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 same, the positive electrode active material for a sodium secondary battery comprising: layered oxide particles containing at least sodium and a transition metal; and a coating layer on the layered oxide particles, the coating layer containing at least two compounds selected from the group consisting of compounds having crystal structures belonging to space groups P421C, Fd-3m, and Pnma, respectively.

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 of insertion / extraction reaction similar to that 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 sodium ion secondary batteries, 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, etc., 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 and output of the positive electrode active material, and reduction of the battery life and stability. In O3 type oxide particles, when water washing is carried out to remove residual Na, the internal Na is all lost, causing the crystal structure to collapse and unable to be used, which will hinder commercialization.

[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] The object of the present invention is to provide a positive electrode active material and a preparation method thereof, wherein a coating is formed by the re-reaction of residual Na on the surface of the positive electrode active material with a coating material.

[0010] The purpose of the present invention is to form a coating comprising at least two space groups, two crystal structures or two compounds on the surface of a positive electrode active material, thereby suppressing electrolyte side reactions and improving electrochemical characteristics during battery operation.

[0011] Means for solving technical problems

[0012] An embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, characterized in that it includes: layered oxide particles containing at least sodium and a transition metal; and a coating located on the layered oxide particles, wherein the coating comprises at least two selected from compounds having crystal structures belonging to space groups P421c, Fd-3m and Pnma, respectively.

[0013] The coating layer may include at least three compounds having crystal structures belonging to P421c, Fd-3m and Pnma, respectively.

[0014] The compound having a crystal structure belonging to the above-mentioned space group P421c, Fd-3m or Pnma may have a tetragonal, cubic or orthorhombic crystal structure.

[0015] The coating layer may include at least two selected from the group consisting of sodium phosphate, transition metal oxide, and sodium phosphate containing transition metal.

[0016] The sodium phosphate, transition metal oxide, and transition metal-containing sodium phosphate can be represented by the following Chemical Formula 2a, Chemical Formula 2b, and Chemical Formula 2c, respectively.

[0017] [Chemical formula 2a]Na x PO y

[0018] In chemical formula 2a, 0 <x<4,2<y<5。

[0019] [Chemical formula 2b] Co x O y

[0020] In chemical formula 2b, 0 <x<4,0<y<5。

[0021] [Chemical formula 2c]Na x Co y PO4

[0022] In chemical formula 2c, 0 <x<2,0<y<2。

[0023] The coating layer may include at least Na3PO4, Co3O4 and NaCoPO4.

[0024] The coating layer may contain the whole compound in an amount greater than 0.5 weight percent and less than 10 weight percent relative to the whole compound contained in the positive electrode active material.

[0025] The layered oxide may be represented by Chemical Formula 1 below.

[0026] [Chemical formula 1]

[0027] Na a Ni x TM y M1 z Mn 1-x-y-z O2

[0028] In the above chemical formula 1, TM is Co or Fe, M1 is at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Co, Fe, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, TM and M1 are different elements, 0.80 <a<1.2,0.01≤x≤0.45,0.01≤y≤0.45,0≤z≤0.1,0.01≤1-x-y-z≤0.45。

[0029] The BET specific surface area of ​​the positive electrode active material can be less than 1m 2 / g.

[0030] In the positive electrode active material, the residual Na content (Total Sodium (TTS)) may be less than 10000 ppm.

[0031] Another example of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery, comprising: a process of mixing a layered oxide containing at least sodium and a transition metal and a coating source containing a transition metal, phosphorus (P) and oxygen (O); and a process of calcining the mixture of the layered oxide and the coating source.

[0032] In the above mixing process, the coating source is contained in an amount greater than 0.5 weight percent and less than 10 weight percent relative to the entire mixture.

[0033] The coating source may include at least one selected from Co3(PO4)2, CoPO4, Co2PO4, Co4PO4 and NH4H2PO4.

[0034] The sintering process is performed at a temperature of 300° C. to 550° C. for 2 hours to 6 hours.

