A sodium-ion cathode material, its preparation method, and a battery

Abstract

This invention relates to a sodium-ion cathode material, its preparation method, and a battery, belonging to the field of sodium battery technology. It addresses at least one of the problems of poor conductivity, structural instability, and poor cycle performance in composite materials of sodium iron pyrophosphate and sodium iron sulfate prepared by existing methods. The sodium-ion cathode material of this invention uses sodium iron pyrophosphate as a three-dimensional porous framework structure. Sodium iron pyrophosphate and sodium iron sulfate are combined to form a "framework-filler" two-phase structure and a porous structure. Through precise control of the material's microstructure, the conductivity, structural stability, and sodium-ion storage performance of the material are significantly improved.

Description

Technical Field

[0001] This invention relates to the field of sodium battery technology, and in particular to a sodium-ion cathode material, its preparation method, and a battery thereof. Background Technology

[0002] With the increasing global demand for renewable energy storage, sodium-ion batteries have become an important alternative technology to lithium-ion batteries due to their abundant resources and low cost. As the core component of sodium-ion batteries, the performance of the cathode material directly affects the battery's energy density, cycle life, and rate performance. Currently, iron-based compounds have become one of the research hotspots for sodium-ion battery cathode materials due to their high theoretical capacity and environmental friendliness. However, traditional iron-based cathode materials suffer from poor conductivity and insufficient structural stability, which limits their practical application.

[0003] Sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7)) has an open crystal structure, which facilitates the insertion and extraction of sodium ions. However, the conductivity and cycling stability of sodium iron pyrophosphate alone still need improvement. Sodium iron sulfate (Na2Fe(SO4)2), as another iron-based active material, has a high redox potential and theoretical capacity, but it also faces challenges in conductivity and structural compatibility.

[0004] Sodium iron pyrophosphate possesses excellent cycling and thermal stability due to its rigid framework structure in an orthorhombic crystal system, but it suffers from low electronic conductivity and insufficient utilization of active sites. Sodium iron sulfate, while possessing a high voltage plateau and three-dimensional ion channels, suffers from poor lattice stability due to Fe-Na antisite defects and is prone to decomposition at high temperatures.

[0005] In the existing technology, a core-shell structured composite material is prepared by first preparing a sodium iron pyrophosphate preform and then sintering the preform with sodium iron sulfate. Due to the differences between the two materials, there are problems such as stress concentration during charging and discharging leading to structural cracking, limited contact area with electrolyte, and long ion transport path, making it difficult to balance conductivity, structural stability and ion transport efficiency. Summary of the Invention

[0006] In view of the above analysis, the present invention aims to provide a sodium ion cathode material, its preparation method and battery, to solve at least one of the problems of poor conductivity, unstable structure and poor cycle performance of composite materials of sodium iron pyrophosphate and sodium iron sulfate prepared by existing methods.

[0007] In a first aspect, the present invention provides a sodium ion cathode material, the cathode material comprising a three-dimensional porous framework structure, wherein a carbon layer is deposited inside the pores of the porous framework, and sodium ferric sulfate is generated in situ on the carbon layer, and the three-dimensional porous framework material is three-dimensional porous sodium ferric pyrophosphate.

[0008] Secondly, a method for preparing a sodium-ion cathode material includes the following steps:

[0009] (1) According to the stoichiometric ratio of sodium, iron and phosphorus in the molecular formula Na4Fe3(PO4)2(P2O7), the sodium source, phosphorus source and iron source are weighed and added to water to obtain a solution. Then, polyurethane sponge and complexing agent are added, heated and dried to obtain a viscous gel.

[0010] (2) The dry gel was calcined under an inert atmosphere to obtain three-dimensional porous sodium iron pyrophosphate.

[0011] (3) According to the stoichiometric ratio of sodium, iron and sulfur in the molecular formula Na2Fe(SO4)2, the sodium source, iron source and sulfur source are weighed and added to water, mixed evenly, and then the three-dimensional porous sodium iron pyrophosphate and complexing agent are added. The mixture is heated, dried and sintered to obtain the sodium ion cathode material.

[0012] Furthermore, in step (1), the phosphorus source includes at least one of phosphoric acid, sodium dihydrogen phosphate, or sodium hydrogen phosphate.

[0013] The sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium nitrate, and sodium hydroxide;

[0014] The iron source includes at least one of ferrous phosphate, ferrous carbonate, and ferrous oxalate.

