Sodium sulfate iron composite sodium ion positive electrode material and preparation method and battery thereof

Abstract

The present application relates to a kind of sodium sulfate iron composite sodium ion positive electrode material and its preparation method and battery, belong to sodium battery technical field, to solve at least one of the problems such as the poor air stability and conductivity of the sodium sulfate iron positive electrode material prepared by the existing method, easily lead to the capacity and cycle stability of battery is low, limit the application of sodium sulfate iron positive electrode material in sodium ion battery.The inner layer of the sodium ion positive electrode material of the present application is boron oxide coated sodium sulfate iron, the middle layer is boron-carbon nanotube transition layer, the outer layer is carbon nanotube coated sodium sodium pyrophosphate phosphate, boron-carbon nanotube transition layer is formed by the covalent bonding reaction of boron oxide and the functional group of CNTs after high-temperature induced decomposition of boric acid, double optimization is carried out in the interface combination and conductivity, improve the ion conductivity of sodium ion positive electrode material, the capacity of positive electrode material, rate and cycle performance, especially air stability.

Description

Technical Field

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

[0002] Sodium iron sulfate (Na2Fe2(SO4)3, abbreviated as NFS) has become a representative technical route for polyanionic cathode materials due to its high voltage platform, low cost and environmental friendliness, as a cathode material for sodium-ion batteries.

[0003] However, its practical applications are still limited by a series of structural stability issues. Specifically, the short Fe-Fe spacing in the crystal structure leads to strong Coulomb repulsion, which in turn causes structural instability and the formation of impurity phases. Currently, there are no reports on pure-phase Na2Fe2(SO4)3. Simultaneously, the presence of sulfate ions also makes the material prone to hydrolysis in air and results in poor stability. Therefore, how to improve the structural and environmental stability of the material while maintaining high electrochemical performance has become a core challenge that urgently needs to be addressed.

[0004] Prior art discloses the use of PO4 3- Partially replaces SO4 2- The strategy involves regulating the Na2Fe2(SO4)3 structure to reduce the Coulomb repulsion within the structure, thereby stabilizing the structure, suppressing the formation of impurity phases, and constructing a novel cathode material – Na. 2.9 Fe 1.7 (SO4) 2.7 (PO4) 0.3 Since the material is a bulk doped material, the surface sulfate ions are still in contact with the air, which fails to fundamentally solve the problem of the material being easily hydrolyzed and having poor stability in the air. At the same time, some capacity is sacrificed. Summary of the Invention

[0005] In view of the above analysis, the present invention aims to provide a sodium iron sulfate composite sodium ion cathode material, its preparation method and battery, to solve at least one of the problems of poor air stability and conductivity of sodium iron sulfate cathode materials prepared by existing methods, which easily leads to low battery capacity and cycle stability, thus limiting the application of sodium iron sulfate cathode materials in sodium ion batteries.

[0006] In a first aspect, the present invention provides a sodium ferric sulfate composite sodium ion cathode material, the cathode material comprising an inner layer, an intermediate layer and an outer layer, wherein the inner layer is sodium ferric sulfate coated with boron oxide, the outer layer is sodium ferric phosphate coated with carbon nanotubes, and the intermediate layer is a boron-carbon nanotube transition layer.

[0007] Secondly, the present invention provides a method for preparing the above-mentioned sodium ferric sulfate composite sodium ion cathode material, comprising the following steps:

[0008] (1) Preparation of carbon nanotube-coated sodium iron pyrophosphate: Sodium iron pyrophosphate was mixed with carboxyl and hydroxyl modified carbon nanotubes and heat-treated under an inert atmosphere to obtain CNTs@NFPP.

[0009] (2) Preparation of boron oxide-coated sodium ferric sulfate: Sodium ferric sulfate was mixed with boric acid and heat-treated under an inert atmosphere to obtain B2O3@NFS;

[0010] (3) The CNTs@NFPP and B2O3@NFS are mixed and heat-treated under an inert atmosphere to obtain the sodium ion cathode material.

[0011] Furthermore, in step (1), the D50 of sodium iron pyrophosphate is 150–500 nm, and the specific surface area is 14–20 m². 2 / g.

