Carbon-coated iron-based polyanionic phosphate, its preparation method and application

Abstract

The present invention provides a carbon-coated iron-based polyanionic phosphate, a preparation method thereof and an application thereof. The preparation method includes adding an iron source, a carbon source, a sodium source and a phosphorus source into water to obtain a mixed solution; drying the mixed solution to obtain a precursor, and under an inert atmosphere, performing three-stage heat treatment on the precursor and then cooling to obtain the carbon-coated iron-based polyanionic phosphate, wherein the average rate of the cooling process is 0.8-1.2 °C / min; the molecular formula of the iron-based polyanionic phosphate in the obtained carbon-coated iron-based polyanionic phosphate is Na x Fe3(PO4)2P y‑2 O7, where 4.0 ≤ x ≤ 4.3 and 3.9 ≤ y ≤ 4.1. The preparation method provided by the present invention has an efficient and simple synthesis process, and can prepare a target product with stable structure and superior electrochemical performance only through the refinement and mixing of materials, drying and three-stage heat treatment.

Description

Technical Field

[0001] The present invention relates to a sodium ion battery, and in particular to a carbon-coated iron-based polyanionic phosphate, a preparation method thereof, and an application thereof. Background Art

[0002] As one of the energy storage systems with excellent performance and mature technology, lithium ion batteries have developed rapidly in many fields. With the widespread application of lithium ion batteries and the shortage and uneven distribution of lithium ore resources, the price of lithium carbonate has increased significantly. Metal sodium and metal lithium have similar electrochemical properties, are rich in resources, widely distributed globally, and inexpensive. At the same time, sodium ion batteries have a working mechanism similar to that of lithium ion batteries. Therefore, as a new type of secondary battery system, sodium ion batteries are one of the ideal supplementary solutions for lithium ion batteries in the energy storage field.

[0003] Iron-based phosphate Na4Fe3(PO4)2P2O7 is a cathode material for sodium ion batteries with a polyanionic structure, belonging to the orthorhombic system. In the crystal structure, PO4 tetrahedrons and FeO6 octahedrons form double chains by sharing vertices, and the layers are bridged by P-O-P bonds of pyrophosphate P2O7 4- to form a layered structure, forming ion diffusion channels extending along the

[010] and

[001] directions. Due to the stable structure of pyrophosphate P2O7 4- which is not prone to thermal decomposition and oxygen evolution reaction at 600 °C, the preparation method of the Na4Fe3(PO4)2P2O7 cathode material is simple and the crystal structure is stable. At the same time, the electronegativity of pyrophosphate P2O7 4- is slightly higher than that of phosphate, showing a relatively high redox potential, making the average working voltage of the composite phosphate Na4Fe3(PO4)2P2O7 cathode material about 3.1 V (vs. Na + / Na). The theoretical reversible specific capacity of this cathode material is 129 mAh g -1 . It has excellent electrochemical sodium storage performance, low raw material cost, safety and environmental protection, etc., and is a cathode material for sodium ion batteries with great development potential. However, the actual specific capacity of the Na4Fe3(PO4)2P2O7 cathode material is much lower than the theoretical specific capacity, and the cycle stability is not very ideal. The discharge medium voltage decays significantly with the increase in the number of cycles. In order to improve the electrochemical stability of the Na4Fe3(PO4)2P2O7 material, it is necessary to not only ensure that the specific capacity does not decay, but also ensure the stability of the discharge medium voltage.

[0004] Based on this, how to overcome the defects of poor electronic conductivity and poor electrochemical stability of the composite phosphate Na4Fe3(PO4)2P2O7 is an urgent problem to be solved in this field. Summary of the Invention

[0005] The main object of the present invention is to provide a carbon-coated iron-based polyanionic phosphate, a preparation method thereof, and an application thereof, so as to solve the problems of poor electronic conductivity and poor electrochemical stability of the existing iron-based phosphate cathode materials for sodium ion batteries.

[0006] To achieve the above object, on the one hand, the present invention provides a preparation method of a carbon-coated iron-based polyanionic phosphate, comprising the following steps: Step S1, adding an iron source, a carbon source, a sodium source, and a phosphorus source into water to obtain a mixed solution; Step S2, performing a drying treatment on the mixed solution to obtain a precursor; Step S3, under an inert atmosphere, performing three-stage heat treatment on the precursor and then cooling, to obtain a carbon-coated iron-based polyanionic phosphate, and the average rate of the cooling process is 0.8-1.2 °C / min; the molecular formula of the iron-based polyanionic phosphate in the carbon-coated iron-based polyanionic phosphate is Na x Fe3(PO4)2P y-2 O7, where 4.0 ≤ x ≤ 4.3 and 3.9 ≤ y ≤ 4.1.

[0007] Further, the three-stage heat treatment in Step S3 includes: the first-stage heat treatment, with a temperature of 150-250 °C and a heat preservation time of 2-5 h; the second-stage heat treatment, with a temperature of 300-400 °C and a heat preservation time of 2-5 h; the third-stage heat treatment, with a temperature of 500-700 °C and a heat preservation time of 10-20 h.

[0008] Furthermore, the heating rate of the first-stage heat treatment is 1-2 °C / min; the heating rate of the second-stage heat treatment is 2-5 °C / min; the heating rate of the third-stage heat treatment is 5-10 °C / min; preferably, the ratio of the heating rate between the second-stage heat treatment and the first-stage heat treatment is (1.5-3):1; preferably, the ratio of the heating rate between the third-stage heat treatment and the second-stage heat treatment is (1.5-3):1.

[0009] Further, the carbon source is glucose, and the three-stage heat treatment is: rising to 180 °C at a heating rate of 1 °C / min and keeping warm for 2 h, rising to 300 °C at a heating rate of 2 °C / min and keeping warm for 5 h, rising to 600 °C at a heating rate of 10 °C / min and keeping warm for 10 h; or, the carbon source is citric acid, and the three-stage heat treatment is: rising to 170 °C at a heating rate of 1 °C / min and keeping warm for 2 h, rising to 280 °C at a heating rate of 2 °C / min and keeping warm for 5 h, rising to 560 °C at a heating rate of 10 °C / min and keeping warm for 10 h; or, the carbon source is soluble starch, and the three-stage heat treatment is: rising to 200 °C at a heating rate of 1 °C / min and keeping warm for 2 h, rising to 320 °C at a heating rate of 2 °C / min and keeping warm for 5 h, rising to 640 °C at a heating rate of 10 °C / min and keeping warm for 10 h.

