NMTP@c material doped with nitrogen-doped graphene, and preparation method therefor and use thereof

Abstract

The present disclosure relates to an NMTP@C material doped with nitrogen-doped graphene, and a preparation method therefor and the use thereof. The material is composed of NMTP@C and nitrogen-doped graphene coated on the surface of the NMTP@C. The NMTP@C is a nanofiber composite material having a core-shell structure, with NMTP being a core and C being a shell. The preparation method comprises: S1, preparing nitrogen-doped graphene; S2, adding citric acid and the nitrogen-doped graphene obtained in step S1 to deionized water, continuously stirring same, and adding an MnC4H6O4•4H2O powder, an NaC2H3O2 powder and an NH4H2PO4 powder, so as to obtain a mixed solution; S3, adding an ethanol solution of C12H28O4Ti to the mixed solution obtained in step S2, and removing deionized water and ethanol, so as to obtain a gel precursor; S4, drying the gel precursor obtained in step S3; and S5, placing the gel precursor dried in step S4 in a protective atmosphere, and heating same, so as to obtain an NMTP@C material doped with nitrogen-doped graphene. Doping the outside of the NMTP@C with the nitrogen-doped graphene can inhibit the growth of NMTP crystals, and promotes the electrochemical performance of the NMTP@C material doped with nitrogen-doped graphene of the present disclosure.

Description

Nitrogen-doped graphene-doped NMTP@C material, and preparation method and application thereof TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of battery electrode composite materials, and particularly relates to a nitrogen-doped graphene-doped NMTP@C material, and a preparation method and application thereof. BACKGROUND

[0002] At present, high-performance sodium ion batteries are widely recommended as a good choice for electrochemical energy storage due to their low price, abundant natural resources, good safety and high energy density. The power density of sodium ion storage is determined by the cathode material. Due to the large radius of Na+, the structural stability of the cathode during Na+ insertion / deinsertion is poor.

[0003] Therefore, it is of great significance to study new cathode materials with high theoretical capacity and high working voltage for improving the power density of sodium ion batteries. Under this background, various new cathode materials such as NaVPO4F, Na4MnV(PO4)3, NaNi1 / 3Fe1 / 3Mn1 / 3O2, Na6Fe5(SO4)8, Na3V2(PO4)3, NaVOPO4 and NMTP are widely studied due to their various advantages.

[0004] Among the above-mentioned cathodes, sodium manganese titanium phosphate (NMTP) with a NASICON structure is considered to be the most promising cathode for sodium ion storage due to its abundant natural resources, stable structural framework and excellent ion conductivity. The crystal structure feature of the NASICON structure is that it has an open three-dimensional framework, which is beneficial to the migration and diffusion of ions. Such an ideal electrode was first reported by a group led by Goodenough. Despite these good advantages, pure NMTP material shows a disadvantage of conductivity, which leads to poor sodium ion storage performance of the battery. In order to solve this obstacle, synthesis with conductive carbon is adopted to form a new structure. For example, a research group prepared NMTP@C material by sol-gel method, and the designed electrode has a high reversible capacity.

[0005] However, although the above-mentioned researches have prepared corresponding battery electrode composite materials, the performances of the above-mentioned several composite materials are still not very ideal.

[0006] SUMMARY

[0007] In order to solve the problems existing in the prior art, the present disclosure aims to provide a nitrogen-doped graphene-doped NMTP@C material, and a preparation method and application thereof, so as to improve the electrochemical performance of the NMTP composite material.

[0008] The nitrogen-doped graphene-doped NMTP@C material comprises NMTP@C and nitrogen-doped graphene, wherein the NMTP@C is a core-shell structure nanofiber composite material with NMTP as the core and C as the shell.

[0009] Preferably, the NMTP is of NASICON structure.

