Sodium ferric pyrophosphate positive electrode material and preparation method and application thereof

Abstract

The application discloses a sodium iron pyrophosphate positive electrode material and a preparation method and application thereof. The sodium iron pyrophosphate positive electrode material is a double-carbon-layer sodium iron pyrophosphate material, which comprises sodium iron pyrophosphate particles, a first carbon layer coated on the surface of the sodium iron pyrophosphate particles, and a second carbon layer coated on the surface of the first carbon layer. The first carbon layer is in-situ coated on the surface of the sodium iron pyrophosphate particles, thereby avoiding large particle agglomeration of the sodium iron pyrophosphate during the generation process, so that the diffusion path of ions in the particle interior is shortened. The second carbon layer is coated on the surface of the first carbon layer, thereby modifying the surface and preventing the sodium iron pyrophosphate from being exposed, and effectively improving the iron ion dissolution problem. The double-carbon-layer sodium iron pyrophosphate electrode material is prepared by adopting a solid phase method and dry ball milling, the synthesis process is simple, and the electrode material can be easily produced in batches in an industrialized manner. The electrode material provided by the application can exhibit good cycle stability and capacity performance in organic electrolyte and aqueous electrolyte.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries, specifically relating to a sodium iron pyrophosphate cathode material, its preparation method, and its application. Background Technology

[0002] As one of the most promising new energy technologies, the research on improving the energy storage capacity and extending the service life of lithium-ion batteries remains highly popular.

[0003] However, lithium resources are severely unevenly distributed globally, and the scarcity of high-quality lithium resources is one of the key factors restricting the development of lithium-ion batteries. In contrast, sodium resources are widely distributed, and sodium-ion batteries have a similar energy storage mechanism to lithium-ion batteries, both belonging to the "rocking chair battery" category. Besides the mechanism, sodium-ion batteries and lithium-ion batteries also share many similarities in process synthesis and preparation, thus sodium-ion batteries can serve as an important reserve technology for lithium-ion batteries. Based on this consensus, research in the field of sodium-ion batteries is increasingly active, with a focus on electrode materials capable of stably intercalating and deintercalating sodium ions.

[0004] Since sodium ions have a larger radius than lithium ions, maintaining material stability while preserving high storage capacity during insertion / extraction is a crucial direction for improving the performance of sodium-ion batteries. Currently, there are four main types of mature sodium-ion battery cathode materials: layered transition metal oxides, polyanionic compounds, Prussian blue and its derivatives, and organic compounds. Among these, polyanionic compounds are widely studied due to their low cost, high safety, and strong structural stability. Sodium iron pyrophosphate (Na2FeP2O7) is increasingly favored due to its low iron resource cost and high safety; its insolubility in water further fuels its widespread research in high-safety aqueous sodium-ion batteries. However, sodium iron pyrophosphate is prone to iron ion dissolution during the reaction process, accelerating capacity decay during charge / discharge and shortening its lifespan. Furthermore, its poor conductivity results in poor performance at high rates, and its high polarization limits its overall performance.

[0005] Patent application CN116845215A discloses a secondary carbon-coated sodium iron pyrophosphate composite material and its preparation method. The synthesized material, like sodium iron pyrophosphate, is a polyanionic compound, relating to the field of sodium-ion battery cathode materials. This invention introduces a carbon source via a sol-gel method, performs carbon coating on the surface of sodium iron pyrophosphate, and then performs secondary carbon coating through wet ball milling and sintering. The prepared secondary carbon-coated sodium iron pyrophosphate composite material has a small grain size and exhibits good rate performance and reversibility. However, the sol-gel method used in this invention is time-consuming, difficult to control, and requires a demanding preparation environment, hindering its large-scale industrial application. Furthermore, in this invention, the carbon source is mixed with the material using a solvent, limiting the selection of carbon sources to soluble carbon sources.

[0006] Improving the carbon coating effect of sodium iron pyrophosphate, enhancing its structural stability and rate performance, and simultaneously achieving wide application range and controllable large-scale production synthesis process are urgent problems to be solved. Summary of the Invention

[0007] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a double carbon layer sodium iron pyrophosphate cathode material to solve the problems of poor stability and poor rate performance of existing sodium iron pyrophosphate.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A double-carbon-layer sodium iron pyrophosphate cathode material includes sodium iron pyrophosphate particles, a first carbon layer coating the surface of the sodium iron pyrophosphate particles, and a second carbon layer coating the surface of the first carbon layer.

