Phosphorus-doped nitrogen-rich porous carbon nanosheet for regulating nitrogen activity, and preparation method and application of phosphorus-doped nitrogen-rich porous carbon nanosheet

Abstract

The invention provides a phosphorus-doped nitrogen-rich porous carbon nanosheet for regulating nitrogen activity as well as a preparation method and application of the phosphorus-doped nitrogen-rich porous carbon nanosheet, and belongs to the technical field of preparation of negative electrode materials. The preparation method of the phosphorus-doped nitrogen-rich porous carbon nanosheet for regulating and controlling the nitrogen activity comprises the following steps: adding a metal source solution into an amine compound solution, stirring to obtain a precipitate, washing and drying to obtain a nanosheet precursor, then heating in an inert gas atmosphere, preserving heat to obtain a two-dimensional N / C nanosheet, and finally phosphating the two-dimensional N / C nanosheet, the amine compound comprises hexamethylenetetramine. The phosphorus-doped nitrogen-rich porous carbon nanosheet for regulating and controlling the nitrogen activity has relatively large interlayer spacing and rich pore defects and edge sites, can adsorb more Na < + >, can effectively reduce the diffusion distance of Na < + >, and has excellent conductivity as a negative electrode material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of negative electrode material preparation, and particularly relates to a phosphorus-doped nitrogen-rich porous carbon nanosheet, a preparation method and application thereof. BACKGROUND

[0002] With the surge in demand for green energy, it is crucial to develop high-capacity and high-rate performance energy storage devices. Sodium-ion batteries (SIBs) are considered as an ideal alternative to lithium-ion batteries (LIBs) due to their abundant sodium reserves, low cost and similar electrochemical properties to lithium. However, due to the larger radius of Na + than Li + , it leads to slow reaction kinetics, significant volume expansion, fast capacity decay and poor rate performance during the cycling process. Therefore, it is still a focus of research in the field to find SIBs negative electrode materials with high structural stability and fast reaction kinetics. Currently reported SIBs negative electrode materials mainly include carbon materials, metal oxides (MOs), metal chalcogenides (MCs), metal phosphides (MPs) and alloy materials. Among them, MOs, MCs, MPs and alloy materials are prone to volume expansion during the cycling process and have poor kinetic performance, which is not conducive to maintaining the stability of structure and performance. In contrast, carbon materials have attracted widespread attention in the field of SIBs due to their low cost, abundant resources, environmental friendliness, high electronic conductivity and adjustable interlayer spacing, but they have the problem of poor sodium storage performance.

[0003] Methods for improving the sodium storage performance of carbon materials usually include constructing pore structures, heteroatom doping and preparing carbon-based nanocomposites. Among them, constructing pore structures can shorten the diffusion path of ions, but may reduce the volumetric energy density; preparing carbon-based composite materials can improve the capacity through composite synergy, but may face problems of interface compatibility and cycling stability; heteroatom doping, especially N doping, can effectively adjust the surface functional groups, local electronic structure and chemical properties, improve the electronic conductivity, electrolyte wettability and produce stronger sodium storage active sites, thereby improving the sodium storage performance. However, conventional doping has limited regulation of defect sites, which affects the further improvement of the energy storage performance of sodium-ion batteries.

[0004] Therefore, it is urgent to provide N-doped carbon materials with sufficient defects and edge sites to improve the sodium storage performance and thereby improve the performance of the prepared batteries. SUMMARY

[0005] ​In view of the deficiencies of the prior art, the present application provides a phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet, a preparation method and application thereof.The phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet has a large interlayer spacing, rich pore defects and edge sites, can adsorb more Na + , and can effectively reduce the diffusion distance of Na + , and has excellent conductivity as a negative electrode material.

[0006] The technical scheme of the present application is as follows:

[0007] The present application provides a preparation method of a phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet, comprising the following steps:

[0008] S1: adding a metal source solution into an amine compound solution, stirring to obtain a precipitate;

[0009] S2: washing and drying the precipitate to obtain a nanosheet precursor;

[0010] S3: placing the nanosheet precursor in a container, heating under an inert gas atmosphere, and then keeping warm to obtain a two-dimensional N / C nanosheet;

[0011] S4: phosphorizing the two-dimensional N / C nanosheet to obtain a phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet;

[0012] In S1, the amine compound comprises hexamethylenetetramine.

[0013] Preferably, in S1, the metal source comprises a metal nitrate;

[0014] Preferably, the metal nitrate is Cd(NO3)2·4H2O.

