A ni-al co-doped composite sodium iron phosphate pyrophosphate material, a preparation method and application thereof

Abstract

This application discloses a niobium-aluminum co-doped composite sodium iron pyrophosphate material, its preparation method, and its application, belonging to the technical field of sodium-ion battery cathode materials. The chemical formula of this niobium-aluminum co-doped composite sodium iron pyrophosphate material is Na. x Fe 3‑y‑z Nb y Al z (PO4)2P2O7, where y+z=0.1, and satisfying 0<y<0.1, 0<z<0.1, x, y, and z satisfy charge balance. Co-doping sodium iron pyrophosphate with niobium and aluminum synergistically enhances the material's electronic conductivity and ion diffusion. Simultaneously, the formation of Al-O-P enhances the local environment and strengthens the material's cycle stability, thereby significantly improving the rate performance and long-cycle stability of sodium-ion batteries.

Description

Technical Field

[0001] This application belongs to the field of phosphate material preparation technology, specifically relating to a niobium-aluminum co-doped composite sodium pyrophosphate material, its preparation method, and its application. Background Technology

[0002] The iron-based mixed phosphate polyanionic compound Na4Fe3(PO4)2P2O7 combines the advantages of iron-based phosphates, exhibiting a high theoretical specific capacity (129 mAh / g) and a high average operating voltage (3.1 mAh / g). vs .Na + Sodium-ion batteries (Na₄Fe₃(PO₄)₂P₂O₇) possess advantages such as good cycle stability, low cost, environmental friendliness, and abundant reserves, making them considered the most promising cathode material for large-scale energy storage. However, Na₄Fe₃(PO₄)₂P₂O₇ exhibits poor electronic conductivity and slow ion diffusion rate. Under high-current charge and discharge conditions, it is highly susceptible to polarization, increasing internal resistance and potentially leading to fire and explosion, severely limiting its application. Therefore, addressing the issues of low electronic conductivity and slow ion diffusion rate in polyanionic sodium-ion batteries is crucial.

[0003] In existing technologies, doping Na₄Fe₃(PO₄)₂P₂O₇ can improve the overall performance of materials, including initial-cycle specific capacity, electronic conductivity, sodium ion transport rate, and electrochemical properties. For example, Chinese patent application CN120072932A discloses a sodium iron pyrophosphate cathode material, where the dopant M is selected from one of Zr, Sc, Hf, Ta, W, Nb, Al, V, Ti, Cr, Co, Ni, Mn, and Cu. However, this technology only uses a single element for doping, resulting in a limited modification effect. It cannot achieve the combined effect of structural support and local environment optimization, thus restricting the potential for improving the overall electrochemical performance of the material, especially rate capability and long-cycle stability. Summary of the Invention

[0004] In view of this, the primary objective of this application is to provide a niobium-aluminum co-doped composite sodium iron pyrophosphate material, which utilizes niobium and aluminum to co-dope sodium iron pyrophosphate, thereby synergistically enhancing the electronic conductivity and ion diffusion of the material. At the same time, the formation of Al-OP enhances the local environment and strengthens the cycle stability of the material, thus significantly improving the rate performance and long-cycle stability of sodium-ion batteries.

[0005] To achieve the above objectives, this application adopts the following technical solution: One aspect of this application discloses a niobium-aluminum co-doped composite sodium iron pyrophosphate material, the general chemical formula of which is Na. x Fe 3-y-z Nb y Alz (PO4)2P2O7, where y+z=0.1, and satisfy 0<y<0.1, 0<z<0.1, and x, y, z satisfy charge balance.

[0006] Another aspect of this application discloses a method for preparing a niobium-aluminum co-doped composite sodium iron pyrophosphate material, wherein sodium source, iron source, niobium source, aluminum source, phosphorus source and carbon source are weighed according to stoichiometric ratio and mixed to prepare a precursor powder; The precursor powder was sintered in a non-oxidizing atmosphere to obtain niobium-aluminum co-doped composite sodium iron pyrophosphate material NFNAPP.

