Composite Prussian blue and preparation and application thereof

Abstract

The invention belongs to the field of sodium-ion battery positive electrode materials, and particularly relates to a method for preparing nickel-iron prussian blue and prussian blue after compounding and application of the nickel-iron prussian blue and the prussian blue in a sodium-ion battery. A series of composite Prussian blue materials are firstly subjected to a coprecipitation method and are calcined in nitrogen to synthesize a Prussian blue precursor material, and then a series of composite Prussian blue materials with different nickel-iron ratios are synthesized through a secondary coprecipitation method. Nickel and water of the composite material jointly adjust ligand field energy to form a sodium storage mechanism tending to capacitance control. And the inert nickel component does not participate in the reaction and serves as a shell to protect deformation of the structure in the charging and discharging process, and the stability is enhanced. Through a half-cell performance test, it is obtained that the material serves as a positive electrode material, the excellent specific capacity can be achieved under the extremely high multiplying power, and the cycling stability is excellent.

Description

Technical Field

[0001] The present invention belongs to the field of sodium ion battery positive electrode materials, and particularly relates to a method for compounding a Prussian blue material and an application thereof in a sodium ion battery. Background Art

[0002] Energy underpins the development and progress of modern society. With the advancement of human society, traditional energy sources are becoming increasingly scarce, and environmental problems are becoming increasingly severe. To address this issue, countries around the world are seeking new energy development pathways. Vigorously developing renewable energy is an effective approach. Renewable energy sources include solar, wind, hydro, and tidal energy. While renewable energy poses minimal environmental risks, its dependence on the natural environment leads to random, intermittent, and unstable power generation, making it difficult to provide a stable supply. Therefore, energy storage technologies, particularly large-scale energy storage technologies, are needed to store electricity generated by renewable energy and ensure a safe, continuous, and stable power supply. Electrochemical energy storage technology plays a key role in the large-scale energy storage market. Lithium-ion batteries are a key component of this technology, but limited lithium reserves and price fluctuations hinder their further development in the large-scale energy storage sector.

[0003] Sodium resources are abundant and cheap, and sodium-ion batteries have similar working principles to lithium-ion batteries, which makes sodium-ion batteries one of the favorable candidates for large-scale energy storage. However, since sodium ions have a larger radius than lithium-ion batteries, it is also necessary to find new electrode materials that can easily store and transport sodium ions when drawing on mature lithium-ion battery systems. Prussian blue and its analogues have a 3D pore structure with large gaps and wide pores, which can fully accommodate sodium ions for embedding and extraction. And this type of material contains M A and M B The two transition metal sites offer great flexibility in designing the material's structure, stability, and active sites through rational selection. However, in practical applications, the presence of defects results in lower specific capacity at high current densities, poor rate performance, and poor cycling performance at high current densities. Summary of the Invention

[0004] In response to the problem of poor rate performance of Prussian blue and its analogues in the prior art positive electrode materials, the present invention fits a composite Prussian blue material and applies it to sodium ion batteries to improve the rate performance and cycle performance of Prussian blue material-based sodium ion batteries under high current density.

[0005] The invention relates to a preparation method of a composite Prussian blue material and application of the composite Prussian blue material in a sodium ion battery.

[0006] The composition of the composite Prussian blue material is NaFe[Fe(CN)6]@NaNi x Fe 1-x [Fe(CN)6];

[0007] The preparation steps of the composite Prussian blue material include:

[0008] 1) After co-precipitation, the precursor material PW-325 (the composition of the Prussian blue material is NaFe[Fe(CN)6]) was synthesized by calcination in nitrogen;

[0009] 2) 0.2-0.6 g (preferably 0.3-0.5 g) of the precursor material PW-325 obtained in step 1), 2-5 g (preferably 2.5-3.5 g) of sodium citrate, and 1-4 g (preferably 2-3 g) of polyvinyl pyrrolidone were dissolved in 50-200 mL (preferably 80-120 mL) of deionized water to form solution A; 0.5-2.5 g (preferably 0.8-1.5 g) of Na4Fe(CN)6·10H2O was dissolved in 20-80 mL (preferably 40-60 mL) of deionized water to form solution B; NiCl2·6H2O and FeSO4·7H2O (a total of 1-2.5 mmol) in different proportions were added. The Ni / Fe molar ratio ranges from 9 / 1 to 1 / 9, preferably from 3 / 7 to 7 / 3), 1-5 g (preferably 2-3.5 g) of sodium citrate, and 0.5-4 g (preferably 1-3 g) of polyvinyl pyrrolidone (PVP) are dissolved in 20-80 mL (preferably 40-60 mL) of deionized water to form a solution C; solution B and solution C are simultaneously added dropwise to solution A, stirred for 15-25 minutes, aged at room temperature for 12-36 hours (preferably 20-28 hours), the supernatant is removed, and the mixture is centrifuged. The solid is washed with deionized water and ethanol in sequence, and the solid is vacuum dried at 60-100° C. (preferably 70-90° C.) for 8-24 hours to obtain a composite Prussian blue material.