[0015] Preferably, in S1, the mass concentration of the metal source solution is 2-10%, and the mass concentration of the amine compound solution is 2-8%.

[0016] Preferably, in S1, the molar ratio of the metal source to the amine compound is 1-1.5:1.

[0017] The stirring temperature is 25-45 DEG C, and the stirring time is 18-30 h.

[0018] Preferably, in S2, the drying temperature is 60-80 DEG C, and the time is 8-12 h.

[0019] Preferably, in S3, the temperature is raised to 850-1000 DEG C at a rate of 2-4 DEG C / min.

[0020] The keeping warm time is 1-3 h.

[0021] Preferably, in S4, the method for phosphorizing the two-dimensional N / C nanosheet is: placing the two-dimensional N / C nanosheet and a phosphorus-containing compound in a calcination device, vacuumizing, then introducing argon, treating at constant temperature for 0.5-1 h, then increasing the temperature to a calcination temperature, and keeping the temperature.

[0022] Preferably, the phosphorus-containing compound comprises NaH2PO2·H2O; the temperature for the constant temperature treatment is 25-35℃; the speed for the temperature increase is 2-5℃ / min; the calcination temperature is 350-450℃; and the time for the temperature keeping is 2-5 h.

[0023] The second aspect of the present application provides a phosphorus-doped nitrogen-rich porous carbon nanosheet prepared by the preparation method of the first aspect, wherein the content of N element in the phosphorus-doped nitrogen-rich porous carbon nanosheet is 6.52-8.52%, and the content of P element is 3.52-5.52%; and / or,

[0024] The specific surface area of the phosphorus-doped nitrogen-rich porous carbon nanosheet is 1242-1343 m 2 / g.

[0025] The third aspect of the present application provides an application of the phosphorus-doped nitrogen-rich porous carbon nanosheet prepared by the preparation method of the first aspect or the phosphorus-doped nitrogen-rich porous carbon nanosheet of the second aspect, wherein the nanosheet is used as a negative electrode material of a sodium ion battery.

[0026] The present application has the following beneficial technical effects:

[0027] The present application obtains a phosphorus-doped nitrogen-rich porous carbon nanosheet through pyrolysis and phosphorization of a nitrogen-rich metal organic framework; the nanosheet with a MOF-like structure is prepared first, the nanosheet has a large specific surface area and more C-N structures, which can increase the number of active sites for storage, and then phosphorus is doped to further expand the interlayer spacing; the two-dimensional porous structure of the prepared nanosheet has a large interlayer spacing and specific surface area, rich pore defects and edges, which can effectively reduce the Na + diffusion distance, promote the Na + diffusion kinetics; as a negative electrode material, the P-N / C nanosheet has a larger specific surface area, excellent electrical conductivity and higher pyrrole-N and pyridine-N content, which can expose more accessible active sites, increase the Na + adsorption reaction on the interface, and also can accelerate the electron transfer, promote the charge transfer kinetics in the cycle process, and improve the specific capacity of the prepared battery; and the prepared battery has excellent electrochemical performance. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1Scanning electron microscope (SEM) image of the two-dimensional porous N / C nanosheets prepared for Example 1 of the present application.

[0029] Figure 2 Transmission electron microscope (TEM) image of the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheets prepared for Example 2 of the present application.

[0030] Figure 3 SEM image of the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheets prepared for Example 1 of the present application.

[0031] Figure 4 TEM-EDS image and Mapping image of C, N and P elements of the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheets prepared for Example 2 of the present application.

[0032] Figure 5 SEM image of the nitrogen-rich porous carbon nanosheets prepared for Comparative Example 1 of the present application.

[0033] Figure 6 SEM image of the nitrogen-rich porous carbon nanosheets prepared for Comparative Example 2 of the present application.

[0034] Figure 7 SEM image of the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheets prepared for Comparative Example 3 of the present application.

[0035] Figure 8 SEM image of the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheets prepared for Comparative Example 4 of the present application.

[0036] Figure 9 SEM image of the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheets prepared for Comparative Example 5 of the present application.

[0037] Figure 10 TEM image of the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheets prepared for Comparative Example 6 of the present application.

[0038] Figure 11 XRD image of the products obtained for Example 1 and Comparative Example 7 of the present application.

[0039] Figure 12 Sodium ion battery rate performance curve prepared by the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet electrode prepared for Example 1 of the present application.

