Sea urchin-shaped FeP-coated NC composite material as well as preparation method and application thereof

Abstract

The invention provides a sea urchin-shaped FeP-coated NC composite material and a preparation method and application thereof, the composite material is formed by wrapping internal FeP nanoparticles with an external nitrogen-doped carbon layer, and presents a unique sea urchin-shaped structure; the preparation method comprises the following steps: preparing a FeOOH precursor by a hydrothermal method, adding the FeOOH precursor into a Tris buffer solution for ultrasonic dispersion, adding dopamine into the solution, stirring for a long time at normal temperature, separating, washing, and drying to obtain FeOOH (at) PDA; carrying out phosphating treatment on FeOOH-coated PDA and sodium hypophosphite at two ends of a porcelain boat according to a certain mass ratio in an atmosphere full of Ar to obtain a FeP-coated NC composite material; as a sodium ion battery negative electrode material, the composite material shows excellent electrochemical performance, and the specific capacity of 174.5 mAh g <-1 > can still be kept after 200 times of circulation under the current density of 2A g <-1 >, so that the material has the electrochemical performance with higher application potential.

Description

Technical Field

[0001] The present invention belongs to the field of electrochemistry technology, and particularly relates to a sea urchin-like FeP@NC composite material, a preparation method thereof and an application thereof. Background Art

[0002] With the development of economy and the improvement of living standards, the reserves of fossil energy such as petroleum, natural gas and coal are gradually decreasing, and the use of fossil energy is often accompanied by environmental pollution problems. Many countries and regions have begun to seek alternative energy sources to reduce their dependence on fossil energy and promote sustainable development. Sodium resources are rich in reserves, wide in source and low in price. In recent years, sodium-ion batteries have gradually come into the view of many research scholars and are also considered to be promising to replace lithium-ion batteries as one of the main devices for new energy storage. The main challenge in the development of sodium-ion batteries is that the radius of Na + is larger than that of Li + , which results in poor diffusion kinetics. Selecting and improving the anode material is one of the important ways to improve the electrochemical performance of sodium-ion batteries. Conversion materials include a variety of non-metal compounds, such as oxides, phosphides, sulfides, selenides, etc., which show high reversible capacity and fast sodium-ion diffusion rate when applied as anode materials for sodium-ion batteries. Among the conversion anode materials, transition metal phosphides have been widely studied due to their high theoretical capacity and relatively low Na + / Li + intercalation potential. So far, many transition metal phosphides, such as Sn4P3, Cu3P, Ni2P, MoP, CoP, VP and ZnP2, have shown good electrochemical performance in lithium / sodium storage. Recently, iron phosphide (FeP), as a transition metal phosphide, has a theoretical capacity of about 926 mAh g -1 , and has been proven to be one of the most promising anode materials for lithium-ion batteries and sodium-ion batteries, and can achieve a reversible two-step insertion / conversion process. Shi et al. successfully synthesized FeP quantum dots doped in carbon nanotube-grafted phosphorus-doped carbon octahedrons to achieve high-rate performance. Jiang et al. synthesized N,P-functionalized carbon skeletons using green phytic acid (PA) as a precursor and successfully synthesized FeP@PNC materials by combining recrystallization self-assembly with FeP nanoparticles, achieving 4500 ultra-long cycles at a current density of 0.5 A g -1 . Therefore, compounding transition metal phosphides with carbon materials is a feasible way to improve their electrochemical performance. In order to improve the problems of the existing FeP composite materials, such as few structural active sites, volume expansion during charge and discharge, which affects the electrochemical performance of the electrode, and the preparation process is relatively complex and the cost is relatively high, the present invention successfully prepares an FeP@NC material with a simple required process, environmental protection and excellent electrochemical performance. Summary of the Invention

[0003] The present invention constructs a sea urchin-like FeP@NC composite material, its preparation method and application, which can effectively solve the problems occurring in the above application process.

