Sodium-ion battery negative electrode material, preparation method thereof and sodium-ion battery

Abstract

The invention provides a sodium ion battery negative electrode material, a preparation method thereof and a sodium ion battery, and belongs to the technical field of secondary batteries. According to the preparation method, loquat peel is taken as a carrier, ferric salt and nickel salt are taken as raw materials, a ferric salt / nickel salt / loquat peel compound is synthesized by adopting a hydrothermal method, an iron / nickel / biomass charcoal precursor is obtained after cleaning and freeze drying, then carbonization is performed through two-stage calcination, the loquat peel is carbonized into graphite carbon, meanwhile, the graphite carbon reduces the ferric salt and the nickel salt at a high temperature, and iron and nickel are formed. The method comprises the following steps: firstly, preparing iron / nickel / biomass charcoal, then using a chemical vapor deposition technology to promote generation of carbon nanotubes, and finally, carrying out tube-sealing vapor phase phosphorization to obtain the phosphide / biomass charcoal / carbon nanotube composite material. The constructed composite material has excellent capacity, rate capability and stability when being used as a sodium-ion battery negative electrode material.

Description

Technical Field

[0001] The present invention belongs to the technical field, and particularly relates to a negative electrode material for a sodium-ion battery, a preparation method thereof, and a sodium-ion battery. Background Art

[0002] The extensive consumption of traditional non-renewable energy sources such as petroleum not only exacerbates the "energy crisis" but also leads to increasingly serious environmental pollution. Therefore, exploring efficient, durable, and inexpensive clean energy storage methods is an important goal for the sustainable development of contemporary society. In recent years, electrochemical energy storage devices have received extensive attention in the academic community due to their high energy conversion efficiency, high energy density, and other advantages. Among them, as the most common secondary battery, lithium-ion batteries have certain limitations in their applications due to the shortage and high cost of lithium resources, although they have characteristics such as high working voltage, long life, and large energy density. To reduce the over-reliance on lithium resources, researchers have found that Na, belonging to the same main group of alkali metals as Li, has similar chemical properties and the same energy storage mechanism, and sodium reserves are abundant. However, the continuously expanding market demand poses greater challenges to sodium-ion batteries, and exploring new electrode materials is the key way to achieve higher-performance sodium-ion batteries.

[0003] The negative electrode material is a crucial link in determining the excellent performance of sodium-ion batteries. Currently, conversion materials (phosphides, selenides, sulfides, etc.) are regarded as promising negative electrode materials due to their high theoretical specific capacity. Among them, phosphides have significant advantages over selenides and sulfides in terms of high theoretical capacity, cycle stability, low potential plateau, and conductivity, which makes them candidates for sodium-ion battery negative electrode materials with great potential. However, phosphides also have the common problem of severe volume expansion.

[0004] So far, researchers have taken various measures to improve and optimize the above problems. For example, the patent application document CN118572059A designs a carbon-coated phosphide material with an embedded structure, and the special carbon layer structure can effectively buffer the volume expansion of the phosphide, and the rich carbon layer can increase the electrode conductivity. In addition, Liu Shuling et al. used the co-precipitation method to dope Fe ions into cobalt phosphide, and the conductivity was significantly improved. [1] In addition, the patent application document CN117720076A compounded phosphide with carbon and obtained excellent lithium storage performance. However, the above modification methods are relatively single, and the improvement of the phosphide performance is also relatively limited.

[0005] [1]Zeyi Wang,Shuling Liu,Jinyu Du,Yichuang Xing,Yanling Hu,Yujie Ma,Xinyi Lu,Chao Wang.Iron-doped nickel phosphide hollow nanospheres synthesizedby solvothermal phosphidization of layered double hydroxides forelectrocatalytic oxygen evolution,Green Chemistry,2024,26:7779-7788. Summary of the Invention

[0006] This invention is based on the inventors' discovery and understanding of the following facts and problems: In order to further expand the application of phosphides in sodium-ion batteries, it is necessary to construct comprehensive modification means to overcome the above problems, thereby significantly improving the performance of phosphides.

[0007] This invention aims to solve at least one of the technical problems in the related art to some extent. For this purpose, an embodiment of this invention provides a negative electrode material for a sodium-ion battery, a preparation method thereof, and a sodium-ion battery.

[0008] In a first aspect, an embodiment of this invention provides a preparation method for a negative electrode material of a sodium-ion battery, including the following steps:

[0009] S1. Wash loquat peel with water, put it in a refrigerator for pre-freezing, and then perform freeze-drying to obtain dried loquat peel;

[0010] S2. Prepare a mixed aqueous solution of iron salt and nickel salt, and then soak the dried loquat peel in the mixed aqueous solution of iron salt and nickel salt to obtain an iron salt / nickel salt / loquat peel composite;

[0011] S3. Wash the iron salt / nickel salt / loquat peel composite with water and ethanol, and then perform freeze-drying to obtain an iron / nickel / biomass carbon precursor;

[0012] S4. Perform two-stage calcination on the iron / nickel / biomass carbon precursor to obtain an iron / nickel / biomass carbon composite;

[0013] S5. Perform chemical vapor deposition of carbon nanotubes on the iron / nickel / biomass carbon composite to obtain an iron / nickel / biomass carbon / carbon nanotube composite;

[0014] S6. Subject the composite of iron / nickel / biomass carbon / carbon nanotubes to sealed-tube gas-phase phosphidation to obtain a composite metal phosphide / biomass carbon / carbon nanotube composite material.