[0040] Figure 13 Sodium ion battery cycle performance curve prepared by the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet electrode prepared for Comparative Example 7 of the present application. DETAILED DESCRIPTION

[0041] The present application will be described in detail below with reference to examples.

[0042] Based on the problems of uncontrollable structure, less active sites and lower reversible capacity existing in the prior art of constructing pore structure, heteroatom doping and preparing carbon-based nanocomposites of carbonaceous materials, the present application provides a phosphorus-doped nitrogen-rich porous carbon nanosheet, a preparation method and application thereof.

[0043] The present application provides a preparation method of a phosphorus-doped nitrogen-rich porous carbon nanosheet in the first aspect, comprising the following steps:

[0044] S1: adding a metal source solution into an amine compound solution, stirring to obtain a precipitate;

[0045] S2: washing and drying the precipitate to obtain a nanosheet precursor;

[0046] S3: placing the nanosheet precursor in a container, heating under an inert gas atmosphere, and then keeping warm to obtain a two-dimensional N / C nanosheet;

[0047] S4: phosphorizing the two-dimensional N / C nanosheet to obtain a phosphorus-doped nitrogen-rich porous carbon nanosheet;

[0048] In S1, the amine compound includes hexamethylenetetramine.

[0049] The amine source hexamethylenetetramine (HMT, Adamas-beta) used in the present application has a cyclic skeleton: the cyclic structure of hexamethylenetetramine is azacycloheptane, that is, one nitrogen atom is inserted into a six-membered carbon ring to form a structure similar to piperidine, and a metal hexamine framework precursor is formed by hydrogen bonding in an ethanol solution.

[0050] It can be understood that the present application is obtained by forming a metal hexamine framework through hydrogen bonding of a metal salt and hexamethylenetetramine (HMT) in a solvent, then washing and drying to obtain a metal hexamine framework precursor, metal salt-HMT MHF (metal hexamine framework); pyrolyzing the metal salt-HMT MHF in an inert atmosphere to obtain a two-dimensional nitrogen-rich porous carbon nanosheet N / C; and finally phosphorizing the nanosheet N / C with a phosphorus-containing compound in an inert atmosphere to obtain a phosphorus-doped nitrogen-rich porous carbon nanosheet with nitrogen activity regulation. Specifically: the present application obtains a phosphorus-doped nitrogen-rich porous carbon nanosheet through pyrolysis and phosphorization of a N-containing metal framework. By first preparing a nanosheet with a MOF-like structure, the nanosheet has a large specific surface area and more C-N structures, which can increase the number of active sites for storage, thereby improving the sodium storage effect, and then doping with phosphorus to further expand the interlayer spacing, the two-dimensional porous structure of the prepared nanosheet has a large interlayer spacing and specific surface area, rich pore defects and edges, which can effectively reduce the Na + diffusion distance and promote Na +Diffusion kinetics; the P-N / C nanosheet as a negative electrode material has a larger specific surface area, excellent electrical conductivity and higher pyrrole-N and pyridine-N content, thereby exposing more accessible active sites, increasing the interface Na + adsorption reaction, and can also accelerate electron transfer, promote charge transfer kinetics during the cycle process, and improve the specific capacity of the prepared battery.

[0051] The preparation method of the application can prepare phosphorus-doped nitrogen-rich porous carbon nanosheets, and can solve the problems of low pyrrole-N and pyridine-N content, poor activity, complex preparation process and environmental pollution.

[0052] In some embodiments of the application, in S1, the metal source includes metal nitrate.

[0053] Preferably, the metal nitrate is Cd(NO3)2.4H2O.

[0054] It can be understood that the purity of Cd(NO3)2.4H2O used in the application is ≥98%, preferably the Cd(NO3)2.4H2O of Aladdin. Compared with other metal salts, this compound can better interact with HMT to form a porous structure.

[0055] In some embodiments of the application, in S1, the mass concentration of the metal salt solution is 2-10%; the mass concentration of the amine compound solution is 2-8%.

[0056] In some embodiments of the application, in S1, the molar ratio of the metal source to the amine compound is 1-1.5:1.

[0057] It can be understood that when the metal source is too much or the amine compound is too much, the obtained product will appear agglomeration due to intermolecular interaction, so the ratio of the metal source and the amine compound needs to be limited to obtain a more optimal porous structure. At the same time, the appropriate ratio can also improve the yield of the obtained metal hexamine precursor.

