Self-supporting electrode, preparation method thereof and sodium ion battery

Abstract

The self-supporting electrode is a film formed by compounding a carbon nanotube and an active material, the molecular formula of the active material is NaxNiy [Fe (CN) 6] z, 0 < x < = 2, 0 < y < = 1 and 0 < z < = 1, the active material is of a one-dimensional hollow nanotube structure, and the specific surface area of the active material is 20 m < 2 > / g-100 m < 2 > / g. The self-supporting electrode does not need a binder and an additional conductive agent, has good flexibility and mechanical stability, and also has high energy density, rate capability and cycling stability.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a self-supporting electrode, a preparation method thereof, and a sodium ion battery. Background Art

[0002] Among the cathode materials of sodium ion batteries, Prussian blue and its analogs have been widely concerned due to their unique three-dimensional open framework structure, low cost, and easy synthesis advantages.

[0003] However, for existing Prussian blue materials, when preparing electrodes, it is usually necessary to add a conductive agent and an electrochemically inert binder (such as polyvinylidene fluoride or polytetrafluoroethylene). The addition of the conductive agent and the binder will reduce the content of active substances, and the binder will also reduce the electronic conductivity of the material, resulting in a very low overall energy density of the electrode and poor mechanical stability of the electrode. Although self-supporting electrodes have emerged on the market, which do not require binders and conductive agents and can improve the energy density of the electrode to a certain extent, the existing self-supporting electrodes still have problems of poor rate performance and poor cycling performance. Summary of the Invention

[0004] Based on this, in view of the above problems, it is necessary to provide a self-supporting electrode, a preparation method thereof, and a sodium ion battery. The self-supporting electrode does not require a binder and an additional conductive agent, has good flexibility and mechanical stability, and at the same time has high energy density, rate performance, and cycling stability.

[0005] A self-supporting electrode, wherein the self-supporting electrode is a film formed by the composite of carbon nanotubes and an active material, and the molecular formula of the active material is Na x Ni y [Fe(CN)6] z , where 0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 1, and the active material is a one-dimensional hollow nanotube structure, and the specific surface area of the active material is 20m 2 / g - 100m 2 / g.

[0006] In one embodiment, the hollow nanotube structure satisfies at least one of the following conditions:

[0007] (1) The diameter of the hollow nanotube structure is 50 nm - 300 nm;

[0008] (2) The average pore diameter of the hollow nanotube structure is 15 nm - 100 nm;

[0009] (3) The length of the hollow nanotube structure is 1000 nm - 5000 nm.

[0010] In one embodiment, the mass ratio of the active material to the carbon nanotubes is 1:1 - 7:4.

[0011] In one embodiment, the diameter ratio of the active material to the carbon nanotubes is 50:1 - 300:1.

[0012] In one embodiment, the thickness of the self - supporting electrode is 25 μm - 40 μm.

[0013] A method for preparing the self - supporting electrode includes the following steps:

[0014] Taking Na x Ni y [Fe(CN)6] z as a reference, where 0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 1, placing nickel molybdate nanorod powder in a Na4Fe(CN)6·10H2O solution, then stirring and carrying out an ion - exchange reaction, and obtaining active material powder through separation and drying;

[0015] Dispersing the active material powder in a carbon nanotube suspension, and obtaining the self - supporting electrode through ultrasonic treatment, vacuum filtration, and drying.

[0016] In one embodiment, in the step of preparing the active material powder, at least one of the following conditions is satisfied:

[0017] (1) The mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is 1:2 - 1:5;

[0018] (2) The temperature of the ion - exchange reaction is 0°C - 50°C, and the time is 2 h - 12 h;

[0019] (3) The drying temperature is 60°C - 120°C, and the drying time is 8 h - 20 h;

[0020] (4) The solvent in the Na4Fe(CN)6·10H2O solution is selected from water or a mixed solution composed of water and ethanol;

[0021] (5) The method for preparing the nickel molybdate nanorod powder includes the following steps: preparing a mixed solution by mixing a nickel salt, a molybdenum salt, and water, then carrying out a hydrothermal reaction on the mixed solution, and obtaining nickel molybdate nanorod powder through separation and drying.

[0022] In one embodiment, the method for preparing the nickel molybdate nanorod powder further satisfies at least one of the following conditions:

[0023] (1) The molar ratio of the nickel salt to the molybdenum salt is 1:1 - 1:3;

[0024] (2) The nickel salt is selected from at least one of nickel chloride, nickel nitrate, and nickel acetate, and the molybdenum salt is sodium molybdate;

[0025] (3) The hydrothermal reaction temperature is 140°C - 180°C, and the hydrothermal reaction time is 6h - 12h;

[0026] (4) The drying temperature is 60°C - 120°C, and the drying time is 8h - 20h.

[0027] In one embodiment, the method for preparing the self-supporting electrode further satisfies at least one of the following conditions:

[0028] (1) The mass ratio of the active material powder to the carbon nanotubes is 1:1 - 7:4;

[0029] (2) In the step of drying to obtain the self-supporting electrode, the drying process includes: first drying at a temperature of 30°C - 60°C for 3h - 12h, and then raising the temperature to 80°C - 120°C and continuing to dry for 8h - 24h.

[0030] A sodium ion battery prepared by using the self-supporting electrode described above.