[0015] Furthermore, in step (1), the mass ratio of the polyurethane sponge to the complexing agent is 5:1 to 5:5, and the amount of the complexing agent added is 2 to 10 times the number of iron moles in Na4Fe3(PO4)2(P2O7).

[0016] Furthermore, in step (1), the heating temperature is 60-90°C, heated until a viscous gel is formed, the drying temperature is 90-120°C, and the drying time is 18-26 hours.

[0017] Furthermore, in step (2), the inert atmosphere is argon and / or nitrogen, the calcination temperature is 450-600℃, and the calcination time is 6-19h.

[0018] Furthermore, in step (3), the sodium source includes at least one of sodium sulfate, sodium bisulfate, sodium carbonate, sodium bicarbonate, sodium nitrate, and sodium hydroxide.

[0019] The iron source includes at least one of ferrous sulfate and ferrous carbonate;

[0020] The sulfur source includes at least one of ferrous sulfate, sodium sulfate, or potassium sulfate.

[0021] Furthermore, in steps (1) and (3), the complexing agent includes at least one of citric acid, ethylene glycol, or polyvinyl alcohol.

[0022] Furthermore, in step (3), the mass ratio of the three-dimensional porous sodium iron pyrophosphate to the complexing agent is 7:2 to 7:6, and the amount of complexing agent added is 2 to 10 times the number of iron moles in Na2Fe(SO4)2.

[0023] Furthermore, in step (3), the sintering temperature is 300-400℃ and the time is 12-18h.

[0024] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0025] (1) The sodium ion cathode material of the present invention uses sodium iron pyrophosphate as a three-dimensional porous framework structure. Sodium iron pyrophosphate and sodium iron sulfate are combined to form a "framework-filled" two-phase structure and a porous structure sodium ion cathode material. Through precise control of the microstructure of the material, the conductivity, structural stability and sodium ion storage performance of the material are significantly improved.

[0026] (2) The method for preparing the positive electrode material of the present invention uses polyurethane sponge as a template. Sodium, phosphorus, and iron sources are mixed in stoichiometric ratio to form precursors that fill the pores of the polyurethane sponge. After calcination, the precursors react to generate sodium iron pyrophosphate, while simultaneously replicating the porous structure of the template to form a three-dimensional porous framework. High-temperature calcination causes the polyurethane sponge to pyrolyze and carbonize, depositing in situ on the inner wall of the pores of the porous framework to form a carbon layer. Sodium, iron, and sulfur sources are mixed in stoichiometric ratio to form a solution, which is then combined with the porous framework containing the carbon layer. After sol-gel and sintering, the precursors react directionally on the surface of the carbon layer to generate sodium iron sulfate in situ, forming a "framework-carbon layer-sodium iron sulfate" structure.

[0027] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0028] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0029] Figure 1 This is a SEM image of the polyurethane foam in step (1) of Embodiment 1 of the present invention;

[0030] Figure 2 SEM image of the three-dimensional porous sodium iron pyrophosphate obtained in step (2) of Example 1 of this invention;

[0031] Figure 3 This is a SEM image of the cathode material obtained in step (3) of Embodiment 1 of the present invention. Detailed Implementation

[0032] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0033] In a specific embodiment of the present invention, a sodium ion cathode material is disclosed. The cathode material includes a three-dimensional porous framework structure. A carbon layer is deposited inside the pores of the porous framework. Sodium iron sulfate (Na2Fe(SO4)2) is generated in situ on the carbon layer. The three-dimensional porous framework material is three-dimensional porous sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7)).

[0034] Compared with the prior art, the sodium ion cathode material of the present invention uses sodium iron pyrophosphate as a three-dimensional porous framework structure. Sodium iron pyrophosphate and sodium iron sulfate are combined to form a "framework-filler" two-phase structure and a porous structure sodium ion cathode material. Through precise control of the microstructure of the material, the conductivity, structural stability and sodium ion storage performance of the material are significantly improved.

[0035] This invention uses sodium iron pyrophosphate as a porous framework structure. Its rigid orthorhombic crystal structure can disperse the volume stress during the charging and discharging process, avoiding the stress concentration problem of traditional composite structures and reducing structural breakage. At the same time, sodium iron sulfate is tightly bonded to the framework through in-situ growth, reducing the risk of interface separation and further improving the overall structural stability.