[0004] The present invention is implemented as follows:

[0005] The present invention further provides a preparation method of a sea urchin-like FeP@NC composite material, comprising the following steps:

[0006] S1, adding an FeOOH precursor to a pH = 8.0 - 9.0 buffer solution containing Tris for ultrasonic dispersion, and then adding dopamine to the solution according to the mass ratio of FeOOH precursor to dopamine of 1 - 1.8:1, and stirring at room temperature for 8 - 20 h to obtain a brownish-red precipitate-like isolate;

[0007] S2, washing the obtained brownish-red precipitate-like isolate with deionized water and ethanol multiple times and then putting it into an oven, and keeping it at a constant temperature of 60°C - 80°C for 5 - 10 h to obtain FeOOH@PDA;

[0008] S3, uniformly mixing FeOOH@PDA and sodium hypophosphite according to a mass ratio of 1:8 - 10, placing them at the upstream and downstream ends of a porcelain boat in a tube furnace filled with an Ar atmosphere, carrying out phosphating treatment at 350 - 450°C for 2 - 2.5 h, and controlling the heating rate at 1 - 3°C / min to finally obtain FeP@NC.

[0009] The present invention constructs a sea urchin-like FeP@NC composite material. The active material of this negative electrode material system is a nitrogen-doped carbon-coated sea urchin-like FeP@NC composite material, and the particle size of the nitrogen-doped carbon-coated sea anemone-like FeP@NC composite material is 1 micrometer - 3 micrometers.

[0010] The present invention further provides a novel sodium-ion battery negative electrode, including the above novel battery negative electrode material.

[0011] The present invention further provides a novel sodium-ion battery, including the above novel sodium-ion battery negative electrode.

[0012] The beneficial effects of the present invention are as follows: The synthesis process of the novel sodium-ion battery negative electrode material, the negative electrode, and the novel sodium-ion battery prepare an FeP material (FeP@NC) with a unique sea urchin-like structure through a convenient and low-cost hydrothermal synthesis method and a highly controllable phosphating process. This material exhibits excellent electrochemical performance when applied to the negative electrode of a sodium-ion battery: it can still maintain 174.5 mAh g after 200 cycles at a current density of 2 Ag -1 and -1Specific capacity. Its excellent performance is inseparable from its unique sea urchin-like structure. This special structure not only provides more active sites for sodium storage applications, but also the nitrogen-doped carbon coating layer on the material surface effectively alleviates the volume expansion of the anode material during charge and discharge, enabling the material to have more potential electrochemical performance. Brief Description of the Drawings

[0013] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and thus should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0014] Figure 1 It is the XRD characterization diagram of the FeOOH precursor provided by the embodiment of the present invention.

[0015] Figure 2 It is the SEM diagram of the FeOOH precursor provided by the embodiment of the present invention.

[0016] Figure 3 It is the SEM diagram of the FeOOH@PDA provided by the embodiment of the present invention.

[0017] Figure 4 It is the XRD characterization diagram of the FeP@NC provided by the embodiment of the present invention.

[0018] Figure 5 It is the SEM diagram of the FeP@NC provided by the embodiment of the present invention.

[0019] Figure 6 It is the CV test curve of FeP provided by the embodiment of the present invention at a scanning rate of 0.1 mV s -1 and a voltage window of 0.01 - 3.0 V vs. (Na / Na + ).

[0020] Figure 7 It is the cycling performance curve of FeP and FeP@NC provided by the embodiment of the present invention cycled 200 times under the condition of a current density of 2 Ag -1 .

[0021] Figure 8 It is the Nyquist plot of FeP and FeP@NC after cycling provided by the embodiment of the present invention, and the inset is the fitted equivalent circuit diagram. Figure 9 It is the electron microscope image of the collapse of the sea urchin-like structure of the product obtained under abnormal selenization conditions. Detailed Embodiments

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0023] In the description of the present invention, the terms "first" and "second" are used only for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality" means two or more, unless otherwise specifically defined.

[0024] The embodiment of the present invention provides a method for synthesizing a sea urchin-like FeP@NC composite material, comprising the following steps:

[0025] S1. Add the FeOOH precursor to a pH = 8.0 - 9.0 buffer solution containing Tris for ultrasonic dispersion, and then add dopamine to the solution at a mass ratio of FeOOH precursor to dopamine of 1 - 1.8:1 and stir at room temperature for 8 - 20 h to obtain a brownish-red precipitate-like isolate;

[0026] Controlling the pH value between 8.0 and 9.0 is weakly alkaline, which is conducive to the subsequent self-polymerization process of PDA to coat on the surface of the FeOOH precursor. Too acidic or too alkaline conditions will result in uneven coating; the best coating effect is achieved at pH 8.4 - 8.6.

[0027] Adding too much dopamine will result in a relatively thick nitrogen-doped carbon layer, which will affect the cycle rate performance. Adding too little may result in the nitrogen-doped carbon layer not being coated.