[0035] The method also includes a process of washing the fired product prepared in the above-mentioned firing process with water.

[0036] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery including a positive electrode active material, and a sodium secondary battery using the positive electrode.

[0037] Effects of the Invention

[0038] In the present invention, the coating layer formed on the surface of the positive electrode active material suppresses the side reaction of the electrolyte and reduces the amount of residual Na.

[0039] In addition, the present invention can express the following effects: the coating contains at least two space groups, crystal structures or compounds to improve the ionic conductivity of the coating, improve the coating uniformity when the coating thickness is increased, inhibit the increase in resistance caused by the coating, and reduce side reactions with the electrolyte on the particle surface.

[0040] Furthermore, in the present invention, the positive electrode active material having the coating layer is prepared only by a dry process, thereby eliminating the water washing-drying process and reducing the process cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figures 1a to 1e The field emission-scanning electron microscope (FE-SEM) images of the surfaces of the positive electrode active material particles with coating layers prepared in Examples 1-1 to 2-3 and Comparative Examples 1-1 to 3-3 are shown respectively. DETAILED DESCRIPTION

[0042] 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.

[0043] 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.

[0044] An embodiment of the present invention provides a positive electrode active material for a sodium secondary battery. The positive electrode active material comprises: layered oxide particles containing at least sodium and a transition metal; and a coating layer located on the layered oxide particles.

[0045] The residual Na on the surface of the positive electrode active material and the coating material react to form a coating layer, thereby reducing the amount of residual Na and suppressing the side reaction of the electrolyte on the particle surface.

[0046] The coating comprises at least two compounds selected from compounds having crystalline structures belonging to space groups P421c, Fd-3m and Pnma, respectively, and more preferably, at least three compounds having crystalline structures belonging to space groups P421c, Fd-3m and Pnma, respectively. The compound having a crystalline structure belonging to the space groups P421c, Fd-3m or Pnma may have a tetragonal, cubic or orthorhombic crystalline structure, and may be a sodium phosphate, a transition metal oxide or a sodium phosphate containing a transition metal, and may specifically be Na3PO4, Co3O4 or NaCoPO4.

[0047] When a compound having space group P421c and tetragonal crystal structure is coated on the surface of the above-mentioned positive electrode active material, the ionic conductivity of the coating is improved, thereby improving the initial charge capacity. However, as the thickness of the coating increases, the coating is formed unevenly, thereby increasing resistance, reducing discharge capacity, and having no effect on improving life.

[0048] When a compound having a space group Fd-3m, a cubic crystal structure or a space group P63 / mmc, a hexagonal crystal structure is coated on the surface of the above-mentioned positive electrode active material, there is a problem of forming an irreversible coating, or a reduction in the initial discharge capacity due to a very low capacity, but the side reaction between the surface coating of the positive electrode active material and the electrolyte can be reduced, and the life span can be slightly improved.

[0049] When a compound having a space group Pnma and an orthorhombic crystal structure is coated on the surface of the positive electrode active material, there are problems such as increased coating concentration, increased resistance, and reduced discharge capacity, but the effect of slightly improving the life can be achieved.

[0050] In the present invention, the coating contains at least two compounds selected from compounds having crystal structures belonging to space groups P421c, Fd-3m, and Pnma, preferably all three compounds. Thus, as a combined effect, the ionic conductivity of the coating is increased to increase the Na + conductivity, improve the irreversible coating resistance, increase the coating concentration, improve the coating uniformity, reduce the resistance, and suppress side reactions of the electrolyte at the interface. Thus, battery performance such as initial capacity expression and life characteristics can be improved.

[0051] The above coating may contain at least two selected from the group consisting of sodium phosphates, transition metal oxides, and sodium phosphates containing transition metals, preferably all three compounds.