[0007] Another aspect of this application discloses a sodium-ion battery cathode containing the niobium-aluminum co-doped composite sodium iron pyrophosphate material described in this application or the niobium-aluminum co-doped composite sodium iron pyrophosphate material prepared by the preparation method described in this application.

[0008] Another aspect of this application discloses a sodium-ion battery, including the sodium-ion battery positive electrode described in this application.

[0009] This application has at least the following beneficial effects: The composite sodium iron pyrophosphate material in this application is co-doped with appropriate amounts of niobium and aluminum. Part of the niobium forms a three-dimensional network structure with carbon on the surface, while the other part replaces a portion of the iron and sodium incorporating into the material's crystal lattice. This has almost no impact on the crystal structure, morphology, and size, and does not disrupt the material's original structure. Through the co-doping of these two elements, the migration channels for sodium ions are broadened, reducing the migration barrier and improving rate performance.

[0010] The composite sodium iron pyrophosphate material in this application has good conductivity, high specific capacity, high rate performance, and long cycle life. After 200 cycles at a 2C current density, the capacity retention rate can reach up to 99.8%. Attached Figure Description

[0011] Figure 1 Na prepared in Example 2 of this application 3.85 Fe 2.9 Nb 0.025 Al 0.075 SEM image of (PO4)2P2O7 material.

[0012] Figure 2 The images show the XRD characterization of the composite materials prepared in Example 2 and Comparative Example 1 of this application.

[0013] Figure 3 This is a voltage-to-capacity curve of the sodium-ion battery prepared from the composite material in Example 2 of this application.

[0014] Figure 4The graph shows the cycle performance of the sodium-ion battery prepared by the composite material in Example 2 of this application under 2C conditions. Detailed Implementation

[0015] The embodiments of this application will be clearly and completely described below. The technical solutions in the embodiments described below are exemplary and only possible technical implementations of this application, not all possible implementations. Those skilled in the art can combine the embodiments of this application to obtain other embodiments without creative effort, and these embodiments are also within the protection scope of this application.

[0016] The first aspect of this application discloses a niobium-aluminum co-doped composite sodium iron pyrophosphate material, the general chemical formula of which is Na. x Fe 3-y-z Nb y Al z (PO4)2P2O7, where y+z=0.1, and satisfying 0<y<0.1, 0<z<0.1, x, y, and z satisfy charge balance. This application utilizes niobium and aluminum co-doped composite iron-sodium pyrophosphate material, wherein a portion of the niobium and aluminum forms a three-dimensional network structure with carbon on the surface, while another portion replaces part of the iron and sodium within the material lattice. Specifically, Nb... 5+ By introducing a large number of free electrons through a charge compensation mechanism, the electronic conductivity is greatly improved. Simultaneously, the Al-dominated structure remains stable. 3+ The strong Al-O bonds stabilize the lattice framework, suppressing volume changes and phase transitions during cycling and inhibiting Jan-Taylor distortion. Nb enhances conductivity, making the electrochemical reactions within the material more uniform and efficient, while Al ensures that the structure remains robust during high-throughput reactions and high-speed ion insertion / extraction. The two complement each other, achieving both high-rate performance and long cycle life.

[0017] As a preferred example, in the composite sodium iron pyrophosphate material, the stoichiometric ratio of Nb and Al satisfies 1:9 ≤ y / z ≤ 9:1. For example, y / z can be any ratio or a range between any two ratios from 1:9, 1:5, 1:3, 1:1, 2:1, 3:1, 5:1, to 9:1. More preferably, the stoichiometric ratio of y to z satisfies 1:9 ≤ x / y ≤ 1:3; even more preferably, y / z = 1 / 3. A suitable doping ratio of Nb and Al can achieve better battery performance. If the Nb doping level is too high, the number of sites that can participate in the reaction will decrease due to over-substitution of active metal sites, resulting in high-valence Nb... 5+Excessive doping with Al can excessively distort the local crystal lattice, introducing lattice defects or stress, and even disrupting the stability of the main crystal phase, resulting in impurity phases. Simultaneously, sodium ion diffusion channels may be blocked by excessively large atoms (Nb), thus reducing ionic conductivity. Excessive Al doping can lead to material conductivity even lower than undoped materials. This is because too many active sites are occupied by inert Al, excessively diluting the carrier concentration and degrading the battery's energy density and rate performance. Therefore, those skilled in the art can experimentally determine the appropriate stoichiometric ratio of Nb and Al to achieve the synergistic effect of co-doping within a suitable range.