[0015] The advantages and technical effects brought by the preparation method of the embodiments of the present invention are as follows:

[0016] (1) The preparation method of the embodiments of the present invention uses loquat peel as a carrier and iron salt and nickel salt as raw materials, synthesizes a composite of iron salt / nickel salt / loquat peel by a hydrothermal method, then obtains an iron / nickel / biomass carbon precursor after washing and freeze-drying, and then undergoes two-stage calcination for carbonization. The loquat peel is carbonized into graphite carbon, and at the same time, the graphite carbon reduces the iron salt and nickel salt at high temperature to form iron and nickel, obtaining iron / nickel / biomass carbon. Then, chemical vapor deposition (CVD) technology is used to generate carbon nanotubes, and finally, sealed-tube gas-phase phosphidation is carried out to obtain a phosphide / biomass carbon / carbon nanotube composite material. The constructed composite material has excellent capacity, rate performance, and stability as the anode material of a sodium-ion battery.

[0017] (2) The phosphide / biomass carbon / carbon nanotube composite material prepared by the preparation method of the embodiments of the present invention combines the advantages of each composite phase. It not only has the excellent stability of biomass carbon but also has the high reversible capacity of phosphide. At the same time, biomass carbon can act as a buffer layer to slow down the volume expansion of phosphide during sodium storage, and also has a certain chemical adsorption effect on sodium phosphide generated during the charge and discharge process, prolonging the life of the electrode. In addition, the external three-dimensional network constructed by highly conductive carbon nanotubes promotes the rapid transmission of electrons, and the large specific surface area provided is conducive to the infiltration of the electrolyte and ion exchange. Therefore, the composite material is not a simple superposition of the three components and exhibits electrochemical performance superior to any single component.

[0018] In some embodiments, in step S1, the freeze-drying time is more than 14 h.

[0019] In some embodiments, in step S2, the soaking temperature is 50 - 90 °C, and the soaking time is 10 - 24 h.

[0020] In some embodiments, in step S2, the mass of the dried loquat peel is 1 - 3 g, and the volume of the mixed aqueous solution of the iron salt and nickel salt is 60 - 150 mL; and / or, in the mixed aqueous solution of the iron salt and nickel salt, the concentration of the iron salt is 0.35 - 0.7 mol / L, and the concentration of the nickel salt is 0.35 - 0.7 mol / L; and / or, in step S2, the iron salt is at least one of iron nitrate, iron sulfate, and iron chloride; and / or, the nickel salt is at least one of nickel nitrate, nickel sulfate, and nickel chloride.

[0021] In some embodiments, in step S3, washing is performed 3 - 5 times with water and ethanol respectively, and / or, in step S3, the freeze-drying time is 14 - 24 h.

[0022] In some embodiments, in step S4, nitrogen or argon is used as the protective atmosphere, and / or, the reaction temperature of the first-stage calcination is 300 - 400 °C, the reaction time of the first-stage calcination is 1 - 4 h, the reaction temperature of the second-stage calcination is 600 - 900 °C, and the reaction time of the second-stage calcination is 1 - 4 h.

[0023] In some embodiments, in step S5, the carbon source for chemical vapor deposition is ethanol or acetylene, and the flow rate of the carbon source is 10 - 30 sccm; and / or, nitrogen or argon is used as the protective atmosphere for chemical vapor deposition, and the flow rate of the protective atmosphere is 100 - 300 sccm; and / or, hydrogen is used as the reducing atmosphere for chemical vapor deposition, and the flow rate of the reducing atmosphere is 10 - 30 sccm; and / or, the reaction temperature of chemical vapor deposition is 600 - 800 °C, and the reaction time is 0.5 - 3 h.

[0024] In some embodiments, in step S6, the mass ratio of the iron / nickel / biomass carbon / carbon nanotube composite to red phosphorus is 3:1 - 1:1, and the phosphidation reaction temperature and time are 500 - 600 °C and 3 - 5 h respectively.

[0025] In a second aspect, an embodiment of the present invention further provides a negative electrode material for a sodium-ion battery, which is obtained by the preparation method of the first aspect.

[0026] The advantages and technical effects brought by the negative electrode material for a sodium-ion battery according to the embodiment of the present invention are as follows:

[0027] (1) The introduction of the iron diphosphide / nickel diphosphide heterostructure has a synergistic effect, provides more active sites, and reduces the migration energy barrier of Na+.

[0028] (2) The porous wrinkled lamellar structure of loquat peel has rich pores and a large specific surface area. Therefore, the unique structure of the biomass carbon in the phosphide / biomass carbon / carbon nanotube composite not only has rich pores and a high specific surface area, ensuring sufficient contact between the active material and the electrolyte, thus generating more electrochemically reactive sites; at the same time, it alleviates the volume expansion during the sodiation / delithiation process of the phosphide, significantly improving the stability.

[0029] (3) The addition of biomass carbon, in addition to forming a unique structure, also successfully performs nitrogen doping on the phosphide. Nitrogen doping can bring rich oxygen vacancies, effectively optimize the electronic structure of the phosphide, improve the internal conductivity of the phosphide, and at the same time, the increased defects are beneficial to sodium ion transport, accelerating the reaction kinetics.

