A sodium iron pyrophosphate cathode material, its preparation method and application

Abstract

This invention discloses a sodium iron pyrophosphate cathode material, its preparation method, and its application, with the chemical formula Na₄Fe₂O₃. 3‑x‑y □ x HE y (PO4)2P2O7; where: □ represents Fe vacancies; HE represents dopant elements, including at least 5 of the following: Ni, Mg, Co, Mn, Cu, Zn, Cd, Al, and Cr, with each dopant element being equimolar; 0.08

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode material technology, and particularly relates to a sodium iron pyrophosphate cathode material, its preparation method and application. Background Technology

[0002] High-efficiency energy storage technology is a prerequisite for utilizing sustainable energy. In recent decades, the advent of lithium-ion batteries has greatly promoted the development of portable electronic devices and electric vehicles; however, a significant gap remains between supply and demand, particularly the limited lithium resources, which largely restrict the development of large-scale energy storage. In contrast, sodium-ion batteries, with their advantages of abundant and widely distributed sodium resources and low cost, have become an important supplement to large-scale energy storage. However, the large radius and heavy atomic mass of sodium ions easily lead to structural failure of electrodes and slow electrochemical reaction kinetics, accompanied by huge volume changes during charge and discharge, all of which hinder the commercialization of sodium-ion batteries. It is well known that the cathode plays a crucial role in determining the cost and electrochemical performance of a battery system; therefore, it is necessary to vigorously explore advanced cathodes with high operating voltage, high specific capacity, and high cycle stability. Among them, polyanionic cathode materials have attracted widespread attention due to their excellent structural stability, long cycle life, high safety, and abundant resources.

[0003] As the most representative iron-based phosphate cathode material, sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) has a high capacity of up to 129 mAh g. -1 Its theoretical capacity and voltage platform approaching 3.1V are due to its three-dimensional open Na... + The diffusion channel allows Na4Fe3(PO4)2P2O7 to exhibit small volume changes and strong structural stability during insertion / extraction. Furthermore, compared to toxic vanadium-based and structurally unstable manganese-based phosphate compounds, Na4Fe3(PO4)2P2O7 cathodes offer the dual advantages of low cost and high performance, showing promising commercialization prospects, especially in high power density applications. However, the electrochemical performance of Na4Fe3(PO4)2P2O7 is somewhat limited due to its poor electronic conductivity and slow sodium ion diffusion behavior, particularly in rate capability and cycle stability. To address these issues, various strategies are frequently employed, such as nanostructuring, carbon coating (e.g., carbon black, graphene, carbon nanotubes), and elemental doping (e.g., Mn). 2+ Co 2+ Mg 2+ Cu 2+Optimization methods such as [list of methods would be inserted here]. Furthermore, the inability to precisely introduce the two anionic groups during the synthesis process results in the presence of unremovable impurity phases in the synthesis of Na4Fe3(PO4)2P2O7. In particular, current research on the synthesis methods of high-performance Na4Fe3(PO4)2P2O7 materials mainly focuses on ball milling and spray drying methods. Developing a simple, efficient, low-cost, and consistent material synthesis method would also have significant practical value.

[0004] Therefore, how to effectively improve the intrinsic conductivity and structural stability of sodium iron pyrophosphate cathode material through simple synthesis methods, enhance the material's tolerance under high current density and long-term cycling stability, and at the same time ensure the phase purity of the material is a technical problem that urgently needs to be solved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the background art above, and to provide a high-performance, low-cost sodium iron pyrophosphate cathode material, its preparation method and application.

[0006] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0007] A sodium iron pyrophosphate cathode material with the chemical formula Na₄Fe₂O₃ 3-x-y □ x HE y (PO4)2P2O7; where: □ represents Fe vacancies; HE represents dopant elements, including at least 5 of the following: Ni, Mg, Co, Mn, Cu, Zn, Cd, Al, and Cr, with each dopant element being equimolar; 0.08 <x≤0.1,0.2≤y≤0.3。

[0008] As a further improvement, the surface of the cathode material is coated with a carbon layer, wherein the carbon content accounts for 5-12 wt% of the cathode material.

[0009] The present invention also provides a method for preparing the sodium iron pyrophosphate cathode material, comprising the following steps:

[0010] Step S1: Dissolve carbon source A and carbon source B in water and sonicate to obtain a carbon source solution;

[0011] Step S2: Add the iron source and the metal dopant source to the solution in step S1 according to the stoichiometric ratio, and heat and stir.

[0012] Step S3: Add sodium source and phosphorus source to the solution in step S2 according to stoichiometric ratio, heat and stir, and then evaporate to dryness to obtain gel;

[0013] Step S4: Dry the gel from step S3, grind it into powder, and then compress it into tablets to obtain a tablet precursor.

[0014] Step S5: Calcine the sheet-like precursor obtained in step S4 under an inert atmosphere, then cool and grind it to obtain the sodium iron pyrophosphate cathode material.

[0015] As a further improvement, the carbon source A in S1 is one or more of polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, sodium alginate, and polyacrylamide; and the carbon source B is one or more of sucrose, glucose, citric acid, ascorbic acid, oxalic acid, and oxalic acid.

[0016] As a further improvement, the concentration of carbon source A in the carbon source solution is controlled at 12-18 mg / mL, and the concentration of carbon source B in the carbon source solution is controlled at 65-75 mg / mL.

[0017] As a further improvement, carbon source A and carbon source B are polyvinylpyrrolidone and citric acid, respectively.

[0018] As a further improvement, the heating and stirring temperature in S2 and S3 is 60-90℃; the tableting pressure in S4 is 20-40MPa.

[0019] As a further improvement, the calcination temperature of S5 is 550-650℃, and the holding time is 8-12h.