[0018] Another aspect of this application discloses a method for preparing the niobium-aluminum co-doped composite iron-sodium pyrophosphate material described in this application, comprising the following steps: Sodium source, iron source, niobium source, aluminum source, phosphorus source and carbon source were weighed according to stoichiometric ratio and mixed to prepare precursor powder; The precursor powder was sintered in a non-oxidizing atmosphere to obtain niobium-aluminum co-doped composite sodium iron pyrophosphate material NFNAPP.

[0019] In this application, the sodium source, iron source, niobium source, aluminum source, phosphorus source, and carbon source may all be of types well known in the art, without any particular limitation. As an example, the sodium source is selected from at least one of sodium acetate, sodium carbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium nitrate, sodium oxalate, and sodium chloride; the iron source is selected from at least one of ferric phosphate, ferric oxide, ferrous oxide, and ferric pyrophosphate; the niobium source is selected from at least one of niobium pentoxide, niobium pentachloride, and niobium nitride; the aluminum source is selected from at least one of aluminum oxide and aluminum isopropoxide; the phosphorus source is selected from at least one of sodium dihydrogen phosphate, diammonium hydrogen phosphate, ferric phosphate, and ferric pyrophosphate; and the carbon source is selected from at least one of corn starch, polyethylene glycol, glucose, ascorbic acid, citric acid, and maltose. However, this application is not limited to the above examples.

[0020] Furthermore, the preparation of the precursor powder specifically includes the following steps: Sodium source, iron source, niobium source, aluminum source, phosphorus source and carbon source are dispersed evenly in a dispersant to obtain a dispersion; The dispersion was milled to obtain a slurry; The slurry is fed at a uniform speed and spray-dried to obtain a dried precursor powder.

[0021] The dispersant used is a medium with good dispersing properties, and specific examples include, but are not limited to, at least one of water and ethanol.

[0022] Furthermore, measured by mass fraction, the solid content of the slurry is 20% to 30%; the particle size D50 of the slurry is 0.1 to 1 μm.

[0023] In this application, the sand milling method can be any well-known method in the art, such as disc type, pin type, turbine type, with a sanding time of 1 to 3 hours and a rotation speed of 500 to 2000 rpm.

[0024] In this application, the inlet air temperature of the spray dryer is 200~300℃ and the outlet air temperature is 100~120℃.

[0025] In this application, by controlling the solid content and particle size of the slurry, the nano-size effect is utilized to significantly shorten the solid-phase diffusion path of sodium ions, alleviating volume strain during charge and discharge, thereby further improving the rate performance and cycle stability of the material. Simultaneously, the uniform and fine particles facilitate the atomic-level uniform mixing of elements in the precursor stage, laying the foundation for subsequent sintering to generate a high-purity crystalline phase.

[0026] In this application, the sintering is a two-stage sintering: In the first stage, the temperature is increased to 200-300℃ at a rate of 2-4℃ / min, and then maintained at this temperature for 3-4 hours. In the second stage, the temperature is increased to 400-600℃ at a rate of 2-4℃ / min, and then held at this temperature for 10-20 hours. And / or, the non-oxidizing atmosphere is at least one of nitrogen, rare gas, and 5%~15% H2 / N2 mixture.

[0027] The composite sodium iron pyrophosphate material in this application employs a two-stage sintering process. First, in a low-temperature pre-calcination stage (200-300℃), the organic carbon source is slowly decomposed, initially forming a porous structure and constructing an amorphous carbon conductive framework, effectively preventing the collapse of the precursor structure due to vigorous gas generation. Subsequently, in a high-temperature sintering stage (400-600℃), a solid-state reaction is driven to generate a high-purity NFPP crystalline phase, and the carbon layer is further graphitized, forming a uniform and dense carbon coating layer. Through the synergistic effect of the two-stage process, not only is the electronic conductivity and structural stability of the material significantly improved, but the generation of impurity phases such as NaFePO4 is also effectively suppressed, thereby endowing the cathode material with higher specific capacity, excellent rate performance, and ultra-long cycle life.