[0030] (4) The carbon nanotubes are compounded with the phosphide, improving the electrical conductivity and structural stability. The carbon nanotube network structure can form a three-dimensional conductive framework, enhancing the overall performance of the anode material.

[0031] In a third aspect, an embodiment of the present invention provides a sodium-ion battery, including a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector, where the negative electrode active layer includes the negative electrode material of the second aspect.

[0032] The advantages and technical effects brought by the sodium-ion battery of the embodiment of the present invention are as follows:

[0033] Due to the adoption of the negative electrode material of the sodium-ion battery in the embodiment of the present invention, the sodium-ion battery in the embodiment of the present invention has excellent capacity, rate performance, and stability. Description of the Drawings

[0034] Figure 1 SEM images of the phosphide / biomass carbon / carbon nanotube composite material of Example 1 at different magnifications.

[0035] Figure 2 XRD diffraction pattern of the phosphide / biomass carbon / carbon nanotube composite material of Example 1.

[0036] Figure 3 XPS spectrum of the N element in the phosphide / biomass carbon / carbon nanotube composite material of Example 1.

[0037] Figure 4 SEM image of the phosphide / biomass carbon composite material of Comparative Example 1.

[0038] Figure 5 Rate performance comparison chart of the composite materials of Example 1 and Comparative Example 1.

[0039] Figure 6 Impedance comparison chart of the composite materials of Example 1 and Comparative Example 1. Detailed Embodiments

[0040] The embodiments of the present invention will be described in detail below. The examples of the embodiments are shown in the drawings. The embodiments described below with reference to the drawings are exemplary and are intended to explain the present invention, and should not be construed as a limitation of the present invention.

[0041] In a first aspect, an embodiment of the present invention provides a method for preparing a negative electrode material of a sodium-ion battery, including the following steps:

[0042] S1. Wash the loquat peel with water, put it into the refrigerator for pre-freezing, and then perform freeze-drying to obtain dried loquat peel;

[0043] S2. Prepare a mixed aqueous solution of iron salt and nickel salt, and then immerse the dried loquat peel in the mixed aqueous solution of iron salt and nickel salt to obtain an iron salt / nickel salt / loquat peel composite;

[0044] S3. Wash the iron salt / nickel salt / loquat peel composite with water and ethanol, and then perform freeze-drying to obtain an iron / nickel / biomass carbon precursor;

[0045] S4. Perform two-stage calcination on the iron / nickel / biomass carbon precursor to obtain an iron / nickel / biomass carbon composite;

[0046] S5. Perform chemical vapor deposition of carbon nanotubes on the iron / nickel / biomass carbon composite to obtain an iron / nickel / biomass carbon / carbon nanotube composite;

[0047] S6. Perform sealed-tube gas-phase phosphidation on the iron / nickel / biomass carbon / carbon nanotube composite to obtain a composite metal phosphide / biomass carbon / carbon nanotube composite material.

[0048] The preparation method of the embodiment of the present invention uses loquat peel as a carrier and iron salt and nickel salt as raw materials, and synthesizes an iron salt / nickel salt / loquat peel composite by a hydrothermal method. After washing and freeze-drying, an iron / nickel / biomass carbon precursor is obtained; then, an iron / nickel / biomass carbon composite is obtained through subsequent carbonization; afterwards, carbon nanotubes are generated using chemical vapor deposition technology to obtain an iron / nickel / biomass carbon / carbon nanotube composite; finally, sealed-tube gas-phase phosphidation is performed on it to obtain a phosphide / biomass carbon / carbon nanotube composite material embedded in a porous carbon with a wrinkled lamellar structure. The constructed composite material has excellent capacity, rate performance, and stability as a negative electrode material for sodium-ion batteries.

[0049] In the negative electrode material for sodium-ion batteries prepared by the preparation method of the embodiment of the present invention, the synergistic effect between the bimetallic heterostructures can increase the active sites and improve the reversible capacity; secondly, the porous wrinkled structure of the biomass carbon (LPC) can buffer the volume expansion of the phosphide and improve the cycle stability of the electrode material. At the same time, the wrinkled structure has abundant pores, which can provide more reactive sites and fast ion / electron transport channels; finally, the external three-dimensional network constructed by highly conductive carbon nanotubes can overall improve the conductivity of the composite material.

[0050] In some embodiments, in step S1, the freeze-drying time is more than 14 h, such as 14 h, 16 h, 18 h, 20 h, 22 h, 24 h, etc. When the freeze-drying time in step S1 is within the above range, the water in the fresh loquat peel can be removed and the structure of the solid matter of the loquat peel can be maintained.

[0051] In some embodiments, in step S2, the soaking temperature is 50 - 90°C, such as 50°C, 60°C, 70°C, 80°C, 90°C, etc., and the soaking time is 10 - 24 h, such as 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h, 24 h, etc. Using this hydrothermal reaction, a complex of iron salt / nickel salt / loquat peel can be synthesized. When the soaking temperature and soaking time are within the above ranges, the iron salt, nickel salt, and loquat peel can be fully complexed.

