CORE-SHELL NICKEL FERRITE AND PREPARATION METHOD THEREOF, NICKEL FERRITE@C MATERIAL AND PREPARATION METHOD AND APPLICATION THEREOF

Abstract

The present disclosure provides core-shell nickel ferrite, a nickel ferrite@C material and preparation methods and application thereof. The preparation method of the core-shell nickel ferrite includes: preparing nickel iron glycerate ball powder by a solvothermal method; and under an air condition, heating the nickel iron glycerate ball powder at a heating rate of lower than 1.5° C./min to not less than 350° C. for performing calcination to obtain the core-shell nickel ferrite. The preparation method of the nickel ferrite@C material includes: performing a phenolic resin condensation reaction on the core-shell nickel ferrite, resorcinol and formaldehyde to obtain a phenolic resin (RF) coated core-shell nickel ferrite@RF composite material; and in an inert atmosphere, calcining and carbonizing the nickel ferrite@RF composite material to obtain the nickel ferrite@C material.

Claims

1. A core-shell nickel ferrite, wherein a core diameter is 425-450 nm, a shell thickness is 25-30 nm, and a core-shell spacing is 25-30 nm.

2. The core-shell nickel ferrite of claim 1, wherein the core diameter is 435-445 nm, the shell thickness is 25-27 nm, and the core-shell spacing is 25-27 nm.

3. A preparation method of core-shell nickel ferrite, comprising: using a nickel salt, an iron salt and a glycerin as raw materials, preparing a nickel iron glycerate ball powder by a solvothermal method; and under an air condition, heating the nickel iron glycerate ball powder at a heating rate of lower than 1.5° C./min to not less than 350° C. for performing calcination to obtain the core-shell nickel ferrite.

4. The preparation method of the core-shell nickel ferrite of claim 3, wherein in the nickel salt and the iron salt, a molar ratio of nickel ions to iron ions is 1:(1.9-2.1); or, a solvent of a solvothermal reaction system is isopropanol; or, a reaction temperature of the solvothermal method is 150-200° C., and a reaction time is 4-8 h; or, a calcination temperature is 350-450° C., a heating rate is 0.9-1.1° C., and a calcination time is 1.5-2.5 h.

5. A nickel ferrite@C material, comprising the core-shell nickel ferrite of claim 1, and the core-shell nickel ferrite is coated with a carbon coating; and a thickness of the carbon coating is 20-25 nm.

6. A preparation method of a nickel ferrite@C material, comprising: performing a phenolic resin condensation reaction on the core-shell nickel ferrite of claim 1, resorcinol and formaldehyde to obtain a phenolic resin (RF) coated core-shell nickel ferrite@RF composite material; and in an inert atmosphere, calcining and carbonizing the nickel ferrite@RF composite material to obtain the nickel ferrite@C material.

7. The preparation method of the nickel ferrite@C material of claim 6, wherein a rate of charge of the core-shell nickel ferrite to the resorcinol to the formaldehyde is 50 mg: (0.9-1.1) mg: (0.11-0.13) mL; or, the phenolic resin condensation reaction is performed under an alkaline condition, and ammonia water is added to a phenolic resin condensation reaction system; or, a solvent of the phenolic resin condensation reaction system is an aqueous solution of ethanol, and a volume ratio of the ethanol to water is 2:(0.9-1.1); or, a temperature of the calcination and carbonization is 550-650° C., and a calcination time is 1.5-2.5 h.

8. Application of the nickel ferrite@C material of claim 5 in lithium ion batteries.

9. A negative electrode of lithium ion batteries, wherein an active material of the negative electrode is the nickel ferrite@C material of claim 5; a binder and a conductive agent are comprised; and a preparation method of the negative electrode comprises: mixing the active material, the binder and the conductive agent uniformly, adding a solvent to prepare a slurry, coating a surface of a current collector with the slurry, and then drying the slurry.