[0012] Furthermore, in step (1), the mass of the carboxyl and hydroxyl-modified carbon nanotubes is 0.5 to 2% of the mass of sodium iron pyrophosphate.

[0013] Furthermore, in step (1), the heat treatment temperature is 300-400℃ and the heat treatment time is 2-6h.

[0014] Furthermore, in step (2), the D50 of sodium ferric sulfate is 3–5 μm, and the specific surface area is 4–8 m². 2 / g.

[0015] Furthermore, in step (2), the mass of boric acid is 1.5 to 3% of the mass of sodium ferric sulfate.

[0016] Furthermore, in step (3), CNTs@NFPP and B2O3@NFS are mixed according to a mass ratio of sodium ferric pyrophosphate to sodium ferric sulfate of 1:9 to 2:3.

[0017] Furthermore, in step (3), the mixing is performed using a mechanical fusion precision mixer.

[0018] Furthermore, the parameters of the mixer are: rotor speed of 500-3000 rpm, stator speed of 50-200 rpm, and mixing time of 20-60 min.

[0019] Thirdly, the present invention provides a sodium-ion battery comprising the above-described positive electrode material.

[0020] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0021] (1) The inner layer of the sodium ion cathode material of the present invention is sodium iron sulfate coated with boron oxide, the middle layer is a boron-carbon nanotube transition layer, and the outer layer is sodium iron phosphate coated with carbon nanotubes. The boron-carbon nanotube transition layer is formed by covalent bonding reaction between boron oxide after decomposition of boric acid induced by high temperature and the functional groups of CNTs. The interface bonding and conductivity are optimized in two aspects, which improves the ionic conductivity, capacity, rate and cycle performance of the sodium ion cathode material, especially the air stability.

[0022] (2) The sodium ion cathode material of the present invention is prepared by mechanical fusion, which completely and uniformly coats sodium iron pyrophosphate with sodium iron sulfate, completely isolating it from air and avoiding structural damage, while also improving electrical performance. In addition, an in-situ chemical reaction occurs between the coating layers of sodium iron pyrophosphate and sodium iron sulfate to generate B-CNTs, forming a chemically bonded interfacial transition layer, thus achieving dual optimization in terms of interfacial bonding and conductivity.

[0023] (3) The preparation method of the present invention significantly reduces the complexity of operation and improves the efficiency of production line by simplifying the process steps. Its process parameter window is wide and highly controllable, and it has excellent scale-up characteristics, which can effectively support the mass production demand of sodium-ion battery cathode materials at the level of tens of thousands of tons.

[0024] (4) In this invention, sodium ferric pyrophosphate and sodium ferric sulfate with specific particle size and specific surface area are used for surface coating, which can significantly improve air stability and suppress side reactions, and the coating efficiency can reach more than 99%. By controlling the amount of raw materials added to regulate the amount of sodium ferric pyrophosphate and sodium ferric sulfate, the mass ratio of this invention can make the material have a higher specific capacity and a uniform coating effect, and can ensure the integrity of the sodium ferric sulfate coating without hindering the diffusion of sodium ions.

[0025] 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

[0026] 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.

[0027] Figure 1 The images show the Raman spectra of the cathode materials prepared in Example 1 and Comparative Example 9 of this invention. Detailed Implementation

[0028] 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.

[0029] In a specific embodiment of the present invention, a sodium ferric sulfate composite sodium ion cathode material is disclosed. The cathode material includes an inner layer, a middle layer and an outer layer. The inner layer is sodium ferric sulfate (NFS) coated with boron oxide, the outer layer is sodium ferric phosphate pyrophosphate (NFPP) coated with carbon nanotubes, and the middle layer is a boron-carbon nanotube (B-CNTs) transition layer.

[0030] Compared with the prior art, the sodium ion cathode material of the present invention has an inner layer of boron oxide-coated sodium iron sulfate, an intermediate layer of boron-carbon nanotube transition layer, and an outer layer of carbon nanotube-coated sodium iron pyrophosphate. The boron-carbon nanotube transition layer is formed by covalent bonding reaction between boron oxide after high-temperature induced decomposition of boric acid and the functional groups of CNTs. This dual optimization in terms of interface bonding and conductivity improves the ionic conductivity, capacity, rate capability, and cycle performance of the sodium ion cathode material, especially its air stability.