[0010] Further, in step S1, the particle size D50 of the iron source is 50 - 200 nm.

[0011] Further, in the mixed solution, the molar ratio of the carbon source to the sum of the sodium source, the phosphorus source, and the iron source is 1:(6.8 - 7.1), and the mass concentrations of the sodium source and the phosphorus source in the mixed solution are independently selected from 0 - 7.5 wt% and are both not 0; preferably, the water content in the mixed solution is 60 - 85 wt%.

[0012] Another aspect of the present invention provides a carbon-coated iron-based polyanionic phosphate, which is prepared by the above preparation method.

[0013] Further, in the X-ray diffraction pattern of the carbon-coated iron-based polyanionic phosphate, the full width at half maximum of the main peak is 0.190 - 0.210 degrees, and the peak-to-background ratio is (10 - 8):1.

[0014] Further, the morphology of the carbon-coated iron-based polyanionic phosphate is an aggregate of nanoparticles covered with a carbon layer. The size of the nanoparticles is 100 - 150 nm, the size of the aggregate is 1 - 10 μm, and the thickness of the carbon layer is 5 - 10 nm; preferably, the carbon content of the carbon-coated iron-based polyanionic phosphate is 3 - 5 wt%.

[0015] Another aspect of the present invention provides a sodium-ion battery, including a positive electrode sheet, and the positive electrode sheet includes the above carbon-coated iron-based polyanionic phosphate.

[0016] Applying the technical solution of the present invention, by reasonably designing the heat treatment process in the preparation process of the carbon-coated iron-based polyanionic phosphate and controlling the average cooling rate when the atmosphere furnace cools after the high-temperature solid-phase reaction ends, a carbon-coated iron-based polyanionic phosphate with a uniform carbon coating layer, high phase purity, high crystallinity, and a micro-nano tissue morphology can be obtained. Using this material as the positive electrode material in a sodium-ion battery can maintain the stability of the crystal structure during the sodium ion insertion and extraction process, and thus effectively improve the discharge medium voltage value and discharge medium voltage stability of the sodium ion positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The specification drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0018] Figure 1 is the X-ray diffraction pattern of Example 1 of the present invention;

[0019] Figure 2 is the scanning electron microscope and transmission electron microscope images of Example 1 of the present invention;

[0020] Figure 3 is the charge-discharge curve of Embodiment 1 of the present invention;

[0021] Figure 4 is the relationship curve between the discharge medium voltage and the number of cycles of Embodiment 1 of the present invention;

[0022] Figure 5 is the X-ray diffraction pattern of Embodiment 2 of the present invention;

[0023] Figure 6 is the scanning electron microscope and transmission electron microscope images of Embodiment 2 of the present invention;

[0024] Figure 7 is the charge-discharge curve of Embodiment 2 of the present invention;

[0025] Figure 8 is the relationship curve between the discharge medium voltage and the number of cycles of Embodiment 2 of the present invention;

[0026] Figure 9 is the X-ray diffraction pattern of Comparative Example 1 of the present invention;

[0027] Figure 10 is the scanning electron microscope and transmission electron microscope images of Comparative Example 1 of the present invention;

[0028] Figure 11 is the charge-discharge curve of Comparative Example 1 of the present invention;

[0029] Figure 12 is the relationship curve between the discharge medium voltage and the number of cycles of Comparative Example 1 of the present invention. Detailed Description of the Invention

[0030] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0031] As described in the background art, there is a problem in the prior art that the electrochemical stability of the cathode material of the iron-based phosphate sodium ion battery is poor. To solve the above technical problem, the present application provides a preparation method of carbon-coated iron-based polyanionic phosphate, comprising the following steps: Step S1, adding an iron source, a carbon source, a sodium source and a phosphorus source into water to obtain a mixed solution; Step S2, drying the mixed solution to obtain a precursor; Step S3, under an inert atmosphere, performing three-stage heat treatment on the precursor and then cooling to obtain carbon-coated iron-based polyanionic phosphate, and the average rate of the cooling process is 0.8-1.2 °C / min; the formula of the iron-based polyanionic phosphate in the carbon-coated iron-based polyanionic phosphate is Na x Fe3(PO4)2P y-2O7, where 4.0 ≤ x ≤ 4.3 and 3.9 ≤ y ≤ 4.1.

[0032] The preparation method of carbon-coated iron-based polyanionic phosphate by spray drying and heat treatment provided by the present invention designs the heat treatment process in three stages and controls the average cooling rate during the cooling of the atmosphere furnace to be 0.8 - 1.2 °C / min to obtain carbon-coated iron-based polyanionic phosphate with stable structure. First, the preparation method of the present invention performs carbon coating treatment on the iron-based polyanionic phosphate to improve its electronic conductivity, thereby enhancing its electrochemical performance. Second, it should be emphasized that the present invention improves the battery performance by controlling the crystallinity. Specifically: the present invention effectively improves the crystallinity of the obtained carbon-coated iron-based polyanionic phosphate by precisely controlling the cooling rate after heat treatment, thereby reducing the non-uniform lattice volume strain caused by the relatively large sodium ion radius during the insertion and extraction process, and ultimately improving the cycle stability of the carbon-coated iron-based polyanionic phosphate during the electrochemical process. At the same time, the present invention controls the ratio of each component in the composite phosphate, so that the iron-based polyanionic phosphate in the obtained carbon-coated iron-based polyanionic phosphate satisfies the chemical formula of Na x Fe3(PO4)2P y-2 O7 (4.0 ≤ x ≤ 4.3, 3.9 ≤ y ≤ 4.1), thereby further enhancing its electrochemical cycle stability. In addition, the synthesis process of the preparation method provided by the present invention is efficient and simple, and the target product can be prepared only through three steps of material refinement and mixing, drying, and heat treatment. No industrial waste water or waste gas is generated during the whole preparation process, and the environment is not polluted, which is green and environmentally friendly.