10. A lithium ion battery, wherein a negative electrode of the lithium ion battery adopts the negative electrode of lithium ion batteries of claim 9.

11. A nickel ferrite@C material, comprising the core-shell nickel ferrite of claim 2, and the core-shell nickel ferrite is coated with a carbon coating; and a thickness of the carbon coating is 20-25 nm.

12. A nickel ferrite@C material, comprising the core-shell nickel ferrite prepared by the preparation method of claim 3, and the core-shell nickel ferrite is coated with a carbon coating; and a thickness of the carbon coating is 20-25 nm.

13. A nickel ferrite@C material, comprising the core-shell nickel ferrite prepared by the preparation method of claim 4, and the core-shell nickel ferrite is coated with a carbon coating; and a thickness of the carbon coating is 20-25 nm.

14. A preparation method of a nickel ferrite@C material, comprising: performing a phenolic resin condensation reaction on the core-shell nickel ferrite of claim 2, resorcinol and formaldehyde to obtain a phenolic resin (RF) coated core-shell nickel ferrite@RF composite material; and in an inert atmosphere, calcining and carbonizing the nickel ferrite@RF composite material to obtain the nickel ferrite@C material.

15. A preparation method of a nickel ferrite@C material, comprising: performing a phenolic resin condensation reaction on the core-shell nickel ferrite prepared by the preparation method of claim 3, resorcinol and formaldehyde to obtain a phenolic resin (RF) coated core-shell nickel ferrite@RF composite material; and in an inert atmosphere, calcining and carbonizing the nickel ferrite@RF composite material to obtain the nickel ferrite@C material.

16. A preparation method of a nickel ferrite@C material, comprising: performing a phenolic resin condensation reaction on the core-shell nickel ferrite prepared by the preparation method of claim 4, resorcinol and formaldehyde to obtain a phenolic resin (RF) coated core-shell nickel ferrite@RF composite material; and in an inert atmosphere, calcining and carbonizing the nickel ferrite@RF composite material to obtain the nickel ferrite@C material.

17. A negative electrode of lithium ion batteries, wherein an active material of the negative electrode is the nickel ferrite@C material prepared by the preparation method of claim 6; a binder and a conductive agent are comprised; and a preparation method of the negative electrode comprises: mixing the active material, the binder and the conductive agent uniformly, adding a solvent to prepare a slurry, coating a surface of a current collector with the slurry, and then drying the slurry.

18. A negative electrode of lithium ion batteries, wherein an active material of the negative electrode is the nickel ferrite@C material prepared by the preparation method of claim 7; a binder and a conductive agent are comprised; and a preparation method of the negative electrode comprises: mixing the active material, the binder and the conductive agent uniformly, adding a solvent to prepare a slurry, coating a surface of a current collector with the slurry, and then drying the slurry.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings constituting a part of the present disclosure are used to provide further understanding of the present disclosure. Exemplary embodiments of the present disclosure and descriptions thereof are used to explain the present disclosure, and do not constitute an improper limitation to the present disclosure.

[0023] FIG. 1 is an X-ray diffraction pattern (XRD) of core-shell NiFe.sub.2O.sub.4 prepared in Embodiment 1 of the present disclosure.

[0024] FIG. 2 is comparison diagrams in transmission electron micrographs of products prepared in Embodiment 1 and Embodiment 2 of the present disclosure; (a) is a transmission electron micrograph (TEM) of core-shell NiFe.sub.2O.sub.4 prepared in Embodiment 1 of the present disclosure; and (b) is a transmission electron micrograph (TEM) of NiFe.sub.2O.sub.4 solid spheres prepared in Embodiment 2 of the present disclosure.

[0025] FIG. 3 is comparison diagrams in scanning electron micrographs of products prepared in Embodiment 1 and Embodiment 2 of the present disclosure; (a) is a scanning electron micrograph (SEM) of core-shell NiFe.sub.2O.sub.4 prepared in Embodiment 1 of the present disclosure; and (b) is a scanning electron micrograph (SEM) of a core-shell NiFe.sub.2O.sub.4@C composite material prepared in Embodiment 1 of the present disclosure.