[0052] In some embodiments, in step S2, the mass of the dried loquat peel is 1 - 3 g, and the volume of the mixed aqueous solution of the iron salt and nickel salt is 60 - 150 mL; and / or, in the mixed aqueous solution of the iron salt and nickel salt, the concentration of the iron salt is 0.35 - 0.7 mol / L, such as 0.35 mol / L, 0.4 mol / L, 0.45 mol / L, 0.5 mol / L, 0.55 mol / L, 0.6 mol / L, 0.65 mol / L, 0.7 mol / L, etc., and the concentration of the nickel salt is 0.35 - 0.7 mol / L, such as 0.35 mol / L, 0.4 mol / L, 0.45 mol / L, 0.5 mol / L, 0.55 mol / L, 0.6 mol / L, 0.65 mol / L, 0.7 mol / L, etc. When the ratio of loquat peel, iron salt, and nickel salt is within the above ranges, the iron salt, nickel salt, and loquat peel can be fully complexed.

[0053] In some embodiments, in step S2, the nickel salt is at least one of nickel nitrate, nickel sulfate, and nickel chloride; and / or, the iron salt is at least one of iron nitrate, iron sulfate, and iron chloride. The above iron salts and nickel salts are easily soluble in water, facilitating the complexation with loquat peel through hydrothermal reaction.

[0054] In some embodiments, in step S3, wash with water and ethanol respectively 3 - 5 times. This is beneficial for removing the residual substances on the surface of the complex of iron salt / nickel salt / loquat peel.

[0055] In some embodiments, in step S3, the freeze-drying time is 14 - 24 h, such as 14 h, 16 h, 18 h, 20 h, 22 h, 24 h, etc. Freeze-drying the complex of iron salt / nickel salt / loquat peel after washing in this step can remove the solvent introduced during the washing process without damaging the structure of the loquat peel, facilitating the subsequent obtaining of biomass carbon with a porous and wrinkled structure after carbonization.

[0056] In some embodiments, in step S4, use nitrogen or argon as the protective atmosphere. This protective atmosphere is to prevent iron and nickel from being oxidized.

[0057] In some embodiments, in step S4, the reaction temperature for the first-stage calcination is 300 - 400 °C, such as 300 °C, 320 °C, 340 °C, 360 °C, 380 °C, 400 °C, etc., and the reaction time for the first-stage calcination is 1 - 4 h, such as 1 h, 2 h, 3 h, 4 h, etc. The reaction temperature for the second-stage calcination is 600 - 900 °C, such as 600 °C, 350 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, etc., and the reaction time for the second-stage calcination is 1 - 4 h, such as 1 h, 2 h, 3 h, 4 h, etc. In the first-stage calcination, organic substances such as cellulose contained in the iron / nickel / biomass carbon precursor can be removed. The second-stage calcination can carbonize the loquat peel to form amorphous biomass carbon with sodium storage activity, and at the same time increase the conductivity of the composite material.

[0058] In some embodiments, in step S5, the carbon source for chemical vapor deposition is ethanol or acetylene, and the flow rate of the carbon source is 10 - 30 sccm. Chemical vapor deposition under the above conditions can form a three-dimensional network structure of carbon nanotubes on the surface of the phosphide / biomass carbon.

[0059] In some embodiments, in step S5, chemical vapor deposition uses nitrogen or argon as the protective atmosphere, and the flow rate of the protective atmosphere is 100 - 300 sccm; and / or, chemical vapor deposition uses hydrogen as the reducing atmosphere, and the flow rate of the reducing atmosphere is 10 - 30 sccm. The above protective atmosphere and reducing atmosphere are to prevent the biomass carbon from being oxidized.

[0060] In some embodiments, in step S5, the reaction temperature for chemical vapor deposition is 600 - 800 °C, such as 600 °C, 650 °C, 700 °C, 850 °C, 800 °C, etc., and the reaction time is 0.5 - 3 h, such as 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, etc. The reaction temperature and reaction time of the above chemical vapor deposition are beneficial to improving the conductivity of the carbon nanotubes.

[0061] In some embodiments, in step S6, the mass ratio of the iron / nickel / biomass carbon / carbon nanotube composite to red phosphorus is 3:1 - 1:1, such as 3:1, 2.5:1, 2:1, 1.5:1, 1:1, etc. Sealing the tube means putting the iron / nickel / biomass carbon / carbon nanotube composite and red phosphorus into a quartz tube together, evacuating and sealing it with a tube sealer, and then the gas-phase phosphidation is carried out in a vacuum state.

[0062] In some embodiments, in step S6, the temperature of the phosphidation reaction is 500 - 600 °C, such as 500 °C, 520 °C, 540 °C, 560 °C, 580 °C, 600 °C, etc., and the time of the phosphidation reaction is 3 - 5 h, such as 3 h, 3.5 h, 4 h, 4.5 h, 5 h, etc. The above phosphidation reaction temperature and time are beneficial to fully phosphidize iron and nickel in the iron / nickel / biomass carbon / carbon nanotube composite to form metal phosphides.

[0063] In a second aspect, an anode material for a sodium-ion battery provided by an embodiment of the present invention is obtained by the preparation method of the first aspect.