[0058] In some embodiments of the application, in S1, the stirring temperature is 25-45 DEG C, including but not limited to 25 DEG C, 30 DEG C, 35 DEG C, 40 DEG C, 45 DEG C, and the stirring time is 18-30 h, including but not limited to 18 h, 20 h, 22 h, 24 h, 26 h, 28 h, 30 h.

[0059] It can be understood that in the application, the metal source solution and the amine compound solution can be mixed immediately to obtain a precipitate, and by further stirring, more metal hexamine framework precursors can be generated.

[0060] In some embodiments of the present application, in S2, the temperature of the drying is 60-80℃, including but not limited to 60℃, 70℃, 80℃, and the time is 8-12h, including but not limited to 8h, 9h, 10h, 11h, 12h.

[0061] It can be understood that, in the present application, the metal source and the amine compound are in an ethanol solution, and a metal hexamine framework is formed by hydrogen bonding, and the metal hexamine framework precursor is obtained by washing and drying; further pyrolysis obtains two-dimensional nitrogen-rich porous carbon nanosheets N / C, and by controlling the pyrolysis temperature, the structure of the obtained material can be well controlled; when the pyrolysis temperature is too low, the intermolecular force cannot be effectively released, and therefore the obtained material may appear to be agglomerated; but when the pyrolysis temperature is too high, the porous structure has been formed or unnecessary energy loss and resource waste are caused.

[0062] In some embodiments of the present application, in S3, the temperature is raised to 850-1000℃ at a rate of 2-4℃ / min, including but not limited to 850℃, 900℃, 950℃, 1000℃; and the holding time is 1-3h, including but not limited to 1h, 1.5h, 2h, 2.5h, 3h.

[0063] In some embodiments of the present application, in S4, the specific method for phosphating the two-dimensional N / C nanosheets is as follows: the two-dimensional N / C nanosheets and a phosphorus-containing compound are placed in a calcination device, vacuum is drawn, argon is introduced, constant-temperature treatment is performed for 0.5-1h, the temperature is raised to a calcination temperature, and holding is performed.

[0064] In some embodiments of the present application, the phosphorus-containing compound includes NaH2PO2·H2O.

[0065] In some embodiments, the two-dimensional N / C nanosheets and the phosphorus-containing compound can be placed at two ends of the same container, and then the container is placed in a calcination device; or the two substances can be placed in different containers, and the two containers are placed in the calcination device. The container includes a boat, and the calcination device can be a tube furnace.

[0066] In some embodiments, regardless of one container or two containers, the phosphorus-containing compound is placed upstream of the gas flow, and the two-dimensional N / C nanosheet powder is placed downstream of the gas flow. When the temperature is raised, the phosphorus-containing compound decomposes to form a phosphorus-containing gas, which can react with the two-dimensional N / C nanosheet downstream to achieve phosphating.

[0067] In some embodiments, the temperature of the constant-temperature treatment is 25-35℃, including but not limited to 25℃, 30℃, 35℃.

[0068] In some embodiments, the temperature increasing rate is 2-5℃ / min, including but not limited to 2℃ / min, 2.5℃ / min, 3.0℃ / min, 3.5℃ / min, 4.0℃ / min, 4.5℃ / min, 5℃ / min. When the temperature increasing rate is 3℃ / min, the obtained material has better effect.

[0069] In some embodiments, the calcination temperature is 350-450℃, including but not limited to 350℃, 400℃, 450℃; the holding time is 3-5h, including but not limited to 3h, 4h, 5h. When the holding time is 4h, the obtained material has better effect.

[0070] The second aspect of the present application provides a phosphorus-doped nitrogen-activated nitrogen-rich porous carbon nanosheet prepared by the preparation method of the first aspect, wherein the content of N element is 6.52-8.52%, and the content of P element is 3.52-5.52%.

[0071] In some embodiments of the present application, the specific surface area of the phosphorus-doped nitrogen-activated nitrogen-rich porous carbon nanosheet is 1242-1343m 2 / g.

[0072] The P-N / C prepared by the present application has a large specific surface area, a unique two-dimensional porous structure, and excellent electrical conductivity. The content of active nitrogen is adjusted by phosphorus atom doping, which provides more active sites for the conduction of electrons, thereby providing excellent electrochemical performance.

[0073] The present application also provides an application of the phosphorus-doped nitrogen-activated nitrogen-rich porous carbon nanosheet prepared by the preparation method of the first aspect or the phosphorus-doped nitrogen-activated nitrogen-rich porous carbon nanosheet of the second aspect, wherein the nanosheet is used as a negative electrode material.