[0036] Furthermore, the three-dimensional porous framework structure increases the contact area between the material and the electrolyte, shortening the sodium ion transport path (the porous framework provides a convenient channel for ion diffusion); the carbon layer deposited within the framework pores (derived from the decomposition of polyurethane sponge) effectively improves electronic conductivity, solving the problem of poor intrinsic conductivity of the two iron-based materials and synergistically enhancing rate performance. The cycling stability of sodium iron pyrophosphate complements that of sodium iron sulfate, achieving a combination of their advantages through the "framework-filler" structure.

[0037] Another specific embodiment of the present invention discloses a method for preparing the above-mentioned sodium ion cathode material, comprising the following steps:

[0038] (1) According to the stoichiometric ratio of sodium, iron and phosphorus in the molecular formula Na4Fe3(PO4)2(P2O7), the sodium source, phosphorus source and iron source are weighed and added to water to obtain a solution. Then, polyurethane sponge and complexing agent are added, heated and dried to obtain a viscous gel.

[0039] (2) The dry gel was calcined under an inert atmosphere to obtain three-dimensional porous sodium iron pyrophosphate.

[0040] (3) According to the stoichiometric ratio of sodium, iron and sulfur in the molecular formula Na2Fe(SO4)2, the sodium source, iron source and sulfur source are weighed and added to water, mixed evenly, and then the three-dimensional porous sodium iron pyrophosphate and complexing agent are added. The mixture is heated, dried and sintered to obtain the sodium ion cathode material.

[0041] It should be noted that in step (1) of this invention, polyurethane sponge is used as a template, and sodium, phosphorus, and iron sources are formed into precursors according to stoichiometric ratios and filled into the pores of the polyurethane sponge. After calcination, the precursors react to generate sodium iron pyrophosphate, and at the same time replicate the porous structure of the template to form a three-dimensional porous framework. In step (2), calcination is carried out in an inert atmosphere, and the polyurethane sponge is pyrolyzed and carbonized, and deposited in situ on the inner wall of the pores of the porous framework to form a carbon layer. In step (3), sodium, iron, and sulfur sources are prepared into a solution according to stoichiometric ratios and combined with the porous framework containing the carbon layer. After sol-gel and sintering, the precursors react directionally on the surface of the carbon layer to generate sodium iron sulfate in situ, forming a "framework-carbon layer-sodium iron sulfate" structure.

[0042] Specifically, in step (1), the mass ratio of the polyurethane sponge to the complexing agent is 5:1 to 5:5, for example, 5:1, 5:2, 5:3, 5:4, 5:5, and the amount of complexing agent added is 2 to 10 times the number of iron moles in Na4Fe3(PO4)2(P2O7), for example, 2 times, 3 times, 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, 10 times.

[0043] Specifically, in step (1), the heating temperature is 60-90℃, for example, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, heated until a viscous gel is formed, the drying temperature is 90-120℃, for example, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, and the drying time is 18-26h, for example, 18h, 19h, 20h, 21h, 22h, 23h, 24h, 25h, 26h.

[0044] It should be noted that the dry gel in this invention is composed of sodium iron pyrophosphate raw material and polyurethane sponge. The method of this invention involves calcination under an inert atmosphere. At the aforementioned calcination temperature and time, the polyurethane sponge decomposes and carbonizes to obtain a three-dimensional porous framework (i.e., three-dimensional porous sodium iron pyrophosphate), and a layer of carbon from the decomposed polyurethane sponge is deposited inside the pores of the framework.

[0045] Specifically, in step (1), the sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium nitrate, and sodium hydroxide;

[0046] The iron source includes at least one of ferrous phosphate, ferrous carbonate, and ferrous oxalate.

[0047] Specifically, in step (1), the phosphorus source includes at least one of phosphoric acid, sodium dihydrogen phosphate, or sodium hydrogen phosphate.

[0048] Specifically, in step (2), the inert atmosphere is argon and / or nitrogen, the calcination temperature is 450-600℃, for example, 450℃, 470℃, 490℃, 510℃, 530℃, 550℃, 570℃, 590℃, 600℃, and the calcination time is 6-19h, for example, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h.

[0049] Specifically, in step (3), the sulfur source includes at least one of ferrous sulfate, sodium sulfate, or potassium sulfate.

[0050] Specifically, in step (3), the sodium source includes at least one of sodium sulfate, sodium bisulfate, sodium carbonate, sodium bicarbonate, sodium nitrate, and sodium hydroxide.