[0031] Another specific embodiment of the present invention discloses a method for preparing the above-mentioned sodium ion cathode material, comprising the following steps:

[0032] (1) Preparation of carbon nanotube-coated sodium iron pyrophosphate: Sodium iron pyrophosphate was mixed with carboxyl and hydroxyl modified carbon nanotubes and heat-treated under an inert atmosphere to obtain CNTs@NFPP.

[0033] (2) Preparation of boron oxide-coated sodium ferric sulfate: Sodium ferric sulfate was mixed with boric acid and heat-treated under an inert atmosphere to obtain B2O3@NFS;

[0034] (3) The CNTs@NFPP and B2O3@NFS are mixed and heat-treated under an inert atmosphere to obtain the sodium ion cathode material.

[0035] Compared with existing technologies, the method of this invention for preparing cathode materials allows for a completely uniform and dense coating of sodium iron pyrophosphate with sodium iron sulfate, completely isolating it from air and preventing structural damage, while simultaneously improving electrical performance. Furthermore, an in-situ chemical reaction occurs between the coating layers of sodium iron pyrophosphate and sodium iron sulfate to generate B-CNTs, forming a chemically bonded interfacial transition layer, thus achieving dual optimization in terms of both interfacial bonding and conductivity.

[0036] The preparation method of the present invention significantly reduces operational complexity and improves production line efficiency by simplifying process steps. Its process parameter window is wide and highly controllable, and it has excellent scalability characteristics, which can effectively support the mass production demand of tens of thousands of tons of sodium-ion battery cathode materials.

[0037] It should be noted that the carboxyl and hydroxyl-modified carbon nanotubes described in this invention are commercially available products or prepared using existing techniques. For example, they are prepared using commercially available carbon nanotubes (multi-walled CNTs, purity ≥98%, model: TNSM0, manufacturer: Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences). The specific method using existing techniques in this invention is as follows:

[0038] Concentrated H2SO4 (98%) and concentrated HNO3 (65%) were mixed at a volume ratio of 3:1 to obtain a mixed acid;

[0039] Add the mixed acid to a 250 mL round-bottom flask, place it in an ice-water bath and stir. Slowly add carbon nanotubes (at a ratio of 0.1-0.5 g CNTs / 100 mL mixed acid to avoid agglomeration). Remove the ice-water bath, heat the reaction system to 120 °C, and reflux and stir for 24 hours.

[0040] After the reaction is complete, cool the flask to room temperature, slowly add a large amount of distilled water to dilute it, transfer it to a centrifuge tube, centrifuge at 8000-12000 rpm for 10-15 minutes, discard the supernatant, and wash the precipitate repeatedly with distilled water (measure the pH of the supernatant after each centrifugation until it is close to neutral).

[0041] The washed product was dried in a vacuum drying oven at 60°C for 12 hours to obtain carboxyl and hydroxyl modified carbon nanotubes.

[0042] Specifically, in step (1), the D50 of sodium iron pyrophosphate is 150–500 nm, for example, 150 nm, 170 nm, 190 nm, 210 nm, 230 nm, 250 nm, 270 nm, 290 nm, 310 nm, 330 nm, 350 nm, 370 nm, 390 nm, 410 nm, 430 nm, 450 nm, 470 nm, 490 nm, and 500 nm, with a specific surface area of ​​14–20 m². 2 / g, for example, 14m 2 / g, 15m 2 / g, 16m 2 / g、17m 2 / g、18m 2 / g、19m 2 / g、20m 2 / g.

[0043] Specifically, in step (1), the mass of the carboxyl and hydroxyl-modified carbon nanotubes is 0.5% to 2% of the mass of sodium iron pyrophosphate, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2%.

[0044] It should be noted that the carbon nanotubes modified with carboxyl and hydroxyl groups have a large number of carboxyl and hydroxyl groups on their surface, making them readily react with boron oxide after the decomposition of boric acid.

[0045] Specifically, in step (1), the inert atmosphere is nitrogen.

[0046] Specifically, in step (1), the heat treatment temperature is 300-400℃, for example, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, and the heat treatment time is 2-6h, for example, 2h, 3h, 4h, 5h, 6h.