[0010] The present disclosure further provides a preparation method of the nitrogen-doped graphene-doped NMTP@C material, comprising the following steps:

[0011] S1, preparing nitrogen-doped graphene;

[0012] S2, adding 0.7-0.9 g of citric acid and 0.03-0.07 g of the nitrogen-doped graphene obtained in step S1 into 40-60 ml of deionized water, continuously stirring, and adding 0.005-0.02 mol of MnC4H6O4·4H2O powder, 0.02-0.04 mol of NaC2H3O2 powder and 0.02-0.04 mol of NH4H2PO4 powder to obtain a mixed solution;

[0013] S3, adding 15-30 ml of an ethanol solution of C12H28O4Ti with a concentration of 0.0005 mol / ml into the mixed solution obtained in step S2, and removing the deionized water and ethanol at 80-90°C to obtain a gel precursor;

[0014] S4, placing the gel precursor obtained in step S3 in an oven at 90-110°C for drying for 6-12 h;

[0015] S5, placing the gel precursor dried for 6-12 h in step S4 in a protective atmosphere, and heating at 650-700°C for 10-14 h to obtain the nitrogen-doped graphene-doped NMTP@C material.

[0016] Preferably, the step S1 comprises:

[0017] S101, preparing graphene oxide by improving the Hummers method;

[0018] S102, adding sodium amide and the graphene oxide obtained in step S101 into benzene, continuously stirring at 20-30°C for 40-60 min to obtain a mixture;

[0019] S103, heating the mixture obtained in step S102 at 170-190°C for 14-16 h to obtain a powdery substance;

[0020] S104, placing the powdery substance obtained in step S103 in a microwave oven, and irradiating at 700 W for 90 s to obtain the nitrogen-doped graphene.

[0021] Preferably, the step S2 comprises:

[0022] 0.8 g of citric acid and 0.05 g of nitrogen-doped graphite are added to deionized water, stirring is continued, and 0.01 mol of MnC4H6O4·4H2O powder, 0.03 mol of NaC2H3O2 powder and 0.03 mol of NH4H2PO4 powder are added to obtain a mixed solution.

[0023] Preferably, the step S3 comprises:

[0024] 20 ml of 0.0005 mol / ml C12H28O4Ti ethanol solution is added to the mixed solution, deionized water and ethanol are removed at 85°C to obtain a gel precursor.

[0025] Preferably, the step S4 comprises:

[0026] The gel precursor is placed in a 100°C oven for drying for 10 h.

[0027] Preferably, the step S5 comprises:

[0028] The dried gel precursor is placed in an argon atmosphere and heated at 670°C for 12 h to obtain a nitrogen-doped graphene-doped NMTP@C material.

[0029] Preferably, the step S5 comprises:

[0030] The dried gel precursor is placed in an argon atmosphere and heated at 670°C for 12 h to obtain a nitrogen-doped graphene-doped NMTP@C material.

[0031] The present disclosure also provides the use of the nitrogen-doped graphene-doped NMTP@C material as described above as a sodium ion battery cathode.

[0032] The present disclosure also provides a sodium ion battery comprising the sodium ion battery cathode as described above.

[0033] The nitrogen-doped graphene-doped NMTP@C material, the preparation method and the use thereof according to the present disclosure have the following advantages:

[0034] In the core-shell structure, C forms a carbon film outside the NMTP, and nitrogen-doped graphene is doped outside the NMTP@C, which can inhibit the growth of NMTP crystals, improve the coulombic efficiency, and the nitrogen-doped graphene can form a network structure with C, thereby promoting the electrochemical performance of the nitrogen-doped graphene-doped NMTP@C material according to the present disclosure.