[0052] The above compounds are coatings formed by reacting with residual Na on the surface of the positive electrode active material, formed on the surface to suppress side reactions of the electrolyte, and reduce the amount of residual Na. And the following effects can be expressed: making the coating contain at least the above two compounds to improve the ionic conductivity of the coating, improving the coating uniformity when the coating thickness increases, suppressing the increase in resistance caused by the coating, and reducing side reactions with the electrolyte on the particle surface.

[0053] In addition, the above sodium phosphate may be a compound having a crystal structure belonging to space group P421 C The above transition metal oxide may be a compound having a crystal structure belonging to space group Fd-3m, and the above sodium phosphate containing transition metal may be a compound having a crystal structure belonging to space group Pnma.

[0054] The above sodium phosphate, transition metal oxide, and sodium phosphate containing transition metal may be compounds represented by the following Chemical Formula 2a, Chemical Formula 2b, and Chemical Formula 2c, respectively.

[0055] [Chemical Formula 2a] Na x PO y

[0056] In Chemical Formula 2a, 0 < x < 4, 2 < y < 5, for example, 1 < x < 4 or 2 < x < 4, 2 < y < 5 or 3 < y < 5.

[0057] [Chemical Formula 2b] Co x O y

[0058] In Chemical Formula 2b, 0 < x < 4, 0 < y < 5, for example, 1 < x < 4 or 2 < x < 4, 2 < y < 5 or 3 < y < 5.

[0059] [Chemical Formula 2c] Na x Co y PO4

[0060] In Chemical Formula 2c, 0 < x < 2 and 0 < y < 2. For example, 0.5 < x < 1.2, 0.67 < x < 1.2, or 0.8 ≤ x ≤ 1, and 0.5 < y < 1.5, or 0.9 < y < 1.1.

[0061] The sodium phosphate represented by Chemical Formula 2a above may contain Na3PO4, the transition metal oxide represented by Chemical Formula 2b above may contain Co3O4, and the sodium phosphate containing a transition metal represented by Chemical Formula 2c above may contain NaCoPO4.

[0062] Preferably, the above coating may contain at least two selected from the group consisting of Na3PO4, Co3O4, and NaCoPO4, and preferably may contain at least the above three compounds. Thus, the above effects can be further improved.

[0063] Relative to the overall compound contained in the positive electrode active material, the overall compound contained in the above coating may be contained in an amount greater than 0.5 wt% and less than 10 wt%. For example, the overall compound contained in the above coating may be contained in an amount of 1 wt% to 8 wt%, 1 wt% to 7 wt%, 1 wt% to 6.5 wt%, or 1 wt% to 6 wt%.

[0064] When the coating content is 10 wt% or more, the resistance increases due to the increased thickness, and the capacity characteristics may deteriorate. On the contrary, when the coating content is 1 wt% or less, since the coating formation is uneven, the BET specific surface area increases, there is no effect of improving the electrolyte side reaction, and it is difficult to sufficiently remove the residual Na.

[0065] Relative to the overall compound contained in the positive electrode active material, the sodium phosphate contained in the above coating may be contained in an amount greater than 0.5 wt% and less than 5 wt%. For example, the sodium phosphate contained in the above coating may be contained in an amount of 1 wt% to 5 wt%, 1 wt% to 4.5 wt%, 2 wt% to 4.5 wt%, 3 wt% to 4.5 wt%, 3.5 wt% to 4.5 wt%, or 3.7 wt% to 4.1 wt%. When the content range is less than the upper limit value, the coating is formed uniformly, and an increase in resistance and a decrease in discharge capacity can be suppressed. On the contrary, when the content range is greater than the lower limit value, the ionic conductivity of the coating is improved, and thus the initial charge capacity can be improved.

[0066] Relative to the overall compound contained in the positive electrode active material, the transition metal oxide contained in the above coating may be included in an amount greater than 0.01 weight percent and less than 1.5 weight percent, for example, 0.05 weight percent to 1.5 weight percent, 0.05 weight percent to 1.3 weight percent, 0.05 weight percent to 1.0 weight percent, 0.1 weight percent to 0.9 weight percent or 0.3 weight percent to 0.7 weight percent. In the case where the content range is less than the upper limit, the side reaction with the electrolyte is reduced and the life is slightly improved. On the contrary, in the case where the content range is greater than the lower limit, the reduction of the initial discharge capacity can be suppressed.

[0067] Relative to the overall compound contained in the positive electrode active material, the sodium phosphate containing a transition metal contained in the above-mentioned coating may be included in an amount greater than 0.1 weight percent and less than 2 weight percent. For example, the sodium phosphate containing a transition metal contained in the above-mentioned coating may be included in an amount of 0.1 weight percent to 1.5 weight percent, 0.3 weight percent to 1.5 weight percent, 0.5 weight percent to 1.5 weight percent, 0.7 weight percent to 1.5 or 0.9 weight percent to 1.3 weight percent. When the content range is less than the upper limit value, the problems of increased coating concentration, increased resistance and reduced discharge capacity can be improved. On the contrary, when the content range is greater than the lower limit value, the lifespan can be slightly improved.

[0068] The coating may include sodium phosphate, sodium phosphate containing a transition metal, and transition metal oxide in order of increasing content.

[0069] The layered oxide may be represented by Chemical Formula 1 below.

[0070] [Chemical formula 1]

[0071] Na a Ni x TM y M1 z Mn 1-x-y-z O2

[0072] In the above chemical formula 1, TM may be Co or Fe, M1 may be at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Co, Fe, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, TM and M1 may be different elements, 0.80 <a<1.2,0.01≤x≤0.45,0.01≤y≤0.45,0≤z≤0.1,0.01≤1-x-y-z≤0.45。

[0073] The above-mentioned TM is preferably a ternary transition metal composed of Ni, Mn, and Fe, where 0.9 < a < 1.1, 0.3 ≤ x ≤ 0.35, 0.3 ≤ y ≤ 0.35, 0.01 ≤ z ≤ 0.05, and 0.3 ≤ 1 - x - y - z ≤ 0.35. Specifically, it can be a Ni-Fe-Mn 111 layered oxide.

[0074] The above-mentioned layered oxide may have an O3-type crystal structure. Compared with P2-type layered oxide particles, the O3-type layered oxide has a higher energy density but has problems of poor air stability and water stability. In the present invention, a coating is formed by the re-reaction of residual Na on the surface of the positive electrode active material and the coating material. Thus, the washing-drying process can be removed, and the problem of crystal structure collapse during washing to remove residual Na can be improved.

[0075] In the above-mentioned positive electrode active material, the BET specific surface area can be reduced to less than 1 m 2 / g. For example, it can be reduced to 0.1 m 2 / g to 0.8 m 2 / g or 0.1 m 2 / g to 0.6 m 2 / g. In the present invention, the coating contains at least two compounds, preferably three compounds. Thus, even when the coating thickness increases, a relatively uniform and smooth coating can be formed on the particle surface, and the BET specific surface area can be controlled within the above design range, thereby suppressing the side reaction of the interfacial electrolyte.

[0076] The content of residual Na (Total Sodium, TTS) in the above-mentioned positive electrode active material can be less than 10000 ppm. For example, it can be less than 9000 ppm, less than 8000 ppm, less than 7000 ppm, or less than 6000 ppm.

[0077] Thus, the gas generated due to residual Na can be suppressed, and the battery life characteristics can be significantly improved.

[0078] In addition, the content of the above-mentioned residual Na (Total Sodium, TTS) can be a value (Total Sodium, TTS) obtained by separately calculating only the total amount of Na in the compound containing residual Na (for example, NaOH or Na2CO3).

[0079] Another example of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery.

[0080] The preparation method comprises: a process of mixing a layered oxide containing at least sodium and a transition metal, a transition metal and a coating source containing phosphorus (P) and oxygen (O); and a process of calcining the mixture of the layered oxide and the coating source.