[0047] Specifically, in step (2), the D50 of sodium ferric sulfate is 3–5 μm, for example, 3 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4.0 μm, 4.2 μm, 4.4 μm, 4.6 μm, 4.8 μm, and 5 μm, and the specific surface area is 4–8 m². 2 / g, for example, 4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g.

[0048] Specifically, in step (2), the mass of boric acid is 1.5% to 3% of the mass of sodium ferric sulfate, for example, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, or 3%.

[0049] Specifically, in step (2), the inert atmosphere is nitrogen.

[0050] Specifically, in step (2), the heat treatment temperature is 250-350℃, for example, 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, and the heat treatment time is 2-6h, for example, 2h, 3h, 4h, 5h, 6h.

[0051] Specifically, in step (3), CNTs@NFPP and B2O3@NFS are mixed according to a mass ratio of sodium ferric pyrophosphate to sodium ferric sulfate of 1:9 to 2:3 (e.g., 1:9, 2:9, 3:9, 4:9, 5:9, 2:3).

[0052] It should be noted that the use of sodium ferric pyrophosphate and sodium ferric sulfate with the above-mentioned particle size and specific surface area for surface coating in this invention can significantly improve air stability and suppress side reactions, with a coating efficiency of over 99%. By controlling the amount of raw materials added to regulate the amounts of sodium ferric pyrophosphate and sodium ferric sulfate, the mass ratio of this invention can achieve a higher specific capacity and a more uniform coating effect, while ensuring the integrity of the sodium ferric sulfate coating without hindering sodium ion diffusion.

[0053] Specifically, in step (3), the mixing is performed using a mechanical fusion precision mixer.

[0054] It should be noted that the mechanical fusion precision mixer of the present invention is an existing device, for example, manufactured by Wuxi Xinguang Powder Technology Co., Ltd., and the equipment model is VS-3.

[0055] Preferably, the mixer's parameters are a rotor speed of 500–3000 rpm, for example, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, 1400 rpm, 1500 rpm, 1600 rpm, 1700 rpm, 1800 rpm, 1900 rpm, 2000 rpm, 2100 rpm, 2200 rpm, 2300 rpm, 2400 rpm, 2500 rpm, 2600 rpm, 2700 rpm, 2800 rpm. pm, 2900rpm, 3000rpm, stator speed is 50~200rpm, for example 50rpm, 60rpm, 70rpm, 80rpm, 90rpm, 100rpm, 110rpm, 120rpm, 130rpm, 140rpm, 150rpm, 160rpm, 170rpm, 180rpm, 190rpm, 200rpm, mixing time is 20~60min, for example 20min, 25min, 30min, 35min, 40min, 45min, 50min, 55min, 60min.

[0056] It should be noted that this invention employs mechanical fusion. When the material rotates at high speed in the rotor, it adheres tightly to the wall of the device under centrifugal force, passing rapidly between the rotor and the stator extrusion head. At this instant, the material is simultaneously subjected to extrusion and shear forces. Due to the high-speed rotation, the material circulates repeatedly between the rotor and stator, continuously subjected to extrusion and shear forces. Under the action of friction, the particle surface reaches a mechanically molten state, thereby coating nanoscale ultrafine powder onto micron-sized particles. This invention utilizes this principle to uniformly and densely coat sodium ferric pyrophosphate onto the surface of sodium ferric sulfate, forming a uniform and dense layer of sodium ferric phosphate on the surface of sodium ferric sulfate. This completely isolates the sodium ferric sulfate from air, fundamentally solving its air stability problem and improving the capacity, rate capability, and cycle life of the cathode material.

[0057] Specifically, in step (3), the inert atmosphere is nitrogen and / or argon.

[0058] Specifically, in step (3), the heat treatment temperature is 350-450℃, for example, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, and the heat treatment time is 3-6h, for example, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h.

[0059] It should be noted that at high temperatures, the boron on the surface of sodium ferric sulfate reacts with the CNTs of sodium ferric pyrophosphate. The carboxyl and hydroxyl functional groups on the carbon nanotubes undergo covalent bonding with B2O3 produced by the decomposition of boric acid to form BOC covalent bonds, thus forming B-CNTs. This achieves in-situ chemical reaction between the coating layers, forming a chemically bonded interfacial transition layer, which enhances the bonding force and ionic conductivity between materials and improves the electrical properties of the materials.