[0028] The third aspect of this application discloses a sodium-ion battery cathode containing either the niobium-aluminum co-doped composite sodium iron pyrophosphate material described in this application or the niobium-aluminum co-doped composite sodium iron pyrophosphate material prepared by the preparation method described in this application.

[0029] The fourth aspect of this application discloses a sodium-ion battery, including the sodium-ion battery positive electrode described in this application.

[0030] The present application will be further illustrated below with reference to specific embodiments. It should be noted that the specific embodiments below are for illustrative purposes only and do not limit the scope of the present application in any way.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0032] In addition, unless otherwise specified, methods without detailed conditions or steps are conventional methods, and the reagents and materials used are commercially available.

[0033] Example 1 This embodiment discloses a Na 3.88 Fe 2.9 Nb 0.01 Al 0.09 The preparation method of (PO4)2P2O7 material, with specific steps as follows: Sodium carbonate, ferric phosphate, sodium dihydrogen phosphate, niobium nitrate, and aluminum isopropoxide were weighed according to the molar ratio of Na, Fe, P, Nb, and Al as 3.88:2.9:4:0.01:0.09. Then, 10% glucose and 5% polyethylene glycol (used as a raw material dispersant) of the total weight of the positive electrode material were added, and a certain amount of water was added to adjust the solid content of the raw material to 25%.

[0034] The above slurry was pre-dispersed evenly and then milled in a sand mill at 2000 rpm for 2 hours. The milled slurry was then spray-dried at an inlet air temperature of 260°C and an outlet air temperature of 100°C.

[0035] The dried precursor powder was subjected to air jet milling. The milled precursor was then pre-calcined at 250°C for 3 hours under a nitrogen atmosphere at a heating rate of 2°C / min, followed by calcination at 500°C for 10 hours at a heating rate of 2°C / min to obtain Na. 3.88 Fe 2.9 Nb 0.01 Al 0.09 (PO4)2P2O7 material.

[0036] Example 2 This embodiment discloses a Na 3.85 Fe 2.9 Nb 0.025 Al 0.075 The specific steps for obtaining (PO4)2P2O7 material are as follows: Weigh out sodium carbonate, ferric phosphate, sodium dihydrogen phosphate, niobium nitrate, and aluminum isopropoxide according to the molar ratio of Na, Fe, P, Nb, and Al as 3.85:2.9:4:0.025:0.075. Then add 10% glucose and 5% polyethylene glycol (used as a carbon source and dispersant) of the total weight of the positive electrode material, and add a certain amount of water to adjust the solid content of the raw materials to 25%.

[0037] The above slurry was pre-dispersed evenly and then milled in a sand mill at 2000 rpm for 2 hours. The milled slurry was then spray-dried at an inlet air temperature of 260°C and an outlet air temperature of 100°C.

[0038] The dried precursor powder was subjected to air jet milling. The milled precursor was then pre-calcined at 250°C for 3 hours under a nitrogen atmosphere at a heating rate of 2°C / min, followed by calcination at 500°C for 10 hours at a heating rate of 2°C / min to obtain Na. 3.85 Fe 2.9 Nb 0.025 Al 0.075 (PO4)2P2O7 material.

[0039] Example 3 This embodiment discloses a Na 3.8 Fe 2.9 Nb 0.05 Al 0.05 The specific steps for obtaining (PO4)2P2O7 material are as follows: Weigh out sodium carbonate, ferric phosphate, sodium dihydrogen phosphate, niobium nitrate, and aluminum isopropoxide according to the molar ratio of Na, Fe, P, Nb, and Al of 3.8:2.9:4:0.05:0.05. Then add 10% glucose and 5% polyethylene glycol (used as a raw material dispersant) of the total weight of the positive electrode material, and add a certain amount of water to adjust the solid content of the raw material to 25%.