[0026] FIG. 4 is a transmission electron micrograph (TEM) of a core-shell NiFe.sub.2O.sub.4@C composite material prepared in Embodiment 1 of the present disclosure.

[0027] FIG. 5 is a comparison diagram of cycle performance of a core-shell NiFe.sub.2O.sub.4@C composite material prepared in Embodiment 1 and solid spherical NiFe.sub.2O.sub.4@C composite material prepared in Embodiment 2 of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

[0028] It should be noted that, the following detailed descriptions are all exemplary, and are intended to provide further descriptions of the present disclosure. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by a person of ordinary skill in the technical field to which the present disclosure belongs.

[0029] It should be noted that terms used herein are only for describing specific implementations and are not intended to limit exemplary implementations according to the present disclosure. As used herein, the singular form is also intended to include the plural form unless the context clearly dictates otherwise. In addition, it should further be understood that, terms “include” and/or “include” used in this specification indicate that there are features, steps, operations, devices, components, and/or combinations thereof.

[0030] In view of serious capacity attenuation, large irreversible capacity, and poor cycle performance caused by the large volume expansion effect of the existing NiFe.sub.2O.sub.4, the present disclosure provides core-shell nickel ferrite and a preparation method thereof, a nickel ferrite@C material and a preparation method and application thereof.

[0031] A typical implementation of the present disclosure provides core-shell nickel ferrite, a core diameter is 425-450 nm, a shell thickness is 25-30 nm, and a core-shell spacing is 25-30 nm.

[0032] In one or more embodiments of the implementation, the core diameter is 435-445 nm, the shell thickness is 25-27 nm, and the core-shell spacing is 25-27 nm.

[0033] Another implementation of the present disclosure provides a preparation method of the core-shell nickel ferrite, which includes: using nickel salt, iron salt and glycerin as raw materials, preparing nickel iron glycerate ball powder by a solvothermal method; and under an air condition, heating the nickel iron glycerate ball powder at a heating rate of lower than 1.5° C./min to not less than 350° C. for performing calcination to obtain the core-shell nickel ferrite.

[0034] The present disclosure has found through experiments that the heating rate during calcination affects the structure of the nickel ferrite. When the heating rate is higher than 1.5° C./min (especially not lower than 2° C./min), solid spherical nickel ferrite is obtained. When the heating rate is lower than 1.5° C./min (especially not higher than 1° C./min), core-shell nickel ferrite with a core-shell spacing is obtained.

[0035] The NiFe.sub.2O.sub.4 core and NiFe.sub.2O.sub.4 shell prepared by the present disclosure have an obvious hollow space, and can shorten a transmission path of ions and electrons and improve the electrochemical performance.

[0036] The nickel salt as described in the present disclosure refers to a compound with nickel ions as positive ions, such as nickel chloride, nickel nitrate and nickel sulfate.

[0037] The iron salt as described in the present disclosure refers to a compound with ferric ions as positive ions, such as ferric chloride, ferric nitrate and ferric sulfate.

[0038] The solvothermal method as described in the present disclosure refers to a synthetic method using an organic substance or a non-aqueous solvent as a solvent, and an original mixture is reacted at a certain temperature and self-generated pressure of the solution (closed condition).

[0039] In one or more embodiments of the implementation, in the nickel salt and the iron salt, a molar ratio of nickel ions to iron ions is 1:(1.9-2.1).

[0040] In one or more embodiments of the implementation, the solvent of a solvothermal reaction system is isopropanol.

[0041] In one or more embodiments of the implementation, a reaction temperature of the solvothermal method is 150-200° C., and a reaction time is 4-8 h. When the solvothermal temperature is 180±2° C. and the reaction time is 5.5-6.5 h, the reaction effect is better.