[0060] Another specific embodiment of the present invention discloses a sodium-ion battery comprising the above-mentioned sodium-ion cathode material.

[0061] The technical solution of the present invention will be further explained below with reference to specific embodiments.

[0062] The carboxyl and hydroxyl modified carbon nanotubes in the following examples were prepared using commercially available carbon nanotubes (multi-walled CNTs, purity ≥98%, model: TNSM0, manufacturer: Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences).

[0063] The specific preparation method is as follows:

[0064] Concentrated H2SO4 (98%) and concentrated HNO3 (65%) were mixed at a volume ratio of 3:1 to obtain a mixed acid;

[0065] Add the mixed acid to a 250 mL round-bottom flask, place it in an ice-water bath and stir. Slowly add carbon nanotubes (at a ratio of 0.1-0.5 g CNTs / 100 mL mixed acid to avoid agglomeration). Remove the ice-water bath, heat the reaction system to 120 °C, and reflux and stir for 24 hours.

[0066] After the reaction is complete, cool the flask to room temperature, slowly add a large amount of distilled water to dilute it, transfer it to a centrifuge tube, centrifuge at 8000-12000 rpm for 10-15 minutes, discard the supernatant, and wash the precipitate repeatedly with distilled water (measure the pH of the supernatant after each centrifugation until it is close to neutral).

[0067] The washed product was dried in a vacuum drying oven at 60°C for 12 hours to obtain carboxyl and hydroxyl modified carbon nanotubes.

[0068] Example 1

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

[0070] (1) Preparation of carbon nanotube-coated sodium iron pyrophosphate: Sodium iron pyrophosphate was mixed with carboxyl and hydroxyl-modified carbon nanotubes and heat-treated at 350℃ for 4 h under a nitrogen atmosphere to obtain CNTs@NFPP; wherein, the D50 of sodium iron pyrophosphate was 150 nm and the specific surface area was 18 m². 2 / g, the mass of carbon nanotubes is 1.5% of the mass of sodium iron pyrophosphate;

[0071] (2) Preparation of boron oxide-coated sodium ferric sulfate: Sodium ferric sulfate and boric acid were mixed and heat-treated at 300℃ for 4 h under a nitrogen atmosphere to obtain B2O3@NFS, wherein the D50 of sodium ferric sulfate was 3 μm and the specific surface area was 6 m². 2 / g, the mass of boric acid is 2% of the mass of sodium ferric sulfate;

[0072] (3) The CNTs@NFPP and B2O3@NFS were mixed in a mechanical fusion precision mixer at a mass ratio of sodium iron pyrophosphate to sodium iron sulfate of 1:3. The mixer parameters were: rotor speed of 3000 rpm, stator speed of 200 rpm, and mixing time of 60 min. The mixture was then heat-treated in a nitrogen atmosphere at a temperature of 450 °C for 4 h to obtain the sodium ion cathode material.

[0073] Example 2

[0074] This embodiment provides a method for preparing a sodium-ion cathode material, comprising the following steps:

[0075] (1) Preparation of carbon nanotube-coated sodium iron pyrophosphate: Sodium iron pyrophosphate was mixed with carboxyl and hydroxyl-modified carbon nanotubes and heat-treated at 300℃ for 6 h under a nitrogen atmosphere to obtain CNTs@NFPP; the CNTs@NFPP had a D50 of 325 nm and a specific surface area of ​​14 m². 2 / g, the mass of carbon nanotubes is 0.5% of the mass of sodium iron pyrophosphate;

[0076] (2) Preparation of boron oxide-coated sodium ferric sulfate: Sodium ferric sulfate and boric acid were mixed and heat-treated at 250℃ for 6 h under a nitrogen atmosphere to obtain B2O3@NFS, wherein the D50 of sodium ferric sulfate was 4 μm and the specific surface area was 4 m². 2 / g, the mass of boric acid is 1.5% of the mass of sodium ferric sulfate;

[0077] (3) The CNTs@NFPP and B2O3@NFS were mixed in a mechanical fusion precision mixer at a mass ratio of sodium iron pyrophosphate to sodium iron sulfate of 1:2. The mixer parameters were: rotor speed of 1750 rpm, stator speed of 125 rpm, mixing time of 40 min, and heat treatment under nitrogen atmosphere at a temperature of 350℃ for 6 h to obtain the sodium ion cathode material.