[0010] The prepared composite Prussian blue material was used as the positive electrode active material, mixed with Super P (superconductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 7:2:1, and an appropriate amount of NMP (N-methylpyrrolidone) was added and stirred. The slurry was then coated on Al foil and dried at 60°C overnight. A disc was cut as the positive electrode; a metallic sodium sheet was used as the negative electrode; a Whatman glass fiber membrane was used as the diaphragm; 120 μL of 1M NaClO4 dissolved in EC / DEC (ethylene carbonate / diethyl carbonate, 1:1 volume ratio) + 2-7% FEC solution (fluoroethylene carbonate, preferably 4%-6%) was used as the electrolyte; a CR2016 button battery was used, and the positive electrode shell, positive electrode sheet, diaphragm, electrolyte, negative electrode sheet, gasket, spring sheet, and negative electrode shell were stacked in this order, and pressed and assembled into a sodium ion battery for performance testing.

[0011] The advantages and benefits of the present invention include simple operation, low energy consumption, and the use of readily available, inexpensive raw materials. The resulting composite Prussian blue material has a high low-spin iron content, and its sodium ion storage mechanism favors a capacitive process. During charge and discharge at high current densities, it is unaffected by sodium ion diffusion, effectively enhancing capacity at high current densities. Furthermore, the inert nickel component does not participate in the reaction, protecting the structural stability in the outer layer and resulting in excellent cycling performance.

[0012] The nickel and water in the composite material jointly regulate the ligand field energy, forming a sodium storage mechanism that tends to be capacitance-controlled. The inert nickel component does not participate in the reaction, but acts as a protective shell that deforms during charge and discharge, enhancing stability. Half-cell performance testing demonstrates that as a positive electrode material, it can deliver excellent specific capacity at very high rates and exhibits excellent cycling stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 The discharge capacity diagram of Examples 1-6 and Comparative Example 1 in the range of 0.2C-10C.

[0014] Figure 2 The discharge capacity diagram of Examples 1-6 and Comparative Example 1 in the range of 0.5C-50C.

[0015] Figure 3 1 is a cycle diagram of Examples 1-6 and Comparative Example 1 at 1C.

[0016] Figure 4 Circulation diagram of Examples 1-6 and Comparative Example 1 at 5°C. DETAILED DESCRIPTION

[0017] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be used to limit the present invention.

[0018] Examples 1-3

[0019] 1. Investigate the effect of different Ni / Fe ratios on battery performance. The steps include:

[0020] 4mmol Na4Fe(CN)6·10H2O was dissolved in 200mL deionized water to form solution A; 6mmol FeSO4·7H2O and 15g sodium citrate were dissolved in 200mL deionized water to form solution B; after stirring for 2h, solution A was directly poured into solution B, stirred for 15min, and aged at room temperature for 6h. After that, the supernatant was removed, and the solid residue was washed with deionized water and ethanol in sequence, and then placed in a vacuum drying oven at 60°C for vacuum drying overnight (12h). The solid was placed in a N2 atmosphere, heated from room temperature to 325°C at a rate of 2°C / min, kept warm for 8h, and then naturally cooled to room temperature to obtain the PW-325 precursor material. 0.4 g PW-325, 10 mmol sodium citrate, and 2 g PVP were dissolved in 100 mL deionized water to form solution A; 2 mmol Na4Fe(CN)6·10H2O was dissolved in 50 mL deionized water to form solution B; a total of 2 mmol NiCl2·6H2O and FeSO4·7H2O (3:7, 5:5, and 7:3, respectively), 10 mmol sodium citrate, and 2 g PVP were dissolved in 50 mL deionized water to form solution C; after stirring evenly, solutions B and C were respectively added dropwise to solution A and aged for 24 h. After that, the supernatant was removed, and the solid residue was washed with deionized water and ethanol in sequence, and then placed in a vacuum drying oven at 80°C for 15 h. They were named PW-325@2NiFe-37 (Example 1), PW-325@2NiFe-55 (Example 2), and PW-325@2NiFe-73 (Example 3), respectively. The molecular formula of Example 1 is Na 1.11 Ni 0.16 Fe 1.73 (CN)6·2.50H2O, the molar ratio of the core to the shell is about 4.3 / 1, wherein the thickness of the core is about 5μm and the thickness of the outer layer is about 360nm. The molecular formula of Example 2 is Na 1.19 Ni 0.09 Fe 1.84 (CN)6·2.62H2O, the molar ratio of the inner core to the outer shell is about 4.4 / 1, wherein the thickness of the inner core is about 5 μm and the thickness of the outer layer is about 350 nm. The molecular formula of Example 3 is Na 1.16 Ni 0.05Fe 1.84 (CN)6·2.63H2O, the amount ratio of the inner core to the outer shell is about 4.7 / 1, the thickness of the inner core is about 5μm, and the thickness of the outer layer is about 330nm.