[0042] In one or more embodiments of the implementation, a calcination temperature is 350-450° C., the heating rate is 0.9-1.1° C., and a calcination time is 1.5-2.5 h.

[0043] A third implementation of the present disclosure provides a nickel ferrite@C material, which includes the above core-shell nickel ferrite, and the core-shell nickel ferrite is coated with a carbon coating.

[0044] The present disclosure uses the core-shell NiFe.sub.2O.sub.4 as a support carrier and a carbon layer as a protective layer, and can alleviate the problem of capacity attenuation caused by volume changes during charging and discharging of a lithium ion battery.

[0045] In one or more embodiments of the implementation, a thickness of the carbon coating is 20-25 nm.

[0046] A fourth implementation of the present disclosure provides a preparation method of a nickel ferrite@C material, which includes: performing a phenolic resin condensation reaction on the above core-shell nickel ferrite, resorcinol and formaldehyde to obtain a phenolic resin (RF) coated core-shell nickel ferrite@RF composite material; and in an inert atmosphere, calcining and carbonizing the nickel ferrite@RF composite material to obtain the nickel ferrite@C material.

[0047] In the present disclosure, the core-shell NiFe.sub.2O.sub.4 is coated with a mesoporous carbon layer formed by high-temperature carbonization of phenolic resin, and the mesoporous carbon layer can effectively prevent electrochemical wear due to elastic properties thereof.

[0048] The inert atmosphere as described in the present disclosure refers to a gas atmosphere that does not contain oxygen and can avoid oxidation reactions, such as nitrogen and argon.

[0049] In one or more embodiments of the implementation, a rate of charge of the core-shell nickel ferrite to the resorcinol to the formaldehyde is 50 mg:(0.9-1.1) g:(0.11-0.13) mL.

[0050] In one or more embodiments of the implementation, the phenolic resin condensation reaction is performed under an alkaline condition.

[0051] In the series of embodiments, ammonia water is added to a phenolic resin condensation reaction system. A rate of charge of the core-shell nickel ferrite to the ammonia water is 50 mg:(0.9-1.1) mL.

[0052] In one or more embodiments of the implementation, a solvent of the phenolic resin condensation reaction system is an aqueous solution of ethanol. When a volume ratio of the ethanol to water is 2:(0.9-1.1), the reaction effect is better.

[0053] In one or more embodiments of the implementation, a temperature of calcination and carbonization is 550-650° C., and a calcination time is 1.5-2.5 h.

[0054] A fifth implementation of the present disclosure provides application of the above nickel ferrite@C material in lithium ion batteries.

[0055] A sixth implementation of the present disclosure provides a negative electrode of lithium ion batteries, and an active material of the negative electrode of lithium ion batteries is the above nickel ferrite@C material.

[0056] In one or more embodiments of the implementation, a binder and a conductive agent are included.

[0057] In one or more embodiments of the implementation, a preparation method of the negative electrode includes: mixing the active material, the binder and the conductive agent uniformly, adding a solvent to prepare a slurry, coating a surface of a current collector with the slurry, and then drying the slurry.

[0058] A seventh implementation of the present disclosure provides a lithium ion battery, and a negative electrode of the lithium ion battery adopts the above negative electrode of lithium ion batteries.

[0059] In one or more embodiments of the implementation, the lithium ion battery is a CR2032 button cell.

[0060] In order to enable those skilled in the art to understand the technical solutions of the present disclosure more clearly, the technical solutions of the present disclosure will be described in detail below with reference to specific embodiments.

Embodiment 1

[0061] (1) First, 8 mL of glycerin was added to 40 mL of isopropanol and the mixture was stirred uniformly. Then 0.0363 g of Ni(NO.sub.3).sub.2.6H.sub.2O and 0.101 g of Fe(NO.sub.3).sub.3.9H.sub.2O were added in sequence, and the mixture was stirred uniformly at room temperature.