[0074] The present application also provides a sodium ion battery negative electrode, which comprises a current collector and an active component, and the active component comprises the phosphorus-doped nitrogen-activated nitrogen-rich porous carbon nanosheet.

[0075] In some embodiments, a preparation method of the above-mentioned sodium ion negative electrode is provided, which comprises: ball-milling the phosphorus-doped nitrogen-activated nitrogen-rich porous carbon nanosheet into a powder, uniformly mixing the powder with a binder and conductive carbon black, adding a solvent, uniformly mixing to form a gel, and rolling the gel onto a current collector to obtain a negative electrode.

[0076] The present application also provides a sodium ion battery, which comprises the above-mentioned sodium ion battery negative electrode, a counter electrode, a separator, and an electrolyte.

[0077] In some embodiments, the counter electrode is metallic sodium.

[0078] In some embodiments, the separator comprises a glass fiber membrane.

[0079] In some embodiments, the electrolyte is a solution of sodium hexafluorophosphate (KPF6) dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC), wherein the volume ratio of EC to DEC is 1:1 and the concentration of KPF6 in the electrolyte is 0.8 mol / L.

[0080] In some embodiments, the sodium-ion battery is a 2025 button cell.

[0081] The application is further illustrated below by examples.

[0082] Example 1

[0083] A phosphorus-doped nitrogen-rich porous carbon nanosheet with nitrogen activity regulation, and a preparation method thereof, comprises the following steps:

[0084] (1) Preparation of two-dimensional N / C nanosheet

[0085] (1-1) 340 mL of 7.2% ethanol solution was added to 25 g of Cd(NO3)2·4H2O to obtain a Cd(NO3)2·4H2O solution; 560 mL of 5.3% ethanol solution was added to 30 g of HMT and stirred uniformly to obtain an HMT solution;

[0086] (1-2) According to the molar ratio of Cd(NO3)2·4H2O to HMT 1:1, the Cd(NO3)2·4H2O solution of step (1-1) was added dropwise to the HMT solution, which immediately produced a white precipitate. In order to obtain sufficient Cd-HMT MHF, the stirring was continued for 24 h after the dropwise addition, and the precipitate was obtained;

[0087] (1-3) The precipitate was washed with ethanol several times and then dried in an 80℃ oven for 12 h to obtain Cd-HMT MHF.

[0088] (1-4) The Cd-HMT MHF was taken out and placed in a boat under Ar atmosphere, and then pyrolyzed at a heating rate of 3℃ / min to 1000℃ under Ar flow, and kept for 2 h to obtain two-dimensional N / C nanosheet precursor powder.

[0089] Figure 1 The scanning electron microscope (SEM) image of the two-dimensional N / C nanosheet precursor powder obtained in this example is shown in the figure. As can be seen from the figure, the two-dimensional nanosheet of the present example has a large pore and a two-dimensional layer structure.

[0090] (2) Preparation of phosphorus-doped nitrogen-rich porous carbon nanosheet with nitrogen activity regulation

[0091] The two-dimensional N / C nanosheet precursor powder obtained in step (1) and excess NaH2PO2·H2O were placed in the same boat at a mass ratio of 1:2, the boat was placed in a tube furnace, the tube furnace was vacuumed and treated with argon, after 1 h of constant temperature at 30 °C in the argon stream, the temperature was raised to 400 °C at a rate of 3 °C / min, and the calcination was carried out at 400 °C for 2 h, to obtain the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet. The mass percentages of C, N, O and P in the obtained material were 77.06%, 7.52%, 10.9% and 4.52%, respectively (determined by XPS semi-quantification).

[0092] Figure 3 The SEM image of the obtained phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet is shown, and it can be seen from the image that the pore structure of the material is increased.

[0093] Example 2

[0094] A phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet, the preparation method thereof comprises the following steps:

[0095] (1) Preparation of two-dimensional N / C nanosheet

[0096] The preparation method and conditions are the same as those of Example 1.

[0097] (2) Preparation of phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet

[0098] The two-dimensional N / C nanosheet precursor powder obtained in step (1) and NaH2PO2·H2O were placed in two boats at a mass ratio of 1:2, in the tube furnace, the boat containing NaH2PO2·H2O was placed upstream of the gas stream, and the boat containing the two-dimensional N / C nanosheet powder was placed downstream of the gas stream, then the tube furnace was vacuumed and treated with argon, after 1 h of constant temperature at 30 °C in the argon stream, the temperature was raised to 400 °C at a rate of 2 °C / min, and the calcination was carried out at 400 °C for 2 h, to obtain the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet.

