Preparation method of iron nickel phosphide / hollow sodium citrate carbon composite material

Abstract

The application discloses a preparation method of a ferrophosphor / nickel hollow sodium citrate carbon composite material, which comprises the following steps: solid block sodium citrate carbon preparation, ultrathin hollow sodium citrate carbon preparation, ferrophosphor / nickel hollow sodium citrate carbon precursor material preparation and ferrophosphor / nickel hollow sodium citrate carbon composite material preparation. The ultrathin hollow sodium citrate carbon is prepared and used as a hollow buffer material, so that the ferrophosphor / nickel particles can be better encapsulated, the volume expansion problem of the ferrophosphor / nickel particles in the circulation process is effectively relieved, and the stability of the material in the circulation process can be effectively improved. On the other hand, the phosphide metal contributes a high capacity to the whole material, and cooperates with the carbon shell, so that the ferrophosphor / nickel hollow sodium citrate carbon composite material has more excellent electrochemical performance in a lithium ion battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of new energy lithium ion battery materials, and relates to a preparation method of a ferrophosphorus-nickel / hollow sodium citrate carbon composite material. BACKGROUND

[0002] With the increasing cost of traditional fossil energy and the gradual depletion of resources, a new generation of sustainable energy has ushered in an unprecedented development prospect. In this context, lithium ion batteries have rapidly developed due to their high safety and higher energy density. At present, the research on the positive electrode material of lithium ion batteries has been quite mature, so researchers focus on the research on the negative electrode material of lithium ion batteries. Researchers have invested a lot of effort to find a lithium ion battery negative electrode material that can replace graphite and have higher performance than graphite. Among them, transition metals have attracted widespread attention from researchers because they contain unoccupied d orbitals and uncoordinated electrons. So far, researchers have studied the electrochemical performance of many transition metal compounds, such as sulfides, borides, nitrides, carbides and phosphides. Transition metal phosphides (TMPs) have become the focus of research due to their high specific capacity, small voltage platform and low lithium deintercalation potential. Several transition metal phosphides (TMPs) include CoP, NiP, FeP, SnP, etc., and have been reported a lot in recent years. A copper-coated hollow nickel phosphide material and a preparation method thereof (CN110707321A) invented by Wang Jian can effectively alleviate the volume expansion of Ni2P during the charging and discharging process and improve the cycle stability of the material. Moreover, the surface of the material is coated with a dense copper layer, which has good structural stability and good conductivity.

[0003] However, there are still few reports on double-metal phosphides. However, the main reason hindering the further development of metal phosphides is the volume expansion of the material itself during the lithium ion battery cycle, which leads to rapid capacity decay of the battery. Even if it has high capacity, stability is ultimately the root of the problem. Therefore, it is necessary to study it to overcome the above problems. SUMMARY

[0004] The application aims to provide a preparation method of a ferrophosphorus-nickel / hollow sodium citrate carbon composite material. By preparing an ultrathin hollow sodium citrate carbon and using it as a hollow buffer material, the ferrophosphorus-nickel particles can be better encapsulated, effectively alleviating the volume expansion problem of the ferrophosphorus-nickel particles during the cycle process, and effectively improving the stability of the material during the cycle process. On the other hand, the phosphide metal contributes a high capacity to the whole material, and cooperates with the carbon shell to make the ferrophosphorus-nickel / hollow sodium citrate carbon composite material have more excellent electrochemical performance in lithium ion batteries.

[0005] The technical scheme of the application is as follows:

[0006] A preparation method of a ferrophosphorus / nickel / hollow sodium citrate carbon composite material is sequentially carried out according to the following steps:

[0007] S1, sodium citrate is weighed and placed in a constant temperature oven for drying at 120-180 DEG C for 15-30h, and the excess crystal water is removed, and the sodium citrate with the crystal water removed is placed in a tube furnace for high-temperature calcination and carbonization under an argon atmosphere to obtain solid block sodium citrate carbon;

[0008] S2, the solid block sodium citrate carbon is soaked in dilute hydrochloric acid with a concentration of 1 mol / L and stirred for 24h, filtered, washed with distilled water until neutral, and dried in an oven to obtain ultra-thin hollow sodium citrate carbon;

[0009] S3, the nine hydrated ferric nitrate, nickel nitrate and ultra-thin hollow sodium citrate carbon are dissolved in deionized water and stirred uniformly, transferred to a reaction kettle and reacted at 120-180 DEG C for 6-12h, and then centrifuged to obtain a ferrophosphorus / nickel / hollow sodium citrate carbon composite material;

[0010] S4, the ferrophosphorus / nickel / hollow sodium citrate carbon is placed in the downstream position of the tube furnace under an argon atmosphere, and sodium hypophosphite is placed in the upstream position of the tube furnace under an argon atmosphere, and high-temperature phosphating is carried out to obtain the final ferrophosphorus / nickel / hollow sodium citrate carbon composite material.