[0078] Example 3

[0079] This embodiment provides a method for preparing a sodium-ion cathode material, comprising the following steps:

[0080] (1) Preparation of carbon nanotube-coated sodium iron pyrophosphate: Sodium iron pyrophosphate was mixed with carboxyl and hydroxyl-modified carbon nanotubes and heat-treated at 400℃ for 2 h under a nitrogen atmosphere to obtain CNTs@NFPP; wherein, the D50 of sodium iron pyrophosphate was 500 nm and the specific surface area was 20 m². 2 / g, the mass of carbon nanotubes is 2% of the mass of sodium iron pyrophosphate;

[0081] (2) Preparation of boron oxide-coated sodium ferric sulfate: Sodium ferric sulfate and boric acid were mixed and heat-treated at 350℃ for 2 h under a nitrogen atmosphere to obtain B2O3@NFS, wherein the D50 of sodium ferric sulfate was 5 μm and the specific surface area was 8 m². 2 / g, the mass of boric acid is 3% of the mass of sodium ferric sulfate;

[0082] (3) The CNTs@NFPP and B2O3@NFS were mixed in a mechanical fusion precision mixer at a mass ratio of sodium iron pyrophosphate to sodium iron sulfate of 0.5:2. The mixer parameters were: rotor speed of 500 rpm, stator speed of 50 rpm, mixing time of 20 min, and heat treatment under nitrogen atmosphere at a temperature of 400℃ for 6 h to obtain the sodium ion cathode material.

[0083] Example 4

[0084] The preparation method of a sodium ion positive electrode material in this embodiment is similar to that in Example 1, except that in step (3), the mass ratio of sodium iron pyrophosphate to sodium iron sulfate is 1:9.

[0085] Example 5

[0086] The preparation method of a sodium ion positive electrode material in this embodiment is similar to that in Example 1, except that in step (3), the mass ratio of sodium iron pyrophosphate to sodium iron sulfate is 2:3.

[0087] Example 6

[0088] The preparation method of the sodium ion cathode material in this embodiment is similar to that in Example 1, except that in step (1), the D50 of sodium iron pyrophosphate is 500 nm and the specific surface area is 14 m². 2 / g.

[0089] Example 7

[0090] The preparation method of the sodium ion cathode material in this embodiment is similar to that in Example 1, except that the mixing time in step (3) is 20 min.

[0091] Comparative Example 1

[0092] The preparation method of the sodium ion cathode material in this comparative example is similar to that in Example 1, except that steps (1) and (2) are omitted, and sodium iron pyrophosphate and sodium iron sulfate are directly mixed in a mechanical fusion precision mixer.

[0093] Comparative Example 2

[0094] The preparation method of the sodium ion cathode material in this comparative example is similar to that in Example 1, except that no heat treatment is performed in step (3).

[0095] Comparative Example 3

[0096] The preparation method of the sodium ion cathode material in this comparative example is similar to that in Example 1, except that steps (1) and (3) are omitted, that is, only the surface of sodium ferric sulfate is coated with boron as the sodium ion cathode material.

[0097] Comparative Example 4

[0098] The preparation method of the sodium ion cathode material in this comparative example is similar to that in Example 1, except that the mechanical mixing and heat treatment in step (3) are removed, that is, carbon nanotube-coated sodium iron phosphate and boron-coated sodium iron sulfate are added when preparing the cathode slurry.

[0099] Comparative Example 5

[0100] The preparation method of the sodium ion cathode material in this comparative example is similar to that in Example 1, except that in step (3), the rotor speed is 400 rpm and the stator speed is 220 rpm.

[0101] Comparative Example 6

[0102] The preparation method of the sodium ion cathode material in this comparative example is similar to that in Example 1, except that in step (3), the heat treatment temperature is 350°C and the heat treatment time is 7h.