[0099] The transmission electron microscope image (TEM) of the obtained nanosheet is shown in Figure 2 It can be seen from the image that a porous material with rich pore structure is obtained. Figure 4 The TEM-EDS image and the Mapping images of C, N and P elements of the phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet prepared in this example are shown. Among them, a is the micrograph of the nitrogen-rich porous carbon nanosheet, and b, c and d are the Mapping images of C, N and P elements, respectively. It can be seen from the images that C, N and P are uniformly distributed in the material.

[0100] Example 3

[0101] A phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet, a preparation method thereof comprises the following steps:

[0102] (1) The prepared two-dimensional N / C nanosheet

[0103] (1-1) 340 mL of an ethanol solution with a mass concentration of 7.2% was added to 29.9 g of Cd(NO3)2·4H2O to obtain a Cd(NO3)2·4H2O solution; 560 mL of an ethanol solution with a mass concentration of 5.3% was added to 9.02 g of HMT and stirred uniformly to obtain an HMT solution;

[0104] (1-2) According to a molar ratio of Cd(NO3)2·4H2O to HMT of 2:1, the Cd(NO3)2·4H2O solution in step (1-1) was added dropwise into the HMT solution, and a white precipitate was immediately generated; in order to obtain sufficient Cd-HMT MHF, after the dropwise addition, the stirring was continued at 45 ℃ for 18 h to obtain a precipitate;

[0105] (1-3) The precipitate was repeatedly washed with ethanol several times and then dried in a 60 ℃ oven for 10 h to obtain Cd-HMT MHF.

[0106] (1-4) The Cd-HMT MHF was taken out, placed in a boat, and heated to 850 ℃ at a heating rate of 2 ℃ / min under an Ar atmosphere and an Ar flow, and then pyrolyzed for 1 h to obtain a two-dimensional N / C nanosheet precursor powder.

[0107] (2) The prepared phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet

[0108] According to a mass ratio of 1:2, the two-dimensional N / C nanosheet precursor powder obtained in step (1) and excess NaH2PO2·H2O were respectively placed at two ends of the same boat, the boat was placed in a tube furnace, the tube furnace was vacuumized and treated with argon gas, and then the two-dimensional N / C nanosheet precursor powder and excess NaH2PO2·H2O were calcined at 350 ℃ for 3 h at a temperature rising rate of 2 ℃ / min under an argon flow to obtain a phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet.

[0109] Example 4

[0110] A phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheet, a preparation method thereof comprises the following steps:

[0111] (1) The prepared two-dimensional N / C nanosheet

[0112] (1-1) To 5.75 g Cd(NO3)2·4H2O, 340 mL of ethanol solution with a mass concentration of 7.2% was added to obtain a Cd(NO3)2·4H2O solution; to 36 g of HMT, 560 mL of ethanol solution with a mass concentration of 5.3% was added and stirred uniformly to obtain an HMT solution;

[0113] (1-2) According to a molar ratio of Cd(NO3)2·4H2O to HMT of 3:1, the Cd(NO3)2·4H2O solution of step (1-1) was added dropwise to the HMT solution, at which time white precipitate was immediately generated; in order to obtain sufficient Cd-HMT MHF, after dropwise addition, stirring was continued at 30℃ for 30 h to obtain precipitate;

[0114] (1-3) After the precipitate was washed repeatedly with ethanol several times, it was dried in a 70℃ oven for 8 h to obtain Cd-HMT MHF.

[0115] (1-4) The Cd-HMT MHF was taken out and placed in a boat, which was placed in an Ar atmosphere and pyrolyzed at a heating rate of 4℃ / min to 900℃ under Ar flow, and then maintained at 900℃ for 3 h to obtain two-dimensional N / C nanosheet precursor powder.

[0116] (2) Preparation of phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheets

[0117] According to a mass ratio of 1:2, the two-dimensional N / C nanosheet precursor powder obtained in step (1) and excess NaH2PO2·H2O were placed at two ends of the same boat, the boat was placed in a tube furnace, the tube furnace was vacuumed and treated with argon gas, and then the two-dimensional N / C nanosheet precursor powder and excess NaH2PO2·H2O were calcined at 450℃ for 5 h under argon flow after being kept at 30℃ for 0.5 h to obtain phosphorus-doped nitrogen-activity-regulated nitrogen-rich porous carbon nanosheets.