[0028] S2. Wash the obtained brownish-red precipitate-like isolate with deionized water and ethanol multiple times and then place it in an oven. After maintaining a constant temperature at 60°C - 80°C for 5 - 10 h, FeOOH@PDA is obtained;

[0029] Too low drying temperature will cause the product to contain moisture, affecting the subsequent selenization step. Too high temperature will cause serious agglomeration of the product, which is not conducive to the selenization reaction.

[0030] S3. Mix FeOOH@PDA and sodium hypophosphite evenly at a mass ratio of 1:8 - 10, place them at both the upstream and downstream ends of a porcelain boat in a tube furnace filled with an Ar atmosphere, keep the temperature constant at 350 - 450 °C for 2 - 2.5 h, control the heating rate at 1 - 3 °C / min, carry out phosphating treatment, and finally obtain FeP@NC.

[0031] If too little sodium hypophosphite is added, the phosphating of FeOOH@PDA will be incomplete; if too much is added, the structure of FeP will be damaged and it will be difficult to remove completely; if the heating rate is too slow, the phosphating reaction will be too slow and the reaction will be insufficient, and if the heating rate is too fast, the morphology structure of the material will be damaged and the structure will collapse. For example, when selenizing at a heating rate of 0.5 °C / min, a large amount of iron oxides will appear in the product, and selenizing at too long a temperature will cause cost problems. When selenizing at 5 °C / min, the sea urchin-like structure will collapse, as Figure 9 shown.

[0032] The materials used in the present invention are ferrous sulfate heptahydrate, glycerol, and deionized water, which are of low price. After mixing the three raw materials evenly and stirring, they can be added to a reaction kettle for reaction, which is relatively convenient; and by controlling the phosphating temperature and the ratio of the precursor and sodium hypophosphite, FeP with different phase compositions can be obtained, so as to obtain the pure-phase FeP required by the present invention. If the reaction temperature or ratio is inappropriate, substances containing iron oxides will be synthesized, and the obtained substances need to meet the requirement that there should be no large number of miscellaneous peaks in their XRD patterns. The reaction method of the present application has strong controllability and high product synthesis quality.

[0033] S11. Add a certain amount of ferrous sulfate heptahydrate to deionized water and stir. After it is fully dissolved, add glycerol and stir until a uniform mixed solution is formed, then transfer it to a 50 ml reaction kettle, place it in an oven, and react at 100 °C - 140 °C for 22 - 26 h to obtain a precipitate. Wash it by centrifugation with deionized water and alcohol multiple times, and then dry it to obtain the FeOOH precursor for use. The particle size and morphology of the FeOOH precursor can be adjusted by strictly controlling the parameter ratio of deionized water and glycerol.

[0034] Ferrous sulfate heptahydrate is used as the Fe source, and the concentration is controlled at 2.6 - 2.9 g / L. Too little will lead to too few reactants and difficult centrifugation operation; too much ferrous sulfate will lead to incomplete reaction and unable to form a sea urchin-like precursor.

[0035] Preferably, in one embodiment, 0.111 g of ferrous sulfate heptahydrate is added to 37 ml of deionized water. After it is fully dissolved, 3 ml of glycerol is added, stirred to form a uniform mixed solution, then transferred to a 50 ml reaction kettle, and placed in an oven to react at 120 °C for 24 h.

[0036] The sea urchin-like precursor can be synthesized at temperatures ranging from 100°C to 140°C. Temperatures that are too high or too low will not result in the formation of a hollow sea urchin-like morphology. The sea urchin-like precursor with good morphology can be synthesized within 22h to 26h. Too long or too short reaction times will lead to uneven particle sizes of the precursor. In this application, the FeOOH precursor can be prepared without using a template, and the method is simple and controllable.

[0037] As a further improvement, the volume ratio of deionized water to glycerol is 12.3 - 7.4:1.

[0038] Adding too much glycerol will result in the formation of irregular impurities, and adding too little will prevent the reaction from occurring.

[0039] As a further improvement, in step S1, the pH of the Tris buffer solution is 8.4 - 8.6.