[0051] The iron source includes at least one of ferrous sulfate and ferric sulfate.

[0052] Specifically, in steps (1) and (3), the complexing agent includes at least one of citric acid, ethylene glycol, or polyvinyl alcohol.

[0053] Specifically, in step (3), the mass ratio of the three-dimensional porous sodium iron pyrophosphate to the complexing agent is 7:2 to 7:6, for example, 7:2, 7:3, 7:4, 7:5, 7:6, and the amount of complexing agent added is 2 to 10 times the number of moles of iron in Na2Fe(SO4)2, for example, 2 times, 3 times, 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, 10 times.

[0054] Specifically, in step (3), the heating temperature is 60-90℃, for example, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, heated until a viscous gel is formed, and dried at 90-120℃ (for example, 90℃, 92℃, 94℃, 96℃, 98℃, 100℃, 102℃, 104℃, 106℃, 108℃, 110℃, 112℃, 114℃, 116℃, 118℃, 120℃) for 18-26h, for example, 18h, 19h, 20h, 21h, 22h, 23h, 24h, 25h, 26h.

[0055] Specifically, in step (3), sintering is carried out in an inert atmosphere, preferably argon and / or nitrogen.

[0056] Specifically, in step (3), the sintering temperature is 300-400℃, for example, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, and the time is 12-18h, for example, 12h, 13h, 14h, 15h, 16h, 17h, 18h.

[0057] It should be noted that, using the above-mentioned sintering temperature and time, sodium ferric sulfate is generated in situ on the framework of the three-dimensional porous sodium ferric pyrophosphate cathode material, forming a uniform composite cathode material with a porous structure.

[0058] Another specific embodiment of the present invention discloses a sodium-ion battery comprising the sodium-ion cathode material described above.

[0059] It should be noted that the polyurethane foam of the present invention is a commercially available product or a porous polyurethane foam prepared by existing methods. For example, the manufacturer is Henan Shuangxin Environmental Protection Technology Co., Ltd., and the model is SXJAZHM-0155.

[0060] The technical solution of the present invention will be further explained below with reference to specific embodiments.

[0061] Example 1

[0062] This embodiment provides a method for preparing a sodium-ion cathode material, comprising the following steps:

[0063] (1) According to the stoichiometric ratio of sodium, iron and phosphorus in the molecular formula Na4Fe3(PO4)2(P2O7) of 4:3:4, sodium carbonate, sodium dihydrogen phosphate and ferrous phosphate were weighed and added to deionized water to obtain a solution. Then, polyurethane sponge and complexing agent citric acid were added. The mass ratio of polyurethane sponge to complexing agent was 5:3. The amount of complexing agent added was 3 times the number of moles of iron in Na4Fe3(PO4)2(P2O7). The solution was heated at 80°C until a viscous gel was obtained. The solution was dried at 100°C for 24 hours to obtain a dry gel.

[0064] (2) The dry gel was calcined at 450°C for 16 h under an argon atmosphere to obtain three-dimensional porous sodium iron pyrophosphate.

[0065] (3) According to the stoichiometric ratio of sodium, iron and sulfur in the molecular formula Na2Fe(SO4)2 of 2:1:2, sodium sulfate and ferrous sulfate were weighed and added to water, mixed evenly, and then the three-dimensional porous sodium pyrophosphate and complexing agent citric acid were added. The mass ratio of the three-dimensional porous sodium pyrophosphate and the complexing agent was 7:4, and the amount of complexing agent added was 3 times the number of moles of iron in Na2Fe(SO4)2. The mixture was heated and evaporated at 80°C until a viscous gel was formed, dried at 100°C for 24 hours, and sintered at 350°C in a nitrogen atmosphere for 12 hours to obtain the sodium ion cathode material.

[0066] SEM images of the polyurethane foam raw material in step (1) of Test Example 1 are shown below. Figure 1 As shown, the SEM image of the three-dimensional porous sodium ferric pyrophosphate obtained in step (2) is as follows. Figure 2 As shown, the SEM image of the cathode material obtained in step (3) is as follows. Figure 3 As shown.