[0118] Comparative Example 1

[0119] The same as Example 1, except that the type of metal salt was changed, and Cd(NO3)2·4H2O in step (1-1) was replaced by Cu(NO3)2·4H2O. The rest was the same as Example 1.

[0120] Comparative Example 2

[0121] The same as Example 1, except that the type of metal salt was changed, and Cd(NO3)2·4H2O in step (1-1) was replaced by Zn(NO3)2·4H2O. The rest was the same as Example 1.

[0122] The structure of the products obtained from different metal salts in Example 1 and Comparative Examples 1-2 was investigated, wherein Figure 3 、 Figure 5 ,Figure 6 SEM images of the final products obtained in Example 1, Comparative Example 1, and Comparative Example 2 are shown respectively. Comparing the three images, it can be seen that when copper nitrate (Cu(NO3)2·4H2O) or Zn(NO3)2·4H2O is used to replace Cd(NO3)2·4H2O, the products obtained by sintering at the same temperature (1000℃) do not show obvious porous structures; and when Zn(NO3)2·4H2O is used, the obtained material will show cracks and damage.

[0123] Comparative Example 3

[0124] This is basically the same as Example 1, except that the temperature of the heating in steps (1-4) has been changed. In this comparative example, the temperature is raised to 800°C, and the rest is the same as Example 1.

[0125] Comparative Example 4

[0126] This is basically the same as Example 1, except that the temperature of the heating in steps (1-4) has been changed. In this comparative example, the temperature is raised to 1200°C, and the rest is the same as Example 1.

[0127] This invention investigated the structures of the products obtained at different pyrolysis temperatures in Example 1 and Comparative Examples 3-4, wherein... Figure 3 , Figure 7 , Figure 8 SEM images of the final products obtained in Example 1, Comparative Example 3, and Comparative Example 4 are shown respectively. Comparing the three images, it can be seen that the product obtained at a pyrolysis temperature of 800℃ has relatively fewer pores. In addition, excessively high pyrolysis temperatures do not have a more significant effect on the structure and morphology of the material and will also cause unnecessary energy loss.

[0128] Comparative Example 5

[0129] This is basically the same as Example 1, except that the molar ratio of Cd(NO3)2·4H2O to HMT in steps (1-2) has been changed. In this comparative example, the molar ratio of Cd(NO3)2·4H2O to HMT is 2:1.

[0130] Comparative Example 6

[0131] This is basically the same as Example 1, except that the molar ratio of Cd(NO3)2·4H2O to HMT in steps (1-2) has been changed. In this comparative example, the molar ratio of Cd(NO3)2·4H2O to HMT is 1:2.

[0132] This invention investigated the structures of the products obtained at different calcination temperatures in Example 1 and Comparative Examples 5-6, wherein... Figure 3 Corresponding to Example 1 Figures 9-10 The morphology of the final products obtained in Comparative Examples 5-6 shows that when the raw material ratio is changed, the products obtained in the comparative examples will agglomerate.

[0133] Comparative Example 7

[0134] The process is basically the same as in Example 1, except that step (2) is not included, i.e., two-dimensional N / C nanosheets are directly prepared. In the obtained material, the mass percentages of C, N, and O are 85.62%, 12.26%, and 2.12%, respectively (obtained by semi-quantitative determination using XPS).

[0135] Figure 11 XRD patterns of the products obtained in Example 1 and Comparative Example 7 are given, where undoped corresponds to Example 7 and doped corresponds to Example 1. As can be seen from the figures, the phosphorus-doped material of Example 1 of this invention exhibits increased interlayer spacing and increased carbon disorder, which is beneficial for Na… + The adsorption of phosphorus can improve the electrochemical performance of the prepared battery. Although the N content in Comparative Example 7 is higher than that in Example 1, this is because some P elements replace N and C after phosphorus doping. This invention achieves the highest possible N content while increasing the overall proportion of doped elements through P doping synergy.

[0136] Furthermore, the specific surface area of ​​the final products obtained in Example 1 and Comparative Example 7 was measured. The results showed that the specific surface area of ​​the PN / UCS of the final product in Example 1 was 1292.69 m². 2 g -1 This is much larger than the N / UCS (928.26m) of Comparative Example 7. 2 g -1 ).

[0137] Application Example 1

[0138] The nitrogen-rich porous carbon nanosheets with phosphorus doping and nitrogen activity regulated as prepared in Example 1 above were used to prepare a button cell.