[0010] The double-carbon-layer sodium iron pyrophosphate cathode material provided by this invention has a first carbon layer that is in situ coated on the surface of sodium iron pyrophosphate particles, preventing large particle agglomeration of sodium iron pyrophosphate during sintering, thereby shortening the diffusion path of ions inside the particles. The first carbon layer formed by one-time carbon coating often has the problem of uneven coating leading to material exposure. However, the second carbon layer is coated on the surface of the first carbon layer, modifying the surface and preventing sodium iron pyrophosphate exposure, effectively improving the problem of iron ion dissolution.

[0011] Preferably, the double-carbon-layer sodium iron pyrophosphate cathode material has a particle size of 100~200nm.

[0012] The present invention also provides a method for preparing the above-mentioned double-carbon layer sodium iron pyrophosphate cathode material.

[0013] The method for preparing the double-carbon-layer sodium iron pyrophosphate cathode material provided by the present invention includes the following steps:

[0014] (1) Sodium source, iron source, phosphorus source and carbon source are mixed in a certain proportion to obtain mixed powder; wherein the molar ratio of element sodium: element iron: element phosphorus is 2:1:2;

[0015] (2) The mixed powder from step (1) is annealed under an inert protective atmosphere, cooled to room temperature and ground.

[0016] (3) Place the powder material obtained in step (2) under an inert protective atmosphere again for annealing;

[0017] (4) The powder material obtained in step (3) is mixed with the carbon source in a certain proportion and dry ball milled to perform secondary carbon coating;

[0018] (5) The powder material obtained by ball milling in step (4) is annealed under an inert protective atmosphere to obtain the double carbon layer sodium iron pyrophosphate cathode material.

[0019] In the above method, the molar ratio of sodium, iron and phosphorus in step (1) is provided based on the chemical formula of sodium iron pyrophosphate, Na2FeP2O7.

[0020] In the above method, grinding after annealing in step (2) can improve the uniformity of the secondary sintering reaction; secondary sintering in step (3) ensures the fullness of the reaction and improves the crystallinity; ball milling in step (4) can fully coat the carbon source on the powder surface and reduce the exposed surface of the powder during the two-step sintering process; annealing again in step (5) restores the crystallinity of the material; in order to avoid the oxidation of iron and carbon, the sintering process should be in an inert protective atmosphere, specifically nitrogen, argon, hydrogen-argon mixture, etc.

[0021] Preferably, the sodium source includes at least one of sodium carbonate, sodium oxalate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium acetate, sodium citrate, and sodium pyrophosphate.

[0022] Preferably, the iron source includes at least one of ferrous phosphate, ferric phosphate, ferric nitrate, ferrous oxalate, and ferrous carbonate.

[0023] Preferably, the phosphorus source includes at least one of iron phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0024] Preferably, the carbon source includes at least one of glucose, starch, citric acid, conductive carbon black, carbon nanotubes, and graphene. The types of carbon sources added in the two feedings can be the same or different, and the two feedings do not need to be consistent.

[0025] Preferably, the carbon content provided by the carbon source accounts for 1% to 10% of the mass of the prepared double-layer carbon pyrophosphate sodium phosphate electrode material.

[0026] Preferably, in step (1), the carbon source accounts for 1% to 5% of the total mass of the added substances (sodium source, iron source, phosphorus source, carbon source).

[0027] Preferably, the annealing conditions in step (2) are as follows: a heating rate of 0.5... o C / min ~10 o Heat to 250 °C / min o C~400 o C, heat preservation time is 2 h to 6 h.

[0028] Preferably, the annealing conditions in step (3) are as follows: a heating rate of 0.5... o C / min ~10 o Temperature increased to 550 °C / min o C~650 o C, heat preservation time is 4 h to 10 h.

[0029] Preferably, in step (4), the carbon source contains 1% to 5% of the total mass of the powder material obtained by ball milling.

[0030] As a preferred option, the conditions for dry ball milling in step (4) are as follows: rotation speed 300 rpm~800 rpm, ball milling time 2 h~12 h.