[0020] The present invention also provides a sodium-ion battery positive electrode sheet, which comprises the sodium iron pyrophosphate positive electrode material, or the sodium iron pyrophosphate positive electrode material prepared by the method.

[0021] The present invention also provides a sodium-ion battery comprising the positive electrode of the sodium-ion battery.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] The applicant's research found that both Fe vacancy control strategies and high-entropy doping strategies have certain limitations in improving phase purity. The introduction of Fe vacancies reduces the stability of the material, and high-entropy doping can easily cause irreversible capacity loss.

[0024] To address this, this invention provides a high-performance, low-cost, multi-defect sodium iron pyrophosphate cathode material synergistically regulated by high-entropy doping and Fe vacancies. This material significantly improves intrinsic conductivity and structural stability, exhibits a high-purity phase structure and high power density, and enables ultra-stable cycling of sodium-ion batteries at high current densities. The introduction of Fe vacancies effectively suppresses the formation of the NaFePO4 impurity phase; however, excessive Fe vacancies can easily lead to structural collapse and reduce material stability. Therefore, the addition of various high-entropy dopant ions further stabilizes the unfavorable lattice distortion caused by Fe vacancies, improving the material's cycling stability. On one hand, the introduction of Fe vacancies not only ensures phase purity but also further increases the entropy value of sodium iron pyrophosphate; on the other hand, appropriate concentrations of high-entropy doping not only provide a flexible local environment but also stabilize the structural instability caused by Fe vacancies.

[0025] The sodium iron pyrophosphate cathode material provided by this invention exhibits good reversibility and small volume strain during charge and discharge. In in-situ XRD testing, the maximum volume change of the electrode material is only 3.8%, and the lattice parameters can return to their initial state after the test. It also has excellent high-power characteristics and exhibits an ultra-long cycle life (15,000 cycles at 5 A g) under high current density. -1 (under certain conditions) and excellent rate performance (61.1 mAh g) -1 In 10A g -1 Under the given conditions, this indicates that it has good application prospects.

[0026] Compared to the traditional sol-gel method, this invention employs a dual-carbon assisted sol-gel method. By controlling the concentration of two carbon sources with different pyrolysis characteristics, it achieves uniform in-situ carbon coating, microsphere morphology control, and uniform distribution of dopant elements. Simultaneously, it constructs an interwoven carbon matrix network, improving electronic conductivity and shortening the Na... + The diffusion path improved Na + The diffusion rate exhibits smaller volume changes and better structural stability during the insertion / extraction process, demonstrating significant practical value. This invention features a simple and easy-to-operate process, low cost, large-scale production capability, and good material consistency and stability. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1This is a macroscopic SEM image of the sodium-ion battery cathode material prepared in Example 1 of this invention;

[0029] Figure 2 This is a partial SEM image of the sodium-ion battery cathode material prepared in Example 1 of the present invention;

[0030] Figure 3 This is a macroscopic SEM image of the sodium-ion battery cathode material prepared in Comparative Example 0 of this invention;

[0031] Figure 4 This is a partial SEM image of the sodium-ion battery cathode material prepared in Comparative Example 0 of this invention;

[0032] Figure 5 The XRD spectra of the sodium-ion battery cathode materials prepared in Comparative Examples 1, 2, 3 and 1 of this invention are shown below.

[0033] Figure 6 The XRD patterns of the sodium-ion battery cathode materials prepared in Comparative Examples 1, 2, 3 and Example 1 of this invention are locally magnified.

[0034] Figure 7 The sodium-ion battery cathode materials prepared in Comparative Examples 1, 2, 3 and Example 1 of this invention are used in 10 mA g -1 Initial charge-discharge curves at current density;

[0035] Figure 8 The rate performance diagrams are for the sodium-ion battery cathode materials prepared in Comparative Examples 1, 2, 3 and Example 1 of this invention.

[0036] Figure 9 The sodium-ion battery cathode material prepared in Example 1 of this invention was subjected to a reaction at 5 A g. -1 Graphs of long-cycle electrochemical performance at current density;

[0037] Figure 10 The sodium-ion battery cathode materials obtained in Comparative Examples 1, 2, and 3 of this invention were tested at 5 A g. -1 Graphs of long-cycle electrochemical performance at current density;

[0038] Figure 11 The image shows the in-situ XRD pattern of the sodium-ion battery cathode material prepared in Example 1 of this invention during the initial two charge-discharge cycles. Detailed Implementation

[0039] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0040] Unless otherwise defined, all technical terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0041] Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment, etc. used in the present invention can be obtained through market purchase or can be prepared by existing methods.

[0042] The sodium iron pyrophosphate phosphate cathode material of the present invention has the chemical formula Na4Fe 3-x-y □ x HE y (PO4)2P2O7; wherein:

[0043] 0.08 < x ≤ 0.1, 0.2 ≤ y ≤ 0.3. In some embodiments, the value of x can be 0.085, 0.09, 0.095 or 0.1, or other values within the range of 0.08 < x ≤ 0.1; the value of y can be 0.2, 0.25 or 0.3, or other values within the range of 0.2 ≤ y ≤ 0.3.

[0044] HE is a doping element, including at least 5 of Ni, Mg, Co, Mn, Cu, Zn, Cd, Al, Cr, and each doping element is in equimolar amounts. In some embodiments, the types of doping elements can be any combination of five or six of Ni, Mg, Co, Mn, Cu, Zn, Cd, Al, Cr.

