additional support, the film obtained by peeling the positive electrode active material layer from the support is laminated on the positive electrode collector, thereby preparing the positive electrode.

[0083] According to another example of the present invention, an electrochemical element comprising the above-mentioned positive electrode is provided, wherein the electrochemical element may be a battery, a capacitor, etc., and more specifically, may be a sodium secondary battery.

[0084] The sodium secondary battery includes: a positive electrode, a negative electrode disposed opposite to the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte (electrolyte). In addition, the sodium secondary battery may include: a battery container (housing) for accommodating an electrode assembly including the positive electrode, the negative electrode, and the separator; and a sealing member for sealing the battery container.

[0085] At this time, sodium secondary batteries can be divided into can type sodium secondary batteries in which an electrode assembly is placed in a metal can and pouch type sodium secondary batteries in which an electrode assembly is placed in a pouch formed of a sheet such as an aluminum layer, according to the shape of a battery container (casing).

[0086] In particular, in the case of a pouch-type sodium secondary battery using a positive electrode containing a positive electrode active material in various embodiments of the present invention, the possibility of a side reaction between the positive electrode active material and the electrolyte is low, thereby having the advantages of improving stability during storage and / or operation and reducing gas generation.

[0087] Hereinafter, the present invention will be described in detail by way of examples. This is for the purpose of further illustrating the present invention, but the scope of rights of the present invention is not limited to the following examples.

[0088] Example

[0089] (Example 1)

[0090] Ni 0.33 Fe 0.33 Mn 0.33 The (OH)2NFM11 precursor was added to an alumina crucible, oxidatively calcined at 950°C for 6 hours in an air atmosphere, and then cooled to room temperature to prepare a calcined precursor (Ni-Fe-Mn)O4.

[0091] The prepared calcined precursor and Cu(OH)2 were mixed into Cu / M (M=Ni+Fe+Mn+Cu)2 at mol% using a hand mixer and dry coated.

[0092] The prepared Cu 2at mol% coated calcined precursor and Na2CO3 were mixed at Na / (Ni+Fe+Mn+Cu)=0.85 equivalents to obtain a mixture. The prepared mixture was added to an alumina crucible, calcined at 950°C for 6 hours under an O2 atmosphere, and then cooled to room temperature to prepare an O3 type Na 0.85 Ni 0.33 Fe 0.31 Mn 0.33 Cu 0.02 O2 positive electrode active material.

[0093] The prepared positive active material was added to a reactor filled with distilled water, washed with water at a temperature of 5° C. to 50° C. with a stirring speed of 350 rpm for 1 hour, and dried at a temperature of 120° C. under vacuum conditions for 12 hours.

[0094] 85 wt% of the prepared positive electrode active material, 10 wt% of carbon black, and 5 wt% of PVdF binder were dispersed in 30 g of methyl pyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was evenly coated on an aluminum film with a thickness of 15 μm and vacuum dried at 135° C., thereby preparing a positive electrode for a sodium secondary battery.

[0095] For the above-mentioned positive electrode, a sodium metal plate was used as a counter electrode, a porous glass fiber (thickness: 200 μm) was used as a separation membrane, and an electrolyte containing NaPF6 at a concentration of 1.0 M in a solvent of propylene carbonate and fluoroethylene carbonate mixed in a volume ratio of 98:2 was used to prepare a sodium secondary battery (button stick).

[0096] (Comparative Example 1)

[0097] A positive electrode active material and a sodium secondary battery were prepared in the same manner as in Example 1 except that the oxidation roasting process was not performed.

[0098] (Comparative Example 2)

[0099] Without performing the oxidation roasting process, the NFM11 precursor and Cu(OH)2 were mixed at a Cu 2at.mol% and Na2CO3 was mixed at a Na / (Ni+Fe+Mn+Cu)=0.85 equivalents, added into an alumina crucible, and calcined at 950°C for 6 hours in an O2 atmosphere. Except for this, the positive electrode active material and the sodium secondary battery were prepared in the same manner as in Example 1.

[0100] (Comparative Example 3)

[0101] A positive electrode active material and a sodium secondary battery were prepared in the same manner as in Example 1 except that the oxidation baking process and the Cu dry coating process were not performed.