[0033] In order to adapt to the preparation method of the carbon-coated iron-based polyanionic phosphate provided by the present invention to obtain a composite material with high phase purity and crystallinity, so as to further improve the performance of the cathode material, in a typical embodiment, the three-stage heat treatment in step S3 includes: the first-stage heat treatment, the temperature is 150 - 250 °C, and the heat preservation time is 2 - 5 h; the second-stage heat treatment, the temperature is 300 - 400 °C, and the heat preservation time is 2 - 5 h; the third-stage heat treatment, the temperature is 500 - 700 °C, and the heat preservation time is 10 - 20 h.

[0034] Among them, the reaction occurring during the first-stage heat treatment at a temperature of 150 to 250 °C is the low-temperature solid-phase reaction of the precursor. Within this temperature range, the carbon source can be better pre-carbonized, which is conducive to forming a uniform carbon coating layer during the subsequent reaction process, thereby improving its electrochemical stability. The reaction occurring during the second-stage heat treatment at a temperature of 300 to 400 °C is the medium-temperature solid-phase reaction. This temperature range enables the solid-phase reactions involved in this stage to proceed more stably and completely, which is conducive to forming a complete and stable microstructure. The reaction occurring during the third-stage heat treatment at a temperature of 500 to 700 °C is the high-temperature solid-phase reaction. At this time, the material morphology transforms into a more stable spherical shape. Insulating for 10 to 20 h under this condition, the long-time insulation within this range can further improve the crystallinity and structural stability of the carbon coating, thereby enhancing the medium-voltage stability of the finally obtained sodium-ion battery, which has a better promoting effect on improving the comprehensive battery performance such as the battery cycle performance.

[0035] Furthermore, the heating rate of the first-stage heat treatment is 1 to 2 °C / min; the heating rate of the second-stage heat treatment is 2 to 5 °C / min; the heating rate of the third-stage heat treatment is 5 to 10 °C / min. The heating rates of the three-stage heat treatments are respectively set within the above ranges to better adapt to the states and changes of the materials corresponding to the three-stage heat treatment processes, so as to obtain a carbon-coated iron-based polyanionic phosphate material with excellent phase purity and crystallinity. Specifically, in the first-stage heat treatment, a heating rate of 1 to 2 °C / min is selected. Compared with a heating rate above 2 °C / min, the carbonization of the carbon source conducive to forming the carbon coating layer can proceed more completely, and at the same time, effectively prevent the iron ions in the precursor from being oxidized, which is more conducive to obtaining a better structure; in the second-stage heat treatment, a heating rate of 2 to 5 °C / min is selected. Compared with a heating rate above 5 °C / min, it is more conducive to avoiding segregation or lattice distortion caused by too high a temperature during the crystal formation process; in the third-stage heat treatment, a heating rate of 5 to 10 °C / min is selected. Compared with a heating rate below 5 °C / min, this heating rate can reduce heat diffusion and reach the temperature required for the high-temperature solid-phase reaction faster, thereby improving the phase purity and structural uniformity.

[0036] On this basis, in order to improve the coordination between the three-stage heat treatment processes, the inventor preferably selects the ratio of the heating rate between the second-stage heat treatment and the first-stage heat treatment to be (1.5 - 3):1 through a large number of experiments; preferably, the ratio of the heating rate between the third-stage heat treatment and the second-stage heat treatment is (1.5 - 3):1, so as to more greatly eliminate the internal stress generated by the solid-phase reaction of the material during the previous heat treatment stage in the current heat treatment stage, and at the same time enable the reactions involved in the entire heat treatment process to proceed stably and completely, thereby obtaining a carbon-coated iron-based polyanionic phosphate material with a more stable and uniform structure.

[0037] In several preferred embodiments, the inventor selects different three-stage heat treatment process parameters through a large number of experiments and creative designs for different carbon sources. Exemplarily: when the carbon source is glucose, the three-stage heat treatment is: heating to 180°C at a heating rate of 1°C / min and holding for 2 h, heating to 300°C at a heating rate of 2°C / min and holding for 5 h, heating to 600°C at a heating rate of 10°C / min and holding for 10 h; or, when the carbon source is citric acid, the three-stage heat treatment is: heating to 170°C at a heating rate of 1°C / min and holding for 2 h, heating to 280°C at a heating rate of 2°C / min and holding for 5 h, heating to 560°C at a heating rate of 10°C / min and holding for 10 h; or, when the carbon source is soluble starch, the three-stage heat treatment is: heating to 200°C at a heating rate of 1°C / min and holding for 2 h, heating to 320°C at a heating rate of 2°C / min and holding for 5 h, heating to 640°C at a heating rate of 10°C / min and holding for 10 h. That is, for different carbon sources, further design and adjustment are carried out on the basis of the three-stage heat treatment, so as to form a more complete scheme to obtain a carbon-coated iron-based polyanionic phosphate material with high crystallinity, stable structure and electrochemical performance when different carbon sources are selected for carbon coating.

[0038] Furthermore, in order to improve the mixing uniformity of each component in the mixed solution, the particle size D50 of the iron source is selected to be 50 - 200 nm in step S1, so that the iron source component can be dispersed in the mixed system faster, which is beneficial to improving the dispersion uniformity of each element in the whole system, and further improving the structural integrity and uniformity of the precursor, providing a prerequisite for subsequent heat treatment crystallization.

[0039] During the synthesis process of the preparation method provided by the present invention, the particle size D50 of the iron source is achieved to be 50 - 200 nm by ball milling. Specifically, a ball mill is used to refine FePO4·2H2O particles. The ball milling rate is 400 revolutions per minute, the ball-to-material ratio is 4:1, and the ball milling time is 16 h. After ball milling, in order to further improve the dispersion uniformity of each component so as to obtain a more complete, continuous and consistent precursor, the preparation method provided by the present invention further includes transferring the above-mentioned slurry to a beaker before drying and dispersing it with a high-speed emulsifying shear machine for 4 h to obtain a uniform off-white opaque suspension slurry.