[0045] □ is an Fe vacancy, which means adding less than the stoichiometric ratio of Fe source to make some Fe sites vacant. The Fe vacancy is introduced by controlling the Fe content in the material system. The molar concentration of the Fe vacancy accounts for 2.7% - 3.3% of the Fe sites, that is, 96.7 - 97.3% of the Fe sites are occupied by Fe and doping elements, and the rest are Fe vacancies. The present invention obtains a high-purity Na4Fe3(PO4)2P2O7 single-phase structure through the synergistic regulation of Fe vacancies and high-entropy doping. A variety of metal elements (i.e., doping element HE) on the Fe sites are incorporated into the lattice in equimolar ratios to achieve the maximum entropy value. The introduction of Fe vacancies can inhibit the formation of the impurity phase NaFePO4 and further increase the entropy value of the system.

[0046] The matrix structure of the sodium iron pyrophosphate phosphate cathode material of the present invention is a Na4Fe3(PO4)2P2O7 single-phase structure with the space group Pn21a. All metal doping elements are incorporated into the Fe sites in equimolar amounts, and the doping elements have 2+ similar radii and good chemical compatibility, and no charge transfer occurs between elements.

[0047] This invention achieves configuration entropy regulation by introducing equimolar amounts of metal doping elements, and further alleviates the unfavorable lattice distortion caused by Fe vacancies through entropy stabilization effect, thereby improving the structural stability of the material.

[0048] The Fe vacancies, Fe elements, and various metal dopants in the material exhibit good chemical compatibility, effectively occupying Fe sites and providing the phosphate pyrophosphate material with a flexible and variable local environment, thus effectively mitigating the Na+ degradation. + Volumetric strain during the insertion / extraction process.

[0049] Therefore, the synergistic regulation of Fe vacancies and high-entropy doping in this invention can effectively improve the ionic / electronic conductivity of the material while ensuring the pure phase structure and cycle stability of the material. The two strategies achieve a synergistic effect of 1+1>2.

[0050] This invention successfully synthesizes a multi-defect sodium iron pyrophosphate cathode material with a perfect high-purity phase structure by introducing 2.7%-3.3% Fe vacancies (calculated based on Fe content).

[0051] The multi-defect sodium iron pyrophosphate cathode material of the present invention not only has a high-purity phase structure, but also the Fe vacancies and high entropy doping significantly improve the ionic / electronic conductivity and structural stability of the material, thus enabling stable cycling of more than 15,000 cycles at high rates.

[0052] In some embodiments, the cathode material of the present invention is sodium iron pyrophosphate spherical particles with a uniform thin carbon layer on the surface, and the carbon content is controlled at 5-12 wt% of the cathode material.

[0053] In some specific embodiments of the present invention, the preparation method of sodium iron pyrophosphate cathode material includes the following steps:

[0054] Step S1: Dissolve carbon source A and carbon source B in water and perform ultrasonic treatment to obtain a uniformly dispersed transparent carbon source solution.

[0055] In some embodiments, water-soluble carbon sources A and B are selected. Carbon source A is one of polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, sodium alginate, and polyacrylamide; carbon source B is one of sucrose, glucose, citric acid, ascorbic acid, oxalic acid, and oxalic acid. Carbon source A and carbon source B are preferably polyvinylpyrrolidone (PVP) and citric acid. The concentration of carbon source A is controlled at 12-18 mg / mL (preferably 14-16 mg / mL), and the concentration of carbon source B is controlled at 65-75 mg / mL (preferably 67-73 mg / mL).

[0056] This invention achieves uniform carbon coating and microsphere morphology control by adjusting the concentrations of two carbon sources (A and B) with different pyrolysis characteristics (unlike the irregular nanoparticles synthesized by traditional sol-gel methods). A uniform thin carbon layer is constructed in situ on the material surface, along with an interwoven carbon matrix network, forming a unique microsphere embedded in a continuous carbon matrix morphology. This improves electronic conductivity and shortens the Na+ pyrolysis time. + The diffusion path improved Na + Diffusion rate.

[0057] In some embodiments, the ultrasonic treatment time is 0.3-0.6 h, specifically 0.3, 0.4, 0.5 or 0.6 h, or other values ​​within this range.

[0058] Step S2: Add the iron source and the metal dopant source (HE) to the solution in step S1 according to the stoichiometric ratio, and heat and stir thoroughly.

[0059] In some embodiments, the iron source is one or more of ferric acetate, ferrous sulfate, ferric phosphate, ferric nitrate, triferric citrate, and ferric acetylacetone; the metal dopant source is one or more of the corresponding metal sulfate, nitrate, and acetate.

[0060] In some embodiments, the heating and stirring temperature is 60-90℃, the rotation speed is 300-500 rpm, and the processing time is 2-5 hours. The stirring temperature can be 60℃, 70℃, 80℃, or 90℃, or other values ​​within this range; the rotation speed can be 300, 350, 400, 450, or 500 rpm, or other values ​​within this range; and the processing time can be 2, 3, 4, or 5 hours, or other values ​​within this range.

[0061] Step S3: Add sodium source and phosphorus source to the solution in step S2 according to stoichiometric ratio, heat and stir until uniform, and then evaporate to dryness to obtain a light yellow gel.

[0062] In some embodiments, the sodium source is one or more of sodium acetate, sodium dihydrogen phosphate, sodium nitrate, and sodium pyrophosphate; the phosphorus source is one or more of diammonium hydrogen phosphate, diammonium dihydrogen phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate.

[0063] In some embodiments, the heating and stirring temperature is 60-90℃, the rotation speed is 300-500 rpm, and the processing time is 2-4 hours. The stirring temperature can be 60℃, 70℃, 80℃, or 90℃, or other values ​​within this range; the rotation speed can be 300, 350, 400, 450, or 500 rpm, or other values ​​within this range; and the processing time can be 2, 3, or 4 hours, or other values ​​within this range.

[0064] Step S4: Vacuum dry the gel from step S3, then grind it into powder, and compress it into tablets to obtain a tablet precursor.