[0031] The self-supporting electrode of the present invention is a film formed by the composite of carbon nanotubes and an active material. Utilizing the characteristics of carbon nanotubes, the electrode does not require a binder and an additional conductive agent, improving the energy density of the electrode, and at the same time having good flexibility and mechanical stability; meanwhile, by setting the active material as a Prussian blue compound and the active material having a one-dimensional hollow nanotube structure, the active material of this structure has a large specific surface area. On the one hand, it can increase the contact area between it and the carbon nanotubes, further improving the binding property between the carbon nanotubes and the active material and enhancing the structural stability of the electrode; on the other hand, it can construct a three-dimensional through-ion and electron transport channel between it and the carbon nanotubes, reducing the electrolyte ion diffusion resistance and charge transfer resistance, and improving the ion and electron conductivity of the electrode; at the same time, it can effectively increase the contact area between the electrolyte and the active material, increase the reaction active sites, and shorten the transport distance of electrons and sodium ions, so that the electrode still has high mechanical stability and rate performance without a binder and an additional conductive agent; moreover, the hollow structure of the active material can effectively inhibit the volume expansion of the active material during the charge and discharge process, significantly improving the cycle stability of the electrode. In addition, further controlling the specific surface area of the active material can further improve the rate performance and cycle stability of the electrode.

[0032] Therefore, the self-supporting electrode of the present invention does not require a binder and an additional conductive agent, has good flexibility and mechanical stability, and at the same time has high energy density, rate performance, and cycle stability. Description of the Drawings

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0034] Figure 1 FIG. 4 is a scanning electron microscope image of the self-supporting electrode of Embodiment 1 of the present invention. Among them, in the figure, A is a low-magnification scanning electron microscope image (magnified 20,000 times), and B is a high-magnification scanning electron microscope image (magnified 150,000 times);

[0035] Figure 2 FIG. 5 is a scanning electron microscope image of the active material in Embodiment 1 of the present invention. Among them, in the figure, A is a low-magnification scanning electron microscope image, and B is a high-magnification scanning electron microscope image;

[0036] Figure 3 FIG. 6 is a transmission electron microscope image of the active material in Embodiment 1 of the present invention;

[0037] Figure 4 FIG. 7 is an X-ray diffraction pattern of the active material in Embodiment 1 of the present invention;

[0038] Figure 5 FIG. 8 is a graph of the specific surface area change of the active material in Embodiment 1 of the present invention;

[0039] Figure 6 FIG. 9 is a scanning electron microscope image of the nickel molybdate nanorod powder in Embodiment 1 of the present invention;

[0040] Figure 7 FIG. 10 is an X-ray diffraction pattern of the nickel molybdate nanorod powder in Embodiment 1 of the present invention;

[0041] Figure 8 FIG. 11 is a scanning electron microscope image of the active material in Comparative Example 1 of the present invention;

[0042] Figure 9 FIG. 12 is a graph of the specific surface area change of the active material in Comparative Example 1 of the present invention. Detailed Embodiments

[0043] To facilitate the understanding of the present invention, the present invention will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. On the contrary, the purpose of providing these embodiments or examples is to make the understanding of the disclosure content of the present invention more thorough and comprehensive.

[0044] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this invention belongs. The terms used in the description of this invention herein are for the purpose of describing specific embodiments or examples only, and are not intended to limit the invention.

[0045] The self-supporting electrode provided by the present invention is a film formed by the composite of carbon nanotubes and an active material. It can be understood that in the present invention, by utilizing the characteristics of carbon nanotubes such as good flexibility, conductivity, mechanical properties, and large specific surface area, a film can be formed by the composite of carbon nanotubes and an active material. This film has good flexibility, conductivity, and mechanical stability, and can be directly used as an electrode, that is, a self-supporting electrode, thus eliminating the need for a binder and additional conductive agents, greatly increasing the content of the active material in the electrode, improving the energy density of the electrode, and at the same time making the electrode have good flexibility and mechanical stability.

[0046] However, through in-depth research, the applicant has found that although the self-supporting electrode is a film formed by the composite of carbon nanotubes and an active material and can increase the energy density of the electrode to a certain extent without a binder and additional conductive agents, the binding force between the carbon nanotubes and the active material in this electrode is still weak, affecting the structural stability of the electrode, and then affecting the mechanical stability of the electrode. At the same time, when applied to a sodium-ion battery, especially during multiple charge and discharge processes of the active material, there is still a risk of structural collapse and hindered electron and ion transport, resulting in poor rate performance and cycle stability of the electrode and the battery.

[0047] Therefore, in the present invention, the molecular formula of the active material is Na x Ni y [Fe(CN)6] z , where 0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 1, and the active material is a one-dimensional hollow nanotube structure, and the specific surface area of the active material is 20m 2 / g - 100m 2 / g.

[0048] In the present invention, by setting the active material as a Prussian blue compound and the active material having a one-dimensional hollow nanotube structure, compared with the solid structures such as cubes or octahedrons of traditional Prussian blue materials, the active material of this structure has a larger specific surface area. On the one hand, it can increase the contact area between it and the carbon nanotubes, further improving the binding property between the carbon nanotubes and the active material and enhancing the structural stability of the electrode; on the other hand, it can construct a three-dimensional through-ion and electron transport channel between it and the carbon nanotubes, reducing the electrolyte ion diffusion resistance and charge transfer resistance, improving the ionic and electronic conductivity of the electrode, and at the same time effectively increasing the contact area between the electrolyte and the active material, increasing the reaction active sites, and shortening the transport distance of electrons and sodium ions. As a result, the electrode still has high mechanical stability and rate performance without a binder and an additional conductive agent; moreover, the hollow structure of the active material can effectively inhibit the volume expansion of the active material during charge and discharge, significantly improving the cycle stability of the electrode, and at the same time allowing part of the carbon nanotubes to be embedded therein to form a more compact interlocking structure, further enhancing the binding force between the two and improving the overall stability of the electrode. In addition, by further controlling the specific surface area of the active material, the reaction active sites can be further increased, effectively inhibiting the structural strain of the active material during charge and discharge, thereby further improving the rate performance and cycle stability of the electrode.

[0049] Therefore, the self-supporting electrode of the present invention does not require a binder and an additional conductive agent, has good flexibility and mechanical stability, and at the same time has high energy density, rate performance, and cycle stability.

[0050] Optionally, the diameter of the hollow nanotube structure is 50 nm - 300 nm. Specifically, the diameter of the hollow nanotube structure includes but is not limited to 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, etc. By setting it like this, by regulating the diameter of the hollow nanotube structure, the specific surface area of the hollow nanotube material can be effectively regulated, the reaction active sites can be increased, and the electrochemical capacity can be improved; at the same time, it is beneficial to improve its own structural strength and further enhance the structural stability of the electrode.

[0051] Optionally, the average pore diameter of the hollow nanotube structure is 15 nm - 100 nm. Specifically, the average pore diameter of the hollow nanotube structure includes but is not limited to 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, etc. With such a setting, by regulating the average pore diameter of the hollow nanotube structure, the hollow volume of the hollow nanotube structure and the thickness of the tube wall of the hollow nanotube structure can be effectively regulated, effectively avoiding the aggregation between active materials, facilitating the transport of sodium ions, further shortening the sodium ion diffusion distance, and thus improving the rate performance of the electrode; at the same time, it can further improve the structural stability of the active material, and thus improve the cycle stability of the electrode.

[0052] Optionally, the length of the hollow nanotube structure is 1000 nm - 5000 nm. Specifically, the length of the hollow nanotube structure includes but is not limited to 1000 nm, 1500 nm, 2000 nm, 2500 nm, 3000 nm, 3500 nm, 4000 nm, 4500 nm, 5000 nm, etc. With such a setting, it is beneficial to make the active material more uniformly dispersed in the three-dimensional conductive network composed of carbon nanotubes, which is beneficial to further improving the rate performance and cycle stability of the electrode.

[0053] Optionally, the mass ratio of the active material to the carbon nanotubes is 1:1 - 7:4. With such a setting, by regulating the dosage between the two, on the one hand, it is beneficial to form a film with good flexibility through compounding, improving the overall mechanical stability of the electrode; on the other hand, it can further improve the energy density, rate performance and cycle stability of the electrode.

[0054] Optionally, the diameter ratio of the active material to the carbon nanotubes is 50:1 - 300:1. With such a setting, it is beneficial for the active material and the carbon nanotubes to form a uniform film through compounding, further improving the structural stability and electrochemical performance of the electrode.

[0055] Optionally, the thickness of the self-supporting electrode is 25 μm - 40 μm. With such a setting, it is beneficial to further improve the rate performance and cycle stability of the electrode while ensuring that the self-supporting electrode has good flexibility and mechanical stability.

[0056] In one embodiment, the carbon nanotubes are selected from single-walled carbon nanotubes and / or multi-walled carbon nanotubes, preferably single-walled carbon nanotubes.

[0057] Meanwhile, the present invention also provides a preparation method of the self-supporting electrode as described above, including the following steps:

[0058] S1, using Nax Ni y [Fe(CN)6] z Taking Ni y [Fe(CN)6] z as a reference, where 0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 1, the nickel molybdate nanorod powder is placed in an aqueous solution containing Na4Fe(CN)6·10H2O, and then stirred to carry out an ion exchange reaction. After separation and drying, the active material powder is obtained. It can be understood that in step S1, the nickel molybdate nanorod powder is used as a self-sacrificial template and a nickel source, and then assisted by the ion exchange method, the nickel ions in the nickel molybdate nanorods are exchanged with the sodium ions in the reaction system to generate a chemical formula of Na x Ni y [Fe(CN)6] z (0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 1) and an active material with a one-dimensional hollow nanotube structure.

[0059] Optionally, the mass ratio of the nickel molybdate nanorod powder to the Na4Fe(CN)6·10H2O is 1:2 - 1:5; specifically, the mass ratio of the nickel molybdate nanorod powder to the Na4Fe(CN)6·10H2O includes but is not limited to 1:2, 1:3, 1:4, 1:5. By setting it in this way, by regulating the mass ratio of the two, it is beneficial to better form an active material with a one-dimensional hollow nanotube structure.

[0060] Optionally, the temperature of the ion exchange reaction is 0°C - 50°C, and the time is 2h - 12h. By setting it in this way, by controlling the temperature and time of the ion exchange reaction, the diameter and pore size of the hollow nanotube structure can be better regulated, and an active material with a larger specific surface area and uniform structure can be obtained.

[0061] Furthermore, the temperature of the ion exchange reaction is preferably room temperature, which is beneficial to simplifying the process and reducing costs.

[0062] Optionally, the drying temperature is 60°C - 120°C, and the drying time is 8h - 20h; by setting it in this way, it is beneficial to obtain an active material with uniform particle size and stable quality.

[0063] Optionally, the preparation method of the nickel molybdate nanorod powder includes the following steps: A mixed solution is prepared by mixing a nickel salt, a molybdenum salt, and water, and then the mixed solution is subjected to a hydrothermal reaction. After separation and drying, the nickel molybdate nanorod powder is obtained.

[0064] Optionally, the molar ratio of the nickel salt to the molybdenum salt is 1:1 - 1:3; by setting it in this way, it is beneficial to ensure that all nickel is converted into nickel molybdate, better form a nickel molybdate nanorod template, and the yield of this template is high.

[0065] Furthermore, the nickel salt is selected from at least one of nickel chloride, nickel nitrate, and nickel acetate, and the molybdenum salt is selected from sodium molybdate.

[0066] Optionally, the hydrothermal reaction temperature is 140°C - 180°C, and the hydrothermal reaction time is 6h - 12h; such settings are conducive to better formation of nickel molybdate nanorods.

[0067] Optionally, in the step of preparing the nickel molybdate nanorod powder, the drying temperature is 60°C - 120°C, and the drying time is 8h - 20h.

[0068] In one embodiment, in the step of preparing the nickel molybdate nanorod powder, the separation step includes centrifugation and washing.

[0069] In one embodiment, the preparation method of the Na4Fe(CN)6·10H2O solution is as follows: Na4Fe(CN)6·10H2O is dissolved in a solvent, and after stirring for 8 min - 12 min, an aqueous solution containing Na4Fe(CN)6·10H2O can be obtained. Among them, the solvent is selected from water or a mixed solution composed of water and ethanol, preferably a mixed solution composed of water and ethanol, and the volume ratio of water to ethanol is 1:2 - 4:1.

[0070] In one embodiment, in step S1, the separation step includes centrifugation and washing to remove impurities and templates.

[0071] S2. Dispersing the active material powder in the carbon nanotube suspension, and obtaining the self-supporting electrode through ultrasonic treatment, vacuum filtration, and drying.

[0072] In step S2, the active material powder is dispersed in the carbon nanotube suspension, and after ultrasonic treatment for 10 min - 60 min to ensure that the active material is uniformly dispersed in the carbon nanotube suspension to obtain a dispersion; the dispersion is subjected to vacuum filtration, where the vacuum degree is 0.05 MPa - 0.2 MPa, and a mixed cellulose matrix membrane is used as the filtration membrane. After vacuum filtration is completed, the water mixed cellulose matrix membrane is removed by dissolving with N-methylpyrrolidone to obtain a wet electrode; finally, the wet electrode is placed in a vacuum drying oven for drying to obtain a membrane formed by the composite of nanotubes and the active material, that is, the self-supporting electrode.

[0073] Optionally, the mass ratio of the active material powder to the carbon nanotubes is 1:1 - 7:4. Such settings can better construct a flexible three-dimensional network structure composed of carbon nanotubes, and the active material is distributed in the pores of the flexible three-dimensional network structure.

[0074] Optionally, in the step of drying the wet self-supporting electrode in a vacuum drying oven, the drying process includes: first drying at a temperature of 30°C - 60°C for 3h - 12h, and then raising the temperature to 80°C - 120°C and continuing to dry for 8h - 24h. Such settings are conducive to ensuring the integrity and structural stability of the self-supporting electrode.

[0075] Optionally, the method for preparing the carbon nanotube suspension includes the following steps: mixing carbon nanotube powder, a surfactant, and water to obtain a carbon nanotube suspension. Specifically, the carbon nanotube powder, the surfactant, and water are mixed, and after ultrasonic treatment for 20 min - 60 min, a carbon nanotube suspension is obtained, wherein the mass ratio of the carbon nanotubes to water is 1:3 - 1:10.

[0076] Furthermore, the mass ratio of the carbon nanotube powder to the surfactant is 1:10 - 1:50. With such a setting, it is beneficial to obtain a carbon nanotube suspension with uniform dispersion and stability.

[0077] In one embodiment, the carbon nanotubes are selected from single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0078] In one embodiment, the surfactant is selected from sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, etc.

[0079] In addition, the present invention also provides a sodium-ion battery prepared by using the self-supporting electrode as described above. Specifically, the self-supporting electrode is used as a positive electrode in the sodium-ion battery. This sodium-ion battery has excellent rate performance and cycling stability.

[0080] Hereinafter, the self-supporting electrode, its preparation method, and the sodium-ion battery will be further described through the following specific examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. For those not specified in the examples, they are carried out under conventional conditions or conditions recommended by the manufacturer. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0081] Example 1

[0082] 4 mmol of NiSO4·6H2O and 4 mmol of Na2MoO4·2H2O are dissolved in 70 mL of deionized water to prepare a mixed solution, and then the mixed solution is transferred to a 90 mL Teflon-lined stainless steel autoclave and subjected to a hydrothermal reaction at 170 °C for 6 h to obtain a reaction product. The reaction product is centrifuged, washed with deionized water, and then placed in a vacuum drying oven at 60 °C for 18 h to obtain nickel molybdate nanorod powder; single-walled carbon nanotube powder (with a diameter of about 1 nm), sodium dodecyl sulfate, and water are mixed, and after ultrasonic treatment for 30 min, a carbon nanotube suspension is obtained, wherein the mass ratio of the carbon nanotube powder to the sodium dodecyl sulfate is 1:10.

[0083] The obtained nickel molybdate nanorod powder is placed in a Na4Fe(CN)6·10H2O solution. Among them, the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is 1:3.6. The solvent in the Na4Fe(CN)6·10H2O solution is deionized water. Then, it is stirred at room temperature and undergoes an ion exchange reaction for 6 h to obtain a reaction solution. After the reaction solution is centrifuged and washed, it is placed in a vacuum drying oven at 60 °C and dried for 12 h to obtain an active material powder with the chemical formula Na2Ni[Fe(CN)6] and a one-dimensional hollow nanotube structure.

[0084] The obtained active material powder is dispersed in the obtained carbon nanotube suspension and ultrasonicated for 30 min to obtain a dispersion. Among them, the mass ratio of the active material powder to the carbon nanotubes is 1:1. Then, vacuum filtration is carried out through a mixed cellulose matrix membrane. Among them, the vacuum degree is 0.1 MPa. During the vacuum filtration process, the filtration membrane is washed with deionized water. Then, the mixed cellulose matrix membrane is dissolved and removed using N-methylpyrrolidone to obtain a wet electrode. Finally, the wet electrode is placed in a vacuum drying oven. First, it is dried at 40 °C for 6 h, and then the temperature is raised to 80 °C and dried for another 18 h to obtain a self-supporting electrode with a thickness of 30 μm.

[0085] From Figures 6 - 7 it can be seen that the nickel molybdate nanorod template has been successfully synthesized in Example 1 of the present invention; from Figures 2 - 5 it can be seen that the synthesized active material in Example 1 of the present invention has a high crystallinity and has a typical Prussian blue structure (No: 52-1907 face-centered cubic structure, space group Fm3m), indicating that the active material powder with the chemical formula Na2Ni[Fe(CN)6] has been successfully synthesized in Example 1. At the same time, the active material is a one-dimensional hollow nanotube structure, and the specific surface area of the active material is as high as 54.4 m 2 / g, the diameter is about 250 nm, the length is 2062 nm, and the average pore diameter is 17 nm.

[0086] From Figure 1 it can be seen that the self-supporting electrode in Example 1 of the present invention includes carbon nanotubes and an active material. The carbon nanotubes form a flexible three-dimensional network structure, and the active material is distributed in the pores of the flexible three-dimensional network structure.

[0087] Example 2

[0088] 4 mmol of NiCl2·6H2O and 8 mmol of Na2MoO4·2H2O were dissolved in 50 mL of deionized water to prepare a mixed solution. Then, the mixed solution was transferred to a 90 mL Teflon-lined stainless steel autoclave and subjected to hydrothermal reaction at 140 °C for 10 h to obtain a reaction product. The reaction product was centrifuged, washed with deionized water, and then dried in a vacuum drying oven at 90 °C for 12 h to obtain nickel molybdate nanorod powder. Single-walled carbon nanotube powder (with a diameter of about 1 nm), sodium dodecyl sulfate, and water were mixed and ultrasonically treated for 40 min to obtain a carbon nanotube suspension. Among them, the mass ratio of the carbon nanotube powder to the sodium dodecyl sulfate was 1:20.

[0089] The obtained nickel molybdate nanorod powder was placed in a Na4Fe(CN)6·10H2O solution. Among them, the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O was 1:3. The solvent in the Na4Fe(CN)6·10H2O solution was deionized water. Then, it was stirred at room temperature and subjected to an ion exchange reaction for 8 h to obtain a reaction solution. After the reaction solution was centrifuged and washed, it was dried in a vacuum drying oven at 80 °C for 10 h to obtain a powder of active material with the chemical formula Na 1.75 Ni1[Fe(CN)6] 0.94 and a one-dimensional hollow nanotube structure, where the specific surface area of the active material was 62.3 m 2 / g, the diameter was 274 nm, the length was 2360, and the average pore diameter was 27 nm.

[0090] The obtained active material powder was dispersed in the obtained carbon nanotube suspension and ultrasonically treated for 50 min to obtain a dispersion. Among them, the mass ratio of the active material powder to the carbon nanotubes was 3:2. Then, vacuum filtration was carried out through a mixed cellulose matrix membrane, where the vacuum degree was 0.1 MPa. During vacuum filtration, the filtration membrane was washed with deionized water. Then, the mixed cellulose matrix membrane was dissolved and removed with N-methylpyrrolidone to obtain a wet electrode. Finally, the wet electrode was placed in a vacuum drying oven and first dried at 50 °C for 4 h, and then the temperature was raised to 100 °C and dried for another 12 h to obtain a self-supporting electrode with a thickness of 28 μm.

[0091] Example 3

[0092] 4 mmol of Ni(CH3COO)2·4H2O and 12 mmol of Na2MoO4·2H2O were dissolved in 90 mL of deionized water to prepare a mixed solution. Then, the mixed solution was transferred to a 90 mL Teflon-lined stainless steel autoclave and subjected to hydrothermal reaction at 160 °C for 8 h to obtain a reaction product. The reaction product was centrifuged, washed with deionized water, and then dried in a vacuum drying oven at 120 °C for 8 h to obtain nickel molybdate nanorod powder. Single-walled carbon nanotube powder (with a diameter of about 1 nm), sodium dodecyl sulfate, and water were mixed and ultrasonicated for 40 min to obtain a carbon nanotube suspension. Among them, the mass ratio of the carbon nanotube powder to the sodium dodecyl sulfate was 1:40.

[0093] The obtained nickel molybdate nanorod powder was placed in a Na4Fe(CN)6·10H2O solution. Among them, the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O was 1:5. The solvent in the Na4Fe(CN)6·10H2O solution was deionized water. Then, it was stirred at room temperature and subjected to an ion exchange reaction for 10 h to obtain a reaction solution. The reaction solution was centrifuged and washed, and then dried in a vacuum drying oven at 120 °C for 8 h to obtain an active material powder with the chemical formula Na2Ni[Fe(CN)6] and a one-dimensional hollow nanotube structure. Among them, the specific surface area of the active material was 83.1 m 2 / g, the diameter was 298 nm, the length was 2783 nm, and the average pore diameter was 30 nm.

[0094] The obtained active material powder was dispersed in the obtained carbon nanotube suspension and ultrasonicated for 50 min to obtain a dispersion. Among them, the mass ratio of the active material powder to the carbon nanotubes was 7:4. Then, vacuum filtration was carried out through a mixed cellulose matrix membrane, where the vacuum degree was 0.1 MPa. During vacuum filtration, the filtration membrane was washed with deionized water. Then, the mixed cellulose matrix membrane was dissolved and removed using N-methylpyrrolidone to obtain a wet electrode. Finally, the wet electrode was placed in a vacuum drying oven, first dried at 60 °C for 3 h, and then heated to 120 °C and continued to be dried for 8 h to obtain a self-supporting electrode with a thickness of 29 μm.

[0095] Example 4

[0096] Example 4 is different from Example 1 only in that the solvent in the Na4Fe(CN)6·10H2O solution is a mixed solution composed of deionized water and ethanol, wherein the volume ratio of deionized water to ethanol is 1:2; the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is 1:3.7; the ion exchange reaction is carried out for 2 h; the remaining conditions are the same, and an active material powder with the chemical formula Na2Ni[Fe(CN)6] and a one-dimensional hollow nanotube structure and a self-supporting electrode are obtained. Among them, the specific surface area of the active material is 33 m 2 / g, the diameter is 158 nm, the average pore diameter is 15 nm, the length is 3000 nm, and the thickness of the self-supporting electrode is 27 μm.

[0097] Example 5

[0098] Example 5 is different from Example 1 only in that the solvent in the Na4Fe(CN)6·10H2O solution is a mixed solution composed of deionized water and ethanol, wherein the volume ratio of deionized water to ethanol is 2:1; the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is 1:3.7; the ion exchange reaction is carried out for 2 h; the remaining conditions are the same, and an active material powder with the chemical formula Na2Ni[Fe(CN)6] and a one-dimensional hollow nanotube structure and a self-supporting electrode are obtained. Among them, the specific surface area of the active material is 60 m 2 / g, the diameter is 297 nm, the average pore diameter is 25 nm, the length is 4500 nm, and the thickness of the self-supporting electrode is 35 μm.

[0099] Example 6

[0100] Example 6 is different from Example 1 only in that the solvent in the Na4Fe(CN)6·10H2O solution is a mixed solution composed of deionized water and ethanol, wherein the volume ratio of deionized water to ethanol is 4:1; the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is 1:3.7; the ion exchange reaction is carried out for 2 h; the remaining conditions are the same, and an active material powder with the chemical formula Na2Ni[Fe(CN)6] and a one-dimensional hollow nanotube structure and a self-supporting electrode are obtained. Among them, the specific surface area of the active material is 42 m 2 / g, the diameter is 177.5 nm, the average pore diameter is 15 nm, the length is 1793 nm, and the thickness of the self-supporting electrode is 28 μm.

[0101] Example 7

[0102] Example 7 is different from Example 1 only in that the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is 1:1, and the remaining conditions are the same, and Na with the chemical formula is obtained 0.42Ni 0.29 [Fe(CN)6] 0.25 and the active material powder and self - supporting electrode with a one - dimensional hollow nanotube structure, wherein the specific surface area of the active material is 28 m 2 / g, the diameter is 85 nm, the average pore diameter is 18 nm, the length is 1231 nm, and the thickness of the self - supporting electrode is 28 μm.

[0103] Example 8

[0104] Compared with Example 1, Example 8 is only different in that the mass ratio of the nickel molybdate nanorod powder and Na4Fe(CN)6·10H2O is 1:6, and the rest of the conditions are the same, obtaining an active material powder with the chemical formula Na2Ni1[Fe(CN)6] and a one - dimensional hollow nanotube structure and a self - supporting electrode, wherein the specific surface area of the active material is 26 m 2 / g, the diameter is 275 nm, the average pore diameter is 16 nm, the length is 3500 nm, and the thickness of the self - supporting electrode is 35 μm.

[0105] Example 9

[0106] Compared with Example 1, Example 9 is only different in that the mass ratio of the active material powder and the carbon nanotubes is 1:2, and the rest of the conditions are the same, obtaining an active material powder with the chemical formula Na2Ni[Fe(CN)6] and a one - dimensional hollow nanotube structure and a self - supporting electrode, wherein the specific surface area of the active material is 54.4 m 2 / g, the diameter is 250 nm, the average pore diameter is 17 nm, the length is 2062 nm, and the thickness of the self - supporting electrode is 68 μm.

[0107] Example 10

[0108] Compared with Example 1, Example 10 is only different in that the mass ratio of the active material powder and the carbon nanotubes is 3:1, and the rest of the conditions are the same, obtaining an active material powder with the chemical formula Na2Ni[Fe(CN)6] and a one - dimensional hollow nanotube structure and a self - supporting electrode, wherein the specific surface area of the active material is 54.4 m 2 / g, the diameter is 250 nm, the average pore diameter is 17 nm, the length is 2062 nm, and the thickness of the self - supporting electrode is 15 μm.

[0109] Example 11

[0110] Compared with Example 1, Example 11 is only different in that the wet electrode is placed in a vacuum drying oven and dried at 80 °C for another 24 h, and the rest of the conditions are the same, obtaining a self - supporting electrode with a thickness of 28 μm.

[0111] Example 12

[0112] Example 12 is different from Example 1 only in that, in the step of preparing the carbon nanotube suspension, multi-walled carbon nanotube powder (with a diameter of 5 nm) is used to replace the carbon nanotube powder, and the other conditions are the same, and a self-supporting electrode with a thickness of 33 μm is obtained.