[0040] The above slurry was pre-dispersed evenly and then milled in a sand mill at 2000 rpm for 2 hours. The milled slurry was then spray-dried at an inlet air temperature of 260°C and an outlet air temperature of 100°C.

[0041] The dried precursor powder was subjected to air jet milling. The milled precursor was then pre-calcined at 250°C for 3 hours under a nitrogen atmosphere at a heating rate of 2°C / min, followed by calcination at 500°C for 10 hours at a heating rate of 2°C / min to obtain Na. 3.8 Fe 2.9 Nb 0.05 Al 0.05 (PO4)2P2O7 material.

[0042] Example 4 This embodiment discloses a Na 3.75 Fe 2.9 Nb 0.075 Al 0.025 The specific steps for obtaining (PO4)2P2O7 material are as follows: Weigh out sodium carbonate, ferric phosphate, sodium dihydrogen phosphate, niobium nitrate, and aluminum isopropoxide according to the molar ratio of Na, Fe, P, Nb, and Al as 3.75:2.9:4:0.075:0.025. Then add 10% glucose and 5% polyethylene glycol (used as a raw material dispersant) of the total weight of the positive electrode material, and add a certain amount of water to adjust the solid content of the raw material to 25%.

[0043] The above slurry was pre-dispersed evenly and then milled in a sand mill at 2000 rpm for 2 hours. The milled slurry was then spray-dried at an inlet air temperature of 260°C and an outlet air temperature of 100°C.

[0044] The dried precursor powder was subjected to air jet milling. The milled precursor was then pre-calcined at 250°C for 3 hours under a nitrogen atmosphere at a heating rate of 2°C / min, followed by calcination at 500°C for 10 hours at a heating rate of 2°C / min to obtain Na. 3.75 Fe 2.9 Nb 0.075 Al 0.025 (PO4)2P2O7 material.

[0045] Example 5 This embodiment discloses a Na 3.72 Fe 2.9 Nb 0.09 Al 0.01 The specific steps for obtaining (PO4)2P2O7 material are as follows: Weigh out sodium carbonate, ferric phosphate, sodium dihydrogen phosphate, niobium nitrate, and aluminum isopropoxide according to the molar ratio of Na, Fe, P, Nb, and Al of 3.72:2.9:4:0.09:0.01. Then add 10% glucose and 5% polyethylene glycol (used as a raw material dispersant) of the total weight of the positive electrode material, and add a certain amount of water to adjust the solid content of the raw material to 25%.

[0046] The above slurry was pre-dispersed evenly and then milled in a sand mill at 2000 rpm for 2 hours. The milled slurry was then spray-dried at an inlet air temperature of 260°C and an outlet air temperature of 100°C.

[0047] The dried precursor powder was subjected to air jet milling. The milled precursor was then pre-calcined at 250°C for 3 hours under a nitrogen atmosphere at a heating rate of 2°C / min, followed by calcination at 500°C for 10 hours at a heating rate of 2°C / min to obtain Na. 3.72 Fe 2.9 Nb 0.09 Al 0.01 (PO4)2P2O7 material.

[0048] Comparative Example 1 This comparative example discloses a method for preparing Na4Fe3(PO4)2P2O7 material, the specific steps of which are as follows: Sodium carbonate, ferric phosphate, and sodium dihydrogen phosphate were weighed according to a molar ratio of Na, Fe, and P of 4:3:4. Then, 10% glucose and 5% polyethylene glycol (used as a raw material dispersant) of the total weight of the positive electrode material were added, followed by a certain amount of water to adjust the solid content of the raw materials to 25%. The subsequent preparation method was the same as in Example 2, thus obtaining the Na4Fe3(PO4)2P2O7 material.

[0049] Comparative Example 2 This comparative example discloses a Na 3.7 Fe 2.9 Nb 0.1 The specific steps for obtaining (PO4)2P2O7 material are as follows: Sodium carbonate, ferric phosphate, sodium dihydrogen phosphate, and niobium nitrate were weighed according to the molar ratio of Na, Fe, P, and Nb of 3.7:2.9:4:0.1. Then, 10% glucose and 5% polyethylene glycol (used as a raw material dispersant) of the total weight of the positive electrode material were added, followed by a certain amount of water to adjust the solid content of the raw materials to 25%. The subsequent preparation method is the same as in Example 2, thus obtaining Na… 3.7 Fe 2.9 Nb 0.1 (PO4)2P2O7 material.

[0050] Comparative Example 3 This comparative example discloses a Na 3.9 Fe 2.9 Al 0.1 The specific steps for obtaining (PO4)2P2O7 material are as follows: Sodium carbonate, ferric phosphate, sodium dihydrogen phosphate, and aluminum isopropoxide were weighed according to the molar ratio of Na, Fe, P, and Al of 3.9:2.9:4:0.1. Then, 10% glucose and 5% polyethylene glycol (used as a raw material dispersant) of the total weight of the positive electrode material were added, followed by a certain amount of water to adjust the solid content of the raw materials to 25%. The subsequent preparation method is the same as in Example 2, thus obtaining Na… 3.9 Fe2.9 Al 0.1 (PO4)2P2O7 material.

[0051] Comparative Example 4 This comparative example discloses a Na 3.325 Fe 2.725 Nb 0.2 Al 0.075 The specific steps for obtaining (PO4)2P2O7 material are as follows: Sodium carbonate, ferric phosphate, sodium dihydrogen phosphate, and aluminum isopropoxide were weighed according to the molar ratio of Na, Fe, P, Nb, and Al of 3.325:2.725:4:0.2:0.075. Then, 10% glucose and 5% polyethylene glycol (used as a raw material dispersant) of the total weight of the positive electrode material were added, followed by a certain amount of water to adjust the solid content of the raw materials to 25%. The subsequent preparation method is the same as in Example 2, thus obtaining Na… 3.325 Fe 2.725 Nb 0.2 Al 0.075 (PO4)2P2O7 material.

[0052] Comparative Example 5 This comparative example discloses a Na 3.725 Fe 2.775 Nb 0.025 Al 0.2 The specific steps for obtaining (PO4)2P2O7 material are as follows: Sodium carbonate, ferric phosphate, sodium dihydrogen phosphate, and aluminum isopropoxide were weighed according to the molar ratio of Na, Fe, P, Nb, and Al of 3.725:2.775:4:0.025:0.2. Then, 10% glucose and 5% polyethylene glycol (used as a raw material dispersant) of the total weight of the positive electrode material were added, followed by a certain amount of water to adjust the solid content of the raw materials to 25%. The subsequent preparation method is the same as in Example 2, thus obtaining Na. 3.725 Fe 2.775 Nb 0.025 Al 0.2 (PO4)2P2O7 material.

[0053] Comparative Example 6 Compared with Example 2, the only difference is the calcination process. Specifically, the dried precursor powder is subjected to airflow crushing, and the crushed precursor is calcined at 500°C for 10 hours under a nitrogen atmosphere at a heating rate of 2°C / min to obtain Na. 3.85 Fe 2.9 Nb 0.025 Al 0.075 (PO4)2P2O7 material.

[0054] Comparative Example 7 Compared with Example 2, the only difference lies in the calcination process. Specifically, the dried yellow precursor powder is subjected to airflow crushing, and the crushed precursor is pre-calcined at 150°C for 3 hours under a nitrogen atmosphere at a heating rate of 2°C / min, and then calcined at 650°C for 10 hours at a heating rate of 2°C / min to obtain Na. 3.85 Fe 2.9 Nb 0.025 Al 0.075 (PO4)2P2O7 material.

[0055] Material characterization and performance testing (1) Material characterization in, Figure 1 The SEM image of the sintered material of Example 2 is shown, which shows that the particle size distribution is uniform and the coating effect is good.

[0056] Figure 2 The XRD patterns of Example 2 and Comparative Example 1 are shown. It can be seen that the small amount of Nb and Al doping has little effect on the crystal structure and does not destroy the crystal structure of the NFPP material itself.

[0057] (2) Performance testing The composite sodium iron pyrophosphate materials from Examples 1-5 and Comparative Examples 1-7 were assembled into CR2025 button batteries according to the following process: A homogeneous slurry was prepared by mixing sodium iron pyrophosphate, acetylene black, and polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidone (NMP) in a mass ratio of 8:1:1 and coated onto aluminum foil. After drying, the aluminum foil loaded with active material was cut into small circular pieces with a diameter of 11 mm using a cutting machine to serve as the positive electrode. A sodium metal sheet was used as the negative electrode of the half-cell, Celgard2300 was used as the separator, and sodium perchlorate solution was used as the electrolyte. The CR2025 button cell was assembled in an argon glove box.

[0058] The assembled CR2025 coin cells were subjected to constant current charge-discharge tests at different current densities using a CT2001A LAND battery tester. The 1C current density was defined as 129 mAh / g, the charge-discharge voltage range was 2.0V to 3.6V, and the test temperature was 25℃. The test results are shown in Table 1. Furthermore, Figure 3 and Figure 4 The charge-discharge performance curves and cycle performance diagrams under 2C rate conditions of Example 2 are shown.

[0059] Table 1 Test Results of CR2025 Button Cells

[0060] Table 1 shows that the niobium-aluminum co-doped composite sodium iron pyrophosphate materials prepared in Examples 1-5 exhibit significantly better electrochemical performance than the comparative examples. Comparative Example 1, without niobium and aluminum doping, resulted in more side reactions, leading to poor rate performance and cycling performance. Comparative Examples 2 and 3, with single Nb and Al doping respectively, showed some improvement in electrical performance compared to the undoped samples, but were still inferior to the mixed-doped samples. Nb doping significantly reduced the initial charge transfer impedance, enabling excellent fast-charging performance, while Al doping stabilized the crystal structure, suppressing phase transitions and volume changes during cycling, ensuring the integrity and stability of the electrode material structure. A stable structure also leads to a more stable electrode / electrolyte interface, indirectly benefiting charge transfer. Comparative Example 4 also underwent niobium-aluminum co-doping, but with excessive Nb doping, its theoretical capacity was significantly reduced because excessive substitution of active metal sites reduced the number of reaction sites. 5+ Excessive doping with Al can excessively distort the local crystal lattice, introducing lattice defects or stress, and even disrupting the stability of the main crystal phase, resulting in impurity phases. Simultaneously, sodium ion diffusion channels may be blocked by excessively large atoms (Nb), thus reducing ionic conductivity. In Comparative Example 5, excessive Al doping led to a material conductivity even lower than the undoped material. Too many active sites were occupied by inert Al, excessively diluting the carrier concentration and degrading the battery's energy density and rate performance. In Comparative Example 6, a single heating to 500℃ resulted in uneven carbonization on the precursor powder surface, forming an uneven carbon coating layer, affecting the material's conductivity. In Comparative Example 7, a first heating to a lower temperature followed by a second heating to a higher later temperature caused uneven phase transitions and the accumulation of internal stress, making the material's internal structure and morphology unstable, leading to poor electrochemical performance.

[0061] Furthermore, by Figure 3 and Figure 4 As can be seen, the niobium-aluminum co-doped sodium iron pyrophosphate material prepared in Example 2 has excellent electrical properties and almost no capacity decay under 2C cycling for 200 cycles. This is due to the successful incorporation of Nb and Al elements into the lattice of the sodium iron pyrophosphate material, achieving a synergistic effect.

[0062] In summary, this application involves niobium-aluminum co-doping in the material, Nb 5+ By introducing a large number of free electrons through a charge compensation mechanism, the electronic conductivity is greatly improved. The Al-dominated structure is stable. 3+ The strong Al-O bonds stabilize the lattice framework, suppressing volume changes and phase transitions during cycling and inhibiting Jamie-Taylor distortion. Nb enhances conductivity, making the electrochemical reactions within the material more uniform and efficient, while Al ensures that the structure remains robust during high-throughput reactions and high-speed ion insertion / extraction. The two complement each other, synergistically achieving high-rate performance and long cycle life.

[0063] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A niobium-aluminum co-doped composite sodium iron pyrophosphate material, characterized in that, The chemical formula of the niobium-aluminum co-doped composite sodium iron pyrophosphate material is Na. x Fe 3-y-z Nb y Al z (PO4)2P2O7, where y+z=0.1, and satisfy 0<y<0.1, 0<z<0.1, and x, y, z satisfy charge balance.

2. The niobium-aluminum co-doped composite iron pyrophosphate sodium material as described in claim 1, characterized in that, y and z satisfy 1:9≤y / z≤9:

1.

3. The niobium-aluminum co-doped composite sodium iron pyrophosphate material as described in claim 1, characterized in that, in, Part of the niobium and aluminum form a three-dimensional network structure with carbon on the surface, while another part replaces some of the iron and sodium in the material lattice.

4. A method for preparing a niobium-aluminum co-doped composite sodium iron pyrophosphate material, characterized in that, Includes the following steps: Sodium source, iron source, niobium source, aluminum source, phosphorus source and carbon source were weighed according to stoichiometric ratio and mixed to prepare precursor powder; The precursor powder is sintered in a non-oxidizing atmosphere to obtain the niobium-aluminum co-doped composite sodium iron pyrophosphate material as described in any one of claims 1-3.

5. The preparation method according to claim 4, characterized in that, The sodium source is selected from at least one of sodium acetate, sodium carbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium nitrate, sodium oxalate, and sodium chloride. And / or, the iron source is selected from at least one of ferric phosphate, ferric oxide, ferrous oxide, and ferric pyrophosphate; And / or, the phosphorus source is selected from at least one of sodium dihydrogen phosphate, diammonium hydrogen phosphate, ferric phosphate, and ferric pyrophosphate; And / or, the carbon source is selected from at least one of corn starch, polyethylene glycol, glucose, ascorbic acid, citric acid, and maltose.

6. The preparation method according to claim 4, characterized in that, The niobium source is selected from at least one of niobium pentoxide, niobium pentachloride, and niobium nitride; the aluminum source is selected from at least one of aluminum oxide and aluminum isopropoxide.

7. The preparation method according to claim 4, characterized in that, The preparation of the precursor powder includes the following steps: Sodium source, iron source, niobium source, aluminum source, phosphorus source and carbon source are dispersed evenly in a dispersant to obtain a dispersion; The dispersion was milled to obtain a slurry; The slurry is fed at a uniform speed and spray-dried to obtain a dried precursor powder. Preferably, the dispersant is at least one of water and ethanol; Preferably, the sand mill is one of disc type, pin type, or turbine type, and the sand milling time is 1~3 hours and the rotation speed is 500~2000 rpm. Preferably, the solid content of the slurry, measured by mass fraction, is 20% to 30%; the particle size D50 of the slurry is 0.1 to 1 μm. Preferably, the inlet air temperature of the spray dryer is 200~300℃ and the outlet air temperature is 100~120℃.

8. The preparation method according to claim 4, characterized in that, The sintering is a two-stage sintering process: In the first stage, the temperature is increased to 200-300℃ at a rate of 2-4℃ / min, and then maintained at this temperature for 3-4 hours. In the second stage, the temperature is increased to 400-600℃ at a rate of 2-4℃ / min, and then held at this temperature for 10-20 hours. And / or, the non-oxidizing atmosphere is at least one of nitrogen, rare gas, and 5%~15% (v / v) H2 / N2 mixture.

9. A sodium-ion battery positive electrode, characterized in that, The material contains the niobium-aluminum co-doped composite sodium iron pyrophosphate material according to any one of claims 1-3 or the niobium-aluminum co-doped composite sodium iron pyrophosphate material prepared by the preparation method according to any one of claims 4-8.

10. A sodium-ion battery, characterized in that, Includes the sodium-ion battery positive electrode as described in claim 9.

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