[0081] In the above-mentioned mixing process, the dry method is applied to the mixing, whereby the coating source can be uniformly mixed on the surface and inside of the layered oxide particles, and then the water washing-drying process can be eliminated, thereby simplifying the process. In addition, in the case of the wet method, the selection of the coating compound is limited, and there is a problem of increased cost due to the complexity of the process, which is not preferred.

[0082] Relative to the entire mixture, the above-mentioned coating source may be included in an amount greater than 0.5 weight percent and less than 10 weight percent, for example, 0.5 weight percent to 7 weight percent, 0.5 weight percent to 6 weight percent, 1 weight percent to 6 weight percent, 3 weight percent to 6 weight percent or 4 weight percent to 6 weight percent of the above-mentioned coating source may be included.

[0083] The coating source is a compound that can form a coating material that reacts with residual Na, and is a compound containing transition metals, phosphorus (P) and oxygen (O), for example, it can contain cobalt phosphate. The coating source can specifically include at least one selected from Co3(PO4)2, CoPO4, Co2PO4, Co4PO4 and NH4H2PO4, preferably, it can include Co3(PO4)2 or NH4H2PO4, and more preferably, it can be Co3(PO4)2. Thus, a coating that reacts with residual Na is formed and formed on the surface to suppress electrolyte side reactions, and the amount of residual Na can be reduced.

[0084] The sintering process forms a coating layer by the re-reaction of the residual Na on the surface of the layered oxide with the coating source, and sintering the mixture of the layered oxide and the coating source.

[0085] The sintering process may be performed at a temperature of 300°C to 550°C for 2 to 6 hours. Specifically, it may be performed at a temperature of 300°C to 500°C or 350°C to 450°C for 2 to 6 hours, 3 to 5 hours, or 3.5 to 4.5 hours. Thus, the ratio of the three compounds in the coating may be optimized with reference to the following reaction formulas 1 to 4.

[0086] [Reaction formula 1]

[0087]

[0088] In Reaction Formula 1, the sintering process is performed at the designed reaction temperature and reaction time, and a coating can be formed by synthesizing sodium phosphate, transition metal oxide, and sodium phosphate compound containing transition metal.

[0089] [Reaction 2]

[0090]

[0091] In reaction formula 2, when the reaction time is too long, Na3PO4 and Co3O4 can be advantageously generated.

[0092] [Reaction 3]

[0093]

[0094] In reaction formula 3, if the reaction time is insufficient, Co3O4 may not be fully synthesized, and part of Co may be oxidized in the coating, thereby enabling synthesis in the form of a CoCO3 / Co(OH)2 mixture, but it may exist at an impurity level.

[0095] [Reaction 4]

[0096]

[0097] In reaction formula 4, when the sintering process is performed at a relatively high reaction temperature, Co3O4 in the reaction product can be synthesized into Na-metal oxide (Metal Oxide), and NH4H2PO4 reacts with the Co source to synthesize NaCOPO4.

[0098] In addition, the method may further include a step of washing the positive electrode active material (layered oxide) having the coating layer formed thereon prepared in the above-mentioned sintering step.

[0099] The above-mentioned water washing process is a process for removing unreacted substances, impurities and residual sodium. In a reactor added with one or more selected from deionized water, distilled water and ethanol, the positive electrode active material with a coating prepared in the above-mentioned sintering process is added, and the positive electrode active material is washed with water at a temperature of 1°C to 80°C or 5°C to 50°C, at a stirring speed of 200rpm to 500rpm, 200rpm to 400rpm or 300rpm to 400rpm for 0.5 hour to 5 hours, 0.5 hour to 4 hours, 0.5 hour to 3 hours or 0.5 hour to 1.5 hours.

[0100] In the present invention, as the coating is formed on the O3 type layered oxide to improve the structural stability, when the conventional water washing process for the O3 type layered oxide is performed, the residual Na on the surface can be effectively removed without causing the collapse of the crystal structure.