[0064] Due to the adoption of the preparation method of the embodiment of the present invention, the anode material for a sodium-ion battery of the embodiment of the present invention has the advantages of each composite phase. Among them, iron diphosphide / nickel diphosphide, as a substance with a high theoretical capacity, greatly improves the reversible capacity of the composite material; in addition, the heterostructure formed by the two phosphides has a synergistic effect, which can increase the active sites and improve the reversible capacity; the unique structure of biomass carbon not only has rich pores and a high specific surface area, but also can effectively increase the contact area between the active substance and the electrolyte, thereby accelerating electron transfer. At the same time, the external three-dimensional network constructed by highly conductive carbon nanotubes can overall improve the conductivity of the composite material. The above-mentioned various modification means can optimize the practical application value of phosphides from two aspects of electrochemical performance and electrode reaction kinetics.

[0065] In a third aspect, an embodiment of the present invention provides a sodium-ion battery, including a negative electrode current collector and a negative electrode active layer provided on at least one surface of the negative electrode current collector, and the negative electrode active layer includes the negative electrode material of the second aspect.

[0066] Due to the adoption of the negative electrode material of the embodiment of the present invention, the sodium-ion battery of the embodiment of the present invention has excellent capacity, rate performance and stability.

[0067] The present invention will be described in detail below with reference to examples and drawings.

[0068] Example 1

[0069] (1) Peel the skin of loquats, wash it with deionized water, put it in the refrigerator for pre-freezing, and then transfer it to a freeze dryer for freeze drying for 14 h;

[0070] (2) Prepare a 70 mL mixed aqueous solution of ferric chloride and nickel chloride with a concentration of 0.35 mol / L. After stirring it evenly and keeping it at 50 °C, soak 3 g of the loquat peel obtained in step (1) in it for 24 h;

[0071] (3) Wash the loquat peel obtained in step (2) three times with water and ethanol respectively to remove the residual substances on the surface, transfer it to a freeze dryer for freeze-drying, and obtain yellow-green tablets α after 14 h of freeze-drying;

[0072] (4) Obtain black tablets β after calcining the yellow-green tablets α at two-stage temperatures. Among them, using argon as the protective atmosphere, the reaction temperature and time for the first-stage calcination are 400 °C and 1 h respectively; the reaction temperature and time for the second-stage calcination are 900 °C and 1 h respectively;

[0073] (5) Under the argon-hydrogen mixed gas, use the ethanol bubbling method to introduce the carbon source ethanol, and perform chemical vapor deposition (CVD) growth of carbon nanotubes on the black tablets β at 650 °C for 2.5 h to obtain the iron / nickel / biomass carbon / carbon nanotube composite γ; among them, argon is the protective atmosphere, and the argon flow rate is 200 sccm, hydrogen is the reducing atmosphere, and the hydrogen flow rate is 20 sccm;

[0074] (6) Perform sealed-tube gas-phase phosphidation on the iron / nickel / biomass carbon / carbon nanotube composite γ, and the conditions are a mass ratio of 1:1 with red phosphorus, a phosphidation temperature of 600 °C, and a phosphidation time of 3 h to obtain the final product phosphide / biomass carbon / carbon nanotube composite material. Based on the total mass of this composite material being 100%, among them, the content of iron diphosphide is 25%, the content of nickel diphosphide is 25%, the content of CNTs is 15%, and the remaining is 35% of biomass carbon.

[0075] Figure 1 Figure 13 is the SEM images of the phosphide / biomass carbon / carbon nanotube composite material of Example 1 at different magnifications. It can be seen that there are many phosphide nanoparticles after phosphidation, and after the CVD technology, carbon nanotubes grow significantly on the surface of the phosphide particles. Figure 2 Figure 14 is the XRD diffraction pattern of the phosphide / biomass carbon / carbon nanotube composite material of Example 1. From Figure 2 it can be seen that the phase of the phosphide / biomass carbon / carbon nanotube composite material is further identified as a mixed phase of iron phosphide / nickel phosphide (JCPDS 74-1383, JCPDS 71-2234) by X-ray diffraction, and the existence of carbon nanotubes can be confirmed by the above SEM images. By detecting the valence state of N on the surface of the phosphide / biomass carbon / carbon nanotube composite material of Example 1 by X-ray photoelectron spectroscopy (XPS) ([ Figure 3 ), the N 1s high-resolution spectrum reveals the characteristic peaks of pyridine N (397.8 eV), pyrrole N (400.2 eV) and graphitic N (402.8 eV), indicating that N is significantly present in the composite material, proving the successful incorporation of N.

[0076] Example 2

[0077] The preparation method of the embodiment of the present invention is the same as that of Example 1, except that the phosphating temperature in step (6) is different. The specific method is as follows:

[0078] (1) Peel the skin of loquat, wash it with deionized water, put it in the refrigerator for pre-freezing, and then transfer it to a freeze dryer for freeze-drying for 14 h;

[0079] (2) Prepare a mixed aqueous solution of 70 mL of ferric sulfate and nickel sulfate with a concentration of 0.35 mol / L. After stirring it evenly, keep it at 50 °C, and soak 3 g of the loquat peel obtained in step (1) in it for 24 h;

[0080] (3) Wash the loquat peel obtained in step (2) 3 times with water and ethanol respectively to remove the residual substances on the surface, transfer it to a freeze dryer for freeze-drying, and obtain yellow-green flakes α after 14 h of freeze-drying;

[0081] (4) Calcinate the yellow-green flakes α at two-stage temperatures to obtain black flakes β. Among them, using argon as the protective atmosphere, the reaction temperature and time of the first-stage calcination are 400 °C and 1 h respectively; the reaction temperature and time of the second-stage calcination are 900 °C and 1 h respectively;

[0082] (5) Under the argon-hydrogen mixed gas, introduce the carbon source ethanol by the ethanol bubbling method, and carry out chemical vapor deposition (CVD) growth of carbon nanotubes on the black flakes β at 650 °C for 2.5 h to obtain a composite γ of iron / nickel / biomass carbon / carbon nanotubes; among them, argon is the protective atmosphere, and the argon flow rate is 200 sccm, hydrogen is the reducing atmosphere, and the hydrogen flow rate is 20 sccm;

[0083] (6) Carry out sealed-tube gas-phase phosphating on the composite γ of iron / nickel / biomass carbon / carbon nanotubes. The conditions are that the mass ratio to red phosphorus is 1:1, the phosphating temperature is 500 °C, and the phosphating time is 3 h to obtain the final product phosphide / biomass carbon / carbon nanotube composite material. Based on the total mass of this composite material being 100%, among them, the content of iron diphosphide is 25%, the content of nickel diphosphide is 25%, the content of CNTs is 15%, and the remaining is 35% of biomass carbon.

[0084] Example 3

[0085] The preparation method of the embodiment of the present invention is the same as that of Example 1, except that the types and concentrations of iron salts and nickel salts in step (3) are different. The specific method is as follows:

[0086] (1) Peel the skin of loquat, wash it with deionized water, put it in the refrigerator for pre-freezing, and then transfer it to a freeze dryer for freeze-drying for 14 h;

[0087] (2) Prepare a mixed aqueous solution of 70 mL containing 0.53 mol / L ferric nitrate and 0.18 mol / L nickel nitrate. After stirring it evenly, maintain the temperature at 50 °C, and soak 3 g of the loquat peel obtained in step (1) in it for 24 h;

[0088] (3) Wash the loquat peel obtained in step (2) 3 times each with water and ethanol to remove the residual substances on the surface, transfer it to a freeze dryer for freeze drying, and obtain yellow-green flakes α after 14 h of freeze drying;

[0089] (4) Obtain black flakes β by calcining the yellow-green flakes α at two-stage temperatures. Among them, using argon as the protective atmosphere, the reaction temperature and time for the first-stage calcination are 400 °C and 1 h respectively; the reaction temperature and time for the second-stage calcination are 900 °C and 1 h respectively;

[0090] (5) Under an argon-hydrogen mixed gas, use the ethanol bubbling method to introduce the carbon source ethanol, and carry out chemical vapor deposition (CVD) growth of carbon nanotubes on the black flakes β at 650 °C for 2.5 h to obtain a composite γ of iron / nickel / biomass carbon / carbon nanotubes; among them, argon is the protective atmosphere, and the argon flow rate is 200 sccm, hydrogen is the reducing atmosphere, and the hydrogen flow rate is 20 sccm;

[0091] (6) Carry out sealed-tube gas-phase phosphidation on the composite γ of iron / nickel / biomass carbon / carbon nanotubes. The conditions are that the mass ratio to red phosphorus is 1:1, the phosphidation temperature is 600 °C, and the phosphidation time is 3 h to obtain the final product phosphide / biomass carbon / carbon nanotube composite material. Based on the total mass of this composite material being 100%, among them, the content of iron diphosphide is 37.5%, the content of nickel diphosphide is 12.5%, the content of CNTs is 15%, and the remaining is 35% biomass carbon.

[0092] Example 4

[0093] The preparation method of the embodiment of the present invention is the same as that of Example 1, the difference is that in step (3), the nickel salt is omitted, and the specific method is as follows:

[0094] (1) Peel the loquat fruit, wash it with deionized water, put it in the refrigerator for pre-freezing, and then transfer it to a freeze dryer for freeze drying for 14 h;

[0095] (2) Prepare a 70 mL aqueous solution of ferric chloride with a concentration of 0.7 mol / L. After stirring it evenly, maintain the temperature at 50 °C, and soak 3 g of the loquat peel obtained in step (1) in it for 24 h;

[0096] (3) Wash the loquat peel obtained in step (2) 3 times each with water and ethanol to remove the residual substances on the surface, transfer it to a freeze dryer for freeze drying, and obtain yellow-green flakes α after 14 h of freeze drying;

[0097] (4) The yellow-green flakes α are calcined at two-stage temperatures to obtain black flakes β. Among them, with argon as the protective atmosphere, the reaction temperature and time of the first-stage calcination are 400 °C and 1 h respectively; the reaction temperature and time of the second-stage calcination are 900 °C and 1 h respectively.

[0098] (5) Under an argon-hydrogen mixture, carbon source ethanol is introduced by the ethanol bubbling method, and chemical vapor deposition (CVD) is carried out on the black flakes β at 650 °C for 2.5 h to grow carbon nanotubes, obtaining a composite γ of iron / biomass carbon / carbon nanotubes; among them, argon is the protective atmosphere, the argon flow rate is 200 sccm, hydrogen is the reducing atmosphere, and the hydrogen flow rate is 20 sccm.

