Positive electrode active material for sodium secondary battery, method for producing the same, positive electrode for sodium secondary battery, and sodium secondary battery including the same

Abstract

To suppress electrolyte side reactions during battery operation and improve electrochemical properties.SOLUTION: The present invention provides a positive electrode active material for a sodium secondary battery, including layered oxide particles containing at least sodium and a transition metal, and a coating layer located on the layered oxide particles, where the coating layer includes at least two selected from compounds having crystal structures belonging to space groups P421c, Fd-3m and Pnma, respectively.SELECTED DRAWING: Figure 1a

Description

Technical Field

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

Background Art

[0002] Lithium ion secondary batteries have been widely used as energy storage devices in various electronic technology fields. In recent years, the demand for lithium ion secondary batteries has increased rapidly, and sodium ion secondary batteries have attracted attention in order to replace expensive metal lithium.

[0003] Sodium ion secondary batteries have an operating principle of an insertion / desorption reaction similar to that of lithium ion secondary batteries, and thus are one of the next-generation materials with high potential for application to secondary batteries. However, they show low performance in terms of capacity, life characteristics, rate characteristics, etc. compared to lithium ion secondary batteries, and there are difficulties in commercialization. In order to commercialize sodium ion secondary batteries, it is essential to develop a positive electrode active material with high performance.

[0004] As the positive electrode active material of a sodium ion secondary battery, a layered structure transition metal oxide having a simple structure but excellent electrochemical performance and being easy to synthesize is 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 has 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 Nax(TM)O2(x ≦ 2 / 3).

[0005] Generally, O3-type layered oxides have a higher energy density than P2-type layered oxide particles, but have the disadvantage of causing a larger structural change during the charge / discharge process and a decrease in cycle stability. P2-type layered oxides have relatively excellent cycle stability, but due to the disadvantages of low sodium content and relatively low energy density, there are difficulties in commercial application.

[0006] However, O3-type oxide particles have problems such as reduced battery life and stability due to sodium by-products present on the particle surface in the form of Na2CO3 and NaOH, which cause gas generation due to electrolyte side reactions during battery operation and reduce the capacity and output of the positive electrode active material.When O3-type oxide particles are washed with water to remove residual sodium, all of the internal sodium escapes, destroying the crystalline structure and making them unusable, which hinders their commercialization.

[0007] The present invention aims to realize high capacity and excellent life characteristics by improving the structural stability of the O3-type positive electrode active material. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Registration KR 2120071 B1 [Patent Document 2] China Patent Publication CN 115275156 A Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a positive electrode active material in which a coating layer is formed by re-reaction between residual Na on the surface of the positive electrode active material and a coating material, and a method for producing the same. An object of the present invention is to suppress side reactions of the electrolyte during battery operation and improve the electrochemical properties by forming a coating layer containing at least two types of space groups, two types of crystal structures, or two types of compounds on the surface of a positive electrode active material. [Means for solving the problem]

[0010] One embodiment of the present invention provides a 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 disposed on the layered oxide particles, wherein the coating layer comprises at least two compounds selected from compounds having crystal structures belonging to the space groups P421c, Fd-3m, and Pnma, respectively.

[0011] The coating layer may contain at least three compounds having crystal structures belonging to P421c, Fd-3m, and Pnma, respectively.

[0012] The compound having a crystal structure belonging to the space group P421c, Fd-3m, or Pnma may have a tetragonal, cubic, or orthorhombic crystal structure.

[0013] The coating layer may contain at least two kinds selected from the group consisting of sodium phosphate, transition metal oxide, and transition metal-containing sodium phosphate.

[0014] The sodium phosphate, transition metal oxide, and transition metal-containing sodium phosphate may be represented by the following chemical formulas 1a, 1b, and 1c, respectively.

[0015] [ka]

[0016] In Chemical Formula 1a, 0 <x<4、2<y<5である。

[0017] [ka]

[0018] In Chemical Formula 1b, 0 <x<4、0<y<5である。

[0019] [ka]

[0020] In Chemical Formula 1c, 0 <x<2、0<y<2である。

[0021] The coating layer may include at least Na3PO4, Co3O4, and NaCoPO4.

[0022] The total amount of compounds contained in the coating layer may be more than 0.5 wt % and less than 10 wt % of the total amount of compounds contained in the positive electrode active material.

[0023] The layered oxide may be represented by the following formula 2.