[0031] Preferably, the annealing conditions in step (5) are as follows: a heating rate of 0.5... o C / min ~10 o Temperature increased to 550 °C / min o C~650 o C, heat preservation time is 4 h to 10 h.

[0032] This invention also protects a cathode material.

[0033] The cathode material includes the aforementioned double-carbon-layer sodium iron pyrophosphate cathode material.

[0034] This invention also protects a sodium-ion battery.

[0035] The sodium-ion battery includes a positive electrode, and the material of the positive electrode includes the above-mentioned sodium iron pyrophosphate positive electrode material containing a double carbon layer.

[0036] Furthermore, the sodium-ion battery also includes an electrolyte, which is an organic electrolyte and / or an aqueous electrolyte.

[0037] According to an embodiment of the present invention, the aqueous electrolyte is a 1 mol / L sodium chloride aqueous solution.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) The double carbon layer sodium iron pyrophosphate cathode material provided by the present invention has a double carbon layer sequentially coated on the surface of sodium iron pyrophosphate particles. The first carbon layer is coated on the particle surface in situ to avoid agglomeration of particles during sintering, improve the conductivity of the material and enhance the ion diffusion performance. The second carbon layer modifies the particle surface to avoid direct contact between sodium iron pyrophosphate and electrolyte, thereby reducing the ion dissolution problem of sodium iron pyrophosphate in organic electrolyte and aqueous electrolyte and improving the cycle stability of the material.

[0040] (2) The double-carbon-layer sodium iron pyrophosphate particles provided by this invention have a particle size of 100~200nm, which effectively shortens the lithium-ion transport channel and promotes the reaction between lithium ions and sodium iron pyrophosphate. At the same time, the double carbon layer optimizes the conductivity of the material and reduces the resistivity of the material. The lithium-ion transport performance and electronic conduction performance are optimized, thereby reducing the polarization phenomenon in the material reaction process and improving the rate performance of the material.

[0041] (3) The solid-phase method and dry ball milling method adopted in this invention are simple, have low raw material costs, and have low environmental requirements, which have obvious advantages in promoting industrialization. Attached Figure Description

[0042] Figure 1 This is a TEM image of the double-carbon layer sodium iron pyrophosphate cathode material prepared in Example 1 of this application;

[0043] Figure 2 This is a charge-discharge curve of the double-carbon layer sodium iron pyrophosphate cathode material prepared in Example 1 of this application in an organic electrolyte (sodium perchlorate electrolyte);

[0044] Figure 3 This is a charge-discharge curve of the double-carbon-layer sodium iron pyrophosphate cathode material prepared in Example 1 of this application in an aqueous electrolyte (sodium chloride aqueous solution). Detailed Implementation

[0045] The present invention will now be described in further detail with reference to specific embodiments. The given embodiments are merely illustrative of the invention and not intended to limit its scope. The embodiments provided below can serve as a guide for further improvements by those skilled in the art and do not constitute a limitation on the invention in any way.

[0046] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0047] Example 1

[0048] The preparation method of the double-carbon-layer sodium iron pyrophosphate cathode material in this embodiment is as follows:

[0049] (1) Sodium oxalate, ferrous phosphate, ammonium dihydrogen phosphate and citric acid are mixed in a certain proportion, wherein the molar ratio of elemental sodium: elemental iron: elemental phosphorus is 2:1:2, and citric acid accounts for 5% of the total mass of the added substances;

[0050] (2) Place the powder mixed in step (1) under an inert protective atmosphere and heat it at a rate of 5. o Temperature increased to 300 °C / min o C, heat preservation time is 4 hours, cool to room temperature and grind;

[0051] (3) Place the powder material ground in step (2) again under an inert protective atmosphere and heat it at a rate of 5. o Temperature increased to 550 °C / min o C, heat preservation time is 4 hours;

[0052] (4) The powder material obtained in step (3) is mixed with glucose in a certain proportion, wherein glucose accounts for 3% of the total mass of the powder material, and dry ball milling is performed at a speed of 600 rpm for 6 hours to perform secondary carbon coating.

[0053] (5) The powder material obtained by ball milling in step (4) is then annealed under an inert protective atmosphere at a heating rate of 5. o Temperature increased to 550 °C / min o C, the heat preservation time is 4 h, and the double carbon layer sodium iron pyrophosphate material is obtained.