[0062] (2) The homogeneous liquid obtained in step (1) was transferred to a 100 mL polytetrafluoroethylene lined autoclave, and underwent a solvothermal reaction at 180° C. for 6 h. After natural cooling to room temperature, centrifugation, washing and drying were performed to obtain yellow nickel iron glycerate ball powder. In the air, the temperature was raised to 400° C. at 1° C./min, and the nickel iron glycerate ball powder was calcined for 2 h. Then the product was cooled to room temperature naturally to obtain core-shell NiFe.sub.2O.sub.4.

[0063] (3) The core-shell NiFe.sub.2O.sub.4 obtained in step (2) was dispersed ultrasonically in a mixed solution of 10 mL of water and 20 mL of ethanol. 1 mL of NH.sub.3.H.sub.2O (28 wt %), 1 g of resorcinol and 0.12 mL of formaldehyde were added, and the mixture was stirred for 2 h. Centrifugation, washing and drying were performed to obtain a core-shell NiFe.sub.2O.sub.4@RF composite material. In an argon atmosphere, the temperature was raised to 600° C. and the core-shell NiFe.sub.2O.sub.4@RF composite material was calcined for 2 h to obtain a core-shell NiFe.sub.2O.sub.4@C composite material.

Embodiment 2

[0064] (1) First, 8 mL of glycerin was added to 40 mL of isopropanol and the mixture was stirred uniformly. Then 0.0363 g of Ni(NO.sub.3).sub.2.6H.sub.2O and 0.101 g of Fe(NO.sub.3).sub.3.9H.sub.2O were added in sequence, and the mixture was stirred uniformly at room temperature.

[0065] (2) The homogeneous liquid obtained in step (1) was transferred to a 100 mL polytetrafluoroethylene lined autoclave, and underwent a solvothermal reaction at 180° C. for 6 h. After natural cooling to room temperature, centrifugation, washing and drying were performed to obtain yellow nickel iron glycerate ball powder. In the air, the temperature was raised to 400° C. at 2° C./min, and the nickel iron glycerate ball powder was calcined for 2 h. Then the product was cooled to room temperature naturally to obtain solid spherical NiFe.sub.2O.sub.4.

[0066] (3) The solid spherical NiFe.sub.2O.sub.4 obtained in step (2) was dispersed ultrasonically in a mixed solution of 10 mL of water and 20 mL of ethanol. 1 mL of NH.sub.3.H.sub.2O (28 wt %), 1 g of resorcinol and 0.12 mL of formaldehyde were added, and the mixture was stirred for 2 h. Centrifugation, washing and drying were performed to obtain a core-shell NiFe.sub.2O.sub.4@RF composite material. In an argon atmosphere, the temperature was raised to 600° C. and the solid spherical NiFe.sub.2O.sub.4@RF composite material was calcined for 2 h to obtain a solid spherical NiFe.sub.2O.sub.4@C composite material.

Embodiment 3

[0067] (1) First, 8 mL of glycerin was added to 40 mL of isopropanol and the mixture was stirred uniformly. Then 0.0363 g of Ni(NO.sub.3).sub.2.6H.sub.2O and 0.101 g of Fe(NO.sub.3).sub.3.9H.sub.2O were added in sequence, and the mixture was stirred uniformly at room temperature.

[0068] (2) The homogeneous liquid obtained in step (1) was transferred to a 100 mL polytetrafluoroethylene lined autoclave, and underwent a solvothermal reaction at 160° C. for 8 h. After natural cooling to room temperature, centrifugation, washing and drying were performed to obtain yellow nickel iron glycerate ball powder. In the air, the temperature was raised to 400° C. at 1° C./min, and the nickel iron glycerate ball powder was calcined for 2 h. Then the product was cooled to room temperature naturally to obtain core-shell NiFe.sub.2O.sub.4.