[0011] As a limitation of the present application, in step S1, the high-temperature carbonization conditions are calcination at 500-800 DEG C for 1-4h under an inert atmosphere.

[0012] In this step, the high temperature is used for carbonizing sodium citrate, and the temperature and calcination time affect the structure of the ultra-thin hollow carbon shell and the overall carbon skeleton, which in turn affects the morphology of the final product, which has an important influence on the subsequent loading of metal phosphide particles and encapsulation. On the one hand, it ensures its carrier function, making it not easy to agglomerate and collapse, and can play an encapsulation role, on the other hand, it also has more network structure to facilitate the attachment and nucleation reduction of metal ions, which is further beneficial to the subsequent ion transmission. Compared with the traditional commonly used non-biomass carbon, the present application uses biomass sodium citrate carbon as a skeleton for loading and encapsulation, which has the advantages of wide source, low cost and easy modification of biomass structure.

[0013] As a second limitation of the present application, in step S2, the drying temperature is 40-80 DEG C, and the drying time is 12h.

[0014] As a third limitation of the present application, in step S3, the molar ratio of the nine hydrated ferric nitrate and nickel nitrate is 1:1, and the mass of the ultra-thin hollow sodium citrate carbon accounts for 10% of the total mass of the nine hydrated ferric nitrate and nickel nitrate.

[0015] As a fourth limitation of the application, in step S4, the high-temperature phosphating conditions are: annealing at 300-600 DEG C for 1-6 h under an inert atmosphere.

[0016] As a fifth limitation of the application, in step S4, the mass ratio of sodium hypophosphite to iron-nickel / hollow sodium citrate carbon is (5-10):1.

[0017] In the application, iron ions and nickel ions are first loaded on the ultrathin hollow sodium citrate carbon, and then reduced under high-temperature hydrothermal process conditions. During reduction, the two kinds of metal ions compete for sites, and phosphides are directly grown on the carbon skeleton with a three-dimensional structure. The heterojunction interface constructed between the carbon layer and the bimetallic phosphide is conducive to electron transfer, which is one of the important reasons for high specific capacity. On the one hand, by controlling the ratio of the three, it is necessary to ensure uniform reduction nucleation to form nanoparticles of a certain size to prepare for the subsequent phosphating process, and on the other hand, it is necessary to ensure uniform distribution of the two kinds of metal particles. In the subsequent phosphating process, two kinds of metal particles react with phosphorus atoms to form hollow sodium citrate carbon shells encapsulating Fe-Ni-P nanoparticles. In this process, the amount ratio of iron ions, nickel ions, ultrathin hollow sodium citrate carbon, and sodium hypophosphite is critical. In the low-temperature phosphating process, in order to further ensure the uniformity and phosphating effect, the ratio of sodium hypophosphite to metal is much greater than 1:1, the three-dimensional conductive network improves the electron / ion transport capacity, provides an effective transmission channel for the electrolyte, and plays the characteristics of high conductivity and rich reaction sites of FeNiP bimetallic phosphide. The coexistence of metal and P sites helps to regulate the interface electron structure, and the synergistic effect of the two inhibits the volume expansion and powdering of the electrode material during the reaction process, thereby improving the electrochemical performance of the lithium ion battery.

[0018] The application also has a limitation, in step S4, the Fe-Ni-P nanoparticles in the phosphating iron-nickel / hollow sodium citrate carbon composite material are loaded and encapsulated in the hollow sodium citrate carbon shell, and are connected to each other to form an interpenetrating network structure, and the Fe-Ni-P nanoparticles have a particle size of about 50 nm.

[0019] The above technical solutions of the application are closely related and cannot be separated, and the whole determines the morphology and electrochemical performance of the final composite material.