[0035] Meanwhile, the nitrogen-doped graphene-doped NMTP@C material described in the present disclosure has a larger surface area than the ordinary NMTP@C material, so that the nitrogen-doped graphene-doped NMTP@C material described in the present disclosure helps to promote the contact area between the electrolyte solution and the active material, thereby improving the reaction kinetics. BRIEF DESCRIPTION OF DRAWINGS

[0036] FIG. 1 is a preparation flowchart of a nitrogen-doped graphene-doped NMTP@C material described in the present disclosure;

[0037] FIG. 2a is a comparison of XRD images of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material described in the present disclosure; FIG. 2b is a nitrogen adsorption / desorption isotherm of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material described in the present disclosure; FIG. 2c is a TGA curve of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material described in the present disclosure; and FIG. 2d is a Raman spectrum of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material described in the present disclosure;

[0038] FIG. 3 is an XPS image of the nitrogen-doped graphene-doped NMTP@C material described in the present disclosure;

[0039] FIG. 4a and FIG. 4b are SEM images of the NMTP@C material, and FIG. 4c and FIG. 4d are SEM images of the nitrogen-doped graphene-doped NMTP@C material described in the present disclosure;

[0040] FIG. 5 is a comparison of TEM images of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material described in the present disclosure, in which FIG. 5a, FIG. 5b, and FIG. 5c correspond to the NMTP@C material, and FIG. 5d, FIG. 5e, and FIG. 5f correspond to the nitrogen-doped graphene-doped NMTP@C material;

[0041] FIG. 6a is a charge-discharge curve of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material described in the present disclosure at 0.1C; FIG. 6b is a cycle stability image of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material described in the present disclosure; FIG. 6c is a performance evaluation of the electrode obtained at different rates of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material described in the present disclosure; and FIG. 6d is a charge-discharge characteristic of the NG-NMTP@C at different rates;

[0042] Figure 7a is a cyclic voltammetry curve of the electrode prepared using the nitrogen-doped graphene-doped NMTP@C material of the present disclosure and the electrode prepared using the NMTP@C material at 0.1 mV / s; Figure 7b is an EIS curve of the electrode prepared using the nitrogen-doped graphene-doped NMTP@C material of the present disclosure and the electrode prepared using the NMTP@C material; Figure 7c is a charge-discharge curve of the electrode prepared using the nitrogen-doped graphene-doped NMTP@C material of the present disclosure and the electrode prepared using the NMTP@C material; Figure 7d is a long cycle performance of the electrode prepared using the nitrogen-doped graphene-doped NMTP@C material of the present disclosure and the electrode prepared using the NMTP@C material at 10.0 C;

[0043] Figure 8 is a rate capability comparison chart of the example and Comparative Examples 1-6. DETAILED DESCRIPTION

[0044] As shown in Figure 1, the nitrogen-doped graphene-doped NMTP@C material of the present disclosure is prepared by doping NMTP@C and nitrogen-doped graphene, and the NMTP@C is a core-shell structure nanofiber composite material with NMTP as the core and C as the shell.

[0045] Further, the NMTP is of NASICON structure, and the crystal structure feature of the NASICON structure is that it has an open three-dimensional framework, which is conducive to the migration and diffusion of ions. The nitrogen-doped graphene belongs to a two-dimensional graphene sheet, which generally has good electrical conductivity, structural flexibility and chemical stability. Doping it in the NMTP@C material to form a new material is conducive to improving the electrochemical performance of the NMTP material.

[0046] Further, the nitrogen-doped graphene-doped NMTP@C material is prepared by the following method:

[0047] S1, preparing nitrogen-doped graphene;

[0048] S2, adding 0.7-0.9 g of citric acid and 0.03-0.07 g of the nitrogen-doped graphene obtained in step S1 to 40-60 ml of deionized water, continuously stirring, and adding 0.005-0.02 mol of MnC4H6O4·4H2O powder, 0.02-0.04 mol of NaC2H3O2 powder and 0.02-0.04 mol of NH4H2PO4 powder to obtain a mixed solution;

[0049] S3, adding 15-30 ml of an ethanol solution of C12H28O4Ti with a concentration of 0.0005 mol / ml to the mixed solution obtained in step S2, and removing the deionized water and ethanol at 80-90°C to obtain a gel precursor;

[0050] S4, drying the gel precursor obtained in step S3 in an oven at 90-110°C for 6-12h;

[0051] S5, placing the gel precursor dried in step S4 in a protective atmosphere and heating at 650-700°C for 10-14h to obtain a nitrogen-doped graphene-doped NMTP@C material.