[0101] Next, a drying process may be performed to remove moisture from the positive electrode active material after the water washing process. The drying process may be performed under a vacuum condition at a temperature of 100° C. to 300° C. for more than 12 hours.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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).

[0112] 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.

[0113] 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.

[0114] Example

[0115] (Example 1-1)

[0116] Use a hand mixer to mix Na 1.0 Ni 0.33 Fe 0.33 Mn 0.33 The O2(NFM111)O3 type layered oxide and 1 weight percent of the coating source Co3(PO4)2 are dry mixed.

[0117] The prepared mixture was placed in an alumina crucible, fired at 400° C. for 4 hours in an air atmosphere, and then cooled to room temperature to prepare a positive electrode active material having a coating layer formed thereon.

[0118] 85 weight percent of the prepared positive electrode active material, 10 weight percent of carbon black, and 5 weight percent 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.

[0119] 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).

[0120] (Example 1-2 to Example 1-3 and Comparative Example 1-1 to Comparative Example 4-3)

[0121] A positive electrode active material and a sodium secondary battery having a coating layer formed thereon were prepared in the same manner as in Example 1-1 except that the coating source and the calcination conditions were as shown in Table 1 below.

[0122] Experimental example

[0123] Experimental Example 1: Analysis of compounds contained in coatings

[0124] The content of the coating material (i.e., the second compound) on the surface of the positive electrode active material prepared according to Examples 1-1 to 2-3 and Comparative Examples 1-1 to 3-3 was quantitatively analyzed by screening the compounds described in Tables 1, 2 and 3 below from XRD raw data measured using Bruker's EVA program.

[0125] The analysis results of the coating material on the surface of the positive electrode active material analyzed by the above method are shown in Tables 1 to 3 below.

[0126] Table 1

[0127]

[0128] Table 2

[0129] Classification Space group Crystal structure <![CDATA[Na3PO4]]> <![CDATA[P421 C ]]> Tetragonal <![CDATA[Co3O4]]> Fd-3m Cubic <![CDATA[Na 0.6 CoO2]]> P63 / mmc Hexagonal <![CDATA[NaCoPO4]]> Pnma Orthorhombic

[0130] Referring to Table 1, it was confirmed that the coating of the positive electrode active material prepared in Example 1 of the present invention contained three compounds of Na3PO4, Co3O4 and NaCoPO4, and contained two compounds in the case of Example 2. In contrast, in the case of the comparative example, it was confirmed that one compound among Na3PO4, Co3O4 and NaCoPO4 was contained.

[0131] Referring to Table 2, it is analyzed that the three compounds contained in the coating prepared in the example have space group P421 C , Fd-3m or Pnma and tetragonal, cubic or orthorhombic crystal structure.

[0132] Analysis: Na3PO4 (space group P421 C , tetragonal crystal structure) has the advantage of improving the initial charge capacity by increasing the ion conductivity of the coating. As the coating thickness increases, the coating becomes uneven, thereby increasing the resistance and reducing the discharge capacity, and there is no life improvement effect.

[0133] Analysis: Co3O4 (space group Fd-3m, cubic crystal structure) or Na 0.6 CoO2 (space group P63 / mmc, hexagonal crystal structure) has the problem of forming an irreversible coating (Co3O4) and expressing very low capacity (Na 0.6 The problem of reduced initial discharge capacity caused by CoO2) is solved, but the side reaction between the surface coating of the positive electrode active material and the electrolyte is reduced, which slightly improves the life.

[0134] Analysis showed that NaCoPO4 (space group Pnma, orthorhombic crystal structure) had problems such as increased coating concentration, increased resistance, and reduced discharge capacity, but slightly improved lifespan.

[0135] Table 3

[0136]

[0137] Referring to Table 3, the optimal content compositions of Na3PO4, Co3O4 and NaCoPO4 compounds in the coating were confirmed in Examples 1-1 to 1-3.