[0054] Figure 1 This is a TEM image of the double-carbon-layer sodium iron pyrophosphate cathode material prepared in Example 1. The TEM image shows that the particle surface is rounded, with a double carbon shell uniformly coating the particle surface. The outer carbon shell is approximately 3 nm thick, and the inner carbon shell is approximately 2 nm thick. The particle size of the double-carbon-layer sodium iron pyrophosphate particles is approximately 150 nm.

[0055] The double-carbon-layer sodium iron pyrophosphate cathode material prepared in Example 1 was coated onto aluminum foil as the cathode, and a sodium sheet was used as the counter electrode and reference electrode. A 1 mol / L sodium perchlorate electrolyte was selected to assemble a coin cell. The sodium perchlorate electrolyte was prepared by dissolving sodium perchlorate in a solvent with a volume ratio of 1:1:1 of ethylene carbonate (EC) / dimethyl carbonate (DMC):methyl ethyl carbonate (EMC) and adding 5.0% by mass of fluoroethylene carbonate (FEC) to form a 1 mol / L sodium perchlorate electrolyte.

[0056] The assembled coin cells were subjected to constant rate charge-discharge cycles at 0.2 C, with the charge-discharge window set to 1.5 V to 3.5 V. The results showed that the initial specific capacity of the double carbon layer iron sodium pyrophosphate electrode material prepared in Example 1 could reach 89.5 mAh / g at 0.2 C, and the capacity could still maintain 99.78% of the initial capacity after 50 charge-discharge cycles.

[0057] Figure 2 This is a graph showing the charge-discharge rate of the double-carbon-layer sodium iron pyrophosphate cathode material prepared in Example 1 in sodium perchlorate electrolyte at a rate of 0.2C.

[0058] The assembled coin cells were subjected to three charge-discharge cycles at rates of 0.2 C, 0.5 C, and 1 C, with the charge-discharge window set to 1.5 V to 3.5 V. The results showed that the double carbon layer iron sodium pyrophosphate electrode material had a specific capacity of 87.9 mAh / g at 0.5 C and 83.1 mAh / g at 1 C, exhibiting good rate performance.

[0059] The double-layer carbon pyrophosphate sodium cathode material prepared in Example 1 was coated on carbon paper as the cathode, conductive carbon black was coated on carbon paper as the counter electrode, and a saturated Ag / AgCl electrode was used as the reference electrode. A 1 mol / L sodium chloride aqueous solution was selected as the electrolyte to assemble a three-electrode system with an aqueous electrolyte.

[0060] The assembled three-electrode electrolytic cell was subjected to constant rate charge-discharge cycles at 0.2 C, with the charge-discharge window set to -0.6 V to 0.6 V. The results showed that the initial specific capacity of the double carbon layer iron sodium pyrophosphate electrode material prepared in Example 1 could still reach 81.3 mAh / g at 0.2 C, and the capacity could still maintain 92.55% of the initial capacity after 50 charge-discharge cycles.

[0061] Figure 3 This is a graph showing the charge-discharge curve of the double-carbon-layer sodium iron pyrophosphate cathode material prepared in Example 1 applied to an aqueous sodium chloride solution at a charge-discharge rate of 0.2 C.

[0062] Comparative Example 1

[0063] The difference from Example 1 is that no carbon source is added in step (1), and the secondary carbon coating in steps (4) and (5) is not performed, so as to obtain carbon-free sodium iron pyrophosphate material.

[0064] With other conditions remaining constant, the prepared carbon-free sodium iron pyrophosphate material was used as the positive electrode, the sodium sheet was used as the counter electrode, and a 1 mol / L sodium perchlorate electrolyte was selected to assemble a coin cell.

[0065] The assembled coin cells were subjected to constant rate charge-discharge cycles at 0.2 C, with the charge-discharge window set to 1.5 V to 3.5 V. The results showed that the uncoated sodium iron pyrophosphate material had a specific capacity of only 35.6 mAh / g at 0.2 C, and the specific capacity decayed to 21.1 mAh / g after 20 charge-discharge cycles. The material exhibited poor stability and rapid capacity decay during cycling.