[0040] As a further improvement, in step S1, the mass ratio of the FeOOH precursor to dopamine is 1 - 1.8:1. The step of adding dopamine to the solution containing the precursor and stirring at room temperature for 8 - 20h includes:

[0041] Preferably, the mass ratio of the FeOOH precursor to dopamine is 1.7:1. Dopamine is added to the solution containing the precursor and continuously stirred at room temperature for 12h. In step S2, the precipitate is washed several times with deionized water and ethanol, and then dried at 70°C for 8h to obtain FeOOH@PDA for later use;

[0042] As a further improvement, in step S3, the step of mixing FeOOH@PDA and sodium hypophosphite evenly at a mass ratio of 1:8 - 10, placing them in a tube furnace filled with an Ar atmosphere, and maintaining a constant temperature at 350 - 450°C for 2 - 2.5h with a heating rate controlled at 1 - 3°C / min includes:

[0043] Preferably, FeOOH@PDA and sodium hypophosphite are mixed evenly at a mass ratio of 1:8, placed in a tube furnace filled with an Ar atmosphere, maintained at a constant temperature of 380 - 420°C for 2 - 2.5h, and the heating rate is controlled at 2 - 3°C / min.

[0044] The active material of this composite material is a nitrogen-doped carbon-modified sea anemone-like FeP@NC composite material, and the particle size of the nitrogen-doped carbon-modified sea anemone-like FeP@NC composite material is 1 micron to 3 microns.

[0045] An embodiment of the present invention further provides a novel anode for a sodium-ion battery, including the above composite material.

[0046] An embodiment of the present invention further provides a novel sodium-ion battery, including the above anode for a sodium-ion battery.

[0047] Embodiment 1:

[0048] Add 0.111g of ferrous sulfate heptahydrate to 37ml of deionized water, add 3ml of glycerol after it is fully dissolved, stir to form a uniform mixed solution, transfer to a 50ml reactor, put it in an oven, and react at 120℃ for 24h. Wash the precipitate several times with deionized water and ethanol, then dry it to obtain the FeOOH precursor for standby use. Prepare 100mL of a buffer solution containing Tris, adjust the pH value of the solution to 8.5 by controlling the amount of Tris added, accurately weigh 0.1g of the precursor and dissolve it in the buffer solution, ultrasonically disperse it for half an hour, and then add 60mg of dopamine (PDA) at a mass ratio of 1.7:1 between the precursor and dopamine, stir continuously for 12h in a constant temperature environment at 30℃, wash and centrifuge it several times with deionized water and ethanol, and then put the precipitate obtained by centrifugation into a blast drying oven with a set temperature, keep it at 70℃ for 8h, and dry it to obtain FeOOH@PDA. The prepared FeOOH@PDA and sodium hypophosphite were uniformly mixed in a mass ratio of 1:8, placed in a tubular furnace filled with Ar atmosphere, and kept at a constant temperature of 400°C for 2h. The heating rate was controlled at 2°C / min, and finally a nitrogen-doped carbon-modified sea urchin-like FeP@NC composite material, namely, FeP@NC composite material, was obtained.

[0049] Comparative Example 1:

[0050] Basically similar to the embodiment, the difference of the comparative example is that the FeOOH precursor that has not been coated with dopamine and sodium hypophosphite are uniformly mixed in a mass ratio of 1:8, placed in a tubular furnace filled with Ar atmosphere, and kept at a constant temperature of 400oC for 2h. The heating rate is controlled at 2°C / min, and FeP material is obtained as a comparison.

[0051] Preparation and assembly of electrodes: The active material slurry consists of active material, conductive carbon (Super-P) and polyvinylidene fluoride as a binder in a mass ratio of 7:2:1. The active material slurry is fully ground and coated on a copper foil collector to prepare a working electrode. After vacuum drying and rolling, it is cut into pieces. The diameter of the circular electrode piece is 12 mm. Pure metallic sodium sheet is used as the counter electrode. It is assembled into a button cell in a glove box filled with argon. The diaphragm model is Celgard2325.

[0052] Structural characterization: The material phase analysis and crystal structure analysis were performed using a Bruker-D8-Advance X-ray diffractometer (XRD), and the sample morphology and microstructure analysis were performed using a Hitachi High-Tech SU8010 series super-resolution field emission scanning electron microscope (SEM).

[0053] Electrochemical performance characterization: The electrochemical performance of the material was characterized by a constant current charge-discharge test using a Blue Electric battery test system (CT3002CA). The voltage window for the electrochemical test was fixed at 0.01 - 3V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were both performed using a Shanghai Chenhua electrochemical workstation (CHI660E), and the selected scan rate was 0.1 mV s -1 , and the frequency range was 100 kHz - 0.1 Hz.