[0102] (Comparative Example 4)

[0103] A positive electrode active material and a sodium secondary battery were prepared in the same manner as in Example 1 except that Co(OH) 2 was used instead of Cu(OH) 2 .

[0104] Experimental example

[0105] Experimental Example 1: SEM-EDS mapping analysis of positive electrode active material particles

[0106] The surface of the positive electrode active material particles prepared in Example 1 was subjected to SEM-EDS mapping analysis. Figure 1a The results are shown.

[0107] The cross-sections of the positive electrode active material particles prepared in Example 1 and Comparative Example 1 were subjected to SEM-EDS mapping analysis. Figure 1b The results are shown.

[0108] Figure 1a It was confirmed that, on the surface of the positive electrode active material secondary particles of Example 1, Cu partially aggregated, but was coated relatively uniformly on the entire surface of the secondary particles.

[0109] Figure 1b It was confirmed that Cu was unevenly aggregated inside the secondary particles and on the surface of the primary particles, but in the case of Example 1, the size of the primary particles was increased compared to Comparative Example 1 (refer to Figure 2b ), Cu is dispersed on the surface of the primary particles whose size increases, thereby increasing the overall uniformity of Cu coating based on the cross section of the secondary particles.

[0110] Finally, it was confirmed that although Cu was not uniformly coated, the O3 type crystal structure was well maintained when referring to the subsequent experimental examples, and after the positive electrode active material was washed with water, the outflow (detachment) of Na in the lattice structure was prevented.

[0111] Experimental Example 2: Comparison of positive electrode active material particles before and after washing

[0112] SEM and XRD comparative analysis were performed before and after washing the positive electrode active material particles prepared in Example 1 and Comparative Examples 1 to 3. Figure 2a , Figure 2b and Figure 3 The results are shown.

[0113] Reference Figure 2a , Figure 2b It was confirmed that, when the SEM surface shape was confirmed, some of the secondary particles were broken after water washing, and in Example 1, the particles were broken the least, while in Comparative Example 3, most of the secondary particles were broken.

[0114] In addition, refer to Figure 2b It can be seen that, as the oxidation roasting process is carried out in Example 1, the average particle size (D1) (D50) of the plurality of primary particles constituting the secondary particles ranges from 800nm ​​to 2.5μm, the average particle size (D2) (D50) of the secondary particles ranges from 6μm to 12μm, the aspect ratio of the primary particles ranges from 1:1 to 1:2.5, and the average particle size ratio (D2 / D1) ranges from 2.5 to 10. On the contrary, it was confirmed in Comparative Examples 1 to 3 that the average particle size of the primary particles is small and the aspect ratio of the primary particles is high compared to Example 1, thereby presenting a needle-like or rod-like shape, and the average particle size ratio is relatively high, that is, greater than 10.

[0115] Table 1

[0116] (003) Peak FWHM (2θ) Example 1 0.1847 Comparative Example 1 0.2957 Comparative Example 2 0.2729 Comparative Example 3 0.6619

[0117] Reference Figure 3 As shown in Table 1 above, the XRD titration results show that the peak of the O3 type main peak (003) is in the range of 15° to 17.5° at 2θ, and that the O3 type crystalline structure is maintained after water washing in Example 1. Specifically, the (003) half peak width (FWHM (003)) is measured to be 0.1847.

[0118] On the contrary, it was confirmed in Comparative Examples 1 to 3 that the FWHM (003) increased to 0.27 to 0.66, and after water washing, the O3 type crystal structure collapsed. In Comparative Example 3 having the highest FWHM (003), most of the particles were broken.

[0119] In Comparative Example 4 in which Co was applied instead of Cu, it was confirmed that most of the particles of the positive electrode active material were broken after washing with water.

[0120] Experimental Example 3: Determination of residual Na content (TTS)

[0121] In Example 1 and Comparative Examples 1 to 3, after washing with water, the residual Na content on the surface of the positive electrode active material was measured. Figure 4 The results are shown.

[0122] The residual sodium content is determined by measuring the compound containing residual Na (eg, NaOH or Na2CO3) by potentiometric neutralization titration, and then calculating the total amount of Na alone (Total Sodium (TTS)).

[0123] The calculation method is as shown in the following calculation formula 1.