[0065] In some embodiments, the vacuum drying temperature is 80-110℃, the drying time is 24-30h; the tablet diameter is 1-2cm, and the pressure is 20-40MPa. The drying temperature can be 80℃, 90℃, 100℃, or 110℃, or other values ​​within this range; the drying time can be 24, 26, 28, or 30h, or other values ​​within this range. The tablet diameter can be 1, 1.2, 1.5, 1.8, or 2cm, or other values ​​within this range; the pressure can be 20, 30, or 40MPa, or other values ​​within this range.

[0066] Direct sintering of powders can easily lead to unevenness. Using tableting can effectively improve the crystallinity of the material, resulting in a material with better uniformity and consistency.

[0067] Step S5: The sheet-like precursor obtained in step S4 is calcined at high temperature in an inert atmosphere, then cooled to room temperature and ground to obtain the sodium iron pyrophosphate cathode material of the present invention, which is characterized by high entropy doping and synergistic regulation of Fe vacancies.

[0068] In some embodiments, the inert gas is one of argon, nitrogen, an argon-hydrogen mixture, or a nitrogen-hydrogen mixture.

[0069] In some embodiments, the calcination temperature is 550-650℃, the holding time is 8-12h, and the heating rate is 2-5℃ / min. The calcination temperature can be 550, 600, 650℃, or other values ​​within this range; the holding time can be 8, 9, 10, 11, or 12h, or other values ​​within this range; and the heating rate can be 2, 3, 4, or 5℃ / min. -1 It can also be any other value within that range.

[0070] In some specific embodiments of the present invention, the positive electrode sheet of the sodium-ion battery is prepared from the sodium iron pyrophosphate positive electrode material of the present invention, conductive additives, binders and solvents.

[0071] In some embodiments, the conductive additive is one or more of acetylene black, Super P, and Ketjen black; the binder is polyvinylidene fluoride, and the solvent is N-methyl-2-pyrrolidone.

[0072] In some specific embodiments of the present invention, a sodium-ion battery includes a positive electrode sheet. The sodium-ion battery comprises a positive electrode shell, the positive electrode sheet, a separator, an organic electrolyte, metallic sodium, a gasket, a spring, and a negative electrode shell.

[0073] This invention provides a simple, easy-to-operate, and low-cost method for synthesizing dual-carbon assisted sol-gels, which can achieve in-situ carbon coating and morphology control, and achieve uniform distribution of dopant elements. It is an efficient method for preparing multi-defect sodium iron pyrophosphate cathode materials with good consistency.

[0074] The cathode material and its preparation method of the present invention will be described in detail below through specific embodiments.

[0075] Comparative Example 0: Preparation of Na4Fe3(PO4)2P2O7 sodium-ion battery cathode material with low carbon source concentration

[0076] This comparative example uses the sol-gel method to prepare the Na4Fe3(PO4)2P2O7 cathode material, which includes the following steps:

[0077] (1) Dissolve 0.5g polyvinylpyrrolidone (PVP) and 4.7g citric acid (C6H8O7) in deionized water (concentrations of 7.143mg / ml and 67.143mg / ml, respectively), sonicate for 30min, then add 15mmol ferric nitrate nonahydrate (Fe(NO3)3·9H2O), heat and stir at 80℃ and 400rpm for 5h, then add 20mmol sodium dihydrogen phosphate dihydrate (NaH2PO4), heat and stir at 80℃ and 500rpm for 3h, and then heat and evaporate to dryness at 90℃ and 500rpm to obtain a yellow gel precursor.

[0078] (2) The precursor obtained in step (1) was placed in a vacuum oven and dried at 100°C for 25 hours. The dried material was then ground and pressed into sheet material with a diameter of 1 cm under a pressure of 30 MPa. The obtained material sheet was heated to 600°C at a heating rate of 2°C / min under a hydrogen-argon mixture and held at that temperature for 12 hours. It was then naturally cooled to room temperature to obtain blank sample Na4Fe3(PO4)2P2O7 material with low carbon source concentration in situ carbon coating.

[0079] Comparative Example 1: Preparation of blank sample Na4Fe3(PO4)2P2O7 sodium-ion battery cathode material

[0080] This comparative example uses the sol-gel method to prepare the Na4Fe3(PO4)2P2O7 cathode material, which includes the following steps:

[0081] (1) Dissolve 1g of polyvinylpyrrolidone (PVP) and 4.7g of citric acid (C6H8O7) in deionized water (concentrations of 14.286mg / ml and 67.143mg / ml, respectively), sonicate for 30min, then add 15mmol of ferric nitrate nonahydrate (Fe(NO3)3·9H2O), heat and stir at 80℃ and 400rpm for 5h, then add 20mmol of sodium dihydrogen phosphate dihydrate (NaH2PO4), heat and stir at 80℃ and 500rpm for 3h, and then heat and evaporate to dryness at 90℃ and 500rpm to obtain a yellow gel precursor.

[0082] (2) The precursor obtained in step (1) was placed in a vacuum oven and dried at 100°C for 25 hours. The dried material was then ground and pressed into sheet material with a diameter of 1 cm under a pressure of 30 MPa. The obtained material sheet was heated to 600°C at a heating rate of 2°C / min under a hydrogen-argon mixture and held at that temperature for 12 hours. It was then naturally cooled to room temperature to obtain the blank sample Na4Fe3(PO4)2P2O7 material with in-situ carbon coating.