[0138] In addition, it is analyzed in the examples that as the coating content increases within the range of 1.5 weight percent to 5.5 weight percent, the battery characteristics are improved.

[0139] Experimental Example 3: Scanning Electron Microscope (FE-SEM) Analysis and Coating Uniformity Analysis by BET Analysis

[0140] Figures 1a to 1e The FE-SEM images of the surfaces of the positive electrode active material particles with coating layers formed in Examples 1-1 to 2-3 and Comparative Examples 1-1 to 3-3 are shown respectively.

[0141] Table 4 below shows the BET specific surface areas of the positive electrode active materials on which the coating layers were formed, prepared in Examples 1-1 to 1-3 and Comparative Examples 1-1 to 3-3.

[0142] Table 4

[0143] <![CDATA[BET specific surface area (m 2 / g)]]> Pristine 0.64 Example 1-1 0.61 Example 1-2 0.59 Examples 1-3 0.55 Example 2-1 0.60 Example 2-2 0.54 Example 2-3 0.50 Comparative Examples 1-3 0.87 Comparative Example 2-3 2.54 Comparative Example 3-3 0.92

[0144] Reference Figures 1a to 1e Table 4 confirms that in the case of Example 1-1 to Example 2-3, even if the thickness of the coating increases as the proportion of the coating source increases (1 weight percent → 5 weight percent), a relatively uniform and smooth coating is formed on the particle surface. The same result can be confirmed in the BET specific surface area analysis. According to the analysis, this result is caused by the coating containing two or three compounds of Na3PO4, Co3O4 and NaCoPO4.

[0145] On the contrary, it was confirmed that in the case of the comparative example, a compound of one of Na3PO4, Co3O4 or NaCoPO4 was included to deposit a plurality of nanoparticle compounds of 10nm to 100nm in size on the surface of the layered oxide particles, and to form an uneven coating. The same result was also confirmed in the BET specific surface area analysis. As a result, as the coating thickness increases, the coating uniformity decreases, thereby predicting an increase in resistance and the resulting deterioration of the discharge capacity and life characteristics.

[0146] Experimental Example 4: Determination of residual Na content (TTS)

[0147] The residual Na content on the surface of the positive electrode active materials on which the coating layers were formed prepared in Examples 1-1 to 2-3 and Comparative Examples 1-1 to 3-3 was measured, and the results are shown in Table 5 below.

[0148] By potentiometric neutralization titration, after measuring the compound containing residual Na (e.g., NaOH or Na2CO3), the value obtained by calculating the total amount of Na alone (Total Sodium (TTS)) is used as the residual sodium content. The calculation method is as shown in the following calculation formula 1.

[0149] [Calculation formula 1]

[0150] TTS (Total Na) = NaOH analysis value (%) × Na / NaOH + Na2CO3 analysis value (%) × 2Na / Na2CO3

[0151] Table 5

[0152]

[0153] With reference to Table 5, it was confirmed that, in the Examples, the residual Na content (TTS) was removed to 10000 ppm or less, as compared with the Comparative Examples. Preferably, in Examples 1-1 to 1-3, the residual Na was removed to 7000 ppm or less.

[0154] Experimental Example 5: Evaluation of electrochemical performance of sodium secondary batteries

[0155] For the sodium secondary batteries prepared in Examples 1-1 to 2-3 and Comparative Examples 1-1 to 3-3, the ratio of the 50th cycle discharge capacity to the initial charge capacity (CH), the initial discharge capacity (DCH), the initial efficiency (ICE), and the tree cell capacity (cycle capacity retention rate; Retention) was measured, and the results are shown in Table 6 below.

[0156] Table 6

[0157]

[0158] Referring to Table 6, it is analyzed that in Examples 1-1 to 1-3, when the coating contains three compounds, due to Na + The conductivity is increased and the resistance of the irreversible coating is improved, thereby the capacity, initial efficiency and life are improved most.

[0159] It was analyzed that in Example 2-1 to Example 2-3, when the coating contained two compounds, the above effect was reduced.