[0054] Structural characterization:

[0055] The crystal structure of the precursor FeOOH was characterized by XRD ( Figure 1 ). By comparing the spectrum of the synthesized precursor with the FeOOH standard card, it was found that the positions of all characteristic diffraction peaks corresponded to those of the standard card. SEM characterization showed ( Figure 2 ) that the prepared precursor presented a sea urchin-like morphology, and its size was about 1 - 3 microns.

[0056] To compare the phase structures of the original sample and the modified sample, the crystal structures of FeP and FeP@NC composites were first analyzed. The XRD spectra of the two are shown in Figure 1 and 4 . By comparing with the FeP standard card, the characteristic diffraction peaks at each position in the spectrum could be corresponding. The characteristic peaks at 22.9°, 30.9°, 32.7°, 34.5°, 35.5°, 37.2°, 37.9°, 45.5°, 46.3°, 47.0°, 48.3°, 50.4°, 55.3o, 56.0o, 59.1o, 59.6o in the spectrum corresponded to the characteristic diffraction peaks of FeP (PDF#78 - 1443) respectively, indicating that a pure-phase FeP material was successfully prepared by a simple hydrothermal synthesis and a controllable gas-phase phosphating method.

[0057] To further study the morphology and structure of the products, SEM technology was used to characterize the morphologies of FeOOH, FeOOH@PDA, and FeP@NC composites. Figure 3 is the SEM image of the precursor after coating with dopamine hydrochloride. By comparing with Figure 1 , it was found that the surface of the precursor became rough after coating with dopamine, but the overall sea urchin-like morphology of the material was still maintained. Figure 5The SEM image shows the transformation of FeOOH@PDA into FeP@NC through controllable gas-phase selenization. After carbonization, the material can still maintain its original sea urchin-like structure, which fully demonstrates that the outer carbon layer plays a good stabilizing role in the overall structure, sufficient to significantly alleviate the volume change of the material during the charge-discharge cycle, thereby exhibiting excellent cycle stability. At the same time, due to the doping of nitrogen elements derived from dopamine, it is more conducive to increasing the wettability of the electrolyte during the contact between the composite material and the electrolyte, providing a large number of active sites for sodium storage and further accelerating the electron conductivity rate of the material.

[0058] Electrochemical performance characterization:

[0059] The results of the above material structure and morphology characterization preliminarily confirm that the FeP@NC composite material has been successfully prepared by the simple hydrothermal synthesis and controllable gas-phase selenization methods in this invention. To study the improvement of the electrochemical performance of the material by the sea urchin-like structure designed in this invention, the FeP@NC composite material is compared with FeP in this experiment. Performance tests are carried out under the same experimental conditions from several aspects including cycle performance, constant current charge-discharge, and AC impedance, and the results are compared and analyzed. First, the cyclic voltammetry (CV) test method is used to analyze the electrochemical reaction process that occurs during the charge-discharge cycle of the composite material. Figure 6 Shown is the CV curve of the FeP composite material electrode tested at a scanning rate of 0.1 mV s -1 and a voltage window of 0.01 - 3.0 V vs. (Na / Na + ). On the initial CV cathodic scan curve of the material, the peak at 0.75 V corresponds to the insertion of Na + into the material to react with FeP, converting to metallic Fe and Na3P, accompanied by the decomposition of the electrolyte and the formation of the solid electrolyte interface (SEI). Starting from the second cycle, the reduction peak shifts towards the positive potential, which is due to the large volume expansion that occurs during the first cycle of the composite material, resulting in a change in the reduction peak position due to the internal structural change, which is also a common phenomenon of transition metal compounds during sodium storage. In the second and third cycles, the broad oxidation peak at 1.67 - 1.87 V corresponds to the re-conversion of metallic Fe nanocrystals and the Na3P matrix to form FeP. In addition, it is found by comparison that the CV curves of the second and third cycles have good coincidence, and the polarization degree of the FeP composite material is small, indicating that this is conducive to the rapid insertion and extraction of Na + during the electrochemical reaction process, thus significantly improving the cycle stability of the material.

[0060] To further explore the electrochemical performance of the composite material at a larger current density, cyclic tests were carried out on the materials before and after modification. Figure 8 Shown are the FeP and FeP@NC composite materials at a current density of 2 A g -1Performance graph after 200 cycles under certain conditions. It can be found that the modified FeP@NC composite material can still maintain a specific capacity of 174.5 mAh g -1 after 200 cycles, while the unmodified FeP material shows a rapid capacity decay during cycling, and the specific capacity decays to nearly zero after 200 cycles. Combining with the fact that the FeP@NC composite material maintains a good sea urchin-like morphology in the SEM image, it can be shown that it has good structural stability. At the same time, the design of the nitrogen-doped carbon layer helps to alleviate the volume expansion during cycling and slow down the capacity decay rate. Through the above cyclic performance tests, it is further confirmed that this unique sea urchin-like FeP@NC composite material with carbon layer coating has excellent cyclic stability performance.

[0061] Finally, in order to further explore the influence of the nitrogen-doped carbon layer on the Na + diffusion kinetics in the material, AC impedance tests were carried out on the FeP and FeP@NC composite materials in the frequency range of 100 kHz to 0.1 Hz. Figure 9 is the Nyquist plot of the electrode of the FeP@NC composite material after cyclic testing. The small inset is the fitted equivalent circuit. It can be clearly seen that the Nyquist plots of the materials before and after modification are both composed of a typical semicircle in the high-frequency region and a slant line in the low-frequency region. Among them, the semicircle in the high-frequency region corresponds to the charge transfer resistance (R2), and the slant line in the low-frequency region is related to the Warburg impedance (W1) of Na + diffusion. By fitting the Nyquist plot into an equivalent circuit diagram for analysis, the charge transfer resistances (R2) of the FeP and FeP@NC materials are 544.6 Ω and 114.0 Ω respectively, which indicates that after the material is coated with a nitrogen-doped carbon layer, the interfacial impedance has been significantly reduced, which is beneficial to promoting the transmission of internal electronic charges in the material, thus showing more excellent electrochemical performance.

[0062] The above is only the preferred embodiment of the present invention and is not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a sea urchin-shaped FeP@NC composite material, characterized in that: The following steps are involved: S1, synthesizing a FeOOH precursor, adding the precursor to a buffer solution for ultrasonic dispersion, then adding dopamine to the solution and stirring at room temperature to obtain a brown-red precipitate-like separation; S2, washing the brown-red precipitate with deionized water and ethanol for several times and drying it in an oven to obtain FeOOH@PDA; S3, FeOOH@PDA and sodium hypophosphite were placed at both ends of a porcelain boat and phosphated in a tube furnace to obtain FeP@NC with a hollow ring structure.

2. The preparation method according to claim 1, characterized in that In step S1, the FeOOH precursor is obtained by the following method: S11, adding a certain amount of ferrous sulfate heptahydrate to deionized water and stirring, adding glycerol after it is fully dissolved, stirring until a uniform mixed solution is formed, transferring it into a 50 ml reaction kettle, putting it into an oven, reacting it at 120° C. for 24 hours to obtain a precipitate, washing it with deionized water and alcohol by centrifugation several times, and finally drying it to obtain a FeOOH precursor with a sea urchin-like structure.

3. The preparation method according to claim 1, characterized in that: In step S1, the pH of the Tris buffer solution is 8.0-9.

0.

4. The preparation method according to claim 1, characterized in that: In step S1, the FeOOH precursor and dopamine are mixed in a mass ratio of 1 to 1.8:1, dopamine is added to the solution and stirred continuously at room temperature for 8 to 20 hours.

5. The preparation method according to claim 1, characterized in that: In step S3, FeOOH and sodium hypophosphite are uniformly mixed in a mass ratio of 1:8-10, placed at the upstream and downstream ends of the porcelain boat in a tubular furnace filled with Ar atmosphere, and kept at a constant temperature of 350-450° C. for 2-3 hours.

6. The preparation method according to claim 1, characterized in that: In step S3, the heating rate is controlled at 1-3°C / min.

7. A sea urchin-shaped FeP@NC composite material, characterized in that: The preparation method according to any one of claims 1 to 6 is composed of FeP nanoparticles wrapped in an outer nitrogen-doped carbon layer.

8. A sea urchin-shaped FeP@NC composite material, characterized in that: The composite material as claimed in claim 7 has a particle size of 1 micron to 3 microns.

9. A novel sodium ion battery negative electrode, characterized in that: The composite material obtained by the preparation method according to any one of claims 1 to 6 or the composite material according to claim 7 or 8 is used as a negative electrode material.

10. A novel sodium ion battery, characterized in that: The negative electrode material as claimed in claim 9 is used as a negative electrode of a battery.

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