[0083] Comparative Example 2: Preparation of Fe vacancy-modified Na4Fe 2.9 □ 0.1 (PO4)2P2O7 sodium-ion battery cathode material

[0084] This comparative example was prepared using the sol-gel method, Na₄Fe 2.9 □ 0.1 The preparation method of (PO4)2P2O7 cathode material includes the following steps:

[0085] (1) Dissolve 1g of polyvinylpyrrolidone (PVP) and 4.7g of citric acid (C6H8O7) in deionized water (concentrations of 14.286mg / ml and 67.143mg / ml, respectively), sonicate for 30min, then add 14.5mmol of ferric nitrate nonahydrate (Fe(NO3)3·9H2O), heat and stir at 80℃ and 400rpm for 5h, then add 20mmol of sodium dihydrogen phosphate dihydrate (NaH2PO4), heat and stir at 80℃ and 500rpm for 3h, and then heat and evaporate to dryness at 90℃ and 500rpm to obtain a yellow gel precursor.

[0086] (2) The precursor obtained in step (1) was placed in a vacuum oven and dried at 100°C for 25 hours. The dried material was then ground and pressed into sheet material with a diameter of 1 cm under a pressure of 30 MPa. The obtained sheet material was heated to 600°C at a heating rate of 2°C / min under a hydrogen-argon mixture and held at that temperature for 12 hours. It was then naturally cooled to room temperature to obtain in-situ carbon-coated Fe vacancy-regulated Na4Fe.2.9 □ 0.1 (PO4)2P2O7 material.

[0087] Comparative Example 3: Preparation of High-Entropy Doped Na₄Fe 2.7 (Ni,Co,Mn,Cu,Zn,Mg) 0.05 (PO4)2P2O7 sodium-ion battery cathode material (0 Fe vacancies)

[0088] This comparative example was prepared using the sol-gel method, Na₄Fe 2.7 (Ni,Co,Mn,Cu,Zn,Mg) 0.05 The preparation method of (PO4)2P2O7 cathode material includes the following steps:

[0089] (1) Dissolve 1g of polyvinylpyrrolidone (PVP) and 4.7g of citric acid (C6H8O7) in deionized water (concentrations of 14.286mg / ml and 67.143mg / ml, respectively), and sonicate for 30min. Then add 13.5mmol of ferric nitrate nonahydrate (Fe(NO3)3·9H2O), 0.25mmol of nickel acetate tetrahydrate (NiC4H6O4·4H2O), 0.25mmol of cobalt acetate tetrahydrate (CoC4H6O4·4H2O), and 0.25mmol of manganese acetate tetrahydrate (MnC4H6O4). 0.25 mmol of copper acetate monohydrate (C4H6CuO4·H2O), 0.25 mmol of zinc acetate dihydrate (C4H6O4Zn·2H2O), and 0.25 mmol of magnesium acetate tetrahydrate (C4H6MgO4·4H2O) were added to the mixture, and the mixture was heated and stirred at 80 °C and 400 rpm for 5 h. Then, 20 mmol of sodium dihydrogen phosphate dihydrate (NaH2PO4) was added, and the mixture was heated and stirred at 80 °C and 500 rpm for 3 h. Finally, the mixture was heated and evaporated to dryness at 90 °C and 500 rpm to obtain a yellow gel precursor.

[0090] (2) The precursor obtained in step (1) was placed in a vacuum oven and dried at 100°C for 25 hours. The dried material was then ground and pressed into sheets with a diameter of 1 cm under a pressure of 30 MPa. The resulting sheets were heated to 600°C at a heating rate of 2°C / min under a hydrogen-argon mixture and held for 12 hours. After natural cooling to room temperature, in-situ carbon-coated high-entropy doped Na₄Fe was obtained. 2.7 (Ni,Co,Mn,Cu,Zn,Mg) 0.05 (PO4)2P2O7 material.

[0091] Example 1: Preparation of Na₄Fe₄ with synergistic regulation of Fe vacancies and high-entropy doping 2.6 □ 0.1(Ni,Co,Mn,Cu,Zn,Mg) 0.05 (PO4)2P2O7 sodium-ion battery cathode material (x = 0.1; y = 0.3)

[0092] This embodiment uses the sol-gel method for preparation, Na4Fe 2.6 □ 0.1 (Ni,Co,Mn,Cu,Zn,Mg) 0.05 The preparation method of (PO4)2P2O7 cathode material includes the following steps:

[0093] (1) Dissolve 1g of polyvinylpyrrolidone (PVP) and 4.7g of citric acid (C6H8O7) in deionized water (concentrations of 14.286mg / ml and 67.143mg / ml, respectively), and sonicate for 30min. Then add 13mmol of ferric nitrate nonahydrate (Fe(NO3)3·9H2O), 0.25mmol of nickel acetate tetrahydrate (NiC4H6O4·4H2O), 0.25mmol of cobalt acetate tetrahydrate (CoC4H6O4·4H2O), and 0.25mmol of manganese acetate tetrahydrate (MnC4H6O4·9H2O). 0.25 mmol of copper acetate monohydrate (C4H6CuO4·H2O), 0.25 mmol of zinc acetate dihydrate (C4H6O4Zn·2H2O), and 0.25 mmol of magnesium acetate tetrahydrate (C4H6MgO4·4H2O) were added to the mixture, and the mixture was heated and stirred at 80 °C and 400 rpm for 5 h. Then, 20 mmol of sodium dihydrogen phosphate dihydrate (NaH2PO4) was added, and the mixture was heated and stirred at 80 °C and 500 rpm for 3 h. Finally, the mixture was heated and evaporated to dryness at 90 °C and 500 rpm to obtain a yellow gel precursor.

[0094] (2) The precursor obtained in step (1) was placed in a vacuum oven and dried at 100°C for 25 hours. The dried material was then ground and pressed into sheet material with a diameter of 1 cm under a pressure of 30 MPa. The obtained sheet material was heated to 600°C at a heating rate of 2°C / min under a hydrogen-argon mixed gas, held at that temperature for 12 hours, and then naturally cooled to room temperature to obtain Na4Fe with in-situ carbon-coated Fe vacancies and high-entropy doping synergistic regulation. 2.6 □ 0.1 (Ni,Co,Mn,Cu,Zn,Mg) 0.05 (PO4)2P2O7 material.

[0095] Example 2: Preparation of Na₄Fe₄ with synergistic regulation of Fe vacancies and high-entropy doping 2.6 □ 0.1 (Ni,Co,Mn,Cu,Al,Mg) 0.05(PO4)2P2O7 sodium-ion battery cathode material (x = 0.1; y = 0.3)

[0096] This embodiment uses the sol-gel method for preparation, Na4Fe 2.6 □ 0.1 (Ni,Co,Mn,Cu,Al,Mg) 0.05 The preparation method of (PO4)2P2O7 cathode material includes the following steps:

[0097] (1) Dissolve 1g of polyvinylpyrrolidone (PVP) and 4.7g of citric acid (C6H8O7) in deionized water (concentrations of 14.286mg / ml and 67.143mg / ml, respectively), and sonicate for 30min. Then add 13mmol of ferric nitrate nonahydrate (Fe(NO3)3·9H2O), 0.25mmol of nickel acetate tetrahydrate (NiC4H6O4·4H2O), 0.25mmol of cobalt acetate tetrahydrate (CoC4H6O4·4H2O), and 0.25mmol of manganese acetate tetrahydrate (MnC4H6O4·9H2O). 0.25 mmol of copper acetate monohydrate (C4H6CuO4·H2O), 0.25 mmol of aluminum nitrate nonahydrate (Al(NO3)3·9H2O), and 0.25 mmol of magnesium acetate tetrahydrate (C4H6MgO4·4H2O) were added to the mixture, and the mixture was heated and stirred at 80 °C and 400 rpm for 5 h. Then, 20 mmol of sodium dihydrogen phosphate dihydrate (NaH2PO4) was added, and the mixture was heated and stirred at 80 °C and 500 rpm for 3 h. Finally, the mixture was heated and evaporated to dryness at 90 °C and 500 rpm to obtain a yellow gel precursor.

[0098] (2) The precursor obtained in step (1) was placed in a vacuum oven and dried at 100°C for 25 hours. The dried material was then ground and pressed into sheet material with a diameter of 1 cm under a pressure of 30 MPa. The obtained sheet material was heated to 600°C at a heating rate of 2°C / min under a hydrogen-argon mixed gas, held at that temperature for 12 hours, and then naturally cooled to room temperature to obtain Na4Fe with in-situ carbon-coated Fe vacancies and high-entropy doping synergistic regulation. 2.6 □ 0.1 (Ni,Co,Mn,Cu,Al,Mg) 0.05 (PO4)2P2O7 material.

[0099] Example 3: Preparation of Na4Fe with synergistic regulation of Fe vacancies and high-entropy doping 2.6 □ 0.1 (Ni,Co,Mn,Cu,Mg) 0.06 (PO4)2P2O7 sodium-ion battery cathode material (x = 0.1; y = 0.3)

[0100] This embodiment uses the sol-gel method for preparation, Na4Fe 2.6 □ 0.1 (Ni,Co,Mn,Cu,Mg) 0.06 The preparation method of (PO4)2P2O7 cathode material includes the following steps:

[0101] (1) Dissolve 1g of polyvinylpyrrolidone (PVP) and 4.7g of citric acid (C6H8O7) in deionized water (concentrations of 14.286mg / ml and 67.143mg / ml, respectively), and sonicate for 30min. Then add 13mmol of ferric nitrate nonahydrate (Fe(NO3)3·9H2O), 0.3mmol of nickel acetate tetrahydrate (NiC4H6O4·4H2O), 0.3mmol of cobalt acetate tetrahydrate (CoC4H6O4·4H2O), and 0.3mmol of cobalt acetate tetrahydrate. Manganese acetate (MnC4H6O4·4H2O), 0.3 mmol copper acetate monohydrate (C4H6CuO4·H2O), and 0.3 mmol magnesium acetate tetrahydrate (C4H6MgO4·4H2O) were added and heated and stirred at 80 °C and 400 rpm for 5 h. Then, 20 mmol sodium dihydrogen phosphate dihydrate (NaH2PO4) was added and heated and stirred at 80 °C and 500 rpm for 3 h. Finally, the mixture was heated and evaporated to dryness at 90 °C and 500 rpm to obtain a yellow gel precursor.

[0102] (2) The precursor obtained in step (1) was placed in a vacuum oven and dried at 100°C for 25 hours. The dried material was then ground and pressed into sheet material with a diameter of 1 cm under a pressure of 30 MPa. The obtained sheet material was heated to 600°C at a heating rate of 2°C / min under a hydrogen-argon mixed gas, held at that temperature for 12 hours, and then naturally cooled to room temperature to obtain Na4Fe with in-situ carbon-coated Fe vacancies and high-entropy doping synergistic regulation. 2.6 □ 0.1 (Ni,Co,Mn,Cu,Mg) 0.06 (PO4)2P2O7 material.