[0067] This invention uses the three-dimensional interconnected pore structure of polyurethane sponge as a physical template. A sodium iron pyrophosphate precursor is uniformly filled into its pores through solution permeation. After drying, a composite dry gel is formed. At this point, the precursor is spatially confined by the template, replicating the porous morphology of the polyurethane sponge. After calcination in an inert atmosphere, the polyurethane sponge undergoes pyrolysis and carbonization, depositing in situ on the inner walls of the porous framework to form a carbon layer. Figure 2 Through sol-gel and sintering, the precursor undergoes a directional reaction on the carbon layer surface, generating sodium ferric sulfate in situ, forming a "skeleton-carbon layer-sodium ferric sulfate" structure, such as... Figure 3 .

[0068] Example 2

[0069] This embodiment provides a method for preparing a sodium-ion cathode material, comprising the following steps:

[0070] (1) According to the stoichiometric ratio of sodium, iron and phosphorus in the molecular formula Na4Fe3(PO4)2(P2O7) of 4:3:4, sodium bicarbonate, sodium hydrogen phosphate and ferrous phosphate were weighed and added to deionized water to obtain a solution. Then, polyurethane sponge and complexing agent citric acid were added. The mass ratio of polyurethane sponge to complexing agent was 5:1. The amount of complexing agent added was 4 times the number of moles of iron in Na4Fe3(PO4)2(P2O7). The solution was heated at 60°C until a viscous gel was obtained. The solution was dried at 90°C for 26 hours to obtain a dry gel.

[0071] (2) The dry gel was calcined at 600°C for 6 hours under an argon atmosphere to obtain three-dimensional porous sodium iron pyrophosphate.

[0072] (3) According to the stoichiometric ratio of sodium, iron and sulfur in the molecular formula Na2Fe(SO4)2 of 2:1:2, sodium sulfate and ferrous sulfate were weighed and added to water, mixed evenly, and then the three-dimensional porous sodium pyrophosphate and complexing agent citric acid were added. The mass ratio of the three-dimensional porous sodium pyrophosphate and the complexing agent was 7:2, and the amount of complexing agent added was 4 times the number of moles of iron in Na2Fe(SO4)2. The mixture was heated and evaporated at 60°C until a viscous gel was formed, dried at 90°C for 26 hours, and sintered at 400°C in a nitrogen atmosphere for 14 hours to obtain the sodium ion cathode material.

[0073] The present invention tested the SEM of the cathode material prepared in this embodiment, and the results were basically consistent with those of Example 1. Due to space limitations, they will not be listed one by one.

[0074] Example 3

[0075] This embodiment provides a method for preparing a sodium-ion cathode material, comprising the following steps:

[0076] (1) According to the stoichiometric ratio of sodium, iron and phosphorus in the molecular formula Na4Fe3(PO4)2(P2O7) of 4:3:4, sodium carbonate, sodium dihydrogen phosphate and ferrous phosphate were weighed and added to deionized water to obtain a solution. Then, polyurethane sponge and complexing agent citric acid were added. The mass ratio of polyurethane sponge to complexing agent was 5:5. The amount of complexing agent added was 5 times the number of moles of iron in Na4Fe3(PO4)2(P2O7). The solution was heated at 90°C until a viscous gel was obtained. The solution was dried at 120°C for 18 hours to obtain a dry gel.

[0077] (2) The dry gel was calcined at 500°C for 12 hours under an argon atmosphere to obtain three-dimensional porous sodium iron pyrophosphate.

[0078] (3) According to the stoichiometric ratio of sodium, iron and sulfur in the molecular formula Na2Fe(SO4)2 of 2:1:2, sodium sulfate and ferrous sulfate were weighed and added to water, mixed evenly, and then the three-dimensional porous sodium pyrophosphate and complexing agent citric acid were added. The mass ratio of the three-dimensional porous sodium pyrophosphate and the complexing agent was 7:6, and the amount of complexing agent added was 5 times the number of moles of iron in Na2Fe(SO4)2. The mixture was heated and evaporated at 90°C until a viscous gel was formed, dried at 120°C for 18 hours, and sintered at 300°C under a nitrogen atmosphere for 18 hours to obtain the sodium ion cathode material.

[0079] The present invention tested the SEM of the cathode material prepared in this embodiment, and the results were basically consistent with those of Example 1. Due to space limitations, they will not be listed one by one.

[0080] Example 4

[0081] The preparation method of this embodiment is similar to that of Example 1, except that in steps (1) and (3), the amount of complexing agent added is twice the number of moles of iron source.

[0082] The present invention tested the SEM of the cathode material prepared in this embodiment, and the results were basically consistent with those of Example 1. Due to space limitations, they will not be listed one by one.