[0052] Example 1

[0053] The nitrogen-doped graphene-doped NMTP@C material described in the present disclosure is prepared by the following steps:

[0054] S1, preparing nitrogen-doped graphene;

[0055] S2, adding 0.8g of citric acid and 0.05g of nitrogen-doped graphene to deionized water, continuously stirring, and adding 0.01mol of MnC4H6O4·4H2O powder, 0.03mol of NaC2H3O2 powder and 0.03mol of NH4H2PO4 powder to obtain a mixed solution;

[0056] S3, adding 20ml of 0.0005mol / ml C12H28O4Ti ethanol solution to the mixed solution, removing the deionized water and ethanol at 85°C to obtain a gel precursor;

[0057] S4, placing the gel precursor in a 100°C oven for drying for 10h;

[0058] S5, placing the dried gel precursor in an argon atmosphere and heating at 670°C for 12h to obtain a nitrogen-doped graphene-doped NMTP@C material.

[0059] In step S1, the following steps are included:

[0060] S101, preparing graphene oxide by improving the Hummers method;

[0061] S102, adding sodium amide and the graphene oxide obtained in step S101 to benzene, continuously stirring at 20-30°C for 40-60min to obtain a mixture;

[0062] S103, heating the mixture obtained in step S102 at 170-190°C for 14-16h to obtain a powder-like substance;

[0063] S104, placing the powder-like substance obtained in step S103 in a microwave oven and irradiating at 700W for 90s to obtain nitrogen-doped graphene.

[0064] In the step S101, the improved Hummers method can refer to the paper published by Wang Lu in Packaging Journal, DOI: 10.3969 / j.issn.1674-7100.2015.02.006.

[0065] Comparative Example 1

[0066] The NMTP@C material is prepared by the following steps:

[0067] S1, 0.8g of citric acid and 0.05g of graphite powder are added into deionized water, continuously stirred, and 0.01mol of MnC4H6O4·4H2O powder, 0.03mol of NaC2H3O2 powder and 0.03mol of NH4H2PO4 powder are added to obtain a mixed solution;

[0068] S2, 20ml of 0.0005mol / ml C12H28O4Ti ethanol solution is added into the mixed solution, and deionized water and ethanol are removed at 85℃ to obtain a gel precursor;

[0069] S3, the gel precursor is placed in a 100℃ oven for drying for 10h;

[0070] S4, the dried gel precursor is placed in an argon atmosphere and heated at 670℃ for 12h to obtain the NMTP@C material.

[0071] Meanwhile, according to the retrieved papers, several disclosed NMTP materials are prepared; including the paper entitled “Over three-electron reaction in Na3+2xMn1+xTi1-x(PO4)3NASICON cathode with high energy density for sodium-ion batteries”, DOI: 10.1039 / d1ta01148k, which is taken as Comparative Example 2; the paper entitled “Symmetric sodium-ion batteries with dual-electron reaction based on NASICON-structured Na3MnTi(PO4)3 material”, DOI: 10.1021 / acsami.0c05784, which is taken as Comparative Example 3; the paper entitled “Highly efficient, fast and reversible multi-electron reaction of Na3MnTi(PO4)3 cathode for sodium-ion batteries”, DOI: 10.1016 / j.ensm.2019.11.004, which is taken as Comparative Example 4; the paper entitled “Yeast-template derived multi-electron reaction NASICON-structured Na3MnTi(PO4)3 for high-performance sodium-ion batteries”, DOI: 10.1021 / acsami.1c17700, which is taken as Comparative Example 5, and the paper entitled “In-situ preparation of novel carbon nanotube-promoted Na3MnTi(PO4)3@C electrode for high-performance sodium-ion storage”, DOI: 10.1016 / j.jpcs.2024.111911, which is taken as Comparative Example 6, and Comparative Examples 2-6 are compared with the nitrogen-doped graphene-doped NMTP@C material of the present disclosure in terms of rate capability and rate capability curves are drawn.

[0072] And the nitrogen-doped graphene-doped NMTP@C material of the present disclosure is compared with the conventional NMTP@C material in various properties.

[0073] The comparison includes: comparison of the properties of the materials themselves, and the nitrogen-doped graphene-doped NMTP@C material and the conventional NMTP@C material are made into battery electrodes, and the performance of the electrodes made of the above two materials are tested by CR2025 battery test and electrochemical workstation.

[0074] The comparison of the properties of the materials themselves includes comparison of XRD (X-ray diffraction), TGA (thermogravimetric analysis), Raman spectrum, XPS (X-ray photoelectron spectroscopy), SEM (scanning electron microscope), TEM (transmission electron microscope) and other experiments;

[0075] As shown in FIG. 2a, the XRD images of the nitrogen-doped graphene-doped NMTP@C material of the present disclosure and the NMTP@C material are compared, FIG. 2c is a TGA curve of the nitrogen-doped graphene-doped NMTP@C material of the present disclosure and the NMTP@C material, and FIG. 2d is a Raman spectrum of the nitrogen-doped graphene-doped NMTP@C material of the present disclosure and the NMTP@C material;

[0076] The carbon content of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material was evaluated by TGA. As can be seen from FIG. 2c, the carbon content of the NMTP@C material is 4.98 wt%, and the carbon content of the nitrogen-doped graphene-doped NMTP@C material is 5.76 wt%.

[0077] As can be seen from the Raman spectrum of FIG. 2d, the D peak and the G peak of the nitrogen-doped graphene-doped NMTP@C material are relatively flat compared to the D peak and the G peak of the NMTP@C. The ID / IG of the nitrogen-doped graphene-doped NMTP@C material is 1.11, and the ID / IG of the NMTP@C material is 1.15, as calculated by software.

[0078] The ID / IG ratio provides important information about the defect density and the degree of graphitization in the material. The larger the ratio, the higher the proportion of defects or amorphous carbon structures in the sample relative to ordered graphite or graphene structures, indicating that the material has a lower degree of graphitization and a higher abundance of defects. On the contrary, a smaller ID / IG ratio indicates that the material is relatively pure, with fewer defects and a higher degree of graphitization. The more abundant the defects, the greater the impact on the transmission efficiency of electrons and thus the conductivity of the material. Therefore, according to the above data, it can be seen that the nitrogen-doped graphene-doped NMTP@C material of the present disclosure has better conductivity than the NMTP@C material of Comparative Example 1.

[0079] In addition, FIG. 2b shows the nitrogen adsorption / desorption isotherm of the nitrogen-doped graphene-doped NMTP@C material and the NMTP@C material of the present disclosure. In both samples, a typical IV curve with a hysteresis loop can be seen, proving that the designed composite material has a mesoporous nanostructure.

[0080] In the nitrogen adsorption / desorption isotherm, according to the BET theory (Brunauer-Emmett-Teller theory), the surface area of the NMTP@C material is about 25.8 m2 / g, and the surface area of the nitrogen-doped graphene-doped NMTP@C material of the present disclosure is about 29.6 m2 / g. That is, the surface area of the nitrogen-doped graphene-doped NMTP@C material of the present disclosure is greater than that of Comparative Example 1, and a larger surface area helps to promote the contact area between the electrolyte solution and the active material, thereby improving the reaction kinetics and ultimately improving the conductivity.

[0081] FIG. 3 is an XPS graph of the nitrogen-doped graphene-doped NMTP@C material of the present disclosure.