[0066] The assembled coin cells were subjected to three charge-discharge cycles at rates of 0.2 C, 0.5 C, and 1 C, with the charge-discharge window set to 1.5 V to 3.5 V. The results showed that the uncoated sodium iron pyrophosphate material had a specific capacity of 30.1 mAh / g at 0.5 C and only 19.2 mAh / g at 1 C, exhibiting worse electrochemical performance at higher rates.

[0067] Comparative Example 2

[0068] The difference from Example 1 is that the secondary carbon coating in steps (4) and (5) is not performed, so as to obtain a single carbon layer sodium iron pyrophosphate material.

[0069] The preparation of the coin cell made of sodium iron pyrophosphate single-layer material was the same as in Example 1.

[0070] The assembled coin cells were subjected to constant rate charge-discharge cycles at 0.2 C, with the charge-discharge window set to 1.5 V to 3.5 V. The results showed that the monolayer sodium iron pyrophosphate material had a specific capacity of 63.5 mAh / g at 0.2 C, which decreased to 44.3 mAh / g after 50 charge-discharge cycles. Compared with the double carbon layer structure, the monolayer material had poor stability.

[0071] The assembled coin cells were subjected to three charge-discharge cycles at rates of 0.2 C, 0.5 C, and 1 C, with the charge-discharge window set to 1.5 V to 3.5 V. The results showed that the monolayer sodium iron pyrophosphate material had a specific capacity of 56.4 mAh / g at 0.5 C, but only 46.8 mAh / g at 1 C. The higher the charge-discharge rate, the worse the electrochemical performance of the material.

[0072] Comparative Example 3

[0073] The difference from Example 1 is that the two-step sintering of steps (2) and (3) is not performed. Instead, the powder material mixed in step (1) is directly placed under an inert protective atmosphere and heated at a rate of 5. o Temperature increased to 550 °C / min o C, with a holding time of 4 h, thus obtaining a new carbon-containing sodium iron pyrophosphate material.

[0074] The preparation of the coin cell containing carbon-containing sodium iron pyrophosphate material is the same as in Example 1.

[0075] The assembled coin cells were subjected to constant rate charge-discharge cycles at 0.2 C, with the charge-discharge window set to 1.5 V–3.5 V. The results showed that the specific capacity of the carbon-containing iron iron pyrophosphate material at 0.2 C was 46.1 mAh / g, which decreased to 25.8 mAh / g after 50 charge-discharge cycles. Compared to double-layer iron iron pyrophosphate, the material prepared by a single-step sintering process with carbon coating exhibited poor capacity performance. TEM images revealed that the carbon-containing iron iron pyrophosphate had poor surface roundness and uneven carbon coating, resulting in poor electrochemical performance.

[0076] The assembled button cells were subjected to three charge-discharge cycles at rates of 0.2 C, 0.5 C, and 1 C, with the charge-discharge window set to 1.5 V to 3.5 V. The results showed that the specific capacity of the carbon-containing iron pyrophosphate material was 40.4 mAh / g at 0.5 C, but only 33.8 mAh / g at 1 C. The higher the rate, the more pronounced the problems of limited ion transport and poor conductivity caused by uneven carbon coating became, resulting in poor rate performance of the carbon-containing iron pyrophosphate material.

[0077] In summary, the double-carbon-layer sodium iron pyrophosphate cathode material prepared in this application exhibits good cycle stability and rate performance.

[0078] The present invention has been described in detail above. For those skilled in the art, the invention can be practiced in a wide range of ways with equivalent parameters, concentrations, and conditions without departing from its spirit and scope, and without requiring unnecessary experiments. Although specific embodiments have been given, it should be understood that further modifications can be made to the invention. In summary, according to the principles of the invention, this application is intended to include any changes, uses, or improvements to the invention, including changes made using conventional techniques known in the art that depart from the scope disclosed herein. Some of the essential features can be applied within the scope of the following appended claims.