[0024] [ka]

[0025] In the formula 2, TM may be Co or Fe, M1 is at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Co, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, TM and M1 are elements different from each other, and 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であってもよい。

[0026] The positive electrode active material has a BET specific surface area of ​​1 m 2 / g.

[0027] The positive electrode active material may have a residual sodium content (TTS, total sodium) of less than 10,000 ppm.

[0028] Another embodiment of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery, the method comprising: mixing a layered oxide containing at least sodium and a transition metal; and a coating source containing the transition metal, phosphorus (P), and oxygen (O); and firing the mixture of the layered oxide and the coating source.

[0029] In the mixing step, the coating source may be mixed in an amount of more than 0.5 wt % and less than 10 wt % based on the total mixture.

[0030] The coating source may include at least one selected from CO3(PO4)2, CoPO4, Co2PO4, Co4PO4, and NH4H2PO4.

[0031] The firing step may be carried out at a temperature of 300 to 550° C. for 2 to 6 hours.

[0032] The method may further include a step of washing the fired product produced in the firing step with water.

[0033] 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. [Effects of the Invention]

[0034] In the present invention, the coating layer formed on the surface of the positive electrode active material can suppress side reactions in the electrolyte and reduce the amount of residual Na.

[0035] In addition, in the present invention, by including at least two types of space groups, crystal structures, or compounds in the coating layer, the ionic conductivity of the coating layer can be improved, the coating uniformity can be improved when the coating layer thickness is increased, the increase in resistance due to the coating layer can be suppressed, and side reactions with the electrolyte on the particle surface can be reduced.

[0036] In addition, the present invention can reduce process costs by manufacturing a cathode active material having a coating layer formed thereon using only a dry process, thereby eliminating the need for a water washing and drying process. [Brief explanation of the drawings]

[0037] [Figure 1a] 1 is a field emission-scanning electron microscope (FE-SEM) image of the surface of a positive electrode active material particle on which a coating layer is formed, prepared in Examples 1-1 to 1-3. [Figure 1b] 1 is a field emission-scanning electron microscope (FE-SEM) image of the surface of positive electrode active material particles on which coating layers are formed, prepared in Examples 2-1 to 2-3. [Figure 1c] 10 is a field emission-scanning electron microscope (FE-SEM) image of the surface of positive electrode active material particles on which coating layers are formed, prepared in Comparative Examples 1-1 to 1-3. [Figure 1d] 10 is a field emission-scanning electron microscope (FE-SEM) image of the surface of positive electrode active material particles on which coating layers are formed, prepared in Comparative Examples 2-1 to 2-3. [Figure 1e] 10 is a field emission-scanning electron microscope (FE-SEM) image of the surface of positive electrode active material particles on which coating layers are formed, prepared in Comparative Examples 3-1 to 3-3. DETAILED DESCRIPTION OF THE INVENTION

[0038] The advantages and features of the present invention, as well as methods for achieving them, will become more apparent from the following detailed description of the embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms. However, the present invention is defined only by the scope of the claims, so that the disclosure of the present invention will be complete and those skilled in the art will be able to fully understand the scope of the invention.

[0039] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the same manner as commonly understood by a person of ordinary skill in the art to which this invention belongs. Throughout this specification, when a part is referred to as "comprising" a certain element, this does not mean that it excludes other elements, but that it may further include other elements, unless otherwise specified. Furthermore, the singular form includes the plural form unless otherwise specified in the context.

[0040] According to an embodiment of the present invention, there is provided a positive electrode active material for a sodium secondary battery, the positive electrode active material including layered oxide particles containing at least sodium and a transition metal, and a coating layer disposed on the layered oxide particles.

[0041] The residual Na on the surface of the positive electrode active material is re-reacted with the coating material to form a coating layer, thereby reducing the amount of residual Na and suppressing side reactions of the electrolyte occurring on the particle surface.

[0042] The coating layer is characterized by containing at least two compounds selected from compounds having a crystal structure belonging to the space groups P421c, Fd-3m, and Pnma, respectively, and more preferably contains at least three compounds having a crystal structure belonging to the space groups P421c, Fd-3m, and Pnma, respectively. Here, the compound having a crystal structure belonging to the space group P421c, Fd-3m, or Pnma may have a tetragonal, cubic, or orthorhombic crystal structure, and may be sodium phosphate, a transition metal oxide, or a transition metal-containing sodium phosphate, specifically Na3PO4, Co3O4, or NaCoPO4.

[0043] When the surface of the positive electrode active material is coated with a compound having a space group P421c and a tetragonal crystal structure, the ionic conductivity of the coating layer is increased and the initial charging capacity is improved. However, as the thickness of the coating layer increases, the coating layer is formed non-uniformly, which may result in an increase in resistance, a decrease in discharge capacity, and an ineffective improvement in lifespan.

[0044] 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 positive electrode active material, an irreversible coating layer is formed or the initial discharge capacity is reduced due to the problem of very low capacity. However, the surface coating layer of the positive electrode active material may reduce side reactions with the electrolyte, thereby slightly improving the lifespan.

[0045] When a compound having a space group Pnma and a crystal structure Orthorhombic is coated on the surface of the positive electrode active material, the concentration of the coating layer increases, which increases the resistance and reduces the discharge capacity, but it may have the effect of slightly improving the lifespan.

[0046] In the present invention, the coating layer contains at least two compounds selected from compounds having crystal structures belonging to the space groups P421c, Fd-3m, and Pnma, respectively, or preferably all three compounds, and as a result of the combined effect, the ionic conductivity of the coating layer is increased, and Na + The conductivity is increased, the resistance of the irreversible coating layer is improved, the concentration of the coating layer is increased, and the coating uniformity is improved, thereby reducing the resistance and suppressing electrolyte side reactions at the interface, thereby improving battery performance such as initial capacity development and life characteristics.

[0047] The coating layer may contain at least two compounds selected from the group consisting of sodium phosphate, transition metal oxide, and transition metal-containing sodium phosphate, and preferably may contain all three compounds.

[0048] The compounds are coatings formed by reacting with residual Na on the surface of the positive electrode active material, and by forming them on the surface, they can suppress electrolyte side reactions and reduce the amount of residual Na. In addition, by including at least two of the compounds in the coating layer, it is possible to improve the ionic conductivity of the coating layer, improve coating uniformity as the coating layer thickness increases, suppress an increase in resistance due to the coating layer, and reduce side reactions with the electrolyte on the particle surface.

[0049] On the other hand, the sodium phosphate may be a compound having a crystal structure belonging to the space group P421c, the transition metal oxide may be a compound having a crystal structure belonging to the space group Fd-3m, and the transition metal-containing sodium phosphate may be a compound having a crystal structure belonging to the space group Pnma.

[0050] The sodium phosphate, transition metal oxide, and transition metal-containing sodium phosphate may be compounds represented by the following formulas 1a, 1b, and 1c, respectively.

[0051] [ka]

[0052] In Chemical Formula 1a, 0 < x < 4 and 2 < y < 5 may hold. For example, 1 < x < 4 or 2 < x < 4, and 2 < y < 5 or 3 < y < 5 may also hold.

[0053]

Chemical Formula

[0054] In Chemical Formula 1b, 0 < x < 4 and 0 < y < 5 may hold. For example, 1 < x < 4 or 2 < x < 4, and 2 < y < 5 or 3 < y < 5 may also hold.

[0055]

Chemical Formula

[0056] In Chemical Formula 1c, 0 < x < 2 and 0 < y < 2 may hold. 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 may also hold.

[0057] The sodium phosphate represented by Chemical Formula 1a may contain Na3PO4, the transition metal oxide represented by Chemical Formula 1b may contain Co3O4, and the transition metal-containing sodium phosphate represented by Chemical Formula 1c may contain NaCoPO4.

[0058] The coating layer preferably may contain at least two selected from the group consisting of at least Na3PO4, Co3O4, and NaCoPO4, and preferably may contain at least the above three compounds. Thereby, the above-described effects can be further improved.

[0059] The total amount of compounds contained in the coating layer may be more than 0.5 wt % and less than 10 wt % of the total amount of compounds contained in the positive electrode active material, for example, 1 to 8 wt %, 1 to 7 wt %, 1 to 6.5 wt %, or 1 to 6 wt %.

[0060] If the coating layer content is 10 wt % or more, the resistance increases due to the increased thickness, and the capacitance characteristics may deteriorate. Conversely, if the coating layer content is 1 wt % or less, the BET specific surface area increases due to the formation of an uneven coating layer, which is ineffective in improving side reactions of the electrolyte and may make it difficult to sufficiently remove residual sodium.

[0061] The sodium phosphate contained in the coating layer may be more than 0.5 wt% and less than 5 wt% of the total compounds contained in the positive electrode active material, for example, 1 to 5 wt%, 1 to 4.5 wt%, 2 to 4.5 wt%, 3 to 4.5 wt%, 3.5 to 4.5 wt%, or 3.7 to 4.1 wt%. If the content is less than the upper limit of the range, the coating layer is formed uniformly, and an increase in resistance and a decrease in discharge capacity can be suppressed. Conversely, if the content is more than the lower limit of the range, the ionic conductivity of the coating layer can be increased, and the initial charge capacity can be improved.

[0062] The transition metal oxide contained in the coating layer may be contained in an amount of more than 0.01 wt % and less than 1.5 wt % of the total compounds contained in the positive electrode active material, for example, 0.05 to 1.5 wt %, 0.05 to 1.3 wt %, 0.05 to 1.0 wt %, 0.1 to 0.9 wt %, or 0.3 to 0.7 wt %. If the content is less than the upper limit of the range, it may have the effect of reducing side reactions with the electrolyte and slightly improving the lifespan, while if the content is more than the lower limit of the range, it may have the effect of suppressing a decrease in initial discharge capacity.

[0063] The transition metal-containing sodium phosphate contained in the coating layer may be contained in an amount exceeding 0.1% by weight and less than 2% by weight based on the total compounds contained in the positive electrode active material. For example, it may be contained in an amount of 0.1 to 1.5% by weight, 0.3 to 1.5% by weight, 0.5 to 1.5% by weight, 0.7 to 1.5% or 0.9 to 1.3% by weight. When the content range is less than the upper limit value, it is possible to improve the problem that the concentration of the coating layer increases, the resistance increases, and the discharge capacity decreases. Conversely, when the content range exceeds the lower limit value, the life can be slightly improved.

[0064] The coating layer may contain sodium phosphate, transition metal-containing sodium phosphate and transition metal oxide in descending order of high content.

[0065] The layered oxide may be represented by Chemical Formula 2 below.

[0066]

Chemical Formula

[0067] In Chemical Formula 2, TM may be Co or Fe, M1 may be at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Co, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu, TM and M1 may be different elements from each other, and 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 may be satisfied.

[0068] Preferably, TM may be a ternary transition metal composed of Ni, Mn and Fe, 0.9 < a < 1.1, 0.3 ≤ x ≤ 0.35, 0.3 ≤ y ≤ 0.35, 0.01 ≤ z ≤ 0.05, 0.3 ≤ 1 - x - y - z ≤ 0.35 may be satisfied, and specifically, it may be a Ni-Fe-Mn111 layered oxide.

[0069] The layered oxide may have an O3-type crystal structure. O3-type layered oxides have a higher energy density than P2-type layered oxide particles, but suffer from poor air and water stability. In the present invention, the coating layer is formed by re-reaction of the residual Na on the surface of the positive electrode active material with the coating material, eliminating the need for a water washing and drying process and alleviating the problem of the crystal structure being destroyed during water washing to remove the residual Na.

[0070] The positive electrode active material has a BET specific surface area of ​​1 m 2 / g or less, e.g., 0.1 to 0.8 m 2 / g or 0.1~0.6m 2 In the present invention, by including at least two, preferably three, compounds in the coating layer, even if the thickness of the coating layer increases, a relatively uniform and smooth coating layer may be formed on the surface of the particles, and the BET specific surface area may be controlled within the above-mentioned design range, thereby suppressing side reactions of the interfacial electrolyte.

[0071] The positive electrode active material may have a residual sodium content (TTS, total sodium) of less than 10,000 ppm, for example, less than 9,000 ppm, less than 8,000 ppm, less than 7,000 ppm, or less than 6,000 ppm.

[0072] This makes it possible to suppress the generation of gas caused by residual Na, thereby significantly improving the battery life characteristics.

[0073] Meanwhile, the residual sodium content (TTS, Total Sodium) may be a value (TTS, Total Sodium) obtained by separately calculating the total amount of sodium alone among compounds containing residual sodium (e.g., NaOH or Na2CO3).

[0074] Another embodiment of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery.

[0075] The manufacturing method includes a step of mixing a layered oxide containing at least sodium and a transition metal, and a coating source containing the transition metal, phosphorus (P), and oxygen (O), and a step of firing the mixture of the layered oxide and the coating source.

[0076] In the mixing process, by using a dry method, the coating source can be uniformly mixed on the surface and inside of the layered oxide particles, and the subsequent water washing and drying processes can be eliminated, thereby simplifying the process.On the other hand, in the case of a wet method, the selection of coating compounds is limited and there are problems with increased costs due to the complicated process, which may be undesirable.

[0077] The coating source may be mixed in an amount of more than 0.5 wt % and less than 10 wt % of the total mixture, for example, 0.5 to 7 wt %, 0.5 to 6 wt %, 1 to 6 wt %, 3 to 6 wt %, or 4 to 6 wt %.

[0078] The coating source is a compound capable of reacting with residual Na to form a coating material, and is a compound containing a transition metal, phosphorus (P), and oxygen (O), such as cobalt phosphate. Specifically, the coating source may contain at least one selected from CO3(PO4)2, CoPO4, Co2PO4, Co4PO4, and NH4H2PO4, preferably Co3(PO4)2 or NH4H2PO4, and more preferably Co3(PO4)2. This reacts with residual Na to form a coating layer on the surface, suppressing electrolyte side reactions and reducing the amount of residual Na.

[0079] The firing process is a step of firing a mixture of the layered oxide and the coating source to form a coating layer by re-reacting the residual Na on the surface of the layered oxide with the coating source.

[0080] The baking step may be carried out at a temperature of 300 to 550°C for 2 to 6 hours. Specifically, it may be carried out at a temperature of 300 to 500°C or 350 to 450°C for 2 to 6 hours, 3 to 5 hours, or 3.5 to 4.5 hours. This allows the ratio of the three compounds in the coating layer to be optimized with reference to the following reaction formulas 1 to 4. Reaction formula 1

[0081] JPEG0007815364000009.jpg52160

[0082] In Reaction Scheme 1, a baking process may be carried out at a designed reaction temperature and reaction time to synthesize sodium phosphate, transition metal oxide, and transition metal-containing sodium phosphate compound to form a coating layer.

[0083] JPEG0007815364000010.jpg41160

[0084] In Reaction 2, if the reaction time is slightly excessive, Na3PO4 and CO3O4 may be advantageously produced. Reaction 3

[0085] JPEG0007815364000011.jpg44160

[0086] In Reaction Scheme 3, if the reaction time is a little short, Co3O4 may not be synthesized sufficiently, and some of the Co may be oxidized in the coating layer, resulting in a mixed form of CoCO3 / Co(OH)2, but it may also exist at an impurity level. Reaction Scheme 4

[0087] JPEG0007815364000012.jpg37160

[0088] In Reaction Scheme 4, if the calcination process is carried out at a slightly higher reaction temperature, Co3O4 among the reaction products may be synthesized from Na-Metal Oxide, or NaCOPO4 may be synthesized by reacting NH4H2PO4 with a Co source.

[0089] Meanwhile, the method may further include a step of washing the positive electrode active material (layered oxide) having the coating layer formed thereon in the firing step.

[0090] The water-washing step is a step for removing unreacted substances, impurities, and residual sodium. The positive electrode active material having the coating layer formed thereon, which is produced in the baking step, may be placed in a reactor containing at least one selected from deionized water, distilled water, and ethanol, and washed with water at a temperature of 1 to 80°C or 5 to 50°C and a stirring speed of 200 to 500 rpm, 200 to 400 rpm, or 300 to 400 rpm for 0.5 to 5 hours, 0.5 to 4 hours, 0.5 to 3 hours, or 0.5 to 1.5 hours.

[0091] In the present invention, a coating layer is formed on the O3-type layered oxide, improving its structural stability, so that residual Na on the surface can be effectively removed without the problem of the crystal structure collapsing when washing the conventional O3-type layered oxide with water.

[0092] Next, a drying step may be carried out to remove moisture from the positive electrode active material that has been washed with water, and the drying step may be carried out under vacuum conditions at a temperature of 100 to 300°C for 12 hours or more.

[0093] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery and a sodium secondary battery comprising the positive electrode active material.

[0094] The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, and the positive electrode active material according to one aspect of the present invention is present in the positive electrode active material layer.

[0095] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector typically has a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. Such positive electrode current collectors may be provided in various forms, such as films, sheets, foils, nets, porous bodies, foams, nonwoven fabrics, etc.

[0096] The positive electrode active material layer may be a layer containing a conductive material and a binder in addition to the above-mentioned positive electrode active material.

[0097] Here, the conductive material is used to impart conductivity to the electrode and can be any material that is conductive and does not cause chemical changes in the positive electrode active material. Non-limiting examples of conductive materials include graphite such as natural graphite and artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon-based materials such as carbon fiber, metal powder or metal fiber such as copper, nickel, aluminum, or silver, conductive whiskers such as zinc oxide or potassium titanate, conductive metal oxides such as titanium oxide, and conductive polymers such as polyphenylene derivatives. The conductive material may typically be included in an amount of 1 wt % to 30 wt % based on the total weight of the positive electrode active material layer.

[0098] The binder is a substance that improves adhesion between positive electrode active material particles and between the positive electrode active material and the current collector. Non-limiting examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. The binder may typically be included in an amount of 1 wt% to 30 wt% based on the total weight of the positive electrode active material layer.

[0099] The positive electrode according to an embodiment of the present invention may be manufactured by a conventional method for manufacturing a positive electrode for a sodium secondary battery, except that the positive electrode active material described above is used. For example, the positive electrode may be manufactured by coating a positive electrode active material layer-forming slurry containing the positive electrode active material and, optionally, a binder and a conductive material, on a positive electrode current collector, followed by drying and rolling. In another example, the positive electrode may be manufactured by casting the positive electrode active material layer-forming slurry on a separate support, peeling the positive electrode active material layer from the support, and laminating the resulting film on a positive electrode current collector.

[0100] According to yet another aspect of the present invention, there is provided an electrochemical device including the above-described anode. The electrochemical device may be, for example, a battery or a capacitor, and more specifically, a sodium secondary battery.

[0101] A sodium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte (electrolytic solution). The sodium secondary battery may also include a battery container (case) that houses the electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0102] Depending on the shape of the battery container (case), sodium secondary batteries can be classified into can-type sodium secondary batteries, in which the electrode assembly is housed in a metal can, and pouch-type sodium secondary batteries, in which the electrode assembly is housed in a pouch made of a sheet such as an aluminum laminate.

[0103] In particular, in the case of a pouch-type sodium secondary battery using a cathode including the cathode active material according to various embodiments of the present invention, there is a low possibility of a side reaction occurring between the cathode active material and the electrolyte, which has the advantage of improving stability during storage and / or operation and reducing gas generation.

[0104] The present invention will be described in detail below with reference to examples. However, these examples are for the purpose of explaining the present invention in more detail, and the scope of the present invention is not limited to the following examples.

[0105] Example (Example 1-1) Na 1.0 Ni 0.33 Fe 0.33 Mn 0.33 O2(NFM111)O3 type layered oxide and 1 wt% of coating source Co3(PO4)2 were dry mixed using a hand mixer.

[0106] The prepared mixture was placed in an alumina crucible and 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.

[0107] 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 N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was uniformly coated on a 15 μm-thick aluminum foil and dried under vacuum at 135°C to prepare a positive electrode for a sodium secondary battery.

[0108] A sodium secondary battery (coin cell) was fabricated using a sodium metal plate as a counter electrode to the anode, porous glass fiber (thickness: 200 μm) as a separator, and an electrolyte solution containing 1.0 M NaPF6 in a solvent mixed with propylene carbonate and fluoroethylene carbonate in a volume ratio of 98:2.

[0109] (Examples 1-2 to 1-3 and Comparative Examples 1-1 to 4-3) A positive electrode active material and a sodium secondary battery having a coating layer formed thereon were manufactured in the same manner as in Example 1-1, except that the coating source and the firing conditions were as shown in Table 1 below.

[0110] Experimental Example Experimental Example 1: Analysis of compounds contained in the coating layer The contents of coating materials (i.e., second compounds) on the surfaces of the cathode active materials prepared in Examples 1-1 to 2-3 and Comparative Examples 1-1 to 3-3 were quantitatively analyzed by screening the compounds listed in Tables 1, 2, and 3 below from XRD raw data measured using Bruker's EVA program.

[0111] 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.

[0112] [Table 1]

[0113] [Table 2]

[0114] Referring to Table 1, it was confirmed that the coating layer of the positive electrode active material prepared in Example 1 of the present invention contained three compounds, Na3PO4, Co3O4, and NaCoPO4, and that it contained two compounds in Example 2. On the other hand, it was confirmed that the coating layer of the comparative example contained one compound from Na3PO4, Co3O4, and NaCoPO4.

[0115] Referring to Table 2, the three compounds contained in the coating layer prepared in the examples are in the space group P421 C , Fd-3m or Pnma and have a crystal structure of tetragonal, cubic or orthorhombic.

[0116] Na3PO4(space group P421 C The advantage of the tetragonal crystal structure is that the ionic conductivity of the coating layer increases and the initial charging capacity improves. However, as the thickness of the coating layer increases, the coating layer becomes unevenly formed, which increases the resistance and reduces the discharge capacity, resulting in no effect on improving the lifespan.

[0117] Co3O4 (space group Fd-3m, crystal structure Cubic) or Na 0.6 CoO2 (space group P63 / mmc, hexagonal crystal structure) has the problem that an irreversible coating layer is formed (Co3O4) and the expression capacity is very low (Na 0.6 Although there is a problem of a decrease in initial discharge capacity due to CoO2, it is analyzed that the coating layer on the surface of the positive electrode active material reduces side reactions with the electrolyte, slightly improving the lifespan.

[0118] NaCoPO4 (space group Pnma, crystal structure Orthorhombic) has the problem of increasing the concentration of the coating layer, increasing resistance, and decreasing discharge capacity, but is analyzed to slightly improve lifespan.

[0119] [Table 3]

[0120] Referring to Table 3, the optimum content compositions of Na3PO4, Co3O4 and NaCoPO4 compounds in the coating layer were confirmed in Examples 1-1 to 1-3.

[0121] In addition, in the examples, it is analyzed that the battery characteristics are improved as the content of the coating layer increases within the range of 1.5 to 5.5 wt %.

[0122] Experimental Example 3: Analysis of coating uniformity by scanning electron microscope (FE-SEM) analysis and BET analysis 1a and 1b are FE-SEM images of the surfaces of the positive electrode active material particles on which coating layers are formed, prepared in Examples 1-1 to 2-3 and Comparative Examples 1-1 to 3-3, respectively.

[0123] Table 4 below shows the BET specific surface areas of the positive electrode active materials on which coating layers were formed, prepared in Examples 1-1 to 1-3 and Comparative Examples 1-1 to 3-3.

[0124] [Table 4]

[0125] 1a-1e and Table 4, in Examples 1-1 to 2-3, the proportion of coating source increased (1 wt% → 5 wt%), and even though the thickness of the coating layer increased, a relatively uniform and smooth coating layer was formed on the particle surface. The same result was confirmed in the analysis of BET specific surface area. These results are analyzed to be due to the fact that the coating layer contains two or three compounds: Na3PO4, Co3O4, and NaCoPO4.

[0126] On the other hand, in the comparative example, a large number of nanoparticles of 10-100 nm in size, including one of Na3PO4, Co3O4, or NaCoPO4, were deposited on the surface of the layered oxide particles, forming an uneven coating layer. The same results were confirmed in BET specific surface area analysis. As a result, as the coating layer thickness increases, the coating uniformity decreases, which is expected to increase resistance and result in deterioration of discharge capacity and life characteristics.

[0127] Experimental Example 4: Measurement of residual sodium content (TTS) 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.

[0128] The residual sodium content was determined by measuring the residual Na-containing compounds (e.g., NaOH or Na2CO3) separately using potentiometric titration, and then calculating the total amount of Na alone (TTS, Total Sodium). The calculation method is the same as Equation 1 below. Equation 1

[0129] JPEG0007815364000017.jpg9160

[0130] [Table 5]

[0131] Referring to Table 5, it was confirmed that the residual Na content (TTS) in the Examples was 10,000 ppm or less compared to the Comparative Examples, and preferably, in Examples 1-1 to 1-3, the residual Na was reduced to 7,000 ppm or less.

[0132] Experimental Example 5: Evaluation of the electrochemical performance of sodium secondary batteries For the sodium secondary batteries manufactured in Examples 1-1 to 2-3 and Comparative Examples 1-1 to 3-3, the initial charge capacity (CH), initial discharge capacity (DCH), initial efficiency (ICE), and the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention rate; Retention) were measured, and the results are shown in Table 6 below.

[0133] [Table 6]

[0134] Referring to Table 6, in Examples 1-1 to 1-3, when the coating layer contained all three compounds, Na + It is analyzed that the increase in conductivity improves the resistance of the irreversible coating layer, resulting in the greatest improvements in capacity, initial efficiency, and lifespan.

[0135] In Examples 2-1 to 2-3, when the coating layer contains two types of compounds, it is analyzed that the above-mentioned effects are slightly reduced.

[0136] In Comparative Examples 1-1 to 1-3, when the coating layer contains one compound, Na3PO4, it is analyzed that as the content (thickness) of the coating layer increases, the coating layer is formed more non-uniformly, resulting in an increase in resistance and a decrease in discharge capacity, which is ineffective in improving the lifespan.

[0137] In Comparative Examples 2-1 to 2-3, the coating layer contained one type of compound Na 0.6 When CoO2 is included, the Na equivalent is low, which reduces the energy density, resulting in the lowest initial discharge capacity and initial efficiency.

[0138] In Comparative Examples 3-1 to 3-3, when the coating layer contains one compound, NaCoPO4, it is analyzed that the concentration of the coating layer increases, which increases the resistance and decreases the discharge capacity.

[0139] As mentioned above, although the present invention has been illustrated and described with reference to specific embodiments, it will be obvious to those skilled in the art that the present invention can be modified and changed in various ways without departing from the technical spirit of the present invention as defined by the following claims.

Claims

1. Layered oxide particles containing at least sodium and a transition metal; a coating layer located on the layered oxide particles, The coating layer contains at least two compounds selected from compounds having crystal structures belonging to the space groups P421c, Fd-3m, and Pnma, respectively.

2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the coating layer contains at least three compounds having crystal structures belonging to P421c, Fd-3m, and Pnma, respectively.

3. 2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein 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. 2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the coating layer contains at least two kinds of materials selected from the group consisting of sodium phosphate, transition metal oxide, and transition metal-containing sodium phosphate.

5. 5. The positive electrode active material for a sodium secondary battery according to claim 4, wherein the sodium phosphate, the transition metal oxide, and the transition metal-containing sodium phosphate are represented by the following chemical formulas 1a, 1b, and 1c, respectively. 【Chemistry 1a】 In Chemical Formula 1a, 0<x<4 and 2<y<5. 【Chemistry 1b】 In Chemical Formula 1b, 0<x<4 and 0<y<5. 【Chem.1c】 In Chemical Formula 1c, 0<x<2 and 0<y<2.

6. The coating layer contains at least Na ３ P.O. ４ , Co ３ O ４ and NaCoPO ４ The positive electrode active material for a sodium secondary battery according to claim 4, comprising:

7. 2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the total amount of compounds contained in the coating layer is more than 0.5 wt % and less than 10 wt % of the total amount of compounds contained in the positive electrode active material.

8. The positive electrode active material for a sodium secondary battery according to claim 1 , wherein the layered oxide is represented by the following Chemical Formula 2: 【Chemistry 2】 In the above formula 2, TM is Co or Fe, M1 is at least one selected from P, Sr, Ba, Ti, Zr, Al, W, Co, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu; TM and M1 are different elements, The following may be satisfied: 0.80<a<1.2, 0.01≦x≦0.45, 0.01≦y≦0.45, 0≦z≦0.1, and 0.01≦1−x−y−z≦0.

45.

9. The positive electrode active material has a BET specific surface area of ​​1 m ２ The positive electrode active material for a sodium secondary battery according to claim 1, wherein the SiO2 content is less than 1 / g.

10. 2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the positive electrode active material has a residual Na content (TTS, Total Sodium) of less than 10,000 ppm.

11. mixing a layered oxide containing at least sodium and a transition metal, a coating source containing the transition metal, phosphorus (P) and oxygen (O); and firing the mixture of the layered oxide and the coating source.

12. 12. The method of claim 11, wherein the coating source is mixed in an amount of more than 0.5 wt % and less than 10 wt % of the total mixture in the mixing step.

13. In the mixing step, The coating source is CO ３ (P.O. ４ ) ２ , CoPO ４ , Co ２ P.O. ４ , Co ４ P.O. ４ and N.H. ４ H ２ P.O. ４ The method for producing a positive electrode active material for a sodium secondary battery according to claim 11, characterized in that the positive electrode active material contains at least one selected from the following:

14. The method for producing a positive electrode active material for a sodium secondary battery according to claim 11, wherein the calcination process is performed at a temperature of 300 to 550° C. for 2 to 6 hours.

15. The method for producing a positive electrode active material for a sodium secondary battery according to claim 11, further comprising a step of washing the fired product produced in the firing step with water.

16. A positive electrode for a sodium secondary battery, comprising the positive electrode active material according to claim 1.

17. A sodium secondary battery using the positive electrode according to claim 16.

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