[0083] Example 5

[0084] The preparation method of this embodiment is similar to that of Example 1, except that in steps (1) and (3), the amount of complexing agent added is 10 times the number of moles of iron source.

[0085] The present invention tested the SEM of the cathode material prepared in this embodiment, and the results were basically consistent with those of Example 1. Due to space limitations, they will not be listed one by one.

[0086] Comparative Example 1

[0087] The preparation method of this embodiment is similar to that of Example 1, except that polyurethane sponge is not added in step (1).

[0088] Comparative Example 2

[0089] The preparation method of this embodiment is similar to that of Example 1, except that no complexing agent is added in step (3).

[0090] Comparative Example 3

[0091] The preparation method of this embodiment is similar to that of Example 1, except that sodium carbonate, sodium dihydrogen phosphate and ferrous phosphate are not added in step (1).

[0092] Comparative Example 4

[0093] The preparation method of this embodiment is similar to that of Example 1, except that in step (2), the calcination temperature is 300℃ and the time is 4h.

[0094] Comparative Example 5

[0095] The preparation method of this embodiment is similar to that of Example 1, except that in step (3), the calcination temperature is 500℃ and the time is 20h.

[0096] Experimental Example 1

[0097] The cathode materials prepared in Examples 1-7 and Comparative Examples 1-5 were assembled into coin cells, respectively, using the following methods:

[0098] The positive electrode material, acetylene black, and PVDF were weighed and mixed in a mass ratio of 8:1:1. N-methylpyrrolidone solution (NMP) was added to form a slurry, which was then uniformly coated onto aluminum foil and dried in a vacuum drying oven at 80°C for 6 hours. The positive electrode sheet was obtained by rolling and cutting (diameter of 14 mm). A sodium metal sheet was used as the counter electrode, and 1 mol / L NaClO4 was used as the electrolyte. The electrolyte consisted of 1 M sodium perchlorate (NaClO4) dissolved in a 1:1 volume ratio ethylene carbonate / propylene carbonate (EC / PC) system with 5 wt% fluoroethylene carbonate (FEC) added. The separator was made of glass fiber. The cells were assembled into a 2032 type button cell in an argon-filled glove box.

[0099] The carbon layer thickness, pore size, sodium ferric sulfate content in the cathode materials prepared in the examples and comparative examples were tested respectively, and the electrical performance of the assembled batteries were also tested. The results are shown in Table 1.

[0100] The thickness of the carbon layer was measured by transmission electron microscopy, and the pore size was measured by gas adsorption.

[0101] Table 1

[0102]

[0103]

[0104] As shown in Table 1, the carbon layer thickness of the sodium-ion cathode material prepared in this invention is 5–11 nm, the average pore size is 1.12–1.43 μm, the voltage plateau is 3.77–3.82 V, and the conductivity is 5.0 × 10⁻⁶. -3 ~1.2×10 -2 S / m, capacity retention after 100 cycles at 1C is 83-92%, and rate performance (capacity retention at 10C) is 60-75%.

[0105] As can be seen from Example 1 and Comparative Example 1, Comparative Example 1 did not construct a three-dimensional framework and directly physically mixed the materials, which failed to reflect the advantages of porous materials and had poor electrical properties.

[0106] As can be seen from Example 1 and Comparative Example 2, sodium ferric sulfate failed to effectively fill the framework, failed to grow in situ on the framework, failed to generate the structure designed in this invention, and had poor electrical properties.

[0107] As can be seen from Example 1 and Comparative Example 3, the absence of the sodium iron pyrophosphate skeleton (only sodium iron sulfate) resulted in the failure to form a porous structure and poor electrical properties.

[0108] As can be seen from Example 1 and Comparative Example 4, if the calcination temperature is too low, the carbonization will be insufficient, the carbon layer will be discontinuous, and the electrical properties will be poor.

[0109] As can be seen from Example 1 and Comparative Example 5, sintering at excessively high calcination temperature causes the decomposition of sodium ferric sulfate and excessive crystal growth, which may result in the formation of some blind holes and poor electrical properties.

[0110] As can be seen from Examples 1 and 4, reducing the amount of complexing agent will lead to uneven skeleton in step (1) and insufficient filling in step (3), resulting in a decrease in electrical performance.

[0111] As can be seen from Examples 1 and 5, the more complexing agent there is, the more it affects the crystallization of the material, leading to grain coarsening. Furthermore, citric acid itself acts as a carbon source, and carbon residues block the pores, reducing the effective specific surface area and causing a decrease in cycle life.