[0099] (6) The composite γ of iron / biomass carbon / carbon nanotubes is subjected to sealed-tube gas-phase phosphidation. The conditions are that the mass ratio to red phosphorus is 1:1, and the phosphidation temperature is 600 °C, obtaining the final product of phosphide / biomass carbon / carbon nanotube composite material. Calculated based on the total mass of this composite material being 100%, among them, the content of iron diphosphide is 50%, the content of CNTs is 15%, and the remaining is 35% of biomass carbon.

[0100] Example 5

[0101] The preparation method of the embodiment of the present invention is the same as that of Example 1, except that in step (3), the iron salt is omitted, and the specific method is as follows:

[0102] (1) Peel the skin of loquat, wash it with deionized water, put it into the refrigerator for pre-freezing, and then transfer it to a freeze dryer for freeze-drying for 14 h.

[0103] (2) Prepare 70 mL of nickel chloride aqueous solution with a concentration of 0.7 mol / L. After stirring it evenly, soak 3 g of the loquat peel obtained in step (1) in it for 24 h.

[0104] (3) Wash the loquat peel obtained in step (2) 3 times with water and ethanol respectively to remove the residual substances on the surface, transfer it to a freeze dryer for freeze-drying, and obtain yellow-green flakes α after 14 h of freeze-drying.

[0105] (4) The yellow-green flakes α are calcined at two-stage temperatures to obtain black flakes β. Among them, with argon as the protective atmosphere, the reaction temperature and time of the first-stage calcination are 400 °C and 1 h respectively; the reaction temperature and time of the second-stage calcination are 900 °C and 1 h respectively.

[0106] (5) Under an argon-hydrogen mixture, carbon source ethanol is introduced by the ethanol bubbling method, and chemical vapor deposition (CVD) is carried out on the black flakes β at 650 °C for 2.5 h to grow carbon nanotubes, obtaining a composite γ of nickel / biomass carbon / carbon nanotubes; among them, argon is the protective atmosphere, the argon flow rate is 200 sccm, hydrogen is the reducing atmosphere, and the hydrogen flow rate is 20 sccm.

[0107] (6) The composite γ of nickel / biomass carbon / carbon nanotubes was subjected to sealed-tube vapor-phase phosphidation. The conditions were a mass ratio of 1:1 with red phosphorus, a phosphidation temperature of 600 °C, and a phosphidation time of 3 h, to obtain the final product, the phosphide / biomass carbon / carbon nanotube composite material. Based on the total mass of this composite material being 100%, among them, the content of nickel diphosphide was 50%, the content of CNTs was 15%, and the remaining was 35% biomass carbon.

[0108] Comparative Example 1

[0109] According to the steps of Example 1, while removing the CVD process in step (5), the remaining steps remained unchanged to obtain the phosphide / biomass carbon composite material. The specific method was as follows:

[0110] (1) The peel of loquat was peeled off, washed with deionized water, pre-frozen in a refrigerator, and then transferred to a freeze dryer for freeze drying for 14 h;

[0111] (2) Prepare a mixed aqueous solution of 70 mL of 0.35 mol / L ferric chloride and 0.35 mol / L nickel chloride. After stirring it evenly and maintaining at 50 °C, soak 3 g of the loquat peel obtained in step (1) in it for 24 h;

[0112] (3) The loquat peel obtained in step (2) was washed 3 times each with water and ethanol to remove the residual substances on the surface, and then transferred to a freeze dryer for freeze drying. After freeze drying for 14 h, yellow-green flakes α were obtained;

[0113] (4) The yellow-green flakes α were calcined at a two-stage temperature to obtain black flakes β. Among them, with argon as the protective atmosphere, the reaction temperature and time of the first-stage calcination were 400 °C and 1 h respectively; the reaction temperature and time of the second-stage calcination were 900 °C and 1 h respectively;

[0114] (5) The black flakes β were subjected to sealed-tube vapor-phase phosphidation. The conditions were a mass ratio of black flakes β to red phosphorus of 1:1, a phosphidation temperature of 600 °C, and a phosphidation time of 3 h, to obtain the final product, the phosphide / biomass carbon composite material. Based on the total mass of this composite material being 100%, among them, the content of iron diphosphide was 18.5%, the content of nickel diphosphide was 18.5%, and the remaining was 63% biomass carbon.

[0115] Figure 4 It is the SEM image of the phosphide / biomass carbon composite material of Comparative Example 1. It can be seen that on the surface of the wrinkled carbon layer of the sample without CVD, there are only nanoparticles of phosphide, and no rough phenomenon appears, indicating that there are no carbon nanotubes.

[0116] According to the mass ratio of active material: conductive agent: binder of 7:2:1, the finally obtained powdery composite materials in the above-mentioned examples and comparative examples, together with Super P and polyvinylidene fluoride (PVDF), were ground and stirred multiple times to form a slurry, which was coated on a copper foil. After thorough drying, it was punched into circular pieces to serve as the sodium-ion battery electrodes to be assembled. The assembled button-type sodium-ion battery used a sodium sheet as the negative electrode, the above-mentioned electrode sheet as the positive electrode, and glass fiber as the separator. An electrolyte with 1 mol·L -1 NaPF 6 as the solute and ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 as the solvent was dropped on both sides of the separator. After the assembled sodium-ion battery was left standing, electrochemical tests were carried out.