[0139] The prepared nitrogen-rich porous carbon nanosheets with phosphorus doping to regulate nitrogen activity were mixed with PVDF binder and SUPERP LI conductive carbon black in a weight ratio of 70:15:15 and then dissolved in NMP to form a uniform slurry.

[0140] The slurry was coated onto a Cu foil and vacuum dried at 60°C for 12 hours to obtain the negative electrode. The negative electrode was then cut into disc electrode sheets with a diameter of 12 mm. Next, using metallic sodium (Aladdin) as the counter electrode, a glass fiber membrane (GF / Dwhatman) as the separator, and a solution of sodium hexafluorophosphate (NaPF6) dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) as the electrolyte (EC to DEC volume ratio of 1:1, NaPF6 concentration in the electrolyte of 0.8 mol / L), the 12 mm diameter disc electrode sheets were used as the working electrode to assemble a 2025 button cell.

[0141] Comparative Application Example 1

[0142] This is essentially the same as Application Example 1, except that the negative electrode active material has been changed. The product of Comparative Example 7 is used as the negative electrode active material instead of the nitrogen-rich porous carbon nanosheets with phosphorus doping to regulate nitrogen activity. Everything else is the same as Application Example 1.

[0143] The cycle performance curves of the sodium-ion batteries prepared in Application Example 1 and Comparative Application Example 1 were determined. The method was as follows: the batteries were connected using battery clips, and the specific capacity was measured using a Newway battery testing system with a voltage set to 0.01-2V. The results are as follows: Figures 12-13 As shown in the figure, the battery of Example 1 has a better specific capacity at 2Ag. -1 At a current density of 246.6 mA hg, after 500 cycles, it exhibits a reversible capacity of 246.6 mA hg. -1 By measuring the charge transfer resistance of the sodium-ion battery, it can be seen that the Rct (charge transfer resistance) of the PN / UCS negative electrode corresponding to Example 1 (206.2Ω) is significantly lower than that of the N / UCS negative electrode corresponding to the comparative example (468.8Ω).

[0144] The PN / C prepared by this invention has a larger specific surface area, superior electrical conductivity, and higher pyrrole-N and pyridine-N content, which not only exposes more accessible active sites but also increases interfacial Na content. + It exhibits excellent electrochemical performance by not only facilitating adsorption reactions but also accelerating electron transfer and promoting charge transfer kinetics during cycling.

[0145] In the PN / C prepared by this invention, pyridine-N is located at the edge of the carbon layer, which can provide lone pairs of electrons to enhance the local electron density and reduce the adsorption energy barrier of sodium ions; pyrrole-N, through its five-membered ring structure, regulates the electron distribution of the carbon framework, which can improve the overall conductivity of the electrode and accelerate charge transfer. Theoretical calculations show that the adsorption energy of pyridine-N (-2.1 eV) is significantly higher than that of graphite-N (-1.3 eV), which is more conducive to the adsorption of sodium ions. + Anchoring improves the storage performance of sodium, thereby enhancing the performance of the prepared battery.

[0146] The preparation process of the hexamine framework precursor of this invention employs a hydrogen bonding adsorption strategy, which is energy-saving and environmentally friendly. The calcination phosphating process uses low temperatures and short calcination times, which also reduces energy consumption. The phosphorus-doped nitrogen-rich porous carbon nanosheets of this invention, which regulate nitrogen activity, increase the active nitrogen content. The preparation method is simple and highly operable. The synthesized PN / C possesses a large specific surface area, a unique two-dimensional porous structure, excellent electrical conductivity, and more active sites. These properties, through their excellent synergistic effect, result in outstanding electrochemical performance. Simultaneously, phosphorus doping regulates the active nitrogen content, providing more active sites for electron conduction, thus offering a reference for broadening energy material systems.

[0147] The above description is merely a preferred embodiment of the present invention, and the present invention is not limited to the above embodiments. It is understood that other improvements and variations that are directly derived or conceived by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included within the protection scope of the present invention.

Claims

1. A method for preparing nitrogen-rich porous carbon nanosheets with phosphorus doping to regulate nitrogen activity, characterized in that, Includes the following steps: S1: Add the metal source solution to the amine compound solution, stir, and obtain a precipitate; S2: Wash and dry the precipitate to obtain the nanosheet precursor; S3: Place the nanosheet precursor in a container, heat it under an inert gas atmosphere, and then keep it at that temperature to obtain two-dimensional N / C nanosheets. S4: Phosphate two-dimensional N / C nanosheets to obtain nitrogen-rich porous carbