Sodium phosphate-coated sodium ion positive electrode material, preparation method therefor and use thereof

Abstract

A sodium phosphate-coated sodium ion positive electrode material, a preparation method therefor and a use thereof, belonging to the technical field of battery materials. The sodium phosphate-coated sodium ion positive electrode material comprises a sodium ion positive electrode material and a sodium phosphate layer coated on the surface of the sodium ion positive electrode material, the thickness of the sodium phosphate layer being 1-20 nm. The sodium phosphate comprises at least one of sodium calcium phosphate, sodium barium phosphate, sodium strontium phosphate, sodium magnesium phosphate, and sodium aluminum phosphate. The sodium phosphate-coated sodium ion positive electrode material has a round shape, smooth surface, and uniform thickness.

Description

A sodium phosphate-coated sodium ion positive electrode material and its preparation method and application Technical Field

[0001] This article relates to the field of battery material technology, and specifically to a sodium phosphate-coated sodium ion positive electrode material, a preparation method, and an application thereof. Background Art

[0002] Lithium-ion batteries are widely used in various energy storage devices, playing a particularly important role in electric vehicles. However, due to the uneven distribution of lithium in the Earth's crust, its relatively small reserves, and the development of large-scale smart grids, current lithium resources are unable to meet current energy storage needs. Sodium-ion batteries, as naturally abundant and low-cost sodium-containing raw materials, have been considered an alternative solution to my country's lithium shortage. As a result, sodium-ion batteries have experienced rapid development in recent years, and some material systems have already been applied in products.

[0003] There are still some difficulties in the development of positive electrode materials for sodium ion batteries. One of the most difficult difficulties is to form a uniform coating layer with adjustable thickness on the surface of the substrate. At present, the common methods for surface coating of electrode materials in industrial production mainly include spray drying, vapor deposition, liquid phase method and mechanical mixing method. Although the first three methods can form a good coating layer, they require a liquid medium. Most sodium ion layered positive electrode materials are sensitive to water, which can easily lead to the loss of sodium ions in the substrate, reduced structural stability and reduced cycle stability. They are limited in the industrial production of positive electrode materials for sodium ion batteries. Although the mechanical mixing method can achieve good mixing, the coating agent is generally added in a very small amount. After calcination after solid-solid mixing, it is difficult to prepare a uniform coating layer with adjustable thickness, resulting in unsatisfactory coating effect. The newly developed atomic thin layer deposition technology in recent years can form a uniform coating layer with adjustable thickness on the surface of the material, but the expensive equipment will significantly increase the production cost. At the same time, a secondary coating process is still required, which increases the process route and increases the production cost of the material.

[0004] CN112928252A discloses a sodium ion battery positive electrode material, its preparation method and application. The positive electrode material includes a sodium ion positive electrode material oxide and a sodium salt coated on the surface of the sodium ion positive electrode material oxide, and the sodium salt includes any one of sodium borate, sodium phosphate or sodium aluminate or a combination of at least two. The preparation method includes: mixing an acidic solution and a sodium ion positive electrode material oxide to obtain a sodium ion battery positive electrode material; wherein the acidic solution includes any one of boric acid solution, phosphoric acid solution or aluminic acid solution or a combination of at least two. The acidic solution used can fully react with the residual alkali on the surface of the original positive electrode material, thereby reducing the concentration of the residual alkali on the surface and forming a sodium salt with high ionic conductivity, thereby preventing the further generation of residual alkali, and the rate performance and cycle performance of the positive electrode material are improved. At the same time, no high-temperature and high-pressure container equipment is required, the reaction degree is controllable, and it is used for large-scale production practice. However, the sodium salt generated by mixing an acidic solution with an alkali oxide to reduce the residual alkali is only in a surface adsorption state and cannot form a good coating layer. At the same time, the acidic aqueous solution has a significant structural destructive effect on the substrate, especially phosphoric acid has a significant corrosive effect on the instrument, and the converted coating also has the problem of low ionic and electronic conductivity.

[0005] CN114944478A discloses a sodium-ion battery positive electrode material, its preparation method, and application. The present invention provides a method for preparing a sodium-ion battery positive electrode material, comprising the following steps: mixing an oxide positive electrode matrix material with a dihydrogen phosphate solution, heating and stirring, solid-liquid separation, and drying to obtain a metal phosphate-coated matrix material; then mixing the metal phosphate-coated matrix material with a carbon source and sintering to obtain the sodium-ion battery positive electrode material; the dihydrogen phosphate in the dihydrogen phosphate solution is selected from at least one of ferrous dihydrogen phosphate, manganese dihydrogen phosphate, nickel dihydrogen phosphate, cobalt dihydrogen phosphate, and copper dihydrogen phosphate. The preparation method provided herein utilizes a specific dihydrogen phosphate solution for mixing with the oxide positive electrode matrix material, achieving a one-time washing and coating process to optimize residual alkali and improve the coating effect. However, due to the use of secondary sintering, the secondary sintering and inert atmosphere protection significantly increase production costs, and the dihydrogen phosphate aqueous solution also damages the crystal structure of the monoalkoxide positive electrode matrix material. Moreover, its reaction system is a solid-liquid reaction. The sodium ion layered positive electrode material is sensitive to water, which can easily lead to the loss of sodium ions in the substrate, resulting in reduced structural stability and cycle stability, which limits its industrial production in sodium ion battery positive electrode materials.

[0006] Therefore, how to form a coating layer with uniform thickness on the surface of the positive electrode material of a sodium ion battery has become a technical problem that needs to be solved urgently by those skilled in the art.

[0007] Summary of the Invention

[0008] The purpose of this article is to overcome the shortcomings of the existing technology and provide a sodium phosphate-coated sodium ion positive electrode material and its preparation method and application. The sodium phosphate-coated sodium ion positive electrode material has a rounded morphology, a smooth surface, and a uniform thickness, effectively reduces the residual alkali on the surface, improves the structural stability, effectively inhibits the loss of sodium in the crystal structure, maintains better crystallinity, effectively improves the electrochemical performance, and improves the discharge specific capacity and cycle performance.

[0009] To achieve the above objectives, the technical solutions adopted in this paper are:

[0010] A sodium phosphate-coated sodium ion positive electrode material, comprising a sodium ion positive electrode material and a sodium phosphate layer coated on the surface of the sodium ion positive electrode material, wherein the thickness of the sodium phosphate layer is 1 to 20 nm;

[0011] The sodium phosphate includes at least one of sodium calcium phosphate, sodium barium phosphate, sodium strontium phosphate, sodium magnesium phosphate, and sodium aluminum phosphate.

[0012] The sodium phosphate-coated sodium ion positive electrode material in this article has a rounded morphology and a smooth surface. A sodium phosphate layer is creatively coated on the surface of the sodium ion positive electrode material, wherein the thickness of the sodium phosphate layer is uniform and controllable. The coating of the sodium phosphate layer effectively reduces the residual alkali on the surface, improves the structural stability, effectively inhibits the loss of sodium in the crystal structure, maintains better crystallinity, effectively improves the electrochemical performance, and improves the discharge specific capacity and cycle performance.

[0013] Among them, the thickness of the sodium phosphate layer is 1 to 20 nm. When the thickness is within this range, the improvement of discharge specific capacity and cycle performance is more obvious. If the thickness is too low (less than 1 nm), the coating layer cannot play a good role in isolating the electrolyte from the side reactions of the positive electrode material. If the thickness is higher than 20 nm, the kinetics of sodium ion intercalation and deintercalation are significantly inhibited, and the rate performance is significantly reduced. Therefore, this article controls the thickness between 1 and 20 nm.

[0014] At the same time, the sodium phosphate-coated sodium ion cathode material in this paper has a more rounded appearance, a smoother and more uniform surface, and stronger peak intensities in the spectral peaks of the sodium phosphate-coated sodium ion cathode material, especially the (003) peak, which is significantly enhanced. Therefore, compared with sodium phosphate coating, sodium phosphate coating helps to increase the stability of the material, inhibit the loss of sodium in the crystal structure, and maintain better crystallinity. The data also show that sodium phosphate coating has better electrochemical performance than sodium phosphate coating, which can further improve the discharge capacity and cycle performance.

[0015] In one embodiment, the mass ratio of the sodium ion positive electrode material to the sodium phosphate is 1:(0.0001-0.03).

[0016] In one embodiment, the sodium ion positive electrode material is a layered oxide positive electrode material, and its chemical formula is Na x1 MO y1 , wherein M includes at least one of Ni, Mg, Cu, Mn, Co, Zn, Ca, Ba, Fe, Al, B, Ti, Sn, Nb, Sb, Si, and Mo;

[0017] Among them, 0.5<x1<1.2, y1≥2.

[0018] In one embodiment, the sodium ion positive electrode material contains at least one phase structure of O3 phase, P2 phase, O2 phase, and P3 phase.

[0019] This article also provides a method for preparing a sodium phosphate-coated sodium ion positive electrode material, comprising the following steps:

[0020] Weighing the metal oxides according to the molar ratio in the chemical formula of the sodium ion positive electrode material and mixing them uniformly to obtain a precursor;

[0021] The precursor and phosphate are mixed evenly, sintered, cooled, and pulverized by air flow milling to obtain a sodium phosphate-coated sodium ion positive electrode material.

[0022] In this paper, the metal oxides are evenly mixed according to the molar ratio in the chemical formula of the sodium ion positive electrode material, and then directly mixed with phosphate for sintering. During the cooling process, due to the large ionic radius of the tetrahedral phosphate ions and the large ionic radius of the metal salts, the metal salts and phosphate ions are gently precipitated from the positive electrode substrate during the slow cooling process, and further react with the residual sodium on the surface of the substrate to form an in situ sodium phosphate coating layer of uniform thickness on the surface of the sodium ion positive electrode material.

[0023] This article only needs one-step sintering to prepare the sodium phosphate coating layer. Compared with the existing technology (such as CN112928252A), it can effectively shorten the process flow, reduce costs, and has a broader application prospect.

[0024] In addition, the use of phosphate for reaction (solid-solid reaction) in this paper can avoid the use of acidic aqueous solutions (such as phosphoric acid solution) to damage the material structure, avoid corrosion of instruments, and avoid the problem of low ionic and electronic conductivity of the coating layer.

[0025] In one embodiment, the mass ratio of the precursor to the phosphate is 1:(0.0001-0.03). By controlling the mass ratio of the precursor to the phosphate within this range, a sodium phosphate coating layer with a thickness of 1-20 nm can be obtained.

[0026] In one embodiment, the mass ratio of the precursor to the phosphate is 1:(0.002-0.008). In particular, when the mass ratio is within this range, the obtained sodium phosphate coating layer has better performance.

[0027] In one embodiment, the phosphate includes at least one of calcium phosphate, calcium hydrogen phosphate, calcium dihydrogen phosphate, calcium tripolyphosphate, barium phosphate, barium hydrogen phosphate, barium dihydrogen phosphate, barium tripolyphosphate, strontium phosphate, strontium hydrogen phosphate, strontium tripolyphosphate, magnesium hydrogen phosphate, and aluminum phosphate.

[0028] In one embodiment, the sintering temperature is 800-1000°C, and the sintering time is 6-24 hours. Here, the precursor is uniformly mixed with phosphate and sintered. When the sintering temperature is between 800-1000°C, Na is easily separated from the transition metal, forming a well-crystalline layered structure. However, if the sintering temperature is lower than 800°C, the formation of the layered structure is not conducive, and the crystallinity of the material is low. If the sintering temperature is higher than 1000°C, the material will lose oxygen during the sintering process, producing an electrochemically inactive rock salt phase.

[0029] In one embodiment, the sintering temperature is 850-950° C., and the sintering time is 12-18 hours.

[0030] In one embodiment, the sintering is performed in air, oxygen, or a mixture of air and oxygen.

[0031] In one embodiment, the cooling rate is 0.1 to 5°C / min. The inventors of this article found in their research that the cooling rate has a great influence on the effect. By controlling the cooling rate within the range of this article, during the slow cooling process, the metal salt and the phosphate ion are gently precipitated from the positive electrode substrate, and further react with the residual sodium on the surface of the substrate to form a coating layer with uniform thickness and density. The morphology of the coating layer is more lubricated and the surface is smoother. If the cooling rate is too low (lower than 0.1°C / min), the sintering cycle will be significantly extended and energy consumption will increase. If the cooling rate is too high (higher than 5°C / min), due to the different thermal expansion coefficients of the coating layer and the positive electrode material, rapid cooling may cause the coating layer to reduce the bonding strength between the positive electrode material. Therefore, this article controls the cooling rate within the range of 0.1 to 5°C / min to further improve the discharge specific capacity and cycle performance.

[0032] In one embodiment, the cooling rate is 0.1-1° C. / min. In particular, when the cooling rate is controlled within this range, the discharge capacity and cycle performance can be further improved.

[0033] This article also provides a positive electrode material, which includes the sodium phosphate-coated sodium ion positive electrode material described above or the sodium phosphate-coated sodium ion positive electrode material prepared by the preparation method described above.

[0034] Specifically, the positive electrode material is attached to the positive electrode current collector. The material of the positive electrode current collector is not subject to specific restrictions and can be selected according to actual needs.

[0035] This article also provides a sodium ion battery, which includes the positive electrode material described above.

[0036] In one embodiment, the sodium ion battery further includes a negative electrode, a separator and an electrolyte.

[0037] Illustratively, the negative electrode comprises a negative electrode active material capable of inserting and extracting sodium ions.

[0038] Specifically, negative electrode active materials include carbon materials such as hard carbon, natural graphite, artificial graphite, soft carbon, carbon black, acetylene black, carbon nanotubes, graphene, and carbon nanofibers. Other negative electrode active materials include elements such as Si, Ge, Pb, In, Zn, H, Ca, Sr, Ba, Ru, and Rh that alloy with sodium, as well as oxides and carbides containing these elements. However, this document is not limited to these materials; other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0039] More specifically, the negative electrode active material is attached to the negative electrode current collector. The material of the negative electrode current collector is not particularly limited and can be selected according to actual needs.

[0040] Exemplarily, the electrolyte may include an organic solvent and an electrolyte sodium salt. As an example, the organic solvent may be one or more of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and diethyl carbonate; and the electrolyte sodium salt may be one or more of NaPF6, NaClO4, NaBCl4, NaSO3CF3, and Na(CH3)C6H4SO3.

[0041] In the sodium ion battery mentioned in this article, the specific type of the isolation membrane is not subject to specific restrictions and can be any isolation membrane material used in existing batteries, such as polyethylene, polypropylene, polyvinylidene fluoride and their multilayer composite membranes, but is not limited to these.

[0042] In one embodiment, the sodium ion battery may include an outer packaging, which can be used to encapsulate the electrode assembly and the electrolyte.

[0043] In one embodiment, the outer packaging of the sodium-ion battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0044] There is no particular limitation on the shape of the sodium ion battery herein, which may be cylindrical, square, or any other shape.

[0045] The beneficial effects of this article are: (1) The sodium phosphate-coated sodium ion positive electrode material in this article has a rounded morphology and a smooth surface. A sodium phosphate layer is creatively coated on the surface of the sodium ion positive electrode material, wherein the thickness of the sodium phosphate layer is uniform and controllable. The coating of the sodium phosphate layer effectively reduces the residual alkali on the surface, improves the structural stability, effectively inhibits the loss of sodium in the crystal structure, maintains better crystallinity, effectively improves the electrochemical performance, and improves the discharge specific capacity and cycle performance; (2) After the metal oxides are evenly mixed according to the molar ratio in the chemical formula of the sodium ion positive electrode material, they are directly mixed evenly with phosphate and sintered. During the cooling process, since the tetrahedral phosphate ions have a larger ionic radius and the metal salts have a larger ionic radius, the metal salts and phosphate ions are gently precipitated from the positive electrode substrate during the slow cooling process, and further react with the residual sodium on the surface of the substrate to form a sodium phosphate coating layer of uniform thickness in situ on the surface of the sodium ion positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is the sodium calcium phosphate coated Na prepared in Example 1 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 SEM image of O2 layered oxide sodium ion positive electrode material.

[0047] Figure 2 is the Na prepared in Comparative Example 1 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 SEM image of O2 layered oxide sodium ion positive electrode material.

[0048] Figure 3 is the sodium phosphate coated Na prepared in Comparative Example 2 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 SEM image of O2 layered oxide sodium ion positive electrode material.

[0049] FIG4 is an XRD diagram of the positive electrode materials prepared in Example 1, Comparative Example 1, and Comparative Example 2.

[0050] Figure 5 is the sodium barium phosphate coated Na prepared in Example 2 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 SEM image of O2 layered oxide sodium ion positive electrode material. DETAILED DESCRIPTION

[0051] To make the objectives, technical solutions, and advantages of the embodiments herein more clear, the technical solutions in the embodiments herein are described clearly and completely below. Obviously, the described embodiments are only a portion of the embodiments herein, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments herein without inventive effort are also within the scope of protection herein.

[0052] Unless otherwise specified, the experimental reagents and instruments involved in the implementation of this article are commonly used reagents and instruments.

[0053] In this article, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0054] As used herein, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Furthermore, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0055] Example 1

[0056] A sodium calcium phosphate coated Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The preparation method of O2 layered oxide sodium ion positive electrode material comprises the following steps:

[0057] (1) According to the sodium ion layered positive electrode material composition Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10The molar ratio of O2 is 0.90:0.25:0.50:0.15:0.10, and the sodium carbonate, nickel oxide, manganese oxide, ferric oxide, and copper oxide are uniformly mixed to obtain a precursor;

[0058] (2) Take 100g of the precursor, add 0.0045 mol of calcium hydrogen phosphate, mix at 1200rpm for 25min, put the mixture into a sagger, heat it to 900℃ at a rate of 3℃ / min in air atmosphere, sinter it for 12h, cool it to room temperature at a rate of 2℃ / min, and grind it by air flow mill to obtain sodium calcium phosphate coated O3-Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 O2 layered oxide sodium ion positive electrode material.

[0059] The thickness of the sodium calcium phosphate coating layer prepared in this example is 10 nm.

[0060] Example 2

[0061] A sodium barium phosphate coated Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The preparation method of O2 layered oxide sodium ion positive electrode material comprises the following steps:

[0062] (1) According to the sodium ion layered positive electrode material composition Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The molar ratio of O2 is 0.90:0.25:0.50:0.15:0.10, and the sodium carbonate, nickel oxide, manganese oxide, ferric oxide, and copper oxide are uniformly mixed to obtain a precursor;

[0063] (2) Take 100g of the precursor, add 0.0027 mol of barium hydrogen phosphate, mix at 1200rpm for 25min, put the mixture into a sagger, heat it to 900℃ at a rate of 3℃ / min in air atmosphere, sinter it for 12h, cool it to room temperature at a rate of 3℃ / min, and grind it by air flow mill to obtain sodium barium phosphate coated Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 O2 layered oxide sodium ion positive electrode material.

[0064] The thickness of the sodium barium phosphate coating layer prepared in this example is 6 nm.

[0065] Example 3

[0066] A sodium strontium phosphate coated Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The preparation method of O2 layered oxide sodium ion positive electrode material comprises the following steps:

[0067] (1) According to the sodium ion layered positive electrode material composition Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The molar ratio of O2 is 0.90:0.25:0.50:0.15:0.10, and the sodium carbonate, nickel oxide, manganese oxide, ferric oxide, and copper oxide are uniformly mixed to obtain a precursor;

[0068] (2) Take 100g of the precursor, add 0.006 mol of strontium hydrogen phosphate, mix at 1200rpm for 25min, put the mixture into a sagger, heat it to 900℃ at a rate of 3℃ / min in air atmosphere, sinter it for 12h, cool it to room temperature at a rate of 1.5℃ / min, and grind it by air flow mill to obtain Na2PO4 coated with sodium strontium phosphate. 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 O2 layered oxide sodium ion positive electrode material.

[0069] The thickness of the sodium strontium phosphate coating layer prepared in this example is 12 nm.

[0070] Example 4

[0071] The difference between Example 4 and Example 1 is that the sintering temperature of Example 4 is 800° C., and the other aspects are the same.

[0072] Example 5

[0073] The difference between Example 5 and Example 1 is that the sintering temperature of Example 5 is 1000° C., and the other aspects are the same.

[0074] Example 6

[0075] The difference between Example 6 and Example 1 is that the sintering temperature of Example 6 is 850° C., and the other aspects are the same.

[0076] Example 7

[0077] The difference between Example 7 and Example 1 is that the sintering temperature of Example 5 is 950° C., and the other aspects are the same.

[0078] Example 8

[0079] The difference between Example 8 and Example 1 is that the cooling rate of Example 8 is 0.1°C / min, and the other conditions are the same.

[0080] Example 9

[0081] The difference between Example 9 and Example 1 is that the cooling rate of Example 9 is 5°C / min, and the other conditions are the same.

[0082] Example 10

[0083] The difference between Example 10 and Example 1 is that the cooling rate of Example 10 is 1°C / min, and the other conditions are the same.

[0084] Comparative Example 1

[0085] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 is not coated with sodium calcium phosphate, and all other aspects are the same.

[0086] A method for preparing a sodium ion positive electrode material comprises the following steps:

[0087] (1) According to the sodium ion layered positive electrode material composition, Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The molar ratio of O2 is 0.90:0.25:0.50:0.15:0.10, and the sodium carbonate, nickel oxide, manganese oxide, ferric oxide, and copper oxide are uniformly mixed to obtain a precursor;

[0088] (2) The precursor was heated to 900°C at a rate of 3°C / min, sintered at this temperature for 12 hours, cooled to room temperature at a rate of 2°C / min, and pulverized by air jet mill to obtain Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 O2 layered oxide sodium ion positive electrode material.

[0089] Comparative Example 2

[0090] The difference between Comparative Example 2 and Example 1 is that phosphoric acid is used instead of calcium hydrogen phosphate in Comparative Example 2, and all other aspects are the same.

[0091] A sodium phosphate coated Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The preparation method of O2 layered oxide sodium ion positive electrode material comprises the following steps:

[0092] (1) According to the sodium ion layered positive electrode material composition, Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The molar ratio of O2 is 0.90:0.25:0.50:0.15:0.10, and the sodium carbonate, nickel oxide, manganese oxide, ferric oxide, and copper oxide are uniformly mixed to obtain a precursor;

[0093] (2) Take 100g of the precursor, add 0.0045 mol of phosphoric acid, mix at 1200 rpm for 25 min, put the mixture into a sagger, heat it to 900℃ at a rate of 3℃ / min in air atmosphere, sinter it for 12h, cool it to room temperature at a rate of 3℃ / min, and grind it by air flow mill to obtain sodium phosphate-coated Na 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 O2 layered oxide sodium ion positive electrode material.

[0094] Comparative Example 3

[0095] The difference between Comparative Example 3 and Example 1 is that the sintering temperature of Comparative Example 3 is 700° C., and the other conditions are the same.

[0096] Comparative Example 4

[0097] The difference between Comparative Example 4 and Example 1 is that the sintering temperature of Comparative Example 4 is 1100° C., and the other conditions are the same.

[0098] Comparative Example 5

[0099] The difference between Comparative Example 5 and Example 1 is that the cooling rate of Comparative Example 5 is 0.05° C. / min, and the other conditions are the same.

[0100] Comparative Example 6

[0101] The difference between Comparative Example 6 and Example 1 is that the cooling rate of Comparative Example 6 is 8°C / min, and the other conditions are the same.

[0102] Comparative Example 7

[0103] The difference between Comparative Example 7 and Example 1 is that the molar amount of calcium hydrogen phosphate added in Comparative Example 7 is 0.2 mol, and the other conditions are the same.

[0104] Comparative Example 8

[0105] The difference between Comparative Example 8 and Example 1 is that the molar amount of calcium hydrogen phosphate added in Comparative Example 8 is 0.005 mol, and the other aspects are the same.

[0106] Test Example 1

[0107] 1. The materials synthesized in Example 1, Comparative Example 1, and Comparative Example 2 were characterized by scanning electron microscopy (SEM), as shown in Figures 1, 2, and 3, respectively.

[0108] As can be seen from Figures 1 to 3, the Na coated sodium calcium phosphate in Example 1 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 Compared with the positive electrode materials of Comparative Examples 1 and 2, the morphology of O2 is more rounded and the surface is smoother, indicating that the surface of the material coated with sodium calcium phosphate is more uniform.

[0109] 2. XRD characterization was performed on the cathode materials synthesized in Example 1, Comparative Example 1 and Comparative Example 2. As shown in FIG4 , the XRD characterization results showed that the Na coated with sodium calcium phosphate 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The intensity of each spectral peak of O2 is stronger, especially the (003) peak is significantly enhanced, indicating that coating sodium calcium phosphate helps to enhance the structural stability of the material, inhibit the loss of sodium in the crystal structure, and maintain better crystallinity.

[0110] 3. Na coated sodium barium phosphate synthesized in Example 2 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 The O2 scanning electron microscope (SEM) characterization is shown in Figure 5. It can be seen that the Na coated with sodium barium phosphate synthesized in this paper 0.90 Ni 0.25 Mn 0.50 Fe 0.15 Cu 0.10 O2 also has a rounded shape, smooth surface and uniform surface.

[0111] The above results show that the sodium phosphate-coated sodium ion positive electrode material in this article has a rounded morphology and a smooth surface. A sodium phosphate layer is creatively coated on the surface of the sodium ion positive electrode material, wherein the thickness of the sodium phosphate layer is uniform and controllable. The coating of the sodium phosphate layer effectively reduces the residual alkali on the surface, improves the structural stability, effectively inhibits the loss of sodium in the crystal structure, maintains better crystallinity, effectively improves the electrochemical performance, and improves the discharge specific capacity and cycle performance.

[0112] Compared to sodium phosphate-coated sodium ion cathode materials, the sodium phosphate-coated sodium ion cathode material in this paper has a rounder appearance, a smoother and more uniform surface, and stronger peak intensities, especially the (003) peak. Therefore, sodium phosphate-coated sodium is more conducive to increasing the stability of the material, inhibiting sodium loss in the crystal structure, and maintaining better crystallinity than sodium phosphate-coated sodium. Furthermore, data show that sodium phosphate-coated sodium has better electrochemical performance than sodium phosphate-coated sodium, which can further improve the discharge capacity and cycle performance.

[0113] Test Example 2

[0114] The positive electrode materials of Examples 1 to 10 and Comparative Examples 1 to 8 were prepared into button-type full batteries, and the cycle performance (50 cycles at 1C) was tested. The test results are shown in Table 1.

[0115] As can be seen from Table 1, the in-situ phosphate-coated sodium ion cathode material described in this article has excellent cycling performance.

[0116] By comparing Example 1 with Comparative Examples 1 to 2, it can be seen that the phosphate coating in this paper significantly improves the cycle performance, and the phosphate coating can more significantly improve the cycle performance than the phosphoric acid coating.

[0117] Comparison of Example 1 with Comparative Examples 3 to 4 shows that the cycle performance is further improved by controlling the sintering temperature within the range of this article.

[0118] Comparison of Example 1 with Comparative Examples 5 to 6 shows that the cooling rate is controlled within the range of this article, which further improves the cycle performance.

[0119] By comparing Example 1 with Comparative Examples 7 to 8, it can be seen that the cycle performance is further improved by controlling the molar amount of added calcium hydrogen phosphate.

Claims

1. A sodium phosphate coated sodium ion positive electrode material, characterized in that: It comprises a sodium ion positive electrode material and a sodium phosphate layer coated on the surface of the sodium ion positive electrode material, wherein the thickness of the sodium phosphate layer is 1 to 20 nm; The sodium phosphate includes at least one of sodium calcium phosphate, sodium barium phosphate, sodium strontium phosphate, sodium magnesium phosphate, and sodium aluminum phosphate.

2. The sodium phosphate coated sodium ion positive electrode material according to claim 1, characterized in that: The mass ratio of the sodium ion positive electrode material to the sodium phosphate is 1:(0.0001-0.03).

3. The sodium phosphate coated sodium ion positive electrode material according to claim 1, characterized in that: The sodium ion positive electrode material is a layered oxide positive electrode material, and its general chemical formula is Na x1 MO y1 , wherein M includes at least one of Ni, Mg, Cu, Mn, Co, Zn, Ca, Ba, Fe, Al, B, Ti, Sn, Nb, Sb, Si, and Mo; Among them, 0.5<x1<1.2, y1≥2.

4. The sodium phosphate coated sodium ion positive electrode material according to claim 1, characterized in that: The sodium ion positive electrode material contains at least one phase structure of O3 phase, P2 phase, O2 phase and P3 phase.

5. The method for preparing the sodium phosphate coated sodium ion positive electrode material according to any one of claims 1 to 4, characterized in that: The following steps are involved: Weigh the metal oxides according to the molar ratio in the chemical formula of the sodium ion positive electrode material, mix them evenly, and obtain a precursor; The precursor and phosphate are mixed evenly, sintered, cooled, and ground by air flow mill to obtain a sodium phosphate-coated sodium ion positive electrode material.

6. The method for preparing the sodium phosphate coated sodium ion positive electrode material according to claim 5, characterized in that: The mass ratio of the precursor to the phosphate is 1:(0.0001-0.03).

7. The method for preparing the sodium phosphate coated sodium ion positive electrode material according to claim 5, characterized in that: The mass ratio of the precursor to the phosphate is 1:(0.002-0.008).

8. The method for preparing the sodium phosphate coated sodium ion positive electrode material according to claim 5, characterized in that: The phosphate includes at least one of calcium phosphate, calcium hydrogen phosphate, calcium dihydrogen phosphate, calcium tripolyphosphate, barium phosphate, barium hydrogen phosphate, barium dihydrogen phosphate, barium tripolyphosphate, strontium phosphate, strontium hydrogen phosphate, strontium tripolyphosphate, magnesium hydrogen phosphate, and aluminum phosphate.

9. The method for preparing the sodium phosphate coated sodium ion positive electrode material according to claim 5, characterized in that: The sintering temperature is 800-1000° C., and the sintering time is 6-24 hours.

10. The method for preparing the sodium phosphate coated sodium ion positive electrode material according to claim 5, characterized in that: The sintering temperature is 850-950° C., and the sintering time is 12-18 hours.

11. The method for preparing the sodium phosphate coated sodium ion positive electrode material according to claim 5, characterized in that: The sintering is carried out in air, oxygen or a mixed gas of air and oxygen.

12. The method for preparing the sodium phosphate coated sodium ion positive electrode material according to claim 5, characterized in that: The cooling rate is 0.1-5°C / min.

13. The method for preparing the sodium phosphate coated sodium ion positive electrode material according to claim 5, characterized in that: The cooling rate is 0.1-1°C / min.

14. A positive electrode material, characterized in that: The positive electrode material comprises the sodium phosphate-coated sodium ion positive electrode material according to any one of claims 1 to 4 or the sodium phosphate-coated sodium ion positive electrode material prepared by the preparation method according to any one of claims 5 to 13.

15. A sodium ion battery, characterized in that: The sodium ion battery comprises the positive electrode material according to claim 14.

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