[0021] The products obtained in Examples 1-3 (PW-325@2NiFe-37, PW-325@2NiFe-55, and PW-325@2NiFe-73) were used as positive electrode active materials, respectively, and mixed with Super P (superconductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 7:2:1. An appropriate amount of NMP (N-methylpyrrolidone) was added and stirred into a slurry, which was then coated on an Al foil (active material loading of approximately 1.2 mg cm). -2 ), and after drying, cut into discs with a diameter of 14 mm as the positive electrode, 120 μL of 1 M NaClO4 dissolved in EC / DEC (ethylene carbonate / diethyl carbonate, 1:1 volume ratio) + 5% FEC solution (fluoroethylene carbonate, volume ratio) was used as the electrolyte, a whatman glass fiber membrane was used as the separator, and a disc-shaped metallic sodium with a diameter of 16 mm was used as the negative electrode. The positive electrode shell (also known as the end plate), positive electrode sheet, separator, electrolyte, negative electrode sheet, gasket, spring sheet, and negative electrode shell (also known as the end plate) were stacked in this order, and then pressed to form a button battery. The charge and discharge tests were carried out at 0.2C, 0.5C, 0.8C, 1.0C, 2C, 3C, 5C, 8C, 10C, and 0.2C for 5 cycles each to test its rate performance. Charge and discharge tests were conducted at 0.5C, 1C, 5C, 10C, 20C, 30C, 40C, 50C, and 1C for 5 cycles each to test its performance at ultra-high rates. The battery was cycled 500 times at 1C to test its cycling stability at conventional current density. The battery was cycled 500 times at 5C to test its cycling stability at high current density.

[0022] Figure 1 The rate performance of Examples 1-3 at 0.2C-10C is given. Since Ni is not electrochemically active, the specific capacity of the battery gradually decreases with the increase of Ni content. The discharge specific capacities of Examples 1-3 at 0.2C are 133, 127, and 118 mAh / g, respectively. The difference in specific capacity between Example 1 and Example 2 at each rate is small, while the specific capacity of Example 3 is lower, which is mainly due to the excessive inert Ni component affecting the capacity. Among them, at a current density of 10C, the discharge specific capacities of Examples 1-3 are 123, 123, and 92 mAh / g, respectively.

[0023] Figure 2The ultra-high rate performance of Examples 1-3 at 0.5C-50C is given. Due to the lower Ni component in Example 1, it shows the highest discharge specific capacity before 30C. However, at ultra-high current densities of 40C and 50C, Example 2 gives the best performance, which is due to its moderate Ni / Fe content. The Ni component in Example 1 is relatively small, so it is difficult to maintain a stable structure at a higher current density, and the capacity will decrease. The Ni component in Example 3 is too much, and the capacity loss is relatively large.

[0024] Figure 3 The cycling performance of Examples 1-3 at 1C is shown. The initial specific capacities of Examples 1-3 were 123, 122, and 104 mAh / g, respectively. After 500 cycles, the specific capacities were 69, 91, and 68 mAh / g, respectively. The low Ni content of Example 1 made it relatively unstable, while the high Ni content of Example 3 made it more stable, but with a lower specific capacity. Example 2 showed the most balanced performance.

[0025] Figure 4 The cycling performance of Examples 1-3 at 5C is shown. The initial capacities of Examples 1-3 were 123, 115, and 109 mAh / g, respectively. After 500 cycles, the capacities were 84, 93, and 77 mAh / g, respectively. The trends are similar to those at 1C, but the shorter test time results in superior cycling performance compared to 1C.