[0040] In step S2, the drying is achieved by spray drying and / or flash drying, preferably spray drying, to quickly dry the suspension slurry, instantaneously evaporate the moisture, and form a powdery dry precursor; more preferably, the inlet air temperature of the spray drying is 150 - 240 °C, and the outlet air temperature is 90 - 120 °C.

[0041] In a typical embodiment, the molar ratio of the carbon source to the sum of the sodium source, phosphorus source, and iron source in the mixed solution is 1:(6.8 - 7.1), so as to facilitate the smooth progress of carbon coating, so that both the coating amount and coating thickness can better match the structure, size, and morphology of the composite phosphate particles. Since Na2CO3 as the sodium source and NaH2PO4·2H2O as the phosphorus source will both volatilize during the high-temperature sintering process, in order to obtain a sodium-rich carbon-coated Na x Fe3(PO4)2P y- 2O7(4.0 ≤ x ≤ 4.3, 3.9 ≤ y ≤ 4.1) material, make the two appropriately excessive. Preferably, the mass concentration of the sodium source and the phosphorus source in the mixed solution are each independently selected from 0 - 7.5 wt% and are not 0. And in a preferred embodiment, the water content in the mixed solution is set to 60 - 85 wt%, so that each component in the mixed solution system can be evenly dispersed, reducing the possibility of agglomeration, and at the same time being able to form a precursor at a faster reaction rate, shortening the experimental period.

[0042] On the other hand, the present invention provides a carbon-coated iron-based polyanionic phosphate, which is prepared by the above-mentioned preparation method. The obtained carbon-coated iron-based polyanionic phosphate has a uniform carbon coating layer, high phase purity and crystallinity, and a micro-nano tissue morphology, and can maintain the stability of the crystal structure during the sodium ion insertion / extraction process.

[0043] In several typical embodiments, the full width at half maximum of the main peak in the X-ray diffraction pattern of the carbon-coated iron-based polyanionic phosphate is 0.190 - 0.210 degrees, and the peak-to-background ratio is (10 - 8):1. That is, the obtained carbon-coated iron-based polyanionic phosphate product has high phase purity and crystallinity, and there are no impurity phases such as NaFePO4 and Na2FeP2O7. Therefore, it can effectively suppress the lattice volume strain of the Na4Fe3(PO4)2P2O7 bulk phase caused by the insertion and extraction of sodium ions during charge and discharge, and finally exhibit higher electrical cycle stability.

[0044] Furthermore, the morphology of the carbon-coated iron-based polyanionic phosphate is an aggregate of nanoparticles covered with a carbon layer. The size of the nanoparticles is 100 - 150 nm, and the size of the aggregate is 1 - 10 μm. Since the radius of sodium ions is relatively large, the diffusion kinetics in the composite phosphate material is relatively slow. However, the nanosized material with the size within the above range provided by the present invention can effectively shorten the ion diffusion path, thereby improving its diffusion kinetics and enhancing the electrochemical performance. Regarding the carbon coating layer, preferably, its thickness is 5 - 10 nm, so as to form a more complete carbon coating layer without excessively increasing the resistance of sodium ion transmission. On this basis, it is further preferred that the carbon content of the carbon-coated iron-based polyanionic phosphate is 3 - 5 wt%, so as to protect the proportion of the active substance of the iron-based polyanionic phosphate in the composite material to a greater extent, thereby enhancing its electrochemical performance.

[0045] Another aspect of the present invention provides a sodium ion battery, including a positive electrode sheet, and the positive electrode sheet includes the above-mentioned carbon-coated iron-based polyanionic phosphate. When the electrode sheet prepared from the carbon-coated iron-based polyanionic phosphate is used as the positive electrode of a sodium ion battery and charged and discharged at a rate of 0.5C, the discharge medium voltage is 3.05V, and the discharge medium voltage hardly decays after 200 charge-discharge cycles, showing excellent electrical cycle stability.

[0046] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0047] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0048] Example 1

[0049] Preparation of a carbon-coated iron-based polyanionic phosphate positive electrode material:

[0050] NaH2PO4·2H2O, Na2CO3, glucose, and FePO4·2H2O were added to deionized water at a molar ratio of 1.09:1.505:0.8:3.0, where the amount of deionized water accounted for 70 wt% of the total solution mass. The FePO4·2H2O particles were refined by ball milling using a ball mill with a ball milling rate of 400 revolutions per minute, a ball-to-material ratio of 4:1, and a ball milling time of 16 h. After the ball milling was completed, the above slurry was transferred to a beaker and then dispersed with a high-speed emulsifying shear machine for 4 h to obtain a uniform off-white opaque suspension slurry. At this time, the particle size D50 of the FePO4·2H2O particles was 50 - 100 nm. The water in the above opaque suspension slurry was instantaneously evaporated by a spray dryer with an inlet air temperature of 240 °C and an outlet air temperature of 120 °C to obtain a powdery dry light yellow precursor. The precursor was heated to 180 °C at a heating rate of 1 °C / min in a nitrogen atmosphere furnace and held for 2 h; then heated to 300 °C at a heating rate of 2 °C / min and held for 5 h; then heated to 600 °C at a heating rate of 10 °C / min and held for 10 h; subsequently, the average cooling rate of the atmosphere furnace was controlled to cool at an average cooling rate of 1 °C / min. After cooling to room temperature, a carbon-coated Na4Fe3(PO4)2P2O7 cathode material was obtained.

[0051] The X-ray diffraction pattern of the obtained carbon-coated Na4Fe3(PO4)2P2O7 cathode material is as Figure 1 shown. It can be seen from the figure that there are no impurity phases such as NaFePO4 and Na2FeP2O7 in the product, and both the phase purity and crystallinity are very high. The full width at half maximum of the main peak is 0.196 degrees, and the peak-to-background ratio is about 10.0:1. The scanning electron microscope and transmission electron microscope images of the obtained carbon-coated Na4Fe3(PO4)2P2O7 cathode material are as Figure 2 shown. It can be seen that the carbon-coated Na4Fe3(PO4)2P2O7 cathode material is composed of irregularly shaped nanoparticles with a size of 100 - 150 nm aggregated into spherical particles with a size of about 1 - 10 μm, forming a micro-nano structure. The nanoparticles adhere to each other, but the interfaces are clear, and its surface is uniformly coated with a carbon layer with a thickness of about 5 - 10 nm. The carbon content was tested by a carbon-sulfur analyzer to be 3.9 wt%.

[0052] Example 2

[0053] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0054] The differences between this embodiment and Embodiment 1 are as follows: The carbon source glucose is replaced with citric acid, and at the same time, the molar ratio of NaH2PO4·2H2O:Na2CO3:citric acid:FePO4·2H2O is changed to 1.09:1.455:1.8:3.0. Also, to prevent the corrosion of the ball mill by citric acid, citric acid is added to the solution before high-speed emulsification and shearing. Specifically:

[0055] First, NaH2PO4·2H2O:Na2CO3:FePO4·2H2O are added to deionized water according to the molar ratio of 1.09:1.455:3.0, where the amount of deionized water accounts for 75 wt% of the total mass of the solution. The FePO4·2H2O particles are refined by ball milling using a ball mill, with a ball milling rate of 400 revolutions per minute, a ball-to-material ratio of 4:1, and a ball milling time of 16 h. After the ball milling is completed, the above slurry is transferred to a beaker, and then a certain amount of citric acid is added to the beaker according to the molar ratio, and it is dispersed with a high-speed emulsification shearer for 4 h to obtain a uniform off-white opaque suspension slurry. At this time, the particle size D50 of the FePO4·2H2O particles is 50 - 100 nm. The water in the above opaque suspension slurry is instantaneously evaporated by a spray dryer, with an inlet air temperature of 240 °C and an outlet air temperature of 120 °C, to obtain a powdery and dry light yellow precursor. The precursor is heated to 170 °C at a heating rate of 1 °C / min in a nitrogen atmosphere furnace and held for 2 h; then heated to 280 °C at a heating rate of 2 °C / min and held for 5 h; then heated to 560 °C at a heating rate of 10 °C / min and held for 10 h; subsequently, the average cooling rate of the atmosphere furnace is controlled to cool it at an average cooling rate of 1 °C / min. After cooling to room temperature, a carbon-coated Na4Fe3(PO4)2P2O7 cathode material is obtained.

[0056] The X-ray diffraction pattern of the obtained carbon-coated Na4Fe3(PO4)2P2O7 cathode material is shown in Figure 5 It can be seen that there are no impurity phases such as NaFePO4 and Na2FeP2O7 in the product, and both the phase purity and crystallinity are relatively high. The full width at half maximum of the main peak is 0.204 degrees, and the peak-to-background ratio is about 9.0:1. The scanning electron microscope and transmission electron microscope images of the obtained carbon-coated Na4Fe3(PO4)2P2O7 cathode material are as shown in Figure 6 It can be seen that the carbon-coated Na4Fe3(PO4)2P2O7 cathode material is composed of irregularly shaped nanoparticles with a size of 100 - 150 nm aggregated into micron-sized particles of about 1 - 10 μm, forming a micro-nano structure. The nanoparticles adhere to each other, with a blurred interface, and a carbon layer with a thickness of about 10 - 20 nm is coated on its surface. The carbon content measured by a carbon-sulfur analyzer is 4.3 wt%.

[0057] Embodiment 3

[0058] Preparation of carbon-coated iron-based polyanionic phosphate cathode material:

[0059] The difference between this example and Example 1 is that the carbon source is soluble starch, and the relative three-stage heat treatment process is as follows: The precursor is heated to 200 °C at a heating rate of 1 °C / min in a nitrogen atmosphere furnace and held for 2 h; then heated to 320 °C at a heating rate of 2 °C / min and held for 5 h; then heated to 640 °C at a heating rate of 10 °C / min and held for 10 h; subsequently, the average cooling rate of the atmosphere furnace is controlled to cool at an average cooling rate of 1 °C / min.

[0060] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.192 degrees, and the peak-to-background ratio is about 8.4:1.

[0061] Example 4

[0062] Preparation of carbon-coated iron-based polyanionic phosphate cathode material:

[0063] The difference between this example and Example 1 is that the three-stage heat treatment process is different. Specifically, the precursor is heated to 150 °C at a heating rate of 1 °C / min in a nitrogen atmosphere furnace and held for 5 h; then heated to 300 °C at a heating rate of 2 °C / min and held for 5 h; then heated to 500 °C at a heating rate of 10 °C / min and held for 20 h; subsequently, the average cooling rate of the atmosphere furnace is controlled to cool at an average cooling rate of 1 °C / min.

[0064] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.191 degrees, and the peak-to-background ratio is about 8.5:1.

[0065] Example 5

[0066] Preparation of carbon-coated iron-based polyanionic phosphate cathode material:

[0067] The difference between this example and Example 1 is that the three-stage heat treatment process is different. Specifically, the precursor is heated to 250 °C at a heating rate of 1 °C / min in a nitrogen atmosphere furnace and held for 2 h; then heated to 400 °C at a heating rate of 2 °C / min and held for 2 h; then heated to 700 °C at a heating rate of 10 °C / min and held for 10 h; subsequently, the average cooling rate of the atmosphere furnace is controlled to cool at an average cooling rate of 1 °C / min.

[0068] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.194 degrees, and the peak-to-background ratio is about 8.3:1.

[0069] Example 6

[0070] Preparation of carbon-coated iron-based polyanionic phosphate cathode material:

[0071] The differences between this example and Example 1 are as follows: the three-stage heat treatment process is different. Specifically, the precursor is heated to 120 °C at a heating rate of 1 °C / min in a nitrogen atmosphere furnace and held for 8 h; then heated to 280 °C at a heating rate of 2 °C / min and held for 8 h; then heated to 450 °C at a heating rate of 10 °C / min and held for 24 h; subsequently, the average cooling rate of the atmosphere furnace is controlled to cool at an average cooling rate of 1 °C / min.

[0072] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.228 degrees, and the peak-to-background ratio is approximately 7.3:1.

[0073] Example 7

[0074] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0075] The differences between this example and Example 1 are as follows: the three-stage heat treatment process is different. Specifically, the precursor is heated to 270 °C at a heating rate of 1 °C / min in a nitrogen atmosphere furnace and held for 1 h; then heated to 420 °C at a heating rate of 2 °C / min and held for 1 h; then heated to 750 °C at a heating rate of 10 °C / min and held for 8 h; subsequently, the average cooling rate of the atmosphere furnace is controlled to cool at an average cooling rate of 1 °C / min.

[0076] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.213 degrees, and the peak-to-background ratio is approximately 8.4:1.

[0077] Example 8

[0078] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0079] The differences between this example and Example 1 are as follows: the heating rates of the three-stage heat treatment are all 2 °C / min.

[0080] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.212 degrees, and the peak-to-background ratio is approximately 7.0:1.

[0081] Example 9

[0082] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0083] The differences between this example and Example 1 are as follows: the heating rates of the three-stage heat treatment are all 5 °C / min.

[0084] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.241 degrees, and the peak-to-background ratio is approximately 7.3:1.

[0085] Example 10

[0086] Preparation of carbon-coated iron-based polyanionic phosphate cathode material:

[0087] The difference between this example and Example 1 is that the three-stage heat treatment process is different. Specifically, the precursor is heated to 180 °C at a heating rate of 1 °C / min in a nitrogen atmosphere furnace and held for 2 h; then heated to 300 °C at a heating rate of 5 °C / min and held for 5 h; then heated to 600 °C at a heating rate of 10 °C / min and held for 10 h; subsequently, the average cooling rate of the atmosphere furnace is controlled to cool at an average cooling rate of 10 °C / min.

[0088] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.195 degrees, and the peak-to-background ratio is about 8.2:1.

[0089] Example 11

[0090] Preparation of carbon-coated iron-based polyanionic phosphate cathode material:

[0091] The difference between this example and Example 1 is that the three-stage heat treatment process is different. Specifically, the precursor is heated to 180 °C at a heating rate of 3 °C / min in a nitrogen atmosphere furnace and held for 2 h; then heated to 300 °C at a heating rate of 3 °C / min and held for 5 h; then heated to 600 °C at a heating rate of 10 °C / min and held for 10 h; subsequently, the average cooling rate of the atmosphere furnace is controlled to cool at an average cooling rate of 10 °C / min.

[0092] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.196 degrees, and the peak-to-background ratio is about 8.2:1.

[0093] Example 12

[0094] Preparation of carbon-coated iron-based polyanionic phosphate cathode material:

[0095] The difference between this example and Example 1 is that the particle size D50 of FePO4·2H2O after ball milling and dispersion is <50 nm.

[0096] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.220 degrees, and the peak-to-background ratio is about 8.0:1.

[0097] Example 13

[0098] Preparation of carbon-coated iron-based polyanionic phosphate cathode material:

[0099] The difference between this example and Example 1 is that the particle size D50 of FePO4·2H2O after ball milling and dispersion is >200 nm.

[0100] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.218 degrees, and the peak-to-background ratio is about 7.4:1.

[0101] Example 14

[0102] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0103] The difference between this example and Example 1 is that the molar ratio of NaH2PO4·2H2O:Na2CO3:glucose:FePO4·2H2O is 1.09:1.505:1.12:3.0, that is, the molar ratio of the carbon source to the other three is 1:5.

[0104] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.234 degrees, and the peak-to-background ratio is about 7.6:1.

[0105] Example 15

[0106] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0107] The difference between this example and Example 1 is that the molar ratio of NaH2PO4·2H2O:Na2CO3:glucose:FePO4·2H2O is 1.09:1.505:0.62:3.0, that is, the molar ratio of the carbon source to the other three is 1:9.

[0108] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.270 degrees, and the peak-to-background ratio is about 7.9:1.

[0109] Comparative Example 1

[0110] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0111] The difference between this comparative example and Example 1 is that the temperature of the third heat treatment is 500 °C and the holding time is 5 h; and during the cooling process, the average cooling rate of the atmosphere furnace is not controlled and it is allowed to cool naturally. Specifically:

[0112] NaH2PO4·2H2O:Na2CO3:glucose:FePO4·2H2O were added to deionized water at a molar ratio of 1.09:1.505:0.8:3.0, where the amount of deionized water accounted for 70 wt% of the total mass of the solution. The FePO4·2H2O particles were refined by ball milling using a ball mill at a rate of 400 revolutions per minute, with a ball-to-material ratio of 4:1 and a ball milling time of 16 h. After the ball milling was completed, the above slurry was transferred to a beaker and then dispersed using a high-speed emulsifying shear machine for 4 h to obtain a uniform off-white opaque suspension slurry. The water in the above opaque suspension slurry was instantaneously evaporated using a spray dryer at an inlet air temperature of 240 °C and an outlet air temperature of 120 °C to obtain a powdery dry light yellow precursor. At this time, the particle size D50 of the FePO4·2H2O particles was 50 - 100 nm. The precursor was heated in a nitrogen atmosphere furnace at a heating rate of 1 °C / min to 180 °C and held for 2 h; then heated at a heating rate of 2 °C / min to 300 °C and held for 5 h; then heated at a heating rate of 10 °C / min to 500 °C and held for 5 h. The average cooling rate of the atmosphere furnace was not controlled and it was allowed to cool naturally. After cooling to room temperature, a carbon-coated Na4Fe3(PO4)2P2O7 cathode material was obtained.

[0113] The X-ray diffraction pattern of the obtained carbon-coated Na4Fe3(PO4)2P2O7 cathode material is shown in Figure 9 , indicating that there are no impurity phases such as NaFePO4 and Na2FeP2O7 in the product and the phase purity is high. However, the main peak at 2θ = 33.34 degrees is significantly broadened, with a full width at half maximum of 0.278 degrees and a peak-to-background ratio of approximately 6:1. The scanning electron microscope and transmission electron microscope images of the obtained carbon-coated Na4Fe3(PO4)2P2O7 cathode material are as shown in Figure 10 . It can be seen that the carbon-coated Na4Fe3(PO4)2P2O7 cathode material is composed of irregularly shaped nanoparticles with a size of 100 - 150 nm aggregated into spherical particles of about 2 - 10 μm, forming a micro-nano structure. The nanoparticles are adhered to each other, but the interfaces are clear, and a carbon layer with a thickness of about 5 - 10 nm is coated on the surface. The carbon content measured by a carbon-sulfur analyzer is 4.1 wt%.