[0082] As can be seen from FIG. 3a, the nitrogen-doped graphene-doped NMTP@C material contains all elements such as titanium, sodium, manganese, oxygen, carbon, phosphorus, nitrogen, etc. As shown in FIG. 3b, the Mn2p peak shows two typical patterns at 653.5 and 640.7 eV, which correspond to Mn2p1 / 2 and Mn2p3 / 2, respectively. According to FIGS. 3d and 3e, the patterns at about 399.2 and 284.6 eV are assigned to N1 and c1, indicating the presence of nitrogen-doped graphene and carbon film in the NG-NMTP@C nanocomposite.

[0083] For the NMTP@C 6 sample (FIGS. 4a and 4b), the crystal size thereof is about 100-200 nm. The nanoparticles can shorten the transmission path of sodium ions during the process. As can be seen from FIGS. 4c and 4d, the nitrogen-doped graphene sheets are dispersed in the NMTP@C particles. In addition, the crystal size thereof is slightly smaller than that of NMTP@C, indicating that these added nitrogen-doped graphene sheets can inhibit the growth of NMTP crystals at high temperatures. Inhibiting the growth of NMTP crystals can effectively improve the coulombic efficiency and reduce the electron loss. In order to study the nanostructure of the obtained crystals, transmission electron microscopy is used for measurement, as shown in FIGS. 5a-5c. For the NMTP@C material, it can be found that the surface of the NMTP crystal is successfully coated with an amorphous carbon layer. According to FIGS. 5d-5f, in the nitrogen-doped graphene-doped NMTP@C material, carbon film and nitrogen-doped graphene can be detected; and the inventors have unexpectedly found that the nitrogen-doped graphene sheets and the carbon film can form a network structure, and this network structure can promote the conductivity of the electrode.

[0084] The nitrogen-doped graphene-doped NMTP@C material and the conventional NMTP@C material are made into battery electrodes, and the performance of the electrodes made of the above two materials is tested by CR2025 battery test and electrochemical workstation;

[0085] The electrodes are made as follows:

[0086] The weight ratio of acetylene black, polyvinylidene fluoride (PVDF) and nitrogen-doped graphene-doped NMTP@C material is 20:10:70, and a slurry-like mixture is prepared. The slurry-like mixture is poured onto a clean aluminum film and dried at 115°C for 18h to obtain an electrode.

[0087] The method of using NMTP@C material to make electrodes is similar to the above electrode preparation method, and only the nitrogen-doped graphene-doped NMTP@C is replaced by NMTP@C of the same proportion.

[0088] The above two electrodes are used as cathodes of sodium ion batteries, the electrolyte of the sodium ion battery is NaClO4, and metal sodium is used as an anode for testing.

[0089] FIGS. 6 and 7 are images data obtained from corresponding tests of the prepared electrodes, respectively.

[0090] FIG. 6a shows the charge / discharge curves of the prepared electrode at 0.1C. Apparently, two platforms can be clearly detected in the initial charge process, which represents the double electron reaction. However, three flat platforms at about 4.1, 3.6 and 2.1 V were found in the second charge curve [16, 18]. Different from the first and second discharge cycles, three platforms centered at about 4.03, 3.53 and 2.16 V can be found, which are assigned to three Na+insertion processes