Claims

1. A sodium iron pyrophosphate cathode material, comprising sodium iron pyrophosphate particles, a first carbon layer coated on the surface of the sodium iron pyrophosphate particles, and a second carbon layer coated on the surface of the first carbon layer; The particle size of the sodium iron pyrophosphate cathode material is 100~200nm; The thicknesses of both the first carbon layer and the second carbon layer are less than 5 nm; The preparation method of the sodium iron pyrophosphate cathode material includes the following steps: (1) Sodium source, iron source, phosphorus source and carbon source are mixed in a certain proportion to obtain mixed powder; wherein the molar ratio of element sodium: element iron: element phosphorus is 2:1:2; (2) The mixed powder from step (1) is annealed under an inert protective atmosphere, cooled to room temperature and ground. (3) Place the powder material obtained in step (2) under an inert protective atmosphere again for annealing; (4) The powder material obtained in step (3) is mixed with the carbon source in a certain proportion and dry ball milled to perform secondary carbon coating; (5) The powder material obtained by ball milling in step (4) is annealed under an inert protective atmosphere to obtain the sodium iron pyrophosphate cathode material. In step (1), the carbon source accounts for 1% to 5% of the total mass of the sodium source, iron source, phosphorus source, and carbon source; In step (4), the carbon source contains 1% to 5% of the total mass of the powder material obtained by ball milling; The annealing conditions in step (2) are as follows: the heating rate is 0.5... o C / min ~10 o Heat to 250 °C / min o C~400 o C, heat preservation time is 2 h to 6 h; The annealing conditions in step (3) are as follows: with a heating rate of 0.5... o C / min ~10 o Temperature increased to 550 °C / min o C~650 o C, heat preservation time is 4 h~10 h; The annealing conditions in step (5) are as follows: with a heating rate of 0.5... o C / min ~10 o Temperature increased to 550 °C / min o C~650 o C, heat preservation time is 4 h to 10 h.

2. The preparation method of the sodium iron pyrophosphate cathode material according to claim 1, comprising the following steps: (1) Sodium source, iron source, phosphorus source and carbon source are mixed in a certain proportion to obtain mixed powder; wherein the molar ratio of element sodium: element iron: element phosphorus is 2:1:2; (2) The mixed powder from step (1) is annealed under an inert protective atmosphere, cooled to room temperature and ground. (3) Place the powder material obtained in step (2) under an inert protective atmosphere again for annealing; (4) The powder material obtained in step (3) is mixed with the carbon source in a certain proportion and dry ball milled to perform secondary carbon coating; (5) The powder material obtained by ball milling in step (4) is annealed under an inert protective atmosphere to obtain the sodium iron pyrophosphate cathode material. In step (1), the carbon source accounts for 1% to 5% of the total mass of the sodium source, iron source, phosphorus source, and carbon source; In step (4), the carbon source contains 1% to 5% of the total mass of the powder material obtained by ball milling; The annealing conditions in step (2) are as follows: the heating rate is 0.5... o C / min ~10 o Heat to 250 °C / min o C~400 o C, heat preservation time is 2 h to 6 h; The annealing conditions in step (3) are as follows: with a heating rate of 0.5... o C / min ~10 o Temperature increased to 550 °C / min o C~650 o C, heat preservation time is 4 h~10 h; The annealing conditions in step (5) are as follows: with a heating rate of 0.5... o C / min ~10 o Temperature increased to 550 °C / min o C~650 o C, heat preservation time is 4 h to 10 h.

3. The preparation method according to claim 2, characterized in that: The sodium source includes at least one of sodium carbonate, sodium oxalate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium acetate, sodium citrate, and sodium pyrophosphate. And / or, the iron source includes at least one of ferrous phosphate, ferric phosphate, ferric nitrate, ferrous oxalate, and ferrous carbonate; And / or, the phosphorus source includes at least one of iron phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; And / or, the carbon source includes at least one of glucose, starch, and citric acid; And / or, the carbon content provided by the carbon source accounts for 1% to 10% of the mass of the prepared double-layer sodium iron pyrophosphate electrode material.

4. The preparation method according to claim 2, characterized in that: The conditions for dry ball milling in step (4) are as follows: rotation speed 300 rpm~800 rpm, ball milling time 2 h~12 h.

5. A cathode material, comprising the sodium iron pyrophosphate cathode material of claim 1 or the sodium iron pyrophosphate cathode material prepared by any one of claims 2-4.

6. A sodium-ion battery, comprising a positive electrode, wherein the material of the positive electrode comprises the positive electrode material of claim 5.

7. The sodium-ion battery according to claim 6, characterized in that: The sodium-ion battery also includes an electrolyte, which is an organic electrolyte and / or an aqueous electrolyte.

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