[0103] Example 4: Preparation of Na₄Fe₄ with synergistic regulation of Fe vacancies and high-entropy doping 2.7 □ 0.1 (Ni,Co,Mn,Cu,Mg) 0.04 (PO4)2P2O7 sodium-ion battery cathode material (x = 0.1; y = 0.2)

[0104] This embodiment uses the sol-gel method for preparation, Na4Fe 2.7 □ 0.1 (Ni,Co,Mn,Cu,Mg) 0.04 The preparation method of (PO4)2P2O7 cathode material includes the following steps:

[0105] (1) Dissolve 1g of polyvinylpyrrolidone (PVP) and 4.7g of citric acid (C6H8O7) in deionized water (concentrations of 14.286mg / ml and 67.143mg / ml, respectively), and sonicate for 30min. Then add 13.5mmol of ferric nitrate nonahydrate (Fe(NO3)3·9H2O), 0.2mmol of nickel acetate tetrahydrate (NiC4H6O4·4H2O), 0.2mmol of cobalt acetate tetrahydrate (CoC4H6O4·4H2O), and 0.2mmol of... Manganese acetate hydrate (MnC4H6O4·4H2O), 0.2 mmol copper acetate monohydrate (C4H6CuO4·H2O), and 0.2 mmol magnesium acetate tetrahydrate (C4H6MgO4·4H2O) were added and heated and stirred at 80 °C and 400 rpm for 5 h. Then, 20 mmol sodium dihydrogen phosphate dihydrate (NaH2PO4) was added and heated and stirred at 80 °C and 500 rpm for 3 h. Finally, the mixture was heated and evaporated to dryness at 90 °C and 500 rpm to obtain a yellow gel precursor.

[0106] (2) The precursor obtained in step (1) was placed in a vacuum oven and dried at 100°C for 25 hours. The dried material was then ground and pressed into sheet material with a diameter of 1 cm under a pressure of 30 MPa. The obtained sheet material was heated to 600°C at a heating rate of 2°C / min under a hydrogen-argon mixed gas, held at that temperature for 12 hours, and then naturally cooled to room temperature to obtain Na4Fe with in-situ carbon-coated Fe vacancies and high-entropy doping synergistic regulation. 2.7 □ 0.1 (Ni,Co,Mn,Cu,Mg) 0.04 (PO4)2P2O7 material.

[0107] Figure 1 and Figure 2 These are macroscopic SEM images and magnified SEM images of the sodium-ion battery cathode material prepared in Example 1 of this invention. It can be seen that the material prepared in Example 1 exhibits a distinct microspherical morphology with uniform overall particle size. The microsphere diameter is between 300-400 nm. Furthermore, an interwoven carbon matrix network is clearly observed in the magnified SEM images, which significantly shortens the Na... + The diffusion path improved Na + This increases the diffusion rate and also effectively improves the conductivity of the material.

[0108] Figure 3 and Figure 4The images show macroscopic SEM images and magnified SEM images of the sodium-ion battery cathode material prepared in Comparative Example 0 of this invention. It can be seen that when the carbon source concentration is reduced, especially the content of polyvinylpyrrolidone, the prepared sodium-ion cathode material exhibits irregular nanoparticles with significant agglomeration. The particle diameter is relatively large, ranging from 500-1000 nm, and no microsphere morphology was observed, highlighting the importance of carbon source concentration for morphology control.

[0109] Figure 5 and Figure 6 The XRD full-length and partially magnified spectra of the sodium-ion battery cathode materials prepared in Comparative Examples 1, 2, 3, and 1 of this invention are shown. It can be seen that the materials prepared in Comparative Examples 1, 2, 3, and 1 all exhibit high crystallinity, and their diffraction peaks correspond well to Na4Fe3(PO4)2P2O7, confirming the effectiveness of the preparation method and design scheme. It can be seen that the diffraction spectrum of the material prepared in Comparative Example 1 shows obvious NaFePO4 and Na2FeP2O7 impurity peaks. After Fe vacancy modulation alone, the impurity peaks in the material prepared in Comparative Example 2 are significantly weakened, but still noticeable; similarly, after high-entropy doping modification alone, weak impurity peaks can also be observed in the material prepared in Comparative Example 3; in particular, after synergistic modulation of high-entropy doping and Fe vacancy, no impurity peaks were detected in the material prepared in Example 1, exhibiting a good pure-phase Na4Fe3(PO4)2P2O7 structure.

[0110] Example 5: Assembling a button cell sodium-ion battery

[0111] The active material, Super P, and polyvinylidene fluoride (PVDF) were ground uniformly in a molar ratio of 7:2:1. Then, an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added for further wet grinding. After dispersion, the slurry was uniformly coated onto aluminum foil and dried in a vacuum oven at 80℃ for 12 hours. The resulting material was then cut into positive electrode sheets. The active materials were Na4Fe3(PO4)2P2O7 prepared in Comparative Example 1 and Na4Fe2O3 prepared in Comparative Example 2, respectively. 2.9 □ 0.1 (PO4)2P2O7, Na4Fe prepared in Comparative Example 3 2.7 (Ni,Co,Mn,Cu,Zn,Mg) 0.05 (PO4)2P2O7, Na4Fe prepared in Example 1 2.6 □ 0.1 (Ni,Co,Mn,Cu,Zn,Mg) 0.05 (PO4)2P2O7; Super P is a conductive additive, polyvinylidene fluoride (PVDF) is a binder, and aluminum foil is a current collector.

[0112] In an argon-filled glove box, where the concentrations of H2O and O2 were below 0.1 ppm, a sodium metal sheet was used as the negative electrode. A 1M NaClO4 solution dissolved in ethylene carbonate (EC) / diethyl carbonate (DEC) and 5% fluoroethylene carbonate (FEC) was used as the electrolyte. A glass fiber membrane was used as the separator for the half-cell, and the cells were assembled into CR2025 coin cells. Constant current charge-discharge mode was used for testing, with a charging cutoff voltage of 4.25V and a discharging cutoff voltage of 1.75V. The test was conducted at 100 mA g. -1 The experiment was conducted at a current density of [specific value].

[0113] Figure 7 The sodium-ion battery cathode materials prepared in Comparative Examples 1, 2, 3 and Example 1 of this invention are used in 10 mA g -1 Initial charge-discharge curves at current density; among them, the material prepared in Example 1 exhibits a significant plateau capacity improvement due to the improved phase purity and the synergistic effect of multiple cations, with an initial discharge capacity reaching 129 mAh g⁻¹. -1 (Specific capacity calculation is based on the active material of the cathode material), which is significantly better than the materials prepared in Comparative Examples 1, 2, and 3.

[0114] Figure 8 The figures show the rate performance of the sodium-ion battery cathode materials prepared in Comparative Examples 1, 2, 3, and 1 of this invention. It can be seen that the Fe vacancies and high-entropy doping provide a flexible local environment, enhancing the material's tolerance and structural stability, allowing it to better adapt to high-current charge and discharge. The material prepared in Example 1 exhibits excellent rate performance at 10 Ag. -1 It still has more than 60mAh g at high current density -1 The specific capacity, while when the current recovers to 1Ag -1 and 0.05Ag -1 When the current is reduced, the capacity can be basically recovered. In contrast, the materials prepared by Comparative Examples 1, 2, and 3 still exhibit low capacity and poor high current tolerance due to the presence of impurity phases.

[0115] Figure 9 and Figure 10 The sodium-ion battery cathode materials prepared in Example 1 and Comparative Examples 1, 2, and 3 of this invention were subjected to a 5Ag test. -1 Long-cycle electrochemical performance at current density; thanks to the synergistic regulation of high-entropy doping and Fe vacancies, the material prepared in Example 1 not only has high capacity but also exhibits good long-cycle stability, reaching up to 5Ag. -1It can withstand stable high current density cycling for over 6000 cycles with a capacity retention of up to 88.8%, and still maintains approximately 57 mAh g after 15000 cycles. -1 The reversible capacity (capacity retention rate of 74.2%) was achieved. In contrast, Comparative Examples 1, 2, and 3 could not withstand large current densities, exhibiting rapid capacity decay and electrode failure.

[0116] Figure 11 The images show the in-situ XRD patterns of the sodium-ion battery cathode material prepared in Example 1 of this invention during the initial two charge-discharge cycles. It can be seen that Example 1 underwent a highly reversible solid-solution-two-phase structure evolution process throughout the entire charge-discharge process, with the lattice parameters changing reversibly as charging progressed. (The last sentence appears to be incomplete and possibly refers to Na...) + The material exhibits reversible extraction / intercalation, with a maximum volume change of only 3.8% throughout the entire charge-discharge process. All lattice parameters return to their initial state upon completion of the test. This is attributed to the rational design of the Fe vacancy and high-entropy doping scheme, and to the interaction between Fe vacancy and various metal dopant ions with Fe. 2+ It has good chemical compatibility.

[0117] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Therefore, any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention should fall within the protection scope of the present invention.

Claims

1. A sodium iron pyrophosphate cathode material, characterized in that, Its chemical formula is Na₄Fe 3-x-y □ x HE y (PO4)2P2O7; where: □ represents Fe vacancies; HE represents dopant elements, including at least 5 of the following: Ni, Mg, Co, Mn, Cu, Zn, Cd, Al, and Cr, with each dopant element being equimolar; 0.08 <x≤0.1,0.2≤y≤0.3; The sodium iron pyrophosphate cathode material is prepared using the following steps: Step S1: Dissolve carbon source A and carbon source B in water and perform ultrasonic treatment to obtain a carbon source solution; wherein carbon source A and carbon source B are polyvinylpyrrolidone and citric acid, respectively, and the concentration of carbon source A in the carbon source solution is controlled at 12-18 mg / mL, and the concentration of carbon source B in the carbon source solution is controlled at 65-75 mg / mL. Step S2: Add the iron source and the metal dopant source to the solution in step S1 according to the stoichiometric ratio, and heat and stir. Step S3: Add sodium source and phosphorus source to the solution in step S2 according to stoichiometric ratio, heat and stir, and then evaporate to dryness to obtain gel; Step S4: Dry the gel from step S3, grind it into powder, and then compress it into tablets to obtain a tablet precursor. Step S5: The sheet-like precursor obtained in step S4 is calcined in an inert atmosphere, then cooled and ground to obtain the sodium iron pyrophosphate cathode material; the calcination temperature is 550-650℃.

2. The sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The surface of the cathode material is coated with a carbon layer, wherein the carbon content accounts for 5-12 wt% of the cathode material.

3. A method for preparing the sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, Includes the following steps: Step S1: Dissolve carbon source A and carbon source B in water and perform ultrasonic treatment to obtain a carbon source solution; wherein carbon source A and carbon source B are polyvinylpyrrolidone and citric acid, respectively, and the concentration of carbon source A in the carbon source solution is controlled at 12-18 mg / mL, and the concentration of carbon source B in the carbon source solution is controlled at 65-75 mg / mL. Step S2: Add the iron source and the metal dopant source to the solution in step S1 according to the stoichiometric ratio, and heat and stir. Step S3: Add sodium source and phosphorus source to the solution in step S2 according to stoichiometric ratio, heat and stir, and then evaporate to dryness to obtain gel; Step S4: Dry the gel from step S3, grind it into powder, and then compress it into tablets to obtain a tablet precursor. Step S5: The sheet-like precursor obtained in step S4 is calcined in an inert atmosphere, then cooled and ground to obtain the sodium iron pyrophosphate cathode material; the calcination temperature is 550-650℃.

4. The preparation method according to claim 3, characterized in that, The heating and stirring temperature in S2 and S3 is 60-90℃; the tableting pressure in S4 is 20-40MPa.

5. The preparation method according to claim 3, characterized in that, The calcination described in S5 involves holding the temperature at 550-650℃ for 8-12 hours.

6. A positive electrode sheet for a sodium-ion battery, characterized in that, It comprises the sodium iron pyrophosphate cathode material as described in claim 1 or 2, or the sodium iron pyrophosphate cathode material prepared by the method described in any one of claims 3 to 5.

7. A sodium-ion battery, characterized in that, It includes the sodium-ion battery positive electrode sheet as described in claim 6.

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