Sodium ferric pyrophosphate material with sodium storage dominant crystal face regulated and controlled by cation doping as well as preparation method and application of sodium ferric pyrophosphate material

Abstract

The invention discloses a sodium ferric pyrophosphate material with a sodium storage dominant crystal face regulated and controlled by cation doping as well as a preparation method and application of the sodium ferric pyrophosphate material. The chemical formula of the prepared sodium ferric pyrophosphate material with the regulated and controlled crystal structure is Na < 4 > Fe < 2.97 > M < 0.03 > (PO4) < 2 > P < 2 > O < 7 >. Wherein M = Sr, or Sr and a second doping element, and the second doping element is selected from one of Cd, Bi, Mo, Ni and Al. The material is prepared by adopting a sol-gel method, an iron source, a chelating agent, a sodium source, a phosphorus source and doping elements are dissolved in deionized water, and stirring is performed until a gel state is formed; and drying and calcining twice to obtain a final product. The sodium ferric pyrophosphate electrode material prepared by the invention has more sodium storage dominant crystal faces, shows good electrochemical performance, has the advantages of simple preparation process, easily available raw materials and low cost, has remarkable industrial application value, and is easy for large-scale production.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium-ion batteries, and particularly relates to a sodium iron pyrophosphate material with cation doping for regulating sodium storage dominant crystal planes, a preparation method thereof, and an application thereof. Background Art

[0002] Due to advantages such as high energy density and wide working voltage, lithium-ion batteries have been widely used in various portable electronic devices. However, due to the limited reserves of lithium resources and the increasing cost of lithium raw materials year by year, it will not be able to meet the needs of the large-scale energy storage field. Therefore, it is necessary to develop other more inexpensive energy storage systems to supplement the market share of lithium-ion batteries. Sodium-ion batteries have advantages such as rich sodium resources and low prices, and sodium has a similar electrochemical working principle to lithium, making it a highly potential commercial battery energy storage system. However, sodium ions have a larger ionic radius and a heavier molar mass, resulting in slower insertion and extraction in electrode materials. Therefore, it is necessary to find a suitable electrode material to meet the working requirements of sodium-ion batteries.

[0003] Among the cathode materials of sodium-ion batteries with high commercial application potential, pyrophosphates are a good type of material in terms of cost and structural stability, and have many advantages. Compared with phosphate materials such as sodium iron phosphate and lithium iron phosphate, sodium iron pyrophosphate has a lower manufacturing cost because it can be sintered in one step by the high-temperature solid-phase method. Secondly, sodium iron pyrophosphate has a high theoretical specific capacity and a high average working voltage. In addition, sodium iron pyrophosphate has good stability, which gives it great application potential in energy storage.

[0004] Currently, the main problem faced in the preparation of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) is the poor conductivity of phosphates. Crystal plane regulation has important significance in the field of battery materials, especially in energy storage technologies such as lithium-ion batteries, sodium-ion batteries, and magnesium-ion batteries. The influence of different ionic materials on battery performance is often closely related to their crystal plane structure. Therefore, by precisely controlling the crystal plane structure of materials, the capacity, conductivity, cycle stability, and charge and discharge rate of batteries can be significantly improved. At the same time, the current methods for crystal plane regulation are generally hydrothermal methods, etc., which make the material preparation process complex and difficult to achieve large-scale preparation, resulting in increased costs. Summary of the Invention

[0005] In view of the technical problems proposed in the background art, the present invention provides a method for adjusting the exposed dominant sodium storage crystal planes of materials through cation doping. Controlling crystal orientation is a huge challenge in the field of materials science. Current scientific work has confirmed that surface energy can drive crystal growth, and ion transport on different crystal planes is different, both of which are crucial for anode energy storage. Therefore, exposing crystal planes with ion channels and shortening the ion diffusion distance through crystal orientation adjustment are expected to accelerate ion diffusion kinetics and improve the rate performance of the cathode material. At the same time, the addition of active elements can increase the initial discharge specific capacity of the material, greatly improving the electrochemical performance of the material. Therefore, the present invention modifies the material based on the above description.

[0006] The object of the present invention is to provide a sodium iron pyrophosphate material with cation doping to regulate the dominant sodium storage crystal planes, and its preparation method and application.

[0007] In the first aspect of the present invention, a sodium iron pyrophosphate material with crystal structure regulation is provided, and its chemical formula is: Na4Fe 2.97 M 0.03 (PO4)2P2O7. Where M = Sr, or Sr and a second doping element, and the second doping element is selected from one of Cd, Bi, Mo, Ni, Al.

[0008] The present invention induces the growth of sodium iron pyrophosphate crystals along the crystal axis directions of

[011] and

[210] by doping metal salts, exposing more (011) and (210) crystal planes, so that more sodium ions are exposed, and finally a sodium iron pyrophosphate material with crystal structure regulation is obtained.

[0009] Further, the M is Sr + The second doping element, and the second doping element is Cd; the molar ratio of Sr to Cd is 1:1.

[0010] In the second aspect of the present invention, the above-mentioned sodium iron pyrophosphate material with crystal structure regulation is prepared by the sol-gel method, and the specific preparation method includes the following steps:

[0011] (a) The iron source and the chelating agent are placed in deionized water and stirred and dissolved at room temperature to obtain solution A;

[0012] (b) The sodium source, the phosphorus source and the doping element are added to solution A and mixed evenly to obtain solution B; solution B is continuously heated and stirred until it becomes gel-like;

[0013] (c) The gel obtained in step (b) is dried to obtain a dry gel; it is calcined once in an inert gas;

[0014] (d) After taking out, it is pressed into a shape, which can be circular, square, etc.; it is calcined for the second time in an inert gas, taken out and crushed after cooling to obtain the final product, which is the sodium iron pyrophosphate material with crystal structure regulation.

[0015] Furthermore, the iron source is selected from one or more of iron nitrate, iron oxide, and iron phosphate; the chelating agent includes one or more of citric acid, oxalic acid, and ascorbic acid; the sodium source can be selected from one or more of sodium pyrophosphate, sodium dihydrogen phosphate, and sodium acetate; the phosphorus source is selected from one or more of sodium pyrophosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid, and the doping elements are composed of strontium and cadmium, and are added in the form of their soluble salts, such as strontium acetate and cadmium acetate.

[0016] Furthermore, the molar ratio of Fe in the iron source, the chelating agent, Na in the sodium source, P in the phosphorus source, and the doping elements is 10:14-15:13-18:13-18:0.03-0.06.

[0017] Furthermore, in step (a), the stirring time is 30 min and the rotation speed is 500-1000 rpm.

[0018] Furthermore, in step (b), the stirring time is 4-6 h, and the temperature for heating and stirring is 70-80 °C.

[0019] Furthermore, in step (c), the drying is carried out in an oven, the drying temperature in the oven is 80-100 °C, and the time is 6-12 h.

[0020] Furthermore, in step (c), the heating rate of the first calcination is 3 °C / min, the temperature is 250-400 °C, and the heat preservation time is 3-6 h; the inert gas is argon or argon-hydrogen gas (Ar:H2 = 95:5).

[0021] Furthermore, step (c) also includes grinding the dry gel into powder before calcination.

[0022] Furthermore, in step (d), the pressing pressure is 15-20 MPa.

[0023] Furthermore, in step (d), the temperature of the second calcination is 500-600 °C, and the heating rate is 3 °C / min; the inert gas is argon or argon-hydrogen gas (Ar:H2 = 95:5).

[0024] In the third aspect of the present invention, an application of the above-mentioned sodium iron pyrophosphate material with crystal structure regulation in a battery is provided.

[0025] Specifically, after the above-mentioned sodium iron pyrophosphate material with crystal structure regulation is ground and mixed with a conductive agent and a binder, it is coated on a current collector to form a positive electrode of a sodium-ion battery.

[0026] The present invention proposes a simple sol-gel method to prepare Na4Fe 2.97 M 0.03 (PO4)2P2O7, where M is one or more of metals such as Sr and Cd. The crystal plane regulation of the material is successfully realized, and it is expected to commercialize this method in the future. Among them, as an alkaline earth metal, Sr shows significant advantages in many aspects compared with transition metals such as Cd, Ni, and Mn. First of all, Sr has a relatively high crustal abundance, wide resource distribution, and relatively low mining and refining costs, which makes it more economical in industrial applications. Secondly, Sr has relatively stable chemical properties, and its toxicity is much lower than that of heavy metals such as Cd, with less potential harm to the environment and human health, meeting the requirements of modern industry for environmental protection and sustainable development. In addition, compared with the unmodified material, the (011) and (210) sodium storage dominant crystal planes of Na4Fe3(PO4)2P2O7 in the material synthesized by this method are significantly improved. The capacity and cycle stability of Na4Fe3(PO4)2P2O7 are significantly improved. Compared with the prior art, the sodium iron pyrophosphate electrode material prepared by the present invention has more sodium storage dominant crystal planes, exhibits good electrochemical performance, and has a simple preparation process, easily available raw materials, low cost, significant industrial application value, and is easy to mass-produce. Description of the Drawings

[0027] Figure 1 XRD comparison diagrams of the sodium iron pyrophosphate crystal materials prepared in Examples 1, 2, 3 and Comparative Example 1.

[0028] Figure 2 Cycling performance of the doped and modified sodium iron pyrophosphate prepared in Example 2 at 5C.

[0029] Figure 3 Scanning electron microscope images of the doped and modified sodium iron pyrophosphate prepared in Example 2.

[0030] Figure 4 XRD comparison diagrams of the sodium iron pyrophosphate crystal materials prepared in Examples 1 and Comparative Examples 1 and 2.

[0031] Figure 5 First charge-discharge curve diagrams of the sodium iron pyrophosphate crystal materials prepared in Examples 1, 2, 3 and Comparative Examples 1 and 2 at 0.1C. Detailed Embodiments

[0032] The present invention will be described in detail below with reference to specific embodiments. These embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0033] Example 1

[0034] A preparation method of a sodium ion secondary battery cathode material Na4Fe 2.97 Sr 0.03 (PO4)2P2O7. The method includes the following steps:

[0035] Using ferric nitrate nonahydrate as the iron source, sodium dihydrogen phosphate as the sodium source and phosphorus source, and citric acid monohydrate as the chelating agent. 4.04 g of ferric nitrate nonahydrate and 2.8818 g of citric acid monohydrate are added to 20 mL of deionized water, and stirred for 30 min at a rotation speed of 500 rpm. After that, 2.12258 g of sodium dihydrogen phosphate and 0.0524 g of strontium acetate are added. Stir at room temperature for another 2 h, then stir at 80 °C until it becomes a gel state at a rotation speed of 1000 rpm. After completion, it is placed in a vacuum oven and dried at 100 °C for 12 h. The formed dry gel is ground into powder and then placed in a tube furnace, and calcined at 300 °C for 4 h under argon-hydrogen gas. After cooling, it is taken out and pressed into a disc under 20 MPa, and then calcined at 500 °C for 12 h in the tube furnace under argon-hydrogen gas. After cooling, it is taken out and crushed to obtain Na4Fe 2.97 Sr 0.03 (PO4)2P2O7.

[0036] The prepared sodium iron pyrophosphate phosphate material is subjected to physical property tests. Figure 1 is the XRD diffraction pattern of the sodium iron pyrophosphate phosphate material prepared by Sr doping. It can be seen from the figure that after introducing the cation Sr, the relative intensities of the (011) crystal plane and the (210) crystal plane in its diffraction pattern change significantly, and the diffraction peaks of the (011) crystal plane and the (210) crystal plane gradually increase. This indicates that Sr can induce the growth of sodium iron pyrophosphate phosphate crystals along the

[011] crystal plane and the

[210] crystal axis, further exposing the (011) crystal plane and the (210) crystal plane with more sodium ions.

[0037] Example 2

[0038] This example provides a sodium iron pyrophosphate phosphate crystal material Na4Fe 2.97 Cd 0.03 (PO4)2P2O7, and its preparation method is basically the same as that in Example 1, except that: the Sr element is replaced with the Cd element, and cadmium acetate is used to obtain Na4Fe 2.97 Cd 0.03 (PO4)2P2O7, Figure 2 is the cycle performance of the doped and modified sodium iron pyrophosphate phosphate prepared in Example 2 at 5C. Figure 3 is the scanning electron microscope image (SEM image) of the doped and modified sodium iron pyrophosphate phosphate prepared in Example 2.

[0039] Example 3

[0040] This embodiment provides a sodium iron pyrophosphate crystal material Na4Fe 2.97 (Cd,Sr) 0.03 (PO4)2P2O7. Its preparation method is basically the same as that in Examples 1 and 2, except that the doped cations in step (b) are replaced with Cd and Sr elements, and the molar ratio of Cd and Sr elements is 1:1. To prepare Na4Fe 2.97 (Cd,Sr) 0.03 (PO4)2P2O7 can also controllably achieve the effect of inducing crystal growth and has a positive effect on the system as well.

[0041] The XRD diffraction pattern of the sodium iron pyrophosphate material prepared by co-doping Sr and Cd is shown in Figure 1 . Compared with the comparative example and Examples 1 and 2, the most obvious changes occur in the (011) crystal plane and the (210) crystal plane.

[0042] Comparative Example 1

[0043] This comparative example provides a sodium iron pyrophosphate crystal material Na4Fe3(PO4)2P2O7. Its preparation method is basically the same as that in Example 1, except that no additional cations are added, that is, Na4Fe3(PO4)2P2O7 is prepared.

[0044] Comparative Example 2

[0045] This comparative example provides a sodium iron pyrophosphate crystal material Na4Fe 2.94 Sr 0.06 (PO4)2P2O7. Its preparation method is basically the same as that in Example 1, except that the doped Sr element in step (b) is X = 0.06, and Na4Fe 2.94 Sr 0.06 (PO4)2P2O7 is prepared. Figure 4 is the XRD diffraction pattern of Comparative Example 2. No further effect of inducing crystal growth is found, and further increasing the doping amount of Sr has no positive effect on the system.

[0046] Taking the sodium iron pyrophosphate materials obtained in the above examples as active substances, the active substances, SuperP, and PVDF are mixed in a ratio of 7:2:1 to prepare a slurry, which is coated on the current collector aluminum foil. After drying, it is cut into circular electrodes with a diameter of 12 mm for assembling 2032-type button cells for electrochemical performance testing. Figure 5 is the first charge-discharge curve diagram of the sodium iron pyrophosphate materials in Examples 1, 2, 3 and Comparative Examples 1 and 2 at a 0.1C rate.

[0047] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A sodium iron pyrophosphate material with regulated crystal structure, characterized in that: The chemical formula of the sodium iron pyrophosphate material is: Na4Fe 2.97 M 0.03 (PO4)2P2O7, wherein M is Sr, or Sr and a second doping element, and the second doping element is selected from one of Cd, Bi, Mo, Ni, and Al.

2. The sodium iron pyrophosphate material with regulated crystal structure according to claim 1, characterized in that: The M is Sr and a second doping element, the second doping element is Cd; and the molar ratio of Sr to Cd is 1:

1.

3. A method for preparing the sodium iron pyrophosphate material with regulated crystal structure as claimed in claim 1 or 2, characterized in that: The following steps are involved: (a) placing an iron source and a chelating agent in deionized water and stirring to dissolve them at room temperature to obtain a solution A; (b) adding a sodium source, a phosphorus source, and a doping element to solution A and mixing them evenly to obtain solution B; and continuing to heat and stir solution B until it becomes a gel; (c) drying the gel obtained in step (b) to obtain a xerogel; and calcining the xerogel once in an inert gas; (d) After taking out, pressing into a shape, calcining for a second time in an inert gas, taking out and crushing after cooling, to obtain the sodium iron pyrophosphate material with regulated crystal structure.

4. The method for preparing the sodium iron pyrophosphate material with regulated crystal structure according to claim 3, characterized in that: Fe in iron source 3+ , chelating agent, sodium source + 、P2O7 in phosphorus source 4- The molar ratio of the doping element is 10: 14~15: 13~18: 13~18: 0.03~0.

06.

5. The method for preparing the sodium iron pyrophosphate material with regulated crystal structure according to claim 3, characterized in that: The doping elements are added in the form of their soluble salts.

6. The method for preparing the sodium iron pyrophosphate material with regulated crystal structure according to claim 3, characterized in that: The chelating agent includes one or more of citric acid, oxalic acid and ascorbic acid.

7. The method for preparing the sodium iron pyrophosphate material with regulated crystal structure according to claim 3, characterized in that: The primary calcination temperature is 250-400 ℃, and the heat preservation time is 3-6 hours.

8. The method for preparing the sodium iron pyrophosphate material with regulated crystal structure according to claim 3, characterized in that: The secondary calcination temperature is 500-550 ℃, and the heat preservation time is 12-24 hours.

9. Use of the sodium iron pyrophosphate material with controlled crystal structure according to claim 1 or 2 or the sodium iron pyrophosphate material with controlled crystal structure prepared by the method according to any one of claims 3 to 8 in a battery.

10. The use according to claim 9, characterized in that: The sodium iron pyrophosphate material with regulated crystal structure is ground and mixed with a conductive agent and a binder, and then coated on a current collector to form a positive electrode of a sodium ion battery.

Images