[0069] (3) The solid spherical NiFe.sub.2O.sub.4 obtained in step (2) was dispersed ultrasonically in a mixed solution of 10 mL of water and 20 mL of ethanol. 1 mL of NH.sub.3.H.sub.2O (28 wt %), 1 g of resorcinol and 0.12 mL of formaldehyde were added, and the mixture was stirred for 2 h. Centrifugation, washing and drying were performed to obtain a core-shell NiFe.sub.2O.sub.4@RF composite material. In an argon atmosphere and in an inert atmosphere, the temperature was raised to 600° C. and the core-shell NiFe.sub.2O.sub.4@RF composite material was calcined for 2 h to obtain a core-shell NiFe.sub.2O.sub.4@C composite material.

Embodiment 4

[0070] (1) First, 8 mL of glycerin was added to 40 mL of isopropanol and the mixture was stirred uniformly. Then 0.0363 g of Ni(NO.sub.3).sub.2.6H.sub.2O and 0.101 g of Fe(NO.sub.3).sub.3.9H.sub.2O were added in sequence, and the mixture was stirred uniformly at room temperature.

[0071] (2) The homogeneous liquid obtained in step (1) was transferred to a 100 mL polytetrafluoroethylene lined autoclave, and underwent a solvothermal reaction at 180° C. for 6 h. After natural cooling to room temperature, centrifugation, washing and drying were performed to obtain yellow nickel iron glycerate ball powder. In the air, the temperature was raised to 400° C. at 1° C./min, and the nickel iron glycerate ball powder was calcined for 2 h. Then the product was cooled to room temperature naturally to obtain core-shell NiFe.sub.2O.sub.4.

[0072] (3) The core-shell NiFe.sub.2O.sub.4 obtained in step (2) was dispersed ultrasonically in a mixed solution of 10 mL of water and 20 mL of ethanol. 1 mL of NH.sub.3.H.sub.2O (28 wt %), 2 g of resorcinol and 0.24 mL of formaldehyde were added, and the mixture was stirred for 2 h. Centrifugation, washing and drying were performed to obtain a core-shell NiFe.sub.2O.sub.4@RF composite material. In an argon atmosphere, the temperature was raised to 600° C. and the core-shell NiFe.sub.2O.sub.4@RF composite material was calcined for 2 h to obtain a core-shell NiFe.sub.2O.sub.4@C composite material.

Embodiment 5

[0073] (1) First, 8 mL of glycerin was added to 40 mL of isopropanol and the mixture was stirred uniformly. Then 0.0363 g of Ni(NO.sub.3).sub.2.6H.sub.2O and 0.101 g of Fe(NO.sub.3).sub.3.9H.sub.2O were added in sequence, and the mixture was stirred uniformly at room temperature.

[0074] (2) The homogeneous liquid obtained in step (1) was transferred to a 100 mL polytetrafluoroethylene lined autoclave, and underwent a solvothermal reaction at 180° C. for 6 h. After natural cooling to room temperature, centrifugation, washing and drying were performed to obtain yellow nickel iron glycerate ball powder. In the air, the temperature was raised to 400° C. at 1° C./min, and the nickel iron glycerate ball powder was calcined for 2 h. Then the product was cooled to room temperature naturally to obtain core-shell NiFe.sub.2O.sub.4.

[0075] (3) The core-shell NiFe.sub.2O.sub.4 obtained in step (2) was dispersed ultrasonically in a mixed solution of 10 mL of water and 20 mL of ethanol. 1 mL of NH.sub.3.H.sub.2O (28 wt %), 0.5 g of resorcinol and 0.06 mL of formaldehyde were added, and the mixture was stirred for 2 h. Centrifugation, washing and drying were performed to obtain a core-shell NiFe.sub.2O.sub.4@RF composite material. In an argon atmosphere, the temperature was raised to 600° C. and the core-shell NiFe.sub.2O.sub.4@RF composite material was calcined for 2 h to obtain a core-shell NiFe.sub.2O.sub.4@C composite material.