[0020] As a result of using the above technical solutions, the application has the following beneficial effects:

[0021] 1. The sodium citrate carbon with ultra-thin hollow structure is prepared in the application, which acts as a carrier to load Fe-Ni-P nanoparticles, and on the other hand, it can inhibit the volume expansion of iron nickel phosphide in the cycle process by encapsulating Fe-Ni-P nanoparticles.

[0022] 2. The sodium citrate carbon hollow structure can better provide ion / electron transmission channels, and Fe-Ni-P acts as an active material, and the three particles can play a synergistic effect, and the active material and the sodium citrate carbon hollow structure also have a synergistic effect, so as to further improve the electrochemical performance of the composite material in the lithium ion battery, and the composite material has the advantages of good cycle stability and long service life.

[0023] 3. The preparation process is easy to control, the cost is low, and the process is suitable for industrial production.

[0024] The application is suitable for preparing iron nickel phosphide / sodium citrate carbon hollow composite material, and is further applied to lithium ion batteries.

[0025] The specific embodiments of the application will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The XRD pattern of the Fe-Ni-P / H-SCC composite material prepared in Example 1 of the application;

[0027] Figure 2 The SEM image of the SCC not treated with dilute hydrochloric acid in Example 1 of the application;

[0028] Figure 3 The SEM image of the H-SCC obtained under the concentration of 1 mol / L dilute hydrochloric acid in Example 1 of the application;

[0029] Figure 4 The SEM image of the H-SCC obtained under the concentration of 0.1 mol / L dilute hydrochloric acid in the comparative example of the application;

[0030] Figure 5 The SEM image of the H-SCC obtained under the concentration of 10 mol / L dilute hydrochloric acid in the comparative example of the application;

[0031] Figure 6 The SEM image of the Fe-Ni-P / H-SCC composite material prepared in Example 1 of the application;

[0032] Figure 7 The cycle performance diagram of the Fe-Ni-P / H-SCC composite material prepared in Example 1 and the comparative example of the application with different metal ion ratios. DETAILED DESCRIPTION

[0033] In the following examples, the reagents used are commercially available unless otherwise specified. The experimental methods and detection methods used are conventional unless otherwise specified.

[0034] Example 1 A preparation method of a ferrophosphor nickel / hollow sodium citrate carbon composite material

[0035] In this example, a ferrophosphor nickel / hollow sodium citrate carbon composite material is prepared, and the preparation method is performed according to the following sequence of steps:

[0036] S1, 10 g of sodium citrate (SC) is weighed and placed in a constant temperature oven at 150°C for drying for 24 h to remove excess crystal water; the sodium citrate (SC) from which the crystal water is removed is placed in a porcelain boat and the porcelain boat is placed in a tube furnace, and the sodium citrate (SC) is carbonized under an argon atmosphere at 600°C for 2 h in the tube furnace; after cooling to room temperature, a solid block of sodium citrate carbon (SCC) is obtained, and the SEM image of the product is shown in FIG. 1. Figure 2

[0037] S2, the solid block of sodium citrate carbon (SCC) obtained above is placed in 1 mol / L dilute hydrochloric acid and stirred for 24 h, then filtered and washed with distilled water until neutral, and the sample is placed in an oven at 60°C for drying for 12 h to obtain an ultrathin hollow sodium citrate carbon (H-SCC);

[0038] S3, 0.5 mmol of Fe(NO3)3·9H2O and 0.5 mmol of Ni(NO3)2 are weighed respectively, and the ultrathin hollow sodium citrate carbon (H-SCC) is weighed, the mass of the H-SCC being 10% of the total mass of the iron nitrate and the nickel nitrate, the three materials are dissolved in 30 mL of deionized water and stirred for 12 h to obtain a mixed solution; the mixed solution is transferred to a reaction kettle (total capacity 50 mL) and reacted at 150°C for 8 h, after the reaction is completed, a network crosslinked precursor iron nickel / hollow sodium citrate carbon composite material (Fe-Ni / H-SCC) is obtained by centrifugal separation;

[0039] S4, the iron nickel / hollow sodium citrate carbon (Fe-Ni / H-SCC) is placed in the downstream position of the tube furnace under argon, sodium hypophosphite (NaH2PO2) is placed in the upstream position of the tube furnace under argon, the mass ratio of sodium hypophosphite to iron nickel / hollow sodium citrate carbon is 5:1, and the final product, a ferrophosphor nickel / hollow sodium citrate carbon composite material (Fe-Ni-P / H-SCC), is obtained by annealing at 350°C for 2 h under an argon atmosphere.

[0040] The product Fe-Ni-P / H-SCC prepared is subjected to a series of tests, and the specific results are as follows:

[0041] As Figure 1 ​As shown in the XRD pattern of the Fe-Ni / H-SCC composite material prepared in Example 1, it can be seen from the figure that the diffraction peaks of the sample at 2θ = 32.9°, 36.7°, 46.5°, 56.2° are between FeP (PDF #78-1443) and Ni2P (PDF #73-0436); in addition, the peak at 26° is attributed to amorphous carbon, indicating that the material obtained after phosphating is Fe-Ni-P / H-SCC.

[0042] As shown in the scanning electron microscope picture of the sodium citrate carbon material (SCC) prepared in Example 1, it can be seen from the figure that the carbon shell morphology remains relatively complete. Figure 3

[0043] As shown in the scanning electron microscope picture of the final product phosphated iron-nickel / hollow sodium citrate carbon composite material (Fe-Ni-P / H-SCC) prepared in Example 1, it can be clearly observed from the figure that the size of the Fe-Ni-P particles and the interconnecting network structure formed by the hollow carbon shells, the overall morphology of the sample is a hollow carbon shell skeleton loaded with metal particles, the Fe-Ni-P nanoparticles are only 50 nm in size and are surrounded and encapsulated by the network space composed of hollow carbon shells. Figure 6

[0044] As shown in the cycle performance diagram of the Fe-Ni-P / H-SCC composite material prepared in Example 1, it can be seen from the figure that when the mass of H-SCC is 10% of the total mass of ferric nitrate nine hydrate and nickel nitrate and the metal ion ratio is 1:1, the Fe-Ni-P / H-SCC material prepared is assembled into a lithium ion battery and subjected to electrochemical performance test, and the performance diagram after 1000 cycles at a current density of 2A·g -1 , it can be seen from the figure that at the 320th cycle, it tends to be stable, and the capacity reaches 1222mAh·g -1 when stable, and shows an upward trend when cycled to 1000 cycles, which indicates that the material has excellent cycle stability. Figure 7 Preparation method of phosphated iron-nickel / hollow sodium citrate carbon composite material

[0045] Phosphated iron-nickel / hollow sodium citrate carbon composite materials were prepared in Examples 2-4, and the preparation process was similar to that of Example 1, except that the corresponding technical parameters were different during the preparation process, as shown in the following table.

[0046]

[0047] Comparative example

[0048]

[0049] ​​​To investigate the effects of different parameters on the final performance and results of the preparation of nickel phosphide / sodium citrate hollow carbon composite material, a series of experiments were conducted. The preparation process of the material was similar to that in Example 1, except that the preparation parameters were different.

[0050] Group A: The concentration of dilute hydrochloric acid used in step S2 is 0.1 mol / L, and the remaining steps are the same as in Example 1 of this invention.

[0051] The H-SCC product obtained in step S2 of this group was subjected to SEM testing, such as... Figure 4 As shown in the figure, the material inside the carbon block was not completely removed. As a result, metal ions could not adhere to the inside of the shell during the subsequent hydrothermal process, and the effect of encapsulating the metal particles with a carbon shell could not be formed. Consequently, the problem of volume expansion of iron-nickel phosphide during the cycle could not be solved.

[0052] Group B: The concentration of dilute hydrochloric acid used in step S2 is 10 mol / L, and the remaining steps are the same as in Example 1 of this invention.

[0053] The H-SCC product obtained in step S2 of this group was subjected to SEM testing, such as... Figure 5 As shown in the figure, the ultrathin carbon shell structure begins to break and crack, with the edges of the carbon shell being particularly incomplete. As a result, it is ultimately difficult to encapsulate the metal particles, and thus the problem of volume expansion of iron-nickel phosphide during cycling cannot be solved.

[0054] Group C: In step S3, the mass of H-SCC is 5% of the total mass of ferric nitrate nonahydrate and nickel nitrate.

[0055] The performance of the products prepared in this group was tested. The Fe-Ni-P / H-SCC material with 5% carbon content began to stabilize at 576 cycles, and the specific capacity finally stabilized at 220 mAh g. -1 The carbon content is much lower than that of Fe-Ni-P / H-SCC materials with 10% carbon content. This is mainly because the proportion of carbon in the overall material is low, which cannot encapsulate all the metal particles, resulting in some phosphides being exposed on the surface. This neither increases the long-term cycling stability of the material nor solves the problem of volume expansion of metal phosphides.

[0056] Group D: In step S3, the mass of H-SCC is 15% of the total mass of ferric nitrate nonahydrate and nickel nitrate.

[0057] The performance of the products prepared in this group was tested. The Fe-Ni-P / CAC material with 15% carbon content began to stabilize after 700 cycles, and the specific capacity eventually stabilized at 204 mAh g. -1, which is much lower than the Fe-Ni-P / H-SCC material with 10% carbon content. This is mainly because the proportion of carbon is large, and although the stability increases, a large amount of carbon is aggregated, and the combination with metal particles is uneven.

[0058] Group E: In step S3, Fe(NO3)3·9H2O is 1 mmol, and Ni(NO3)2 is 0.5 mmol, that is, the molar ratio of iron ions to nickel ions is 2:1.

[0059] Performance tests were performed on the products prepared in this group, and the results showed that the Fe-Ni-P / H-SCC electrode tended to be stable at the 257th cycle, and the capacity reached 596 mAh g -1 , which is much lower than the capacity of the electrode with a metal ratio of 1:1, which is mainly because the network structure is destroyed, and a small part of the particle accumulation phenomenon occurs, which cannot alleviate the volume expansion problem.

[0060] Group F: In step S3, Fe(NO3)3·9H2O is 0.5 mmol, and Ni(NO3)2 is 1 mmol, that is, the molar ratio of iron ions to nickel ions is 1:2.

[0061] Performance tests were performed on the products prepared in this group, and the results showed that the Fe-Ni-P / H-SCC electrode tended to be stable at the 767th cycle, and the capacity reached 258 mAh g -1 , which is much lower than the electrode with a metal ratio of 1:1, and in addition, the Fe-Ni-P / CAC-1:2 electrode reached a maximum capacity of 362 mAh g -1 at the 423rd cycle, and then the capacity decreased again, which is mainly because the small particles are decomposed again during the cycle, and the structure collapses, resulting in rapid capacity decay.

[0062] Group G: Non-biomass carbon is used for preparation, and the specific process is referred to Example 1, only the sodium citrate carbon in Example 1 is replaced with

[0063] G1, 10 grams of glucose were placed in a porcelain boat and the porcelain boat was placed in a tube furnace, and the glucose was carbonized by calcining at 600°C for 2h in an argon atmosphere. After cooling to room temperature, glucose carbon (GC) was obtained;

[0064] G2, the above obtained glucose carbon (GC) was washed to neutral with distilled water, and the sample was placed in an oven at 60°C and dried for 12h;

[0065] G3, 0.5 mmol of Fe(NO3)3·9H2O and 0.5 mmol of Ni(NO3)2 and glucose carbon (GC) were weighed respectively, the mass of GC was 10% of the total mass of iron nitrate and nickel nitrate, the three materials were dissolved in 30 mL of deionized water and stirred for 12 h to obtain a mixed solution; the mixed solution was transferred to a reaction kettle (total capacity 50 mL) and reacted at 150°C for 8 h, after the reaction was completed, the iron-nickel / glucose carbon (Fe-Ni / GC) composite material was obtained by centrifugal separation;

[0066] G4, the iron-nickel / glucose carbon (Fe-Ni / GC) was placed downstream of the argon atmosphere of the tube furnace, sodium hypophosphite (NaH2PO2) was placed upstream of the argon atmosphere of the tube furnace, the mass ratio of sodium hypophosphite to iron-nickel / hollow sodium citrate carbon was 5:1, and the final product, iron-nickel-phosphide / glucose carbon composite material (Fe-Ni-P / GC), was obtained by annealing at 350°C for 2 h under argon atmosphere.

[0067] The materials prepared in this group were tested for electrochemical performance, and the specific results are as follows: when the current density was 2 Ag -1 , the cycle performance of Fe-Ni-P / GC was evaluated, which provided an initial discharge capacity of 405 mAh g -1 , the capacity retention rate was 88.3% at 800 cycles, and the capacity decreased by 817 mAh g -1 compared with the Fe-Ni-P / H-SCC electrode.

[0068] Group H: no Ni(NO3)2 was added in step S3, and the final product prepared was Fe-P / H-SCC.

[0069] Performance tests were conducted on the product prepared in this group, and the results showed that at a current density of 2 Ag -1 , the specific capacity of Fe-P / H-SCC decreased by 425.5 mAh g -1 compared with the Fe-Ni-P / H-SCC electrode, which was mainly because the bimetallic phosphide had better synergistic effect than the monometallic phosphide, and could exhibit more excellent electrochemical performance.

[0070] Group I: no Fe(NO3)3·9H2O was added in step S3, and the final product prepared was Ni-P / H-SCC.

[0071] Performance tests were conducted on the product prepared in this group, and the results showed that the specific capacity of Ni-P / H-SCC decreased by 715.5 mAh g -1This is mainly because the Fe with rich valence and the NiP form the best atomic ratio of the bimetallic phosphide, which not only has better conductivity, but also is beneficial to the electron transmission, and can further reduce the interface resistance of the charge transfer and the surface reaction kinetic energy barrier, and further improve the electrochemical performance.

[0072] Finally, it should be noted that: the above only for the preferred embodiments of the present application, and is not intended to limit the present application, although in the foregoing detailed description of the present application with reference to the foregoing embodiments, for those skilled in the art, it still can be modified, or the equivalent replacement of the technical solutions recorded in the foregoing embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included within the scope of protection of the claims of the present application.

Claims

1. A method for preparing a nickel-iron phosphide / sodium hollow citrate carbon composite material, characterized in that, Follow these steps in sequence: S1. Weigh sodium citrate and place it in a constant temperature oven to dry at 120-180℃ for 15-30 hours to remove excess water of crystallization. Place the sodium citrate with the removed water of crystallization in a tube furnace and calcine it at high temperature under an argon atmosphere to obtain solid block sodium citrate carbon. S2. The solid block sodium citrate carbon was soaked in dilute hydrochloric acid with a concentration of 1 mol / L and stirred for 24 hours. After filtration, it was washed with distilled water until neutral and then dried in an oven to obtain ultrathin hollow sodium citrate carbon. S3. Weigh out ferric nitrate nonahydrate, nickel nitrate and ultrathin hollow sodium citrate carbon, dissolve them in deionized water and stir evenly. Transfer to a reaction vessel and react at 120-180℃ for 6-12 hours. After the reaction is completed, centrifuge to obtain the iron-nickel / hollow sodium citrate carbon composite material. S4. Under an argon atmosphere, place the iron-nickel / hollow sodium citrate carbon downstream of the argon atmosphere in a tubular furnace, and place the sodium hypophosphite upstream of the argon atmosphere in a tubular furnace. Phosphate at high temperature to obtain the final phosphated iron-nickel / hollow sodium citrate carbon composite material.

2. The method for preparing a nickel phosphide / hollow sodium citrate carbon composite material according to claim 1, characterized in that, In step S1, the high-temperature calcination and carbonization conditions are: calcination at 500-800 ℃ for 1-4 h under an inert atmosphere.

3. The method for preparing a nickel phosphide / hollow sodium citrate carbon composite material according to claim 1, characterized in that, In step S2, the drying temperature is 40-80℃ and the drying time is 12h.

4. The method for preparing a nickel-phosphide / hollow sodium citrate carbon composite material according to claim 1, characterized in that, In step S3, the molar ratio of ferric nitrate nonahydrate and nickel nitrate is 1:1; the mass of ultrathin hollow sodium citrate carbon accounts for 10% of the total mass of ferric nitrate nonahydrate and nickel nitrate.

5. The method for preparing a nickel-phosphide / hollow sodium citrate carbon composite material according to claim 1, characterized in that, In step S4, the high-temperature phosphating conditions are: annealing at 300-600℃ for 1-6 hours under an inert atmosphere.

6. The method for preparing a nickel-phosphide / hollow sodium citrate carbon composite material according to claim 1, characterized in that, In step S4, the mass ratio of sodium hypophosphite to iron-nickel / hollow sodium citrate carbon is (5-10):

1.

7. A method for preparing a nickel phosphide / hollow sodium citrate carbon composite material according to any one of claims 1-6, characterized in that, In step S4, the Fe-Ni-P nanoparticles in the Fe-Ni-P / hollow sodium citrate carbon composite material are loaded and encapsulated in a hollow sodium citrate carbon shell, and are interconnected with the hollow sodium citrate carbon shell to form a cross-linked network structure. The Fe-Ni-P nanoparticles have a particle size of approximately 50 nm.

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