[0103] Comparative Example 7

[0104] The preparation method of the sodium-ion cathode material in this comparative example is similar to that in Example 1, except that in step (1), the D50 of sodium iron pyrophosphate is 600 nm and the specific surface area is 10 m². 2 / g.

[0105] Comparative Example 8

[0106] The preparation method of the sodium ion cathode material in this comparative example is similar to that in Example 1, except that in step (2), the D50 of sodium ferric sulfate is 6 μm and the specific surface area is 10 m². 2 / g.

[0107] Comparative Example 9

[0108] The preparation method of the sodium ion cathode material in this comparative example is similar to that in Example 1, except that in step (2), the mass of boric acid is 0% of the mass of sodium ferric sulfate.

[0109] Experimental Example 1

[0110] (1) The positive electrode materials prepared in Examples 1-7 and Comparative Examples 1-9 were assembled into button cells. The specific method was as follows: 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 for homogenization, and the slurry was uniformly coated on 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). The sodium metal sheet was used as the counter electrode, and 1 mol / L NaClO4 was used as the electrolyte. The electrolyte was 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 glass fiber. The cells were assembled into 2032 type button cells in an argon-filled glove box.

[0111] The electrochemical performance of the button batteries prepared with the cathode materials of Examples 1-7 and Comparative Examples 1-9 were tested respectively, and the results are shown in Table 1.

[0112] Table 1

[0113]

[0114] According to Table 1, the specific capacity of discharge at 0.1C is 98-105 mAh / g at 25℃, the specific capacity of discharge at 1C is 88-98 mAh / g at 25℃, the specific capacity of discharge at 5C is 75-85 mAh / g at 25℃, the specific capacity of discharge at 10C is 58-72 mAh / g at 25℃, and the capacity retention rate after 100 cycles at 1C is 90.1-95.2%.

[0115] As can be seen from Example 1 and Comparative Example 1, sodium ferric pyrophosphate and sodium ferric sulfate were directly mixed without any coating. This indicates that the uncoated material is structurally unstable and easily hydrolyzed upon contact with air, resulting in impaired ion transport and a significant decrease in electrochemical performance.

[0116] As can be seen from Example 1 and Comparative Example 2, the absence of a B-CNTs transition layer indicates that the high-temperature induced in-situ reaction between boron and CNTs is the core of forming the chemical bonding interface. The lack of this step will lead to increased interlayer impedance and reduced ion / electron transport efficiency.

[0117] As can be seen from Example 1 and Comparative Example 3, although the performance is improved compared to the uncoated Comparative Example 1 due to the lack of outer NFPP@CNTs coating, the air stability is still insufficient because the sodium ferric sulfate surface is not completely isolated, and the conductivity optimization is limited due to the lack of B-CNTs transition layer.

[0118] As can be seen from Example 1 and Comparative Example 4, when the coating material is directly mixed during the preparation of the positive electrode slurry, a uniform and dense coating layer is not formed, which proves that mechanical fusion of the pre-coated coating layer can significantly improve the interfacial bonding force and coating uniformity.

[0119] As can be seen from Example 1 and Comparative Example 5, the rotor speed is lower than that of Example 1, resulting in uneven coating. This proves that high-speed mechanical fusion can achieve dense adhesion of nanoscale coating through shear force and extrusion force.

[0120] As can be seen from Example 1 and Comparative Example 6, the heat treatment temperature is lower than that of Example 1, the amount of B-CNTs transition layer is lower than that of Example 1, and the optimization of conductivity is limited.

[0121] As can be seen from Example 1 and Comparative Example 7, the NFSS particle size of Comparative Example 7 is larger than that of Example 1, which affects the coating effect.

[0122] As can be seen from Example 1 and Comparative Example 8, the larger NFS size in Comparative Example 8 increases the ion transport path and affects the electrical performance.

[0123] As can be seen from Example 1 and Comparative Example 9, in the absence of boron oxide, the lack of a chemically bonded interfacial transition layer results in poor electrical performance.

[0124] As can be seen from Examples 1 and 4, insufficient NFPP dosage affects the coating of NFS, and the electrical performance of Example 4 is slightly lower than that of Example 1.

[0125] As can be seen from Examples 1 and 5, excessive NFPP dosage will hinder sodium ion diffusion, and the electrical performance of Example 5 is slightly lower than that of Example 1.