[0160] It was analyzed that in Comparative Examples 1-1 to 1-3, when the coating contained a compound Na3PO4, as the coating content (thickness) increased, the coating became uneven, thereby increasing resistance and reducing discharge capacity, with no effect on improving life.

[0161] Analysis shows that in Comparative Examples 2-1 and 2-3, the coating contains a compound Na 0.6 In the case of CoO2, as the Na equivalent is low, the energy density decreases, and the initial discharge capacity and initial efficiency are the lowest.

[0162] It was analyzed that in Comparative Examples 3-1 to 3-3, when the coating contained a compound NaCoPO4, the coating concentration increased, the resistance increased, and the discharge capacity decreased.

[0163] 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 include: layered oxide particles containing at least sodium and a transition metal; as well as a coating layer disposed on the layered oxide particles; The coating layer includes at least two selected from compounds having crystalline structures belonging to space groups P421c, Fd-3m and Pnma, respectively.

2. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that The coating includes at least three compounds having crystalline structures belonging to P421c, Fd-3m and Pnma respectively.

3. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that The compound having a crystal structure belonging to the space group P421c, Fd-3m or Pnma has a tetragonal, cubic or orthorhombic crystal structure.

4. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that The coating layer includes at least two selected from the group consisting of sodium phosphate, transition metal oxide, and sodium phosphate containing transition metal.

5. The positive electrode active material for sodium secondary battery according to claim 4, characterized in that: The sodium phosphate, transition metal oxide and transition metal-containing sodium phosphate are represented by the following chemical formula 2a, chemical formula 2b and chemical formula 2c respectively: [Chemical formula 2a]Na x PO y , In chemical formula 2a, 0 <x<4,2<y<5, [Chemical formula 2b] Co x O y , In chemical formula 2b, 0 <x<4,0<y<5, [Chemical formula 2c]Na x Co y PO4, In chemical formula 2c, 0 <x<2,0<y<2。 6. The positive electrode active material for sodium secondary battery according to claim 4, characterized in that: The coating at least includes Na3PO4, Co3O4 and NaCoPO4.

7. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that: The overall compound included in the coating layer is included at greater than 0.5 weight percent and less than 10 weight percent relative to the overall compound included in the positive electrode active material.

8. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that: The layered oxide is represented by the following chemical formula 1: [Chemical formula 1] And a It’s x ™ y M1 z Mn 1-x-y-z O2, In the chemical formula 1, TM is Co or Fe, M1 is at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Co, Fe, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, TM and M1 are different elements. 0.80 <a<1.2,0.01≤x≤0.45,0.01≤y≤0.45,0≤z≤0.1,0.01≤1-x-y-z≤0.45。 9. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that: The BET specific surface area of ​​the positive electrode active material is less than 1 m 2 / g.

10. The positive electrode active material for sodium secondary battery according to claim 1, characterized in that: In the positive electrode active material, the content of residual Na is less than 10000 ppm.

11. A method for preparing a positive electrode active material for a sodium secondary battery, characterized in that: include: A process for mixing a layered oxide containing at least sodium and a transition metal and a coating source containing a transition metal, phosphorus P and oxygen O; as well as A process of firing the mixture of the layered oxide and the coating source.

12. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 11, characterized in that: In the mixing process, the coating source is included in an amount greater than 0.5 weight percent and less than 10 weight percent relative to the entire mixture.

13. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 11, characterized in that: In the mixing process, The coating source includes at least one selected from Co3(PO4)2, CoPO4, Co2PO4, Co4PO4 and NH4H2PO4.

14. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 11, characterized in that: The firing process is performed at a temperature of 300° C. to 550° C. for 2 hours to 6 hours.

15. The method for preparing a positive electrode active material for a sodium secondary battery according to claim 11, characterized in that: The method also includes a process of washing the fired product prepared in the firing process with water.

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

17. A sodium secondary battery, characterized in that: The positive electrode according to claim 16 is used.

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