A thin-layer uniform low-carbon sodium iron pyrophosphate phosphate cathode material, its preparation method and application

Abstract

The present invention discloses a thin-layer uniformly low-carbon-content coated sodium iron pyrophosphate phosphate cathode material, its preparation method and application. The thin-layer uniformly low-carbon-content coated sodium iron pyrophosphate phosphate cathode material has a secondary spherical particle structure formed by stacking primary core-shell spherical particles; the primary core-shell spherical particles have a carbon layer as the shell layer and sodium iron pyrophosphate phosphate as the inner core; the particle size of the primary core-shell spherical particles is 200 - 300 nm; the particle size of the secondary spherical particles is 2 - 5 μm; the carbon layer thickness of the primary core-shell spherical particles is 2 - 4 nm; the proportion of carbon in the total content of carbon, hydrogen and oxygen in the carbon layer is > 90 wt%; the carbon content of the thin-layer uniformly low-carbon-content coated sodium iron pyrophosphate phosphate cathode material is 1.0 - 2.5 wt%. The thin-layer uniformly low-carbon-content coated sodium iron pyrophosphate phosphate cathode material can maintain a high capacitance at different charge-discharge rates and has good discharge performance.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials for sodium-ion batteries, and particularly to a sodium iron pyrophosphate cathode material with a thin-layer uniform low-carbon coating, a preparation method thereof, and an application thereof. Background Art

[0002] Iron-based system materials are considered to be one of the most commercially promising cathode material systems for sodium-ion batteries due to their rich and widespread raw materials. However, the poor conductivity of mixed phosphate iron-based materials limits their application in sodium-ion batteries. In addition, the preparation process of the current iron-based system in sodium-ion batteries is relatively cumbersome, and there are still great challenges in the electrochemical performance of the materials, especially the cycling performance.

[0003] Carbon coating is a key method to improve the electronic conductivity of polyanionic phosphate electrode materials. Coating carbon materials on the surface of polyanionic compound particles can not only enhance the material conductivity but also promote ion transport, which helps to improve the diffusion rate of sodium ions in the electrode material. In practice, carbon coating is usually achieved by introducing a carbon source, such as oxalic acid, citric acid, ascorbic acid, etc., during the preparation of the precursor. However, the carbon materials coated by this method have poor uniformity and bonding strength on the surface, which will damage the crystal structure of the cathode material and affect the electrochemical performance of the electrode material. Moreover, the excessive addition of carbon will lead to a decrease in the sodium ion transfer rate and affect the discharge performance of secondary batteries. Therefore, it is very necessary to develop a high-performance carbon-coated sodium iron pyrophosphate material and its preparation method. Summary of the Invention

[0004] The purpose of the present invention is to provide a sodium iron pyrophosphate cathode material with a thin-layer uniform low-carbon coating, a preparation method thereof, and an application thereof, so as to solve the problems existing in the above-mentioned prior art. The present invention provides a sodium iron pyrophosphate cathode material with a thin-layer uniform low-carbon coating to improve the conductivity of the sodium iron pyrophosphate cathode material, thereby improving the long-cycle performance and rate performance of the sodium-ion battery prepared with it as a raw material.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] One of the technical solutions of the present invention: A sodium iron pyrophosphate cathode material with a thin-layer uniform low-carbon coating, wherein the sodium iron pyrophosphate cathode material with a thin-layer uniform low-carbon coating is a secondary spherical particle structure stacked by primary core-shell spherical particles; the primary core-shell spherical particles have a carbon layer as the shell layer and sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) as the inner core; the particle size of the primary core-shell spherical particles is 200 - 300 nm; the particle size of the secondary spherical particles is 2 - 5 μm; the carbon layer thickness of the primary core-shell spherical particles is 2 - 4 nm.

[0007] The secondary spherical particle structure formed by stacking primary core-shell spherical particles is similar to the structure of a pomegranate after removing its rind. Among them, the primary core-shell spherical particles are equivalent to individual pomegranate arils (including pomegranate seeds and the pulp wrapped on the surface of the pomegranate seeds, where the pulp is equivalent to the carbon layer on the surface of the primary core-shell spherical particles, and the internal pomegranate seeds are equivalent to the inner core sodium iron pyrophosphate phosphate of the primary core-shell spherical particles), and the secondary spherical particles are equivalent to the structure of a complete pomegranate formed by stacking individual pomegranate arils.