[0016] . Here, the reversible capacity of NG-NMTP@C (173.1 mAh / g) is superior to that of NMTP@C (166.5 mAh / g). After 50 cycles (FIG. 6b), the high capacity retention of NG-NMTP@C cathode is about 98.3%, while that of NMTP@C electrode is only 96.6%. The high-rate performance of the obtained samples is shown in FIG. 6c. As expected, the NG-NMTP@C cathode has higher specific capacity than NMTP@C at each rate. It accordingly provides capacities of 165.8, 158.2, 147.5 and 125.2 mAh / g at 0.5, 1.0, 2.0 and 5.0 C. FIG. 6d gives the charge-discharge curves of NG-NMTP@C at different densities. Apparently, NG-NMTP@C exhibits less polarization with the increase of current density due to its superior conductivity. FIG. 7a shows the CV curves of NMTP@C and NG-NMTP@C at 0.1 mV / s. Apparently, both electrodes have three pairs of redox peaks. The three peaks are attributed to the redox couples of Mn 3+ / Mn 4+, Mn 2+ / Mn 3+and Ti3+ / Ti4+[13, 17, 19]. Compared with NMTP@C electrode material, the prepared NG-NMTP@C has larger redox mode area, indicating that NG-NMTP@C has higher capacity. At the same time, the oxidation mode of NG-NMTP@C moves to small value, and the reduction mode moves to high value, showing low polarization. EIS test (FIG. 7b) is introduced to study the battery behavior of the obtained electrode. Apparently, the charge transfer resistance of NG-NMTP@C is 144 Ω, which is lower than that of NMTP@C (239 Ω). The low impedance of NG-NMTP@C electrode can be attributed to the presence of conductive nitrogen-doped graphene. This small resistance can well change the transport of electrons in the charge and discharge process

[0030] . The diffusion coefficient of DNa+of NMTP@C and NG-NMTP@C can be calculated according to the following formula:

[0091] D Na+ =R 2 T 2 / 2A 2 F4 n 4 C 2 σ 2 (1)

[0092] Z’=R e +R ct +σω -1 / 2 (2)

[0093] where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode material, F is the Faraday constant, n is the number of electrons in the oxidation process, C is the Na-ion concentration, and σ is a factor related to Z’ and ω-1 / 2. According to the above formula, the Na+ion diffusion coefficient of NMTP@C is 3.96x10 -12 , and the Na+ion diffusion coefficient of nitrogen-doped graphene-doped NMTP@C material is 5.17x10 -10 cm2s-1, indicating that nitrogen-doped graphene has a positive effect on the Na+ion diffusion coefficient of NMTP. The charge-discharge curves and long cycle stability of NMTP@C and nitrogen-doped graphene-doped NMTP@C cathodes at 10.0C are shown in Figures 7c and d. For the nitrogen-doped graphene-doped NMTP@C cathode, a platform in the charge-discharge curve was observed. In contrast, the NMTP@C electrode showed greater polarization due to its lower electrode rate. As can be seen from Figure 7d, the capacity of the NG-NMTP@C electrode was 98.7mAh / g, and the retention rate was about 97.1% after 400 cycles at 10.0C, while the capacity retention rate of the NMTP@C electrode was only 90.5%. The results show that nitrogen-doped graphene can significantly improve the battery performance of NMTP@C material.

[0094] As can be seen from Figure 8, the rate performance of the nitrogen-doped graphene-doped NMTP@C material of the present disclosure is better than that of Comparative Examples 1-6 before 3C, and only slightly worse than that of Comparative Example 4 after 3C. The excellent rate performance of the nitrogen-doped graphene-doped NMTP@C material of the present disclosure can be attributed to the following aspects:

[0095] (1) The small crystals of NMTP can reduce the transport path of Na+in the charge-discharge process;

[0096] (2) The carbon film formed can change the conductivity of NMTP;

[0097] (3) The nitrogen-doped graphene sheets can further improve the conductivity of NMTP, which can significantly promote the transmission of electrons.

[0098] In summary, the nitrogen-doped graphene-doped NMTP@C material has good electrochemical performance, and the growth of the NMTP crystal is inhibited due to the nitrogen-doped graphene, so that the nitrogen-doped graphene-doped NMTP@C material is safer when applied to a sodium ion battery.

[0099] Further, the present disclosure also provides an application of the nitrogen-doped graphene-doped NMTP@C material as described above, which is used as a sodium ion battery cathode.

[0100] Further, the present disclosure also provides a sodium ion battery, which comprises the sodium ion battery cathode as described above.