[0114] Comparative Example 2

[0115] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0116] The difference between this comparative example and Example 1 is that the heat treatment process is a single stage. Specifically: the precursor was heated in a nitrogen atmosphere furnace at a heating rate of 10 °C / min to 600 °C and held for 10 h; then the average cooling rate of the atmosphere furnace was controlled and it was cooled at an average cooling rate of 1 °C / min.

[0117] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.275 degrees, and the peak-to-background ratio is approximately 6.0:1.

[0118] Comparative Example 3

[0119] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0120] The difference between this comparative example and Example 1 is that the heat treatment process is in two stages. Specifically: the precursor is heated to 300 °C at a heating rate of 2 °C / min in a nitrogen atmosphere furnace, held for 3 h, then heated to 500 °C at a heating rate of 2 °C / min, held for 10 h, and then the average cooling rate of the atmosphere furnace is controlled to cool at an average cooling rate of 1 °C / min.

[0121] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.271 degrees, and the peak-to-background ratio is approximately 5.8:1.

[0122] Comparative Example 4

[0123] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0124] The difference between this comparative example and Example 1 is that the heat treatment process is in two stages. Specifically: the precursor is heated to 300 °C at a heating rate of 1 °C / min in a nitrogen atmosphere, held for 5 h, then heated to 650 °C at a heating rate of 5 °C / min, held for 12 h, and then the average cooling rate of the atmosphere furnace is controlled to cool at an average cooling rate of 1 °C / min.

[0125] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.279 degrees, and the peak-to-background ratio is approximately 5.7:1.

[0126] Comparative Example 5

[0127] Preparation of a carbon-coated iron-based polyanionic phosphate cathode material:

[0128] The difference between this comparative example and Example 1 is that the component ratios of the synthesized precursor are different, that is, the molar ratios of the iron source, carbon source, sodium source, and phosphorus source are controlled so that in the finally obtained carbon-coated iron-based polyanionic phosphate, the molecular formula of the composite phosphate satisfies Na 3.5 Fe3(PO4)2P2O7.

[0129] In the XRD pattern of the obtained material, the full width at half maximum of the main peak is 0.265 degrees, and the peak-to-background ratio is approximately 6.2:1.

[0130] Preparation and performance testing of sodium-ion button batteries:

[0131] Weigh the carbon-coated Na4Fe3(PO4)2P2O7 cathode material, acetylene black, and PVDF according to a mass ratio of 85:10:5 and mix them evenly. Add a certain amount of N-methylpyrrolidone and stir evenly to obtain the electrode slurry. Coat the slurry evenly on the aluminum foil and then dry it in a vacuum drying oven at 120 °C for 12 h. Punch it into a circular positive electrode sheet with a diameter of 12 mm, and the coating mass is about 4 mg / cm 2 ; Using a sodium metal sheet as the negative electrode, 1 M NaPF6 in ethylene carbonate / dimethyl carbonate (volume ratio 1:1) as the electrolyte, and cellgard2035 as the separator, assemble it into a CR2025 type button battery in a glove box. Perform a constant current charge-discharge cycle test on the above button battery, with a current density of 0.1 C and 0.5 C (1 C = 129 mAh·g -1 ), and the voltage test range is 1.5 - 4.0 V (vs. Na / Na + ).

[0132] The electrochemical performance test results of each obtained example and comparative example are shown in Table 1. The table includes the initial discharge specific capacity, first-cycle Coulomb efficiency, second discharge specific capacity, and fifth discharge specific capacity during charge and discharge at a rate of 0.1 C, the initial discharge mid-voltage at a rate of 0.5 C, and the discharge mid-voltage after 200 charge-discharge cycles; and:

[0133] The charge-discharge curve of Example 1 is shown in Figure 3 , and the relationship curve between the discharge mid-voltage and the number of cycles is shown in Figure 4 ;

[0134] The charge-discharge curve of Example 2 is shown in Figure 7 , and the relationship curve between the discharge mid-voltage and the number of cycles is shown in Figure 8 ;

[0135] The charge-discharge curve of Comparative Example 1 is shown in Figure 11 , and the relationship curve between the discharge mid-voltage and the number of cycles is shown in Figure 12 .

[0136] Table 1

[0137]

[0138]

[0139] From the above description, it can be seen that: the purity of the Na4Fe3(PO4)2P2O7 phase in Example 1, Example 2, and Comparative Example 1 is very high, and there are no impurity phases such as NaFePO4 and Na2FeP2O7. However, the crystallinity of the Na4Fe3(PO4)2P2O7 phase in Comparative Example 1 without controlling the average cooling rate is not as high as that in Example 1 and Example 2. Its main peak has an obvious broadening phenomenon, and the peak-to-background ratio is also low. This deteriorated structural feature is manifested in the electrochemical stability, and the specific description is as follows.

[0140] The microstructural morphologies of the carbon-coated Na4Fe3(PO4)2P2O7 cathode materials in Example 1, Example 2, and Comparative Example 1 are all spherical particles with a size of 1-10 μm formed by the aggregation of irregular nanoparticles with a size of 100-150 nm, forming a micro-nano structure. The nanoparticles are adhered to each other, and a carbon layer with a thickness of about 5-20 nm is coated on their surface, and the carbon content is in the range of 3.9-4.2 wt%. Among them, the carbon layer coated in Example 1 is the most uniform, with a thickness of about 5-10 nm.