[0126] As can be seen from Examples 1 and 6, the particle size of Example 6 is larger than that of Example 1 (150 nm), which leads to an increase in the porosity of the coating layer. The electrical performance of Example 6 is slightly lower than that of Example 1.

[0127] As can be seen from Examples 1 and 7, the lower the mixing time, the lower the uniformity of the coating layer, and the electrical performance of Example 7 is slightly lower than that of Example 1.

[0128] (2) The Raman spectra of the cathode materials prepared in Example 1 and Comparative Example 9 were tested, such as... Figure 1 As shown in the figure, the ID / IG of Comparative Example 9 is 1.05, which proves that it contains a large number of defects. The free energy at the defect is high and the binding energy is low. Boron preferentially reacts at the defects of CNTs, reducing the number of defects and causing the ID / IG to decrease to 0.83. This proves that an in-situ chemical reaction occurs between the coating layers of sodium iron pyrophosphate and sodium iron sulfate to generate B-CNTs, forming a chemically bonded interfacial transition layer.

[0129] The same experiments were also conducted on other embodiments of the present invention, and the results were basically the same. Due to space limitations, they will not be listed one by one.

[0130] 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 ferric sulfate composite sodium ion cathode material, characterized in that, The cathode material comprises an inner layer, a middle layer, and an outer layer. The inner layer is sodium iron sulfate coated with boron oxide, the outer layer is sodium iron pyrophosphate coated with carbon nanotubes, and the middle layer is a boron-carbon nanotube transition layer. The cathode material is prepared by the following method: (1) Preparation of carbon nanotube-coated sodium iron pyrophosphate: Sodium iron pyrophosphate was mixed with carboxyl and hydroxyl-modified carbon nanotubes and heat-treated under an inert atmosphere to obtain CNTs@NFPP; (2) Preparation of boron oxide coated sodium ferric sulfate: Sodium ferric sulfate was mixed with boric acid and heat-treated under an inert atmosphere to obtain B2O3@NFS; (3) The CNTs@NFPP and B2O3@NFS are mixed and heat-treated under an inert atmosphere to obtain the sodium ion cathode material.

2. A method for preparing the sodium ferric sulfate composite sodium ion cathode material according to claim 1, characterized in that, Includes the following steps: (1) Preparation of carbon nanotube-coated sodium iron pyrophosphate: Sodium iron pyrophosphate was mixed with carboxyl and hydroxyl-modified carbon nanotubes and heat-treated under an inert atmosphere to obtain CNTs@NFPP; (2) Preparation of boron oxide coated sodium ferric sulfate: Sodium ferric sulfate was mixed with boric acid and heat-treated under an inert atmosphere to obtain B2O3@NFS; (3) The CNTs@NFPP and B2O3@NFS are mixed and heat-treated under an inert atmosphere to obtain the sodium ion cathode material.

3. The preparation method according to claim 2, characterized in that, In step (1), the D50 of sodium iron pyrophosphate is 150~500 nm, and the specific surface area is 14~20 m². 2 / g.

4. The preparation method according to claim 2, characterized in that, In step (1), the mass of the carboxyl and hydroxyl modified carbon nanotubes is 0.5 to 2% of the mass of sodium iron pyrophosphate.

5. The preparation method according to claim 2, characterized in that, In step (1), the heat treatment temperature is 300-400℃ and the heat treatment time is 2-6h.

6. The preparation method according to claim 2, characterized in that, In step (2), the D50 of sodium ferric sulfate is 3~5 μm, and the specific surface area is 4~8 m². 2 / g.

7. The preparation method according to claim 2, characterized in that, In step (2), the mass of boric acid is 1.5 to 3% of the mass of sodium ferric sulfate.

8. The preparation method according to claim 2, characterized in that, In step (3), CNTs@NFPP and B2O3@NFS are mixed according to a mass ratio of sodium ferric pyrophosphate to sodium ferric sulfate of 1:9 to 2:

3.

9. The preparation method according to claim 2, characterized in that, In step (3), the mixing is performed using a mechanical fusion precision mixer.

10. A sodium-ion battery, characterized in that, This includes the cathode material of claim 1 or the cathode material prepared by any one of claims 2-9.

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