[0112] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A sodium-ion cathode material, characterized in that, The cathode material includes a three-dimensional porous framework structure, in which a carbon layer is deposited inside the pores of the porous framework, and sodium iron sulfate is generated in situ on the carbon layer. The three-dimensional porous framework material is three-dimensional porous sodium iron pyrophosphate. The sodium ion cathode material is prepared by the following method: (1) According to the stoichiometric ratio of sodium, iron and phosphorus in the molecular formula Na4Fe3(PO4)2(P2O7), the sodium source, phosphorus source and iron source are weighed and added to water to obtain a solution. Then, polyurethane sponge and complexing agent are added, heated and dried to obtain a viscous gel. (2) The dry gel was calcined under an inert atmosphere to obtain three-dimensional porous sodium iron pyrophosphate; (3) According to the stoichiometric ratio of sodium, iron and sulfur in the molecular formula Na2Fe(SO4)2, the sodium source, iron source and sulfur source are weighed and added to water, mixed evenly, and then the three-dimensional porous sodium iron pyrophosphate and complexing agent are added, heated, dried and sintered to obtain the sodium ion cathode material.

2. A method for preparing a sodium-ion cathode material, characterized in that, Includes the following steps: (1) According to the stoichiometric ratio of sodium, iron and phosphorus in the molecular formula Na4Fe3(PO4)2(P2O7), the sodium source, phosphorus source and iron source are weighed and added to water to obtain a solution. Then, polyurethane sponge and complexing agent are added, heated and dried to obtain a viscous gel. (2) The dry gel was calcined under an inert atmosphere to obtain three-dimensional porous sodium iron pyrophosphate; (3) According to the stoichiometric ratio of sodium, iron and sulfur in the molecular formula Na2Fe(SO4)2, the sodium source, iron source and sulfur source are weighed and added to water, mixed evenly, and then the three-dimensional porous sodium iron pyrophosphate and complexing agent are added, heated, dried and sintered to obtain the sodium ion cathode material.

3. The method for preparing a sodium-ion cathode material according to claim 2, characterized in that, In step (1), the phosphorus source includes at least one of phosphoric acid, sodium dihydrogen phosphate, or sodium hydrogen phosphate; The sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium nitrate, and sodium hydroxide; The iron source includes at least one of ferrous phosphate, ferrous carbonate, and ferrous oxalate.

4. The method for preparing a sodium-ion cathode material according to claim 2, characterized in that, In step (1), the mass ratio of the polyurethane sponge to the complexing agent is 5:1 to 5:5, and the amount of the complexing agent added is 2 to 10 times the number of iron moles in Na4Fe3(PO4)2(P2O7).

5. The method for preparing a sodium-ion cathode material according to claim 2, characterized in that, In step (1), the heating temperature is 60~90℃, and the temperature is heated until a viscous gel is formed. The drying temperature is 90~120℃, and the drying time is 18~26h.

6. A method for preparing a sodium-ion cathode material according to any one of claims 2-5, characterized in that, In step (2), the inert atmosphere is argon and / or nitrogen, the calcination temperature is 450~600℃, and the calcination time is 6~19h.

7. A method for preparing a sodium-ion cathode material according to any one of claims 2-5, characterized in that, In step (3), the sodium source includes at least one of sodium sulfate, sodium bisulfate, sodium carbonate, sodium bicarbonate, sodium nitrate, and sodium hydroxide; The iron source includes at least one of ferrous sulfate and ferrous carbonate; The sulfur source includes at least one of ferrous sulfate, sodium sulfate, or potassium sulfate.

8. A method for preparing a sodium-ion cathode material according to any one of claims 2-5, characterized in that, In steps (1) and (3), the complexing agent includes at least one of citric acid, ethylene glycol or polyvinyl alcohol.

9. A method for preparing a sodium-ion cathode material according to any one of claims 2-5, characterized in that, In step (3), the mass ratio of the three-dimensional porous sodium iron pyrophosphate to the complexing agent is 7:2 to 7:6, and the amount of complexing agent added is 2 to 10 times the number of moles of iron in Na2Fe(SO4)2.

10. A method for preparing a sodium-ion cathode material according to any one of claims 2-5, characterized in that, In step (3), the sintering temperature is 300~400℃ and the time is 12~18h.

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