[0141] Example 1 is charged and discharged at a rate of 0.1C, and its first discharge specific capacity is 107 mAh·g -1 , the Coulomb efficiency of the first cycle is 93%. When charged and discharged at a rate of 0.5C, its discharge mid-voltage is 3.05V, and there is almost no attenuation after 200 charge-discharge cycles; Example 2 is charged and discharged at a rate of 0.1C, and its first discharge specific capacity is 105 mAh·g -1 , the Coulomb efficiency of the first cycle is 91.4%. When charged and discharged at a rate of 0.5C, its discharge mid-voltage is also 3.05V, and there is almost no attenuation after 200 charge-discharge cycles; while the comparative example is charged and discharged at a rate of 0.1C, and its first discharge specific capacity is 110 mAh·g -1 , the Coulomb efficiency of the first cycle is 113%. When charged and discharged at a rate of 0.5C, its discharge mid-voltage is 2.83V. However, after 200 charge-discharge cycles, the discharge mid-voltage drops to 2.58V, and the stability is poor.

[0142] In summary, the phase purity, microstructural morphology, and carbon content of the carbon-coated Na4Fe3(PO4)2P2O7 cathode materials obtained in each comparative example are almost the same. However, the high-temperature solid-phase reaction time and the average cooling rate during cooling will significantly affect the crystallinity of the Na4Fe3(PO4)2P2O7 phase, and improving the crystallinity of the Na4Fe3(PO4)2P2O7 phase can effectively improve the discharge mid-voltage stability and the discharge mid-voltage value of the Na4Fe3(PO4)2P2O7 cathode material.

[0143] It should be noted that the terms "first", "second", etc. in the description and claims of this application are used to distinguish similar objects and do not necessarily describe a specific order or sequence. It should be understood that such terms can be interchanged under appropriate circumstances so that the embodiments of the present application described herein can be implemented, for example, in an order other than those described herein.

[0144] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of carbon-coated iron-based polyanionic phosphate, characterized in that, It includes the following steps: Step S1: Add an iron source, a carbon source, a sodium source, and a phosphorus source into water to obtain a mixed solution; Step S2: Conduct a drying treatment on the mixed solution to obtain a precursor; Step S3: Under an inert atmosphere, conduct three-stage heat treatment on the precursor and then cool down to obtain carbon-coated iron-based polyanionic phosphate, and the average rate of the cooling process is 0.8 - 1.2 °C / min; The three-stage heat treatment includes: The first-stage heat treatment: the temperature is 150 - 250 °C, the heat preservation time is 2 - 5 h; the heating rate is 1 - 2 °C / min; The second-stage heat treatment: the temperature is 300 - 400 °C, the heat preservation time is 2 - 5 h; the heating rate is 2 - 5 °C / min; The third-stage heat treatment: the temperature is 500 - 700 °C, the heat preservation time is 10 - 20 h; the heating rate is 5 - 10 °C / min; The molecular formula of the iron-based polyanionic phosphate in the carbon-coated iron-based polyanionic phosphate is Na x Fe3(PO4)2P y-2 O7, where 4.0 ≤ x ≤ 4.3 and 3.9 ≤ y ≤ 4.

1.

2. The preparation method of the carbon-coated iron-based polyanionic phosphate according to claim 1, characterized in that, The ratio of the heating rate between the second-stage heat treatment and the first-stage heat treatment is (1.5 - 3):1; The ratio of the heating rate between the third-stage heat treatment and the second-stage heat treatment is (1.5 - 3):

1.

3. The preparation method of the carbon-coated iron-based polyanionic phosphate according to claim 1, wherein the carbon source is glucose, and the three-stage heat treatment is: raise the temperature to 180 °C at a heating rate of 1 °C / min and keep it warm for 2 h, raise the temperature to 300 °C at a heating rate of 2 °C / min and keep it warm for 5 h, raise the temperature to 600 °C at a heating rate of 10 °C / min and keep it warm for 10 h; or, the carbon source is citric acid, and the three-stage heat treatment is: raise the temperature to 170 °C at a heating rate of 1 °C / min and keep it warm for 2 h, raise the temperature to 280 °C at a heating rate of 2 °C / min and keep it warm for 5 h, raise the temperature to 560 °C at a heating rate of 10 °C / min and keep it warm for 10 h; or, the carbon source is soluble starch, and the three-stage heat treatment is: raise the temperature to 200 °C at a heating rate of 1 °C / min and keep it warm for 2 h, raise the temperature to 320 °C at a heating rate of 2 °C / min and keep it warm for 5 h, raise the temperature to 640 °C at a heating rate of 10 °C / min and keep it warm for 10 h.

4. The preparation method of the carbon-coated iron-based polyanionic phosphate according to any one of claims 1 to 3, characterized in that, In step S1, the particle size D50 of the iron source is 50 - 200 nm.

5. The preparation method of the carbon-coated iron-based polyanionic phosphate according to any one of claims 1 to 3, characterized in that, In the mixed solution, the molar ratio of the carbon source to the sum of the sodium source, the phosphorus source, and the iron source is 1:(6.8 - 7.1), and the mass concentrations of the sodium source and the phosphorus source in the mixed solution are independently selected from 0 - 7.5 wt% and are both not 0.

6. The preparation method of the carbon-coated iron-based polyanionic phosphate according to claim 5, characterized in that, The water content in the mixed solution is 60 - 85 wt%.

7. A carbon-coated iron-based polyanionic phosphate, characterized in that, The carbon-coated iron-based polyanionic phosphate is prepared by the preparation method according to any one of claims 1 to 6.

8. The carbon-coated iron-based polyanionic phosphate according to claim 7, wherein In the X-ray diffraction pattern of the carbon-coated iron-based polyanionic phosphate, the full width at half maximum of the main peak is 0.190 - 0.210 degrees, and the peak-to-background ratio is (10 - 8):

1.

9. The carbon-coated iron-based polyanionic phosphate according to claim 7, wherein, The morphology of the carbon-coated iron-based polyanionic phosphate is an aggregate of nanoparticles covered with a carbon layer. The size of the nanoparticles is 100 - 150 nm, the size of the aggregate is 1 - 10 μm, and the thickness of the carbon layer is 5 - 10 nm.

10. The carbon-coated iron-based polyanionic phosphate according to claim 7, characterized in that The carbon content of the carbon-coated iron-based polyanionic phosphate is 3 to 5 wt%.

11. A sodium-ion battery, comprising a positive electrode sheet, characterized in that, The positive electrode sheet includes the carbon-coated iron-based polyanionic phosphate according to any one of claims 7 to 10.

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