[0117] With the changes in chemical components and parameters during the preparation process, the sodium-ion battery also exhibited different electrochemical performances. After the composite materials of Examples 1-5 and Comparative Example 1 were assembled into sodium-ion batteries as the negative electrode materials of the sodium-ion battery, their maximum discharge capacities at different current densities are shown in Table 1:

[0118] Table 1. Maximum discharge capacities of the composite materials of Examples 1-5 and Comparative Example 1 as the negative electrode materials of the sodium-ion battery at different current densities

[0119]

[0120] It can be seen that changing the parameters during the preparation process has a relatively large impact on the electrochemical performance of the sodium-ion battery. The changed parameters include the phosphating temperature and the iron / nickel ratio. When the phosphating temperature is reduced to 500 °C (Example 2), the capacity is relatively high at low currents, but the capacity decays relatively quickly at high currents. When the iron / nickel ratio is changed (Example 3), the proportion of iron increases appropriately. Although the capacity performance at each current density decreases, the overall stability of the composite material still exists; when using a single metal salt (Examples 4, 5), the overall capacity of the composite material decreases significantly, which is due to the disappearance of the synergistic effect of the heterogeneous structure in the composite material. In the case of not growing carbon nanotubes, the specific capacity of Comparative Example 1 at different current densities is lower than that of Examples 1-3, which shows that the presence of carbon nanotubes can significantly improve the poor conductivity problem in the composite material, thereby improving the rate performance. Figure 5 is the comparison chart of the rate performance of the composite materials of Example 1 and Comparative Example 1, Figure 6 and Figure 5 is the impedance comparison chart of the composite materials of Example 1 and Comparative Example 1. Combining Figure 6 it is not difficult to see that the presence of carbon nanotubes can not only improve the electrochemical performance of the composite material, but also reduce the interfacial impedance, increase the sodium-ion migration rate, and accelerate the reaction kinetics.

[0121] In the present invention, terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0122] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a negative electrode material for a sodium ion battery, characterized in that: The following steps are involved: S1. Wash the loquat peel with water and put it into the refrigerator for pre-freezing, and then freeze-dry it to obtain dried loquat peel; S2. preparing a mixed aqueous solution of an iron salt and a nickel salt, and then immersing the dried loquat peel in the mixed aqueous solution of the iron salt and the nickel salt to obtain an iron salt / nickel salt / loquat peel complex; S3. The iron salt / nickel salt / loquat peel complex is washed with water and ethanol, and then freeze-dried to obtain an iron / nickel / biomass carbon precursor; S4. calcining the iron / nickel / biomass carbon precursor in two stages to obtain an iron / nickel / biomass carbon composite; S5. chemically vapor depositing carbon nanotubes on the iron / nickel / biomass charcoal composite to obtain an iron / nickel / biomass charcoal / carbon nanotube composite; S6. The iron / nickel / biomass charcoal / carbon nanotube composite is subjected to sealed tube gas phase phosphating to obtain a composite metal phosphide / biomass charcoal / carbon nanotube composite material.

2. The preparation method according to claim 1, characterized in that: In step S1, the freeze-drying time is more than 14 hours.

3. The preparation method according to claim 1, characterized in that: In step S2, the soaking temperature is 50-90°C and the soaking time is 10-24h.

4. The preparation method according to claim 1, characterized in that: In step S2, the mass of the dried loquat peel is 1-3g, and the volume of the mixed aqueous solution of the iron salt and the nickel salt is 60-150mL; and / or, in the mixed aqueous solution of the iron salt and the nickel salt, the concentration of the iron salt is 0.35-0.7mol / L, and the concentration of the nickel salt is 0.35-0.7mol / L; and / or, in step S2, the iron salt is at least one of ferric nitrate, ferric sulfate and ferric chloride; and / or, the nickel salt is at least one of nickel nitrate, nickel sulfate and nickel chloride.

5. The preparation method according to claim 1, characterized in that: In step S3, washing is performed with water and ethanol for 3-5 times respectively, and / or, the freeze-drying time in step S3 is 14-24 hours.

6. The preparation method according to claim 1, characterized in that: In step S4, nitrogen or argon is used as the protective atmosphere, and / or the reaction temperature of the first calcination is 300-400°C, the reaction time of the first calcination is 1-4h, the reaction temperature of the second calcination is 600-900°C, and the reaction time of the second calcination is 1-4h.

7. The preparation method according to claim 1, characterized in that: In step S5, the carbon source for chemical vapor deposition is ethanol or acetylene, and the flow rate of the carbon source is 10-30sccm; and / or, nitrogen or argon is used as a protective atmosphere for chemical vapor deposition, and the flow rate of the protective atmosphere is 100-300sccm; and / or, hydrogen is used as a reducing atmosphere for chemical vapor deposition, and the flow rate of the reducing atmosphere is 10-30sccm; and / or, the reaction temperature of chemical vapor deposition is 600-800°C, and the reaction time is 0.5-3h.

8. The preparation method according to claim 1, characterized in that: In step S6, the mass ratio of the iron / nickel / biomass carbon / carbon nanotube composite to red phosphorus is 3:1-1:1, and the phosphating reaction temperature and time are 500-600° C. and 3-5 h, respectively.

9. A sodium ion battery negative electrode material, characterized in that: Obtained by the preparation method according to any one of claims 1 to 8.

10. A sodium ion battery comprising a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector, characterized in that: The negative electrode active layer comprises the negative electrode material according to claim 9.

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