[0124] [Calculation formula 1] TTS (Total Na) = NaOH analysis value (%) × Na / NaOH + Na2CO3 analysis value (%) × 2Na / Na2CO3

[0125] Reference Figure 4 It was confirmed that in Example 1 and Comparative Examples 1 to 3, the residual Na content (TTS) after water washing was as low as about 3000 ppm.

[0126] Experimental Example 4: Evaluation of Battery Performance

[0127] For the sodium secondary batteries prepared in Example 1 and Comparative Examples 1 to 3, the initial charge capacity, initial discharge capacity, initial reversible efficiency and rate characteristics (discharge capacity ratio; rate capability (C-rate)) were measured using an electrochemical analyzer (Toyo, Toscat-3100) through a charge and discharge experiment at 25°C, a voltage range of 2.0V to 4.6V, and a discharge rate of 0.1C to 2.0C.

[0128] In addition, the same sodium secondary battery was charged / discharged 50 times at 25° C. and in the driving voltage range of 2.5 V to 4.3 V at 1 C / 1 C, and the ratio of the 50th discharge capacity to the initial capacity (cycle capacity retention) was measured.

[0129] The above measurement results are shown in Table 2 below.

[0130] Table 2

[0131]

[0132] Referring to Table 2, it can be confirmed that Example 1 has the best electrochemical characteristics.

[0133] As described above, the present invention only shows and illustrates specific embodiments, and it is obvious to those skilled in the art that the present invention can be modified and varied in many ways without departing from the scope of the technical idea of ​​the present invention provided by the attached claims.

Claims

1. A positive electrode active material for a sodium secondary battery, characterized in that It comprises an O3 type sodium composite transition metal oxide containing at least sodium, a transition metal and a doping metal, The sodium composite transition metal oxide is a secondary particle formed by agglomeration of a plurality of primary particles, and the aspect ratio of the primary particles is 1:1 to 1:2.

5.

2. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that A ratio D2 / D1 of an average particle diameter D2 of the secondary particles to an average particle diameter D1 of the primary particles is 2.5 to 10.

3. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that The average particle size D1 of the primary particles is 0.8 μm to 2.5 μm, and the average particle size D2 of the secondary particles is 6 μm to 12 μm.

4. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that The doping metal is copper (Cu).

5. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that: The sodium composite transition metal oxide is represented by the following chemical formula 1: [Chemical formula 1], And a [Cu x I y TM 1-x-y ]O2, In the chemical formula 1, TM is at least one selected from Co, Ni, Mn and Fe, M is at least one selected from P, Sr, Ba, Ti, Zr, W, Co, Mg, Al, Zn, Ce, Hf, Ta, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb and Gd, 0.8 <a<1.0,0.01≤x≤0.1,0≤y≤0.1,0.8≤1-x-y≤0.99。 6. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that For the positive electrode active material, in XRD analysis, the half-value width of (003) at 2θ of 15° to 17.5° is 0.1599 to 0.3399.

7. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that: In the positive electrode active material, the residual Na content (total sodium (TTS)) is 100 ppm to 3000 ppm.

8. A method for preparing a positive electrode active material for a sodium secondary battery, characterized in that: include: A process for calcining a transition metal hydroxide precursor; A process for dry mixing the calcined precursor prepared in the calcination process and the doping metal compound; as well as The metal-doped calcined precursor prepared in the dry mixing process and the sodium compound are mixed at an equivalent ratio of Na / M=greater than 0.8 and less than 1, and then sintered, wherein M is all metals except Na.

9. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 8, characterized in that: The calcination process is performed at a temperature of 700° C. to 1100° C. in an oxidative atmosphere.

10. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 8, characterized in that: In the dry mixing process, The doping metal compound is an acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide or a combination thereof of copper (Cu).

11. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 8, characterized in that: The sintering process is performed at a temperature of 800°C to 1100°C.

12. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 8, characterized in that: The method also includes a process of washing the sodium transition metal oxide prepared in the calcining process.

13. A positive electrode for a sodium secondary battery, characterized in that: The positive electrode active material according to claim 1 is included.

14. A sodium secondary battery, characterized in that: include: The positive electrode of claim 13; and a negative electrode.

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