[0026] Examples 4-6

[0027] 2. Investigate the effect of different coating amounts on battery performance, the steps include:

[0028] 4mmol Na4Fe(CN)6·10H2O was dissolved in 200mL deionized water to form solution A; 6mmol FeSO4·7H2O and 15g sodium citrate were dissolved in 200mL deionized water to form solution B; after stirring for 2h, solution A was directly poured into solution B, stirred for 15min, and aged at room temperature for 6h. After that, the supernatant was removed, and the solid residue was washed with deionized water and ethanol in sequence, and then placed in a vacuum drying oven at 60°C for vacuum drying overnight (12h). The solid was placed in a N2 atmosphere, heated from room temperature to 325°C at a rate of 2°C / min, kept warm for 8h, and then naturally cooled to room temperature to obtain the PW-325 precursor material. 0.4g PW-325, 10mmol sodium citrate, 2g PVP were dissolved in 100mL deionized water to form solution A; 1.2mmol Na4Fe(CN)6·10H2O was dissolved in 50mL deionized water to form solution B; a total of 1.2mmol NiCl2·6H2O and FeSO4·7H2O (3:7, 5:5, 7:3, respectively), 6mmol sodium citrate, 1.2g PVP was dissolved in 50 mL of deionized water to form solution C. After stirring evenly, solutions B and C were added dropwise to solution A, aged for 24 h, and then the supernatant was removed. The solid residue was washed with deionized water and ethanol in sequence, and then placed in a vacuum drying oven at 80 ° C for 15 h. They were named PW-325@1.2NiFe-37 (Example 4), PW-325@1.2NiFe-55 (Example 5), and PW-325@1.2NiFe-73 (Example 6). The molecular formula of Example 1 is Na 1.10 Ni 0.10 Fe 1.78 (CN)6·2.60H2O, the amount of substance ratio of the core to the shell is about 7.1 / 1, wherein the thickness of the core is about 5μm, and the thickness of the outer layer is about 230nm. The molecular formula of Example 2 is Na 1.16 Ni 0.06 Fe 1.83 (CN)6·2.65H2O, the molar ratio of the inner core to the outer shell is about 7.3 / 1, wherein the thickness of the inner core is about 5 μm, and the thickness of the outer layer is about 220 nm. The molecular formula of Example 3 is Na 1.14 Ni 0.03 Fe 1.82 (CN)6·2.62H2O, the amount ratio of the inner core to the outer shell is about 7.8 / 1, the thickness of the inner core is about 5μm, and the thickness of the outer layer is about 200nm.

[0029] The products obtained in Examples 4-6 (PW-325@1.2NiFe-37, PW-325@1.2NiFe-55, and PW-325@1.2NiFe-73) were used as positive electrode active materials, respectively, and mixed with Super P (superconductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 7:2:1. An appropriate amount of NMP (N-methylpyrrolidone) was added and stirred into a slurry, which was then coated on Al foil (active material loading was approximately 1.2 mg cm). -2 ), and after drying, cut into discs with a diameter of 14 mm as the positive electrode, 120 μL of 1 M NaClO4 dissolved in EC / DEC (ethylene carbonate / diethyl carbonate, 1:1 volume ratio) + 5% FEC solution (fluoroethylene carbonate, volume ratio) was used as the electrolyte, a whatman glass fiber membrane was used as the separator, and a disc-shaped metallic sodium with a diameter of 16 mm was used as the negative electrode. The positive electrode shell (also known as the end plate), positive electrode sheet, separator, electrolyte, negative electrode sheet, gasket, spring sheet, and negative electrode shell (also known as the end plate) were stacked in this order, and then assembled into a button battery after being pressed. The charge and discharge tests were carried out at 0.2C, 0.5C, 0.8C, 1.0C, 2C, 3C, 5C, 8C, 10C, and 0.2C for 5 cycles each to test its rate performance. Charge and discharge tests were conducted at 0.5C, 1C, 5C, 10C, 20C, 30C, 40C, 50C, and 1C for 5 cycles each to test its performance at ultra-high rates. The battery was cycled 500 times at 1C to test its cycling stability at conventional current density. The battery was cycled 500 times at 5C to test its cycling stability at high current density.

[0030] Figure 1 The rate performance of Example 4-6 at 0.2C-10C is given. Example 4-6 has a lower coating amount than Example 1-3. Due to the lower composite amount of Ni material in the outer layer, there are some differences in the performance of Example 4-6 and Example 1-3. The discharge specific capacity of Example 4-6 at 0.2C is 136, 129, and 127 mAh / g, respectively. At a current density of 10C, the discharge specific capacity of Example 4-6 is 109, 113, and 112 mAh / g, respectively.

[0031] Figure 2 The ultra-high rate performance of Examples 4-6 at 0.5C-50C is shown. The discharge specific capacities of Examples 4-6 at 50C are 62, 65, and 66 mAh / g, respectively. Due to the low Ni content in Example 1, its rate performance is slightly inferior to that of Examples 5-6. The rate performance of Examples 5-6 is similar.

[0032] Figure 3The cycling performance of Examples 4-6 at 1C is shown. The specific capacities of Examples 4-6 in the first cycle were 123, 120, and 123 mAh / g, respectively. After 500 cycles, the specific capacities were 66, 71, and 74 mAh / g, respectively. The stability of Examples 5-6 was also superior to that of Example 4.

[0033] Figure 4 The cycling performance of Examples 4-6 at 5C is given. The specific capacities of Examples 4-6 in the first cycle are 117, 116, and 119 mAh / g, respectively. After 500 cycles, the specific capacities are 65, 72, and 68 mAh / g, respectively.

[0034] Table 1 shows the discharge specific capacities of Examples 1-6 and Comparative Example 1 at 0.2C, 10C, and 50C, and the discharge specific capacities at 1C and 5C after 500 cycles.

[0035] Table 1

[0036]

[0037] Comparative Example 1:

[0038] The Prussian blue material has not been compounded, and its preparation method is as follows:

[0039] Solution A was formed by dissolving 4 mmol of Na₄Fe(CN)₆·10H₂O in 200 mL of deionized water. Solution B was formed by dissolving 6 mmol of FeSO₄·7H₂O and 15 g of sodium citrate in 200 mL of deionized water. After stirring for 2 hours, solution A was poured directly into solution B, stirred for 15 minutes, and aged at room temperature for 6 hours. The supernatant was then removed, and the solid residue was washed with deionized water and then ethanol, and then dried in a vacuum drying oven at 60°C overnight (12 hours). The solid was then placed in a nitrogen atmosphere and heated from room temperature to 325°C at a rate of 2°C / min. After holding for 8 hours, it was naturally cooled to room temperature to obtain PW-325 material. This material is composed of cubes with primary particles ranging from 0.5 to 1.5 μm stacked together to form secondary particles with a particle size of approximately 5 μm.

[0040] The product obtained in Comparative Example 1 (PW-325) was used as the positive electrode active material. It was mixed with Super P (superconductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 7:2:1, and an appropriate amount of NMP (N-methylpyrrolidone) was added to stir into a slurry. The slurry was then coated on an Al foil (active material loading was about 1.2 mg cm). -2), and after drying, cut into discs with a diameter of 14 mm as the positive electrode, 120 μL of 1 M NaClO4 dissolved in EC / DEC (ethylene carbonate / diethyl carbonate, 1:1 volume ratio) + 5% FEC solution (fluoroethylene carbonate) was used as the electrolyte, a whatman glass fiber membrane was used as the separator, and a disc-shaped metallic sodium with a diameter of 16 mm was used as the negative electrode. The positive electrode shell (also known as the end plate), positive electrode sheet, separator, electrolyte, negative electrode sheet, gasket, spring sheet, and negative electrode shell (also known as the end plate) were stacked in this order, and then pressed to assemble into a button cell. The charge and discharge tests were carried out at 0.2C, 0.5C, 0.8C, 1.0C, 2C, 3C, 5C, 8C, 10C, and 0.2C for 5 cycles each to test its rate performance. Charge and discharge tests were conducted at 0.5C, 1C, 5C, 10C, 20C, 30C, 40C, 50C, and 1C for 5 cycles each to test its performance at ultra-high rates. The battery was cycled 500 times at 1C to test its cycling stability at conventional current density. The battery was cycled 500 times at 5C to test its cycling stability at high current density.

[0041] Figure 1 The rate performance of Comparative Example 1 at 0.2C-10C is shown. Due to the absence of Ni, its specific capacity is relatively high, at 137 mAh / g. However, as the current density increases, the structure exhibits very unstable characteristics, and at 10C, its discharge specific capacity is only 78 mAh / g.

[0042] Figure 2 The ultra-high rate performance of Comparative Example 1 at 0.5C-50C is shown. It can be seen that with the increase of current density, the specific capacity decays very seriously. At 50C, the specific capacity is only 2mAh / g, which is basically unable to work normally.

[0043] Figure 3 The cycling performance of Comparative Example 1 at 1C is shown. Due to the high current density of charge and discharge, the material undergoes irreversible structural transformation, resulting in a specific capacity of only 120 mAh / g in the first cycle. After 500 cycles, the specific capacity is only 42 mAh / g, indicating its structural instability.

[0044] Figure 4 The cycle test performance of Comparative Example 1 at 5C is given. Its specific capacity in the first week is 108 mAh / g, and after 500 cycles, the specific capacity is only 48 mAh / g.

[0045] The method used in the present invention is to carry out a series of composites with different contents and Ni / Fe ratios on the PW-325 material to form a Prussian blue composite material. The performance of the coated material examples 1-6 is significantly improved compared to the uncoated material comparative example 1, especially the specific capacity and cycle performance at high rates. This is because the nickel and water in the composite material jointly regulate the ligand field energy, forming a sodium storage mechanism that tends to be capacitance-controlled, reducing the impact of slow sodium ion diffusion at high rates and improving rate performance. On the other hand, Ni is not electrochemically active and does not participate in the reaction during the charge and discharge process. Therefore, the NiFeHCF material wrapped in the outer layer undergoes less deformation during the electrochemical process. At the same time, it can also suppress the deformation of the inner layer PW-325 material during the reaction process, thereby improving the stability of the material under high pressure and in long cycles. However, when the Ni content in the outer layer of the composite is too low, it will not play a protective role. When the Ni content is too high, the specific capacity of the material will decrease. Therefore, an appropriate composite ratio and an appropriate Ni content are key to regulating performance. Among Examples 1-6, Example 3 exhibits the best comprehensive performance, which is due to the appropriate composite ratio and Ni content. Even so, the composite materials within the scope of the present invention can all exhibit better comprehensive performance than the uncomposite materials.

Claims

1. A method for preparing a composite Prussian blue material, characterized in that: The Prussian blue precursor and nickel-iron Prussian blue are compounded by co-precipitation method. The specific process is as follows: 1) dissolving 0.2-0.6 g (preferably 0.3-0.5 g) of Prussian blue raw material (composed of NaFeFe(CN)6), 2-5 g (preferably 2.5-3.5 g) of sodium citrate, and 1-4 g (preferably 2-3 g) of polyvinyl pyrrolidone in 50-200 mL (preferably 80-120 mL) of deionized water to form solution A; 2) dissolving 0.5-2.5 g (preferably 0.8-1.5 g) of Na4Fe(CN)6·10H2O in 20-80 mL (preferably 40-60 mL) of deionized water to form solution B; 3) dissolving 1-2.5 mmol of NiCl2·6H2O and FeSO4·7H2O (Ni / Fe molar ratio ranges from 9 / 1 to 1 / 9, preferably 3 / 7 to 7 / 3) in different proportions, 1-5 g (preferably 2-3.5 g) of sodium citrate, and 0.5-4 g (preferably 1-3 g) of polyvinylpyrrolidone in 20-80 mL (preferably 40-60 mL) of deionized water to form solution C; 4) Solution B and Solution C are simultaneously added dropwise to Solution A, stirred for 15-25 minutes, and aged at room temperature for 12-36 hours (preferably 20-28 hours). The supernatant is removed and centrifuged, and the solid is washed with deionized water and ethanol in sequence. The solid is vacuum dried at 60-100° C. (preferably 70-90° C.) for 8-24 hours to obtain a composite Prussian blue material.

2. A composite Prussian blue material prepared by the preparation method according to claim 1.

3. The composite Prussian blue material according to claim 2, characterized in that: It has a core-shell structure, with the core being NaFe[Fe(CN)6] and the outer layer being NaNi x Fe 1-x [Fe(CN)6] (where 0<x<1, preferably 0.3-0.7); wherein the inner core is an ion formed by stacking cubes, the primary particle size is between 0.5-1.5μm, the secondary particle size is between 3-7μm, and the outer layer thickness is between 200nm-400nm.

4. Use of the composite Prussian blue material prepared by the method according to claim 2 or 3 in a sodium ion battery.

5. The use according to claim 4, characterized in that: It has excellent rate performance, high specific capacity even at high current density, and good cycle performance.

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