High-crystallinity Prussian blue positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of sodium ion batteries, and particularly discloses a high-crystallinity Prussian blue positive electrode material as well as a preparation method and application thereof. The preparation method comprises the following steps: S1, preparing a sodium ferrocyanide ligand aqueous solution; s2, adding an anion complexing agent phytic acid into the ligand aqueous solution, and uniformly stirring and mixing to obtain a mixed solution; and S3, heating the obtained mixed solution in a water bath, standing, centrifuging and drying to obtain the high-crystallinity Prussian blue positive electrode material. The phytic acid is adopted for anion complexing, crystallization under a single iron source is achieved, the crystallinity of Prussian blue is regulated and controlled, the defect structure in the Prussian blue crystal structure is reduced, and the rate capability and the energy density of the battery are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium ion batteries, and in particular relates to a high-crystalline Prussian blue positive electrode material and a preparation method and application thereof. Background Art

[0002] The structural formula of Prussian blue is A x Fe y [Fe(CN)6]z·nH2O, where x=0-2, A represents an alkali metal element such as sodium or potassium, in which cyanide anions bridge iron ions at different sites to form a double perovskite structure, while sodium ions and water molecules occupy interstitial sites. This open three-dimensional open framework material can accommodate sodium ions with larger ionic radius, while having a directional channel for rapid sodium ion diffusion and a small lattice strain during the reversible sodium ion deintercalation process. In theory, the dual active sites can provide a two-electron redox reaction, so Prussian blue has a 170mAhg -1 The high theoretical capacity of Prussian blue. Its synthesis process usually adopts a relatively simple room temperature precipitation method, which is non-toxic, low-cost and suitable for large-scale synthesis applications. However, due to the rapid precipitation process of Prussian blue, it has many defects, making its specific capacity far lower than the theoretical value. At the same time, the framework becomes unstable during the sodium ion storage process, and the cycle stability is poor, which hinders its large-scale application. Therefore, it is extremely important to design a low-cost and effective synthesis method to prepare a Prussian blue cathode with a complete crystal framework. Summary of the invention

[0003] The purpose of the present invention is to provide a high-crystalline Prussian blue positive electrode material and a preparation method and application thereof in view of the above-mentioned deficiencies in the prior art, so as to solve the technical problems of many defects and low specific capacity of the Prussian blue positive electrode material in the prior art. The method controls the crystallization rate based on the anion complex effect formed by the ligand exchange between anions, thereby obtaining a Prussian blue material with high crystallinity. This high-crystalline Prussian blue material has good electrochemical performance when used as an electrode material for a sodium ion energy storage system.

[0004] To achieve the above object, the present invention adopts the following technical solution:

[0005] The first object of the present invention is to provide a method for preparing a highly crystalline Prussian blue positive electrode material, comprising the following steps:

[0006] S1, preparing a sodium ferrocyanide ligand aqueous solution;

[0007] S2, adding anionic complexing agent phytic acid to the ligand aqueous solution, stirring and mixing evenly to obtain a mixed solution;

[0008] S3, heating the mixed solution obtained in step S2 in a water bath, and then standing and centrifuging to dry to obtain a highly crystalline Prussian blue positive electrode material.

[0009] Furthermore, in step S2, the molar ratio of phytic acid to sodium ferrocyanide is 1:(0.2-5).

[0010] Furthermore, in step S3, the mixed solution is placed in a water bath at 80° C. for 2 hours, then aged for 24 hours, collected by centrifugation, and dried in vacuo.

[0011] The second object of the present invention is to provide a Prussian blue positive electrode material, which is prepared by the above-mentioned preparation method.

[0012] Furthermore, the crystal structure of the Prussian blue positive electrode material is cubic, and the space group is Fm-3m 225.

[0013] Furthermore, the crystal water content in the Prussian blue positive electrode material is not higher than 6%.

[0014] Furthermore, the particle size distribution of the Prussian blue positive electrode material is 50 to 700 nm.

[0015] The third object of the present invention is to provide a positive electrode sheet comprising the above-mentioned Prussian blue positive electrode material.

[0016] The fourth object of the present invention is to provide the use of the above-mentioned Prussian blue positive electrode material or positive electrode sheet in the preparation of a sodium ion battery.

[0017] The fifth object of the present invention is to provide a sodium ion battery, which comprises a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode comprises the above-mentioned Prussian blue positive electrode material.

[0018] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects:

[0019] (1) The present invention provides a highly crystalline Prussian blue positive electrode material, which adopts a coprecipitation synthesis route of anion complexation mechanism. The Prussian blue electrode material synthesized by this synthesis method has high crystallinity, less defect vacancies and coordinated water, and has a stable sodium storage framework. Specifically, water is used as a solvent, a single iron source sodium ferrocyanide is used as a solute, and phytic acid is used as an anion complexing agent. It is prepared by a classic coprecipitation method. The anion complexing effect formed based on the ligand exchange between anions controls the crystallization rate, thereby obtaining a Prussian blue material with high crystallinity.

[0020] (2) The preparation method provided by the present invention has simpler steps, greatly shortens the process, and has the advantages of low cost, simple method, and mass production.

[0021] (3) The high-crystalline Prussian blue electrode material provided by the present invention has higher crystallinity, less crystal water and fewer defect positions. More low-spin iron electrochemical reaction active sites can be activated, which is beneficial to the improvement of electrode capacity and can approach the theoretical capacity of Prussian blue positive electrode material.

[0022] (4) The highly crystalline Prussian blue electrode material provided by the present invention has a stable sodium ion storage and diffusion framework, which can ensure the stability of the structure during the sodium ion insertion and extraction process, and is conducive to improving the cycle stability.

[0023] (5) The high-crystalline Prussian blue electrode material provided by the present invention has a complete conductive path and also has a high conductivity, which reduces the reaction internal resistance of the electrode material and is beneficial to improving the sodium storage performance of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A scanning electron microscope (SEM) image of the highly crystalline Prussian blue provided in Example 1 of the present invention;

[0025] Figure 2 A scanning electron microscope (SEM) image of the highly crystalline Prussian blue provided in Example 2 of the present invention;

[0026] Figure 3 A scanning electron microscope (SEM) image of the highly crystalline Prussian blue provided in Example 3 of the present invention;

[0027] Figure 4 The XRD diagram of the highly crystalline Prussian blue provided by the present invention;

[0028] Figure 5 This is a graph showing the thermogravimetric analysis results of the highly crystalline Prussian blue provided in Example 3 of the present invention;

[0029] Figure 6 When the highly crystalline Prussian blue material prepared in Examples 1 to 3 of the present invention is used as a positive electrode material for a sodium ion battery, the rate performance at a current density of 0.5-20C;

[0030] Figure 7 Cycling performance of the highly crystalline Prussian blue material prepared in Examples 1 to 3 of the present invention as a positive electrode material for a sodium ion battery at a current density of 5C;

[0031] Figure 8 The cycling performance of a full battery assembled with the highly crystalline Prussian blue material prepared in Example 2 of the present invention as the positive electrode material for a sodium ion battery at a current density of 0.5C. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present invention clearer, the specific embodiments of the present invention are further described in detail below in conjunction with specific examples and drawings. If no specific technology or conditions are specified in the examples, the technology or conditions described in the literature in this field or the product instructions are used. If the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be obtained commercially.

[0033] The embodiment of the present invention provides a highly crystalline Prussian blue material, which is synthesized by coprecipitation through an anion complexation synthesis route. Specifically, the embodiment of the present invention uses deionized water as a solvent, a single iron source sodium ferrocyanide as a solute, and phytic acid as an anion complexing agent, and is prepared by a classical coprecipitation method. The crystal structure of the prepared Prussian blue positive electrode material is a cubic crystal system, the space group is Fm-3m 225, and the unit cell parameter a=10.29 angstroms.

[0034] A high-crystalline Prussian blue electrode material and a preparation method thereof according to an embodiment of the present invention are illustrated below through a plurality of specific embodiments.

[0035] Example 1

[0036] This embodiment provides a method for preparing a highly crystalline Prussian blue material, comprising the following steps:

[0037] Step S1, preparing a sodium ferrocyanide aqueous solution: weighing 2 mmol of sodium ferrocyanide and dissolving it in 100 mL of deionized water, and then stirring at 500 rpm for 30 min;

[0038] Step S2, heating the obtained sodium ferrocyanide ligand aqueous solution in a water bath, adding 2 mmol of phytic acid solution dropwise during the water bath, the water bath temperature is 80° C., the rotation speed is 500 rpm, and the time is 2 h.

[0039] Step S3: After aging the mixed solution in the water bath for 24 hours, the mixed solution was centrifuged and washed three times with deionized water at 8000 rpm, and then placed in a vacuum oven at 120° C. for 24 hours. Figure 1 As shown in the SEM images, with the assistance of phytic acid anion complexing agent, the prepared Prussian blue has good crystallinity and the particle size distribution is between 100-300nm.

[0040] Example 2

[0041] This embodiment provides a method for preparing a highly crystalline Prussian blue material, comprising the following steps:

[0042] The method of this embodiment is basically the same as that of embodiment 1, except that the amount of phytic acid complexing agent is changed to 1 mmol. Figure 2As shown in the SEM, the Prussian blue cathode material prepared with the assistance of a smaller amount of phytic acid complexing agent has higher crystallinity and a larger particle size distribution, with the particle size distribution ranging from 400 to 700 nm.

[0043] Example 3

[0044] This embodiment provides a method for preparing a highly crystalline Prussian blue material, comprising the following steps:

[0045] The method of this embodiment is basically the same as that of embodiment 1, except that the water bath temperature is adjusted to 60°C. Figure 3 As shown in the SEM images, at an anion decomplexation temperature of 60°C, the Prussian blue obtained has relatively poor crystallinity and a smaller particle size, with a particle size distribution of 50-200 nm.

[0046] The highly crystalline Prussian blue materials prepared in Examples 1-3 have the same microscopic morphology, a cubic crystal structure, and a space group of Fm-3m 225.

[0047] refer to Figure 4 The XRD of the highly crystalline Prussian blue material prepared in Examples 1-3 matches the standard card PDF#73-0687, proving that the highly crystalline Prussian blue material prepared in the present invention corresponds to the (200), (220), (400), (420), (440), (600) and (620) planes of Fe4[Fe(CN)6]3 (PDF#73-0687).

[0048] refer to Figure 5 The high crystalline Prussian blue material prepared in Example 3 was tested by thermogravimetric analysis, and the crystal water content was 5.8%, and the adsorbed water content was 1.8%. The crystal water contained in Examples 1 and 2 was even less.

[0049] In order to better illustrate the electrochemical performance of the high-crystalline Prussian blue cathode material provided by the present invention, the inventors conducted the following research:

[0050] Example 4

[0051] After mixing the Prussian blue material, Ketjen black, and binder (polyvinylidene fluoride, PVDF) obtained in Examples 1-3 in a mass ratio of 70:20:10, N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry, and the mixture was ground for 30 minutes. A carbon-coated aluminum foil was selected as the current collector, and the slurry was coated on the carbon-coated aluminum foil. The foil was placed in a vacuum drying oven and dried at 80°C for 12 hours. The mass ratio of the electrode active material to the conductive agent and the binder was 7:2:1, and a Prussian blue sodium battery positive electrode was prepared. The sodium ion battery was assembled in an argon-filled glove box, and the electrochemical performance of the assembled battery was tested using the Newway test system. The test results are as follows: Figure 6 and Figure 7 shown.

[0052] from Figure 6 It can be seen that the Prussian blue in Example 2 with the highest crystallinity has the highest reversible specific capacity. Figure 7 It can also be seen that this highly crystalline Prussian blue also has the best cycling performance. It can be seen that there is a positive correlation between crystallinity and capacity and its stability.

[0053] Example 5

[0054] After mixing commercial hard carbon material with Ketjen black and binder (polyvinylidene fluoride, PVDF) in a mass ratio of 80:10:10, add it to N-methylpyrrolidone (NMP) and grind it thoroughly to form a slurry. The slurry is evenly coated on a carbon-coated aluminum foil to form an electrode sheet. In a glove box filled with argon, the electrode sheets are assembled into a sodium half-cell, and then pre-embedded with sodium using a Xinwei testing system. The commercial hard carbon electrode sheet with pre-embedded sodium is used as the negative electrode, and the electrode sheet prepared with the Prussian blue material of Example 1 without pre-embedded sodium is used as the positive electrode. A sodium ion full battery is assembled at a mass ratio of positive and negative active materials of 1:0.5. The electrochemical performance test was carried out using the Xinwei testing system, and the test results are as follows. Figure 8 As shown, at a current density of 0.5C, it can have 70mAh g -1 The reversible capacity of the hard carbon material needs to be regulated to achieve its optimal performance. The data in the figure proves its feasibility as a positive electrode for commercial sodium-ion batteries.

[0055] In the absence of conflict, the above embodiments and features in the embodiments may be combined with each other.

[0056] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a highly crystalline Prussian blue positive electrode material, characterized in that: The following steps are involved: S1, preparing a sodium ferrocyanide ligand aqueous solution; S2, adding anionic complexing agent phytic acid to the ligand aqueous solution, stirring and mixing evenly to obtain a mixed solution; S3, heating the mixed solution obtained in step S2 in a water bath, and then standing and centrifuging to dry to obtain a highly crystalline Prussian blue positive electrode material.

2. The preparation method according to claim 1, characterized in that In step S2, the molar ratio of phytic acid to sodium ferrocyanide is 1:(0.2-5).

3. The preparation method according to claim 1, characterized in that: In step S3, the mixed solution is placed in a water bath at 80° C. for 2 hours, then aged for 24 hours, collected by centrifugation, and dried in vacuo.

4. A Prussian blue positive electrode material, characterized in that: The method is prepared according to any one of claims 1 to 4.

5. The Prussian blue positive electrode material according to claim 4, characterized in that The crystal structure of the Prussian blue positive electrode material is cubic, and the space group is Fm-3m 225.

6. The Prussian blue positive electrode material according to claim 4, characterized in that The crystal water contained in the Prussian blue positive electrode material is no more than 6%.

7. The Prussian blue positive electrode material according to claim 4, characterized in that The particle size distribution of the Prussian blue positive electrode material is 50-700 nm.

8. A positive electrode sheet, characterized in that: The method comprises the Prussian blue positive electrode material according to any one of claims 4 to 7.

9. Use of the Prussian blue positive electrode material as claimed in claim 4 or the positive electrode sheet as claimed in claim 8 in the preparation of a sodium ion battery.

10. A sodium ion battery comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, characterized in that: The positive electrode comprises the Prussian blue positive electrode material according to claim 4.

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