[0076] The electrochemical performance of the core-shell NiFe.sub.2O.sub.4@C composite material as a negative electrode material of lithium-ion batteries was evaluated by a CR2032 button cell. The battery assembly process is as follows: the active material, the binder and the conductive agent were mixed uniformly in a mass ratio of 7:2:1, and a certain amount of N-methylpyrrolidone was added to prepare a uniform slurry. Then, a copper foil was uniformly coated with the slurry and baked at 60° C. under a vacuum condition for 24 h. The battery assembly sequence is: a positive case, a negative pole piece, an electrolyte, a diaphragm, an electrolyte, a lithium sheet, a gasket, a spring sheet and a negative case. The diaphragm is a Celgard 2300 membrane, the electrolyte is 1 mol/L LiPF6 dissolved in a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, and the whole assembly process was performed in a glove box filled with argon. The assembled battery was tested using a Neware battery test system.

[0077] FIG. 1 is an X-ray diffraction pattern (XRD) of the core-shell NiFe.sub.2O.sub.4 prepared in Embodiment 1. There are obvious characteristic diffraction peaks at 30°, 36°, 43°, 57.5° and 63°, and the characteristic diffraction peaks are consistent with NiFe.sub.2O.sub.4 (JCPDS No. 10-0325), and represent NiFe.sub.2O.sub.4 (220), (311), (400), (511) and (440) crystal planes, respectively.

[0078] In FIG. 2, (a) is a transmission electron micrograph of the core-shell NiFe.sub.2O.sub.4 prepared in Embodiment 1, and (b) is a transmission electron micrograph of the solid spherical NiFe.sub.2O.sub.4 prepared in Embodiment 2 of the present disclosure. As shown in the Figure, (a) has an obvious core-shell structure, and a certain spacing exists between the core and the shell, (b) shows a solid ball.

[0079] In FIG. 3, (a) is a scanning electron micrograph of the core-shell NiFe.sub.2O.sub.4 prepared in Embodiment 1. The NiFe.sub.2O.sub.4 spheres are composed of fine particles. The core-shell structure can be clearly seen from the damage. The Figure (b) is a scanning electron micrograph of the core-shell NiFe.sub.2O.sub.4@RF prepared in Embodiment 1 of the present disclosure. As shown in the Figure, the sample has a uniform morphology, no adhesion and good dispersibility.

[0080] FIG. 4 is a transmission electron micrograph of the core-shell NiFe.sub.2O.sub.4@C prepared in Embodiment 1. From the Figure, the radius of the core-shell NiFe.sub.2O.sub.4 is 270 nm, the shell thickness is 25 nm, the core-shell spacing is 25 nm, and the carbon coating is 20 nm.

[0081] FIG. 5 is a comparison diagram of cycle performance of the core-shell NiFe.sub.2O.sub.4@C composite material prepared in Embodiment 1 and the solid spherical NiFe.sub.2O.sub.4@C composite material prepared in Embodiment 2 as negative electrodes of lithium ion batteries. At a current density of 0.5 A g.sup.−1, the core-shell NiFe.sub.2O.sub.4@C composite material prepared in Embodiment 1 has first discharge and charge capacities of 1048 mA h g.sup.−1 and 733 mA h g.sup.−1, respectively, and the first coulombic efficiency is 70%. The second discharge capacity is 735 mA h g.sup.−1, and the irreversible capacity loss may be attributed to the irreversible reaction of the electrolyte and the formation of a solid electrolyte interfacial film (SEI). In the cycle process, the battery capacity shows a trend of decreasing first and then increasing. After 165 cycles, the discharge capacity reached 792.9 mA h g.sup.−1. Compared with the solid spherical NiFe.sub.2O.sub.4@C composite material prepared in Embodiment 2, the cycle performance is greatly improved.

[0082] The foregoing descriptions are merely exemplary embodiments of the present disclosure, but are not intended to limit the present disclosure. The present disclosure may include various modifications and changes for a person skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present disclosure shall fall within the protection scope of the present disclosure.