[0101] In the description of the present disclosure, it should be understood that the orientation words such as "front, back, up, down, left, right", "transverse, vertical, horizontal" and "top, bottom" and the like indicate the orientation or positional relationship shown in the drawings, which are only for the convenience of describing the present disclosure and simplifying the description, and do not indicate and imply that the devices or elements referred to must have a particular orientation or be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the protection scope of the present disclosure.

[0102] For those skilled in the art, other various corresponding changes and modifications can be made according to the technical solutions and concepts described above, and all of these changes and modifications should belong to the protection scope of the claims of the present disclosure.

Claims

1. A nitrogen-doped graphene-doped NMTP@C material, characterized in that, The NMTP@C is a core-shell structure nanofiber composite material with NMTP as the core and C as the shell. 2.The nitrogen-doped graphene-doped NMTP@C material of claim 1, wherein, The NMTP is a NASICON structure.

3. A preparation method of a nitrogen-doped graphene-doped NMTP@C material, characterized in that, The method comprises the following steps: S1, preparing nitrogen-doped graphene; S2, adding 0.7-0.9 g of citric acid and 0.03-0.07 g of the nitrogen-doped graphene obtained in step S1 to 40-60 ml of deionized water, continuously stirring, and adding 0.005-0.02 mol of MnC4H6O4·4H2O powder, 0.02-0.04 mol of NaC2H3O2 powder and 0.02-0.04 mol of NH4H2PO4 powder to obtain a mixed solution; S3, adding 15-30 ml of an ethanol solution of C12H28O4Ti with a concentration of 0.0005 mol / ml to the mixed solution obtained in step S2, and removing the deionized water and ethanol at 80-90 DEG C to obtain a gel precursor; S4, placing the gel precursor obtained in step S3 in an oven at 90-110 DEG C for drying for 6-12 h; S5, placing the gel precursor dried for 6-12 h in step S4 in a protective atmosphere, heating at 650-700 DEG C for 10-14 h to obtain a nitrogen-doped graphene-doped NMTP@C material.

4. The preparation method according to claim 3, characterized in that, The step S1 comprises: S101, preparing graphene oxide by improving the Hummer method; S102, adding sodium amide and the graphene oxide obtained in step S101 to benzene, continuously stirring at 20-30 DEG C for 40-60 min to obtain a mixture; S103, heating the mixture obtained in step S102 at 170-190 DEG C for 14-16 h to obtain a powdery substance; S104, placing the powdery substance obtained in step S103 in a microwave oven, irradiating at 700 W for 90 s to obtain nitrogen-doped graphene.

5. The preparation method according to claim 3, characterized in that, The step S2 comprises: adding 0.8 g of citric acid and 0.05 g of nitrogen-doped graphene to deionized water, continuously stirring, and adding 0.01 mol of MnC4H6O4·4H2O powder, 0.03 mol of NaC2H3O2 powder and 0.03 mol of NH4H2PO4 powder to obtain a mixed solution.

6. The preparation method according to claim 3, characterized in that, The step S3 comprises: adding 20 ml of an ethanol solution of C12H28O4Ti with a concentration of 0.0005 mol / ml to the mixed solution, and removing the deionized water and ethanol at 85 DEG C to obtain a gel precursor.

7. The preparation method according to claim 3, characterized in that, The step S4 comprises: placing the gel precursor in an oven at 100 DEG C for drying for 10 h.

8. The preparation method according to claim 3, characterized in that, The step S5 comprises: placing the dried gel precursor in an argon atmosphere, heating at 670 DEG C for 12 h to obtain a nitrogen-doped graphene-doped NMTP@C material.

9. Use of the nitrogen-doped graphene-doped NMTP@C material according to any one of claims 1 to 2 or of the nitrogen-doped graphene-doped NMTP@C material produced according to any one of claims 3 to 8, characterized in that, It is used as a sodium ion battery cathode.

10. A sodium-ion battery, characterized in that, The sodium ion battery cathode of claim 9 is included.

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