[0113] Comparative Example 1

[0114] 4 mmol of NiSO4·6H2O was dissolved in 40 mL of deionized water to prepare a first solution; 4 mmol of Na4Fe(CN)6·10H2O was dissolved in 40 mL of deionized water to prepare a second solution; then the first solution was added dropwise to the second solution and stirred for 6 h. After the reaction ended, the reaction solution was centrifuged and washed, and then placed in a vacuum drying oven at 60 °C for 12 h to obtain an active material powder with the chemical formula Na2Ni[Fe(CN)6] and in the form of amorphous particles. Among them, the specific surface area of the active material is 1.4 m 2 / g, the diameter is 10 μm, and the average pore diameter is 8 nm; single-walled carbon nanotube powder (with a diameter of about 1 nm), sodium dodecyl sulfate and water were mixed, and after ultrasonic treatment for 40 min, a carbon nanotube suspension was obtained. Among them, the mass ratio of the carbon nanotube powder to the sodium dodecyl sulfate is 1:20.

[0115] The above-obtained active material powder was dispersed in the above-obtained carbon nanotube suspension and ultrasonic-treated for 30 min to obtain a dispersion. Among them, the mass ratio of the active material powder to the carbon nanotubes is 1:1; then vacuum filtration was carried out through a mixed cellulose matrix membrane, where the vacuum degree is 0.1 MPa. During the vacuum filtration process, the filtration membrane was washed with deionized water, and then the mixed cellulose matrix membrane was dissolved and removed with N-methylpyrrolidone to obtain a wet electrode; finally, the wet electrode was placed in a vacuum drying oven, dried at 40 °C for 6 h first, and then heated to 80 °C and continued to be dried for 18 h to obtain a self-supporting electrode with a thickness of 42 μm.

[0116] From Figures 8 - 9 it can be seen that the morphology of the active material powder in Comparative Example 1 is amorphous particles, and the specific surface area is 1.4 m 2 / g.

[0117] Comparative Example 2

[0118] Comparative Example 2 is different from Example 1 only in that the above-obtained active material powder, conductive additive Ketjenblack and binder polytetrafluoroethylene, and N-methylpyrrolidone were mixed to obtain a uniform slurry. Among them, the mass ratio of the active material powder, conductive additive Ketjenblack and binder polytetrafluoroethylene is 7:2:1. The obtained slurry was coated on a copper foil to obtain a non-self-supporting electrode with a thickness of 100 μm.

[0119] The self-supporting electrodes prepared in Examples 1-12 and Comparative Example 1 and the non-self-supporting electrode prepared in Comparative Example 2 were respectively used as the positive electrode sheets to assemble CR2032-type batteries.

[0120] Among them, the method for the sodium-ion battery is as follows: The electrodes prepared in the above examples and comparative examples were used as the positive electrode, metallic sodium was used as the negative electrode, a solution of 1 M NaClO4 dissolved in ethylene carbonate / diethyl carbonate (volume ratio 1:1) (containing 5% fluoroethylene carbonate (FEC)) was used as the electrolyte, and a glass microfiber membrane (GF / D grade) was used as the separator to assemble a CR2032-type battery.

[0121] Subsequently, the constant current charge-discharge tests of each CR2032-type battery at 25 °C were carried out on a Neware battery test system, and its voltage window was between 2.0 V and 4.2 V (vs Na / Na + ), and the electrical performance tests of each CR2032-type battery at different rates were carried out respectively. The test results are shown in Table 1 below.

[0122] Table 1

[0123]

[0124]

[0125] Referring to the data in Table 1, by comparing Example 1 with Examples 7 - 8, it can be seen that if the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is too high, it will lead to insufficient crystal growth of the active material, affecting the size and uniformity of the active material, reducing its specific surface area, thereby reducing the contact area between the active substance and the electrolyte, decreasing the number of active sites, and further affecting the rate performance and cycling performance of the self - supporting electrode; while if the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is too low, the surface of the active material will be covered by excessive unreacted or incompletely reacted Na4Fe(CN)6·10H2O, reducing the number of active sites that actually participate in the electrochemical reaction, thus directly affecting the charge storage capacity and reaction rate of the active material, resulting in a decline in high - rate performance; moreover, the Na4Fe(CN)6·10H2O covering the surface will slowly dissolve in the electrolyte during the cycling process to become impurities, which will also lead to a decline in the cycling performance of the electrode. Therefore, precisely controlling the mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is beneficial to ensuring the optimization of the microstructure of the active material, making its crystal growth more sufficient and uniform, while maintaining a sufficient number of active sites and unobstructed ion channels, improving the cycling performance and rate performance of the electrode. By comparing Example 1 with Examples 9 - 10, it can be seen that whether the mass ratio of the active material powder to the carbon nanotubes is too high or too low, it will affect the rate performance and cycling performance. Among them, when the mass ratio of the active material powder to the carbon nanotubes is too low, that is, when the proportion of carbon nanotubes is relatively high, the thickness of the electrode will increase accordingly, and the thickened electrode means that the diffusion path of sodium ions in the active material becomes longer, which increases the resistance of charge transfer. Especially during high - rate charge - discharge processes, excessive carbon nanotubes will form an unnecessary conductive network, thus affecting the rate performance. In addition, the thicker electrode will also lead to insufficient electrolyte infiltration, and some active materials cannot effectively contact the electrolyte, thereby reducing the number of active sites that can participate in the electrochemical reaction, resulting in a smaller overall capacity. As the cycling progresses, the number of active sites that can participate in the electrochemical reaction becomes fewer and fewer, resulting in poor cycling performance. When the mass ratio of the active material powder to the carbon nanotubes is too high, that is, when the proportion of carbon nanotubes is relatively low, the conductive network formed by the carbon nanotubes is insufficient, and there are large gaps between the active particles. In addition, the electrode is weaker in mechanical strength and is easily deformed or broken under external forces, all of which will affect the rate performance and cycling performance.

[0126] Comparing Comparative Example 1 and Example 11, it can be seen that using segmented drying is beneficial to improving the rate performance and cycling performance. This is because different drying conditions are experienced in different stages, which helps to remove the solvent more uniformly, reduce the accumulation of internal stress, and avoid cracks in the material and deterioration of the pore structure caused by rapid drying, making it more conducive to the infiltration of the electrolyte, and thus maintaining the integrity and stability of the electrode. In addition, as the solvent is gradually removed, the contact between the active material and the carbon nanotubes becomes closer, which helps to reduce the resistance of charge transfer and improve the transport efficiency of electrons and sodium ions inside the electrode, especially during high-rate charge and discharge processes.

[0127] Comparing Example 1 and Example 12, it can be seen that using single-walled carbon nanotubes is beneficial to further improving the rate performance and cycling stability of the electrode.

[0128] Comparing Example 1 and Comparative Example 1, it can be seen that in Comparative Example 1, since the morphology of the active material is amorphous particles, it has a smaller specific surface area, and at the same time, it will affect the rate performance and cycling performance of the electrode.

[0129] Comparing Example 1 and Comparative Example 2, it can be seen that by controlling the active material to be a one-dimensional hollow nanotube structure and introducing carbon nanotubes, the electrochemical performance can be significantly improved. This is because the nanotube structure of the active material has a higher specific surface area and a shorter diffusion path. At the same time, the addition of conductive carbon nanotubes forms a conductive network, which not only provides a channel for electron transport but also enhances the overall conductivity of the electrode, further improving the rate performance and cycling performance of the electrode.

[0130] Thus, it can be seen that the self-supporting electrode of the present invention does not require a binder and an additional conductive agent, has good flexibility and mechanical stability, and at the same time has a high energy density, rate performance, and cycling stability.

[0131] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0132] The above-described embodiments only represent several implementation manners of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent should be subject to the appended claims.

Claims

1. A self-supporting electrode, characterized in that, The self-supporting electrode is a film formed by the composite of carbon nanotubes and an active material, and the molecular formula of the active material is Na x Ni y [Fe(CN)6] z , where 0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 1, and the active material is a one-dimensional hollow nanotube structure, and the specific surface area of the active material is 20 m 2 / g - 100 m 2 / g.

2. The self-supporting electrode according to claim 1, wherein The hollow nanotube structure satisfies at least one of the following conditions: (1) The diameter of the hollow nanotube structure is 50 nm - 300 nm; (2) The average pore diameter of the hollow nanotube structure is 15 nm - 100 nm; (3) The length of the hollow nanotube structure is 1000 nm - 5000 nm.

3. The self-supporting electrode according to claim 1, wherein The mass ratio of the active material to the carbon nanotubes is 1:1 - 7:

4.

4. The self-supporting electrode according to claim 1, wherein The diameter ratio of the active material to the carbon nanotubes is 50:1 - 300:

1.

5. The self-supporting electrode according to claim 1, wherein, The thickness of the self-supporting electrode is 25 μm - 40 μm.

6. A method for preparing a self-supporting electrode according to any one of claims 1-5, characterized in that, It includes the following steps: Taking Na x Ni y [Fe(CN)6] z as a reference, where 0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 1, nickel molybdate nanorod powder is placed in a Na4Fe(CN)6·10H2O solution, then stirred and subjected to an ion exchange reaction, and the active material powder is obtained through separation and drying; Disperse the active material powder in the carbon nanotube suspension, and obtain the self-supporting electrode through ultrasonic treatment, vacuum filtration, and drying.

7. The method for preparing a self-supporting electrode according to claim 6, wherein, In the step of preparing the active material powder, at least one of the following conditions is satisfied: (1) The mass ratio of the nickel molybdate nanorod powder to Na4Fe(CN)6·10H2O is 1:2 - 1:5; (2) The temperature of the ion exchange reaction is 0°C - 50°C, and the time is 2 h - 12 h; (3) The drying temperature is 60°C - 120°C, and the drying time is 8 h - 20 h; (4) The solvent in the Na4Fe(CN)6·10H2O solution is selected from water or a mixed solution composed of water and ethanol; (5) The preparation method of the nickel molybdate nanorod powder includes the following steps: Prepare a mixed solution of nickel salt, molybdenum salt, and water, then perform a hydrothermal reaction on the mixed solution, and obtain the nickel molybdate nanorod powder through separation and drying.

8. The method for preparing a self-supporting electrode according to claim 7, wherein The preparation method of the nickel molybdate nanorod powder also satisfies at least one of the following conditions: (1) The molar ratio of the nickel salt to the molybdenum salt is 1:1 - 1:3; (2) The nickel salt is selected from at least one of nickel chloride, nickel nitrate, and nickel acetate, and the molybdenum salt is selected from sodium molybdate; (3) The hydrothermal reaction temperature is 140°C - 180°C, and the hydrothermal reaction time is 6 h - 12 h; (4) The drying temperature is 60°C - 120°C, and the drying time is 8 h - 20 h.

9. The preparation method of the self-supporting electrode according to claim 6, characterized in that, The preparation method of the self-supporting electrode also satisfies at least one of the following conditions: (1) The mass ratio of the active material powder to the carbon nanotubes is 1:1 - 7:4; (2) In the step of drying to obtain the self-supporting electrode, the drying process includes: First, dry at a temperature of 30°C - 60°C for 3 h - 12 h, then raise the temperature to 80°C - 120°C, and continue to dry for 8 h - 24 h.

10. A sodium-ion battery prepared by using the self-supporting electrode according to any one of claims 1 - 5.

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