[0008] In the sodium iron pyrophosphate phosphate cathode material with a thin-layer uniform low-carbon coating of the present invention, each primary core-shell spherical particle is uniformly coated with a low content of carbon, which can effectively improve the electronic conductivity. In addition, the average particle size of the primary core-shell spherical particles is 200 - 300 nm, and the particle size is small, which is beneficial to shortening the electron and ion transport paths in the material and improving the electronic and ionic conductivities. At the same time, the secondary spherical particle structure with an average particle size of 2 - 5 μm formed by stacking primary core-shell spherical particles is beneficial to the transport of electrons and ions between the spheres. The stacking of primary core-shell spherical particles makes the secondary spherical particles have a porous structure, which helps the penetration of the electrolyte. The synergistic effect of the above multiple factors makes the sodium iron pyrophosphate phosphate cathode material with a thin-layer uniform low-carbon coating of the present invention have excellent electrochemical performance.

[0009] Further, the proportion of carbon in the total content of carbon, hydrogen, and oxygen in the carbon layer > 90 wt%; the carbon content of the sodium iron pyrophosphate phosphate cathode material with a thin-layer uniform low-carbon coating is 1.0 - 2.5 wt%.

[0010] The extremely low carbon content can make the carbon coating uniform and thin, which is beneficial to improving the overall energy density of the material; the uniform and thin carbon coating can improve the electronic conductivity and does not hinder the ion transport. The proportion of carbon in the total content of carbon, hydrogen, and oxygen in the carbon layer > 90 wt%, and the higher carbon content means that there are fewer defects in the carbon coating layer, which further makes the electronic conductivity higher, beneficial to electron transport, and beneficial to the attachment of carbon on the surface of the cathode material.

[0011] Further, the secondary spherical particles are of a porous structure (the stacking of primary core-shell spherical particles makes the formed secondary spherical particles have porous characteristics). That is, the sodium iron pyrophosphate phosphate cathode material with a thin-layer uniform low-carbon coating is a secondary porous spherical particle structure formed by stacking primary core-shell spherical particles.

[0012] Further, the specific surface area of the sodium iron pyrophosphate phosphate cathode material with a thin-layer uniform low-carbon coating is 10 - 20 m 2 / g.

[0013] The second technical solution of the present invention: The preparation method of the above-mentioned sodium iron pyrophosphate phosphate cathode material with a thin-layer uniform low-carbon coating includes the following steps:

[0014] Mix a sodium phosphorus source, an iron carbon source and a solvent to obtain a precursor solution; perform spray drying on the precursor solution to obtain a precursor; perform sintering on the precursor to obtain the thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material (Na4Fe3(PO4)2P2O7 / C).

[0015] Spray drying can form spherical precursors, and further thermal decomposition of the precursors forms uniformly carbon-coated primary core-shell spherical particles and generates a porous structure. The carbon comes from the decomposition of organic iron compounds rather than an additional carbon source. Therefore, the obtained carbon-coated sodium iron pyrophosphate phosphate cathode material has a low carbon content, uniform and thin coating. Generally, when the carbon content is low, it is very difficult to achieve uniform carbon coating, and the obtained product has poor electrochemical performance. However, the method of the present invention can achieve the coating effect with both uniform low carbon.

[0016] Further, the sodium phosphorus source includes sodium dihydrogen phosphate; the iron carbon source includes an organic iron compound; the solvent includes water and / or absolute ethanol.

[0017] Further, the organic iron compound includes iron acetylacetonate (Fe(C5H7O2)3), ferrocene (Fe(C5H5)2) or iron citrate (C6H8O7Fe).

[0018] Further, the molar ratio of the sodium phosphorus source to the iron carbon source is 4:2.95 - 3;

[0019] and / or, the conditions of the spray drying include: the inlet air temperature is 110 - 180 °C, the nozzle diameter is 0.5 - 2.5 mm, and the feed flow rate is 300 - 500 mL·h -1 ;

[0020] and / or, the sintering temperature is 450 - 600 °C and the time is 4 - 20 h.

[0021] Further, the conditions of the spray drying further include: the gas source is compressed air.

[0022] Further, the sintering is carried out in a reducing atmosphere and / or an inert atmosphere.

[0023] Further, the reducing atmosphere includes a hydrogen (H2) atmosphere, a carbon monoxide (CO) atmosphere, an H2 / Ar mixed atmosphere or an H2 / N2 mixed atmosphere; the inert atmosphere includes an Ar (argon) atmosphere, an N2 atmosphere or an Ar / N2 mixed atmosphere.

[0024] Further preferably, the sintering is carried out in two stages. Specifically, it is first sintered in a reducing atmosphere and then in an inert atmosphere; the temperatures of the two stages are the same, both being 450 - 600 °C, and the sintering times of the two stages are the same, and the total sintering time of the two stages is 4 - 20 h.

[0025] Sintering in two stages can better create pores, improve the phase purity and crystal crystallinity, and effectively improve the electrochemical performance of the material, especially the electrochemical performance of the material at high rates.

[0026] Further, the mixing of the sodium phosphorus source, the iron carbon source and the solvent to obtain the precursor solution includes: adding the sodium phosphorus source to a part of the solvent and stirring for 1 - 6 h to obtain a sodium phosphorus source solution; adding the iron carbon source to a part of the solvent and stirring for 1 - 6 h to obtain an iron carbon source solution; mixing the sodium phosphorus source solution and the iron carbon source solution and stirring for 3 - 12 h to obtain the precursor solution.

[0027] Further, the dosage ratio of the sodium phosphorus source to the solvent is 0.04 mol: 100 - 500 mL; the dosage ratio of the iron carbon source to the solvent is 0.03 mol: 100 - 500 mL.

[0028] Technical solution three of the present invention: Application of the above-mentioned thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material in the preparation of sodium-ion batteries.

[0029] Technical solution four of the present invention: A sodium-ion battery cathode, and the raw materials of the sodium-ion battery cathode include the above-mentioned thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material.

[0030] Technical solution five of the present invention: A sodium-ion battery, and the sodium-ion battery uses the above-mentioned sodium-ion battery cathode as the cathode.

[0031] The present invention discloses the following technical effects:

[0032] The thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material of the present invention is a secondary spherical particle structure stacked by primary core-shell spherical particles, having the characteristics of small and uniformly distributed particles and being porous, as well as a carbon coating layer with low content, in-situ and uniform, which can significantly improve the electronic conductivity and sodium ion diffusion kinetics performance of sodium iron pyrophosphate phosphate, thereby effectively improving its long-cycle stability performance and rate performance when used as a sodium-ion battery cathode material.

[0033] The preparation method of the thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material of the present invention is simple, and the obtained thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material can maintain a high capacitance at different charge and discharge rates and has good discharge performance. Description of the Drawings

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0035] Figure 1 It is a schematic structural diagram of the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material of the present invention;

[0036] Figure 2 It is an SEM image of the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in Example 1. Among them, (a) is a low-magnification image, and (b) is a high-magnification image;

[0037] Figure 3 It is a TEM image of the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in Example 1;

[0038] Figure 4 It is an XRD pattern of the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in Example 1;

[0039] Figure 5 It is an SEM image of the carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in Comparative Example 1. Among them, (a) is a low-magnification image, and (b) is a high-magnification image;

[0040] Figure 6 It is an XRD pattern of the carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in Comparative Example 1;

[0041] Figure 7 It is an SEM image of the carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in Comparative Example 2;

[0042] Figure 8 It is a long-cycle performance graph of a sodium-ion battery prepared using the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material of Example 1 at a current density of 20C;

[0043] Figure 9 It is a long-cycle performance graph of a sodium-ion battery prepared using the carbon-coated sodium iron pyrophosphate phosphate cathode material of Comparative Example 1 at a current density of 20C;

[0044] Figure 10 It is a rate performance graph of a sodium-ion battery prepared using the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material of Example 1 at different current densities;

[0045] Figure 11The rate performance graph of a sodium-ion battery prepared using the carbon-coated sodium iron pyrophosphate phosphate cathode material of Comparative Example 1 at different current densities. Detailed Description of the Invention

[0046] The various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation methods of the present invention.

[0047] It should be understood that the terms used in the present invention are only for describing specific embodiments and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0048] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0049] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the present invention's specification, which are obvious to those skilled in the art. Other embodiments obtained from the specification of the present invention are obvious to those skilled in the art. The specification and examples of the present invention are merely exemplary.

[0050] Regarding the use of "comprising", "including", "having", "containing", etc. in this article, they are all open-ended terms, meaning including but not limited to.

[0051] As a first aspect of the present invention, the present invention provides a thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material (the structural schematic diagram is as Figure 1 shown), and the thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material is a secondary spherical particle structure formed by stacking primary core-shell spherical particles; the primary core-shell spherical particles have a carbon layer as the shell layer and sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) as the inner core; the particle size of the primary core-shell spherical particles is 200 - 300 nm; the particle size of the secondary spherical particles is 2 - 5 μm; the carbon layer thickness of the primary core-shell spherical particles is 2 - 4 nm.

[0052] As a preferred embodiment of the present invention, the proportion of carbon in the carbon layer in the total content of carbon, hydrogen and oxygen in the carbon layer > 90 wt%; the carbon content of the thin layer uniformly and low-carbonly coated sodium iron pyrophosphate phosphate cathode material is 1.0 - 2.5 wt%.

[0053] As a preferred embodiment of the present invention, the secondary spherical particles are of a porous structure (the stacking of the primary core-shell spherical particles enables the formed secondary spherical particles to have porous characteristics).

[0054] As a preferred embodiment of the present invention, the specific surface area of the thin layer uniformly and low-carbonly coated sodium iron pyrophosphate phosphate cathode material is 10 - 20 m 2 / g.

[0055] As a second aspect of the present invention, the present invention provides a method for preparing the above-mentioned thin layer uniformly and low-carbonly coated sodium iron pyrophosphate phosphate cathode material, comprising the following steps:

[0056] Mix a sodium phosphorus source, an iron carbon source and a solvent to obtain a precursor solution; perform spray drying on the precursor solution to obtain a precursor; perform sintering on the precursor to obtain the thin layer uniformly and low-carbonly coated sodium iron pyrophosphate phosphate cathode material (Na4Fe3(PO4)2P2O7 / C).

[0057] As a preferred embodiment of the present invention, the sodium phosphorus source includes sodium dihydrogen phosphate; the iron carbon source includes an organic iron compound; the solvent includes water and / or absolute ethanol.

[0058] As a preferred embodiment of the present invention, the organic iron compound includes iron acetylacetonate (Fe(C5H7O2)3), ferrocene (Fe(C5H5)2) or iron citrate (C6H8O7Fe).

[0059] As a preferred embodiment of the present invention, the molar ratio of the sodium phosphorus source to the iron carbon source is 4:2.95 - 3;

[0060] and / or, the conditions of the spray drying include: the inlet air temperature is 110 - 180 °C, the nozzle diameter is 0.5 - 2.5 mm, the feed flow rate is 300 - 500 mL·h -1 , and the gas source is compressed air;

[0061] and / or, the sintering temperature is 450 - 600 °C, and the time is 4 - 20 h.

[0062] As a preferred embodiment of the present invention, the sintering is carried out in a reducing atmosphere and / or an inert atmosphere (that is, entirely in a reducing atmosphere, or entirely in an inert atmosphere, or sintered in a reducing atmosphere for a period of time first and then in an inert atmosphere for a period of time).

[0063] As a preferred embodiment of the present invention, the reducing atmosphere includes a hydrogen (H2) atmosphere, a carbon monoxide (CO) atmosphere, an H2 / Ar mixed atmosphere or an H2 / N2 mixed atmosphere; the inert atmosphere includes an Ar (argon) atmosphere, an N2 atmosphere or an Ar / N2 mixed atmosphere.

[0064] As a preferred embodiment of the present invention, the sintering is carried out in two stages. Specifically, it is first sintered in a reducing atmosphere and then in an inert atmosphere; the temperatures of the two stages are the same, both being 450 - 600 °C, the sintering times of the two stages are the same, and the total sintering time of the two stages is 4 - 20 h.

[0065] As a preferred embodiment of the present invention, the mixing of the sodium phosphorus source, the iron carbon source and the solvent to obtain the precursor solution includes: adding the sodium phosphorus source to a part of the solvent and stirring for 1 - 6 h to obtain a sodium phosphorus source solution; adding the iron carbon source to a part of the solvent and stirring for 1 - 6 h to obtain an iron carbon source solution; mixing the sodium phosphorus source solution and the iron carbon source solution and stirring for 3 - 12 h to obtain the precursor solution.

[0066] As a preferred embodiment of the present invention, the dosage ratio of the sodium phosphorus source to the solvent is 0.04 mol:100 - 500 mL; the dosage ratio of the iron carbon source to the solvent is 0.03 mol:100 - 500 mL.

[0067] As the third aspect of the present invention, the present invention provides the application of the above-mentioned thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material in the preparation of sodium-ion batteries.

[0068] As the fourth aspect of the present invention, the present invention provides a sodium-ion battery cathode, and the raw materials of the sodium-ion battery cathode include the above-mentioned thin-layer uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material.

[0069] As the fifth aspect of the present invention, the present invention provides a sodium-ion battery, and the sodium-ion battery uses the above-mentioned sodium-ion battery cathode as the cathode.

[0070] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0071] All raw materials used in the specific embodiments of the present invention are ordinary commercially available products.

[0072] Example 1

[0073] A thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material is prepared as follows:

[0074] Step 1: Weigh 0.03 mol of iron acetylacetonate and add it to 200 mL of absolute ethanol. Weigh 0.04 mol of sodium dihydrogen phosphate and add it to 200 mL of deionized water. Stir each for 3 h to obtain an iron-carbon source solution and a sodium-phosphorus source solution. After mixing the iron-carbon source solution and the sodium-phosphorus source solution, continue to stir for 3 h to obtain a precursor solution. Spray-dry the precursor solution to obtain a precursor. The conditions for spray drying are: inlet air temperature is 150 °C, nozzle diameter is 1.0 mm, feed flow rate is 350 mL·h -1 and the gas source is compressed air;

[0075] Step 2: Transfer the precursor to a tubular furnace. First, sinter it in a H2 / Ar mixed atmosphere at 550 °C (where the proportion of H2 is 8 vol%) for 4 h, and then sinter it in an Ar atmosphere for 4 h to obtain a thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material (Na4Fe3(PO4)2P2O7 / C).

[0076] Figure 2 This is the SEM image of the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in this example. Among them, (a) is a low-magnification image (scale bar is 10 μm), and (b) is a high-magnification image (scale bar is 1 μm). From Figure 2 it can be seen that the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material in this example is a secondary spherical particle structure composed of stacked primary core-shell spherical particles. There are pores between the primary core-shell spherical particles (that is, the secondary spherical particles formed by stacking the primary core-shell spherical particles have a porous structure). The particle size range of the primary core-shell spherical particles is 200 - 300 nm, and the particle size range of the secondary spherical particles is 2 - 5 μm. The particle size uniformity is relatively high, and there is no obvious agglomeration phenomenon locally. Such uniform fine particles can shorten the transmission distance of sodium ions in the sodium iron pyrophosphate phosphate particles and accelerate the sodium ion migration kinetics. Figure 3 This is the TEM image of the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in this example, mainly showing the TEM detection of the primary core-shell spherical particles. From Figure 3 it can be seen that the average thickness of the carbon layer in the primary core-shell spherical particles is 2 nm, and the carbon layer is uniform and thin.

[0077] Figure 4 This is the XRD pattern of the thin-layer uniform low-carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in this example. The XRD pattern is consistent with the standard spectrum, and basically no impurity phase is generated, and the crystallinity is good.

[0078] Example 2

[0079] A thin-layer uniform low-carbon coated sodium iron pyrophosphate phosphate cathode material, the preparation steps are as follows:

[0080] Step 1: Weigh 0.03 mol of ferrocene and add it to 200 mL of absolute ethanol. Weigh 0.04 mol of sodium dihydrogen phosphate and add it to 200 mL of deionized water. Stir each for 3 h to obtain an iron-carbon source solution and a sodium-phosphorus source solution; after mixing the iron-carbon source solution and the sodium-phosphorus source solution, continue to stir for 3 h to obtain a precursor solution; perform spray drying on the precursor solution to obtain a precursor. The conditions for spray drying are: the inlet air temperature is 150 °C, the nozzle diameter is 1.0 mm, the feed flow rate is 350 mL·h -1 and the gas source is compressed air;

[0081] Step 2: Transfer the precursor to a tubular furnace, first sinter in a mixed atmosphere of H2 / Ar at 550 °C (where the proportion of H2 is 8 vol%) for 4 h, and then sinter in an Ar atmosphere for 4 h to obtain a thin-layer uniform low-carbon coated sodium iron pyrophosphate phosphate cathode material (Na4Fe3(PO4)2P2O7 / C).

[0082] After SEM detection, the thin-layer uniform low-carbon coated sodium iron pyrophosphate phosphate cathode material of this example is a secondary spherical particle structure stacked by primary core-shell spherical particles. There are pores between the primary core-shell spherical particles. The particle size range of the primary core-shell spherical particles is 200-300 nm, and the particle size range of the secondary spherical particles is 2-5 μm. The particle size uniformity is relatively high, and there is no obvious agglomeration phenomenon locally. After TEM detection, the average thickness of the carbon layer in the primary core-shell spherical particles is 2 nm, and the carbon layer is uniform and thin. After XRD detection, the XRD pattern of the thin-layer uniform low-carbon coated sodium iron pyrophosphate phosphate cathode material of this example is consistent with the standard spectrum, and basically no impurity phases are generated, and the crystallinity is good (the SEM image, TEM image and XRD image of the thin-layer uniform low-carbon coated sodium iron pyrophosphate phosphate cathode material prepared in this example are basically the same as those in Example 1 and will not be provided repeatedly).

[0083] Example 3

[0084] A thin-layer uniform low-carbon coated sodium iron pyrophosphate phosphate cathode material, the preparation steps are as follows:

[0085] Step 1: Weigh 0.03 mol of iron citrate and add it to 200 mL of absolute ethanol. Weigh 0.04 mol of sodium dihydrogen phosphate and add it to 200 mL of deionized water. Stir each for 3 h to obtain an iron-carbon source solution and a sodium-phosphorus source solution. After mixing the iron-carbon source solution and the sodium-phosphorus source solution, continue stirring for 3 h to obtain a precursor solution. Spray-dry the precursor solution to obtain a precursor. The conditions for spray drying are as follows: the inlet air temperature is 150 °C, the nozzle diameter is 1.0 mm, the feed flow rate is 350 mL·h -1 , and the gas source is compressed air;

[0086] Step 2: Transfer the precursor to a tubular furnace. First, sinter it in a H2 / Ar mixed atmosphere at 550 °C (where the proportion of H2 is 8 vol%) for 4 h, and then sinter it in an Ar atmosphere for 4 h to obtain a thin-layer and uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material (Na4Fe3(PO4)2P2O7 / C).

[0087] Detected by SEM, the thin-layer and uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material of this example is a secondary spherical particle structure stacked by primary core-shell spherical particles. There are pores between the primary core-shell spherical particles. The particle size range of the primary core-shell spherical particles is 200 - 300 nm, and the particle size range of the secondary spherical particles is 2 - 5 μm. The particle size uniformity is relatively high, and there is no obvious agglomeration phenomenon locally. Detected by TEM, the average thickness of the carbon layer in the primary core-shell spherical particles is 2 nm, and the carbon layer is uniform and thin. Detected by XRD, the XRD pattern of the thin-layer and uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material of this example is consistent with the standard spectrum, with basically no generation of impurity phases and good crystallinity (the SEM image, TEM image, and XRD pattern of the thin-layer and uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in this example are basically the same as those in Example 1 and will not be provided repeatedly).

[0088] Example 4

[0089] Same as Example 1, the only difference is that after transferring the precursor to the tubular furnace, sinter it in an Ar atmosphere at 550 °C for 8 h to obtain a thin-layer and uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material (Na4Fe3(PO4)2P2O7 / C).

[0090] Example 5

[0091] Same as Example 1, the only difference is that after transferring the precursor to the tubular furnace, sinter it in a H2 / Ar atmosphere at 550 °C (where the proportion of H2 is 8 vol%) for 8 h to obtain a thin-layer and uniformly low-carbon-coated sodium iron pyrophosphate phosphate cathode material (Na4Fe3(PO4)2P2O7 / C).

[0092] Comparative Example 1

[0093] A carbon-coated sodium iron pyrophosphate phosphate cathode material, the preparation steps are as follows:

[0094] Step 1: Add 0.04 mol of sodium dihydrogen phosphate, 0.03 mol of iron nitrate and 0.03 mol of citric acid to 500 mL of deionized water, stir for 3 h to obtain a precursor solution; spray-dry the precursor solution to obtain a precursor, and the conditions for spray drying are: the inlet air temperature is 150 °C, the nozzle diameter is 1.0 mm, the feed flow rate is 350 mL·h -1 , and the gas source is compressed air;

[0095] Step 2: Transfer the precursor to a tubular furnace, sinter in a H2 / Ar mixed atmosphere (where the proportion of H2 is 8 vol%) at 550 °C for 4 h, and then sinter in an Ar atmosphere for 4 h to obtain a carbon-coated sodium iron pyrophosphate phosphate cathode material.

[0096] Figure 5 This is the SEM image of the carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in this comparative example. Among them, (a) is a low-magnification image (scale bar is 5 μm), and (b) is a high-magnification image (scale bar is 1 μm). As can be seen from Figure 5 It can be seen that the carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in this comparative example is primary spherical particles, and the particle size range is 1-5 μm, and it does not have a secondary spherical particle structure composed of stacked primary core-shell spherical particles.

[0097] Figure 6 This is the XRD pattern of the carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in this comparative example. The XRD pattern is consistent with the standard spectrum, and the crystallinity is good.

[0098] Comparative Example 2

[0099] A carbon-coated sodium iron pyrophosphate phosphate cathode material, the preparation steps are as follows:

[0100] Step 1: Weigh 0.03 mol of iron acetylacetonate and 0.04 mol of sodium dihydrogen phosphate spheres, mix them with 40 mL of absolute ethanol, and ball-mill them at a speed of 300 rpm for 10 h (the material-ball ratio is 1:20) to obtain a precursor;

[0101] Step 2: Transfer the precursor to a tubular furnace, sinter in a H2 / Ar mixed atmosphere (where the proportion of H2 is 8 vol%) at 550 °C for 4 h, and then sinter in an Ar atmosphere for 4 h to obtain a carbon-coated sodium iron pyrophosphate phosphate cathode material.

[0102] Figure 7 This is the SEM image of the carbon-coated sodium iron pyrophosphate phosphate cathode material prepared in this comparative example. As can be seen from Figure 7It can be seen that it does not have a secondary spherical particle structure stacked by primary core-shell spherical particles.

[0103] Comparative Example 3

[0104] A sodium iron pyrophosphate phosphate cathode material is prepared according to the following steps:

[0105] Step 1: 0.04 mol of sodium dihydrogen phosphate and 0.03 mol of iron nitrate are added to 500 mL of deionized water and stirred for 3 h to obtain a precursor solution; the precursor solution is spray-dried to obtain a precursor. The conditions for spray drying are: the inlet air temperature is 150 °C, the nozzle diameter is 1.0 mm, the feed flow rate is 350 mL·h -1 , and the gas source is compressed air;

[0106] Step 2: The precursor is transferred to a tube furnace and sintered in a H2 / Ar mixed atmosphere (where the proportion of H2 is 8 vol%) at 550 °C for 4 h, and then sintered in an Ar atmosphere for 4 h to obtain a sodium iron pyrophosphate phosphate cathode material.

[0107] Test Example 1

[0108] Measurement of the proportion of carbon in the total carbon, hydrogen and oxygen content in the carbon layer, the overall carbon content of the carbon-coated sodium iron pyrophosphate phosphate cathode material, and the specific surface area

[0109] Test method:

[0110] The proportion of carbon in the total carbon, hydrogen and oxygen content in the carbon layer is analyzed and measured using an elemental analyzer (Elementar Vario EL cube, Germany); the overall carbon content of the cathode material is tested by carbon and sulfur analysis; the specific surface area is obtained by the BET specific surface area test method.

[0111] The test results of Example 1 and Comparative Example 1 are shown in Table 1.

[0112] Table 1

[0113]

[0114] Test Example 2

[0115] Electrochemical performance test

[0116] The cathode materials prepared in the examples and comparative examples are applied to sodium-ion batteries. Specifically, an electrode slurry is prepared according to a mass ratio of cathode material: conductive carbon black: binder (polyvinylidene fluoride) of 8:1:1, and then coated on an aluminum foil by a doctor blade coating method (the coating amount is 2 mg / cm 2). The aluminum foil was dried in a vacuum drying oven at 120 °C for 12 h, and then cut into electrode discs with a diameter of 10 mm. After weighing, it was transferred into a glove box and used as the positive electrode sheet. The CR2032 coin-type battery was assembled in the order of positive electrode sheet, separator, electrolyte, and negative electrode sheet. The sodium metal sheet was used as the negative electrode, and glass fiber GF / D was used as the separator. A commercial sodium-ion electrolyte with a main component of 1M NaClO4 / (EC:DEC = 1:1, v:v) was selected as the electrolyte. The assembled coin battery was connected to a Neware battery test system, and the charge-discharge test of the electrochemical performance of the material was carried out in the range of 2 - 4.2 V.

[0117] Figure 8 Figure showing the long-term cycling performance of a sodium-ion battery prepared using the sodium iron pyrophosphate phosphate positive electrode material with a thin-layer uniform low-carbon coating of Example 1 at a current density of 20C; Figure 9 Figure showing the long-term cycling performance of a sodium-ion battery prepared using the carbon-coated sodium iron pyrophosphate phosphate positive electrode material of Comparative Example 1 at a current density of 20C; Figure 10 Figure showing the rate performance of a sodium-ion battery prepared using the sodium iron pyrophosphate phosphate positive electrode material with a thin-layer uniform low-carbon coating of Example 1 at different current densities; Figure 11 Figure showing the rate performance of a sodium-ion battery prepared using the carbon-coated sodium iron pyrophosphate phosphate positive electrode material of Comparative Example 1 at different current densities. It can be seen from Figures 8 - 11 that the electrochemical performance of the sodium iron pyrophosphate phosphate positive electrode material with a thin-layer uniform low-carbon coating obtained in Example 1 is significantly better than that of the carbon-coated sodium iron pyrophosphate phosphate positive electrode material obtained in Comparative Example 1. Thus, it can be seen that in the present invention, an organic iron compound is used as both a carbon source and an iron source, and a sodium iron pyrophosphate phosphate positive electrode material with a thin-layer uniform low-carbon coating having a secondary spherical particle structure is prepared by spray drying and post-sintering in-situ thermal decomposition, which can effectively improve the cycle stability and capacity retention rate of the battery, proving the superiority of using an organic iron compound as both a carbon source and an iron source, and can effectively enhance the conductivity of the material.

[0118] The initial specific capacity and capacity retention rate after charge-discharge cycling of a sodium-ion battery prepared using the positive electrode material of Example 1 and a sodium-ion battery prepared using the positive electrode material of Comparative Example 1 at different current densities are shown in Table 2.

[0119] Table 2

[0120]

[0121] Note: It can be seen from the comparison of the initial specific capacity that the electrochemical performance of Comparative Example 2 and Comparative Example 3 is significantly worse than that of Example 1 and Comparative Example 1, so the cycle performance tests were not carried out on Comparative Example 2 and Comparative Example 3.

[0122] The initial specific capacity of the sodium-ion battery prepared using the cathode materials of Example 1 and Examples 4-5 at different current densities is shown in Table 3.

[0123] Table 3

[0124]

[0125] As can be seen from Table 3, sintering in a reducing atmosphere first and then in an inert atmosphere is beneficial to improving the electrochemical performance of the material, especially improving the electrochemical performance of the material at high rates (such as 200 C).

[0126] The above-described embodiments are only descriptions of the preferred embodiments of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A thin layer of uniform low carbon coated sodium iron pyrophosphate positive electrode material, characterized in that: The thin layer uniform low carbon coated sodium iron phosphate pyrophosphate positive electrode material is a secondary spherical particle structure formed by stacking primary core-shell spherical particles; the primary core-shell spherical particles have a carbon layer as a shell layer and sodium iron phosphate pyrophosphate as a core; the particle size of the primary core-shell spherical particles is 200-300nm; the particle size of the secondary spherical particles is 2-5μm; the carbon layer thickness of the primary core-shell spherical particles is 2-4nm; The carbon in the carbon layer accounts for more than 90wt% of the total carbon, hydrogen and oxygen content in the carbon layer; the carbon content of the thin layer uniformly low-carbon coated sodium iron pyrophosphate positive electrode material is 1.0-2.5wt%; The method for preparing the thin layer uniform low-carbon coated sodium iron pyrophosphate positive electrode material comprises the following steps: Mixing a sodium phosphorus source, an iron carbon source and a solvent to obtain a precursor solution; spray drying the precursor solution to obtain a precursor; sintering the precursor to obtain the thin layer of uniform low-carbon coated sodium iron pyrophosphate positive electrode material; The iron-carbon source includes an organic iron compound; The sintering is carried out in two stages, specifically including: first sintering in a reducing atmosphere and then sintering in an inert atmosphere; the temperature of the two stages is the same, both 450-600°C, the sintering time of the two stages is the same, and the total sintering time of the two stages is 4-20h.

2. The thin layer uniform low carbon coated sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: The specific surface area of ​​the thin layer uniformly coated with low carbon content sodium iron pyrophosphate positive electrode material is 10-20m 2 / g.

3. The thin layer uniform low carbon coated sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: The sodium-phosphorus source includes sodium dihydrogen phosphate; and the solvent includes water and / or anhydrous ethanol.

4. The thin layer uniform low carbon coated sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: The organic iron compound includes ferric acetylacetonate, ferrocene or ferric citrate.

5. The thin layer uniform low carbon coated sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: The molar ratio of the sodium phosphorus source to the iron carbon source is 4:2.95-3; And / or, the spray drying conditions include: air inlet temperature of 110-180°C, nozzle diameter of 0.5-2.5 mm, feed flow rate of 300-500 mL·h -1 .

6. Use of the thin layer uniform low carbon content coated sodium iron pyrophosphate positive electrode material as described in any one of claims 1 to 5 in the preparation of sodium ion batteries.

7. A sodium ion battery positive electrode, characterized in that: The raw material of the sodium ion battery positive electrode includes the thin layer uniform low carbon content coated sodium iron pyrophosphate positive electrode material according to any one of claims 1 to 5.

8. A sodium ion battery, characterized in that: The sodium ion battery uses the sodium ion battery positive electrode according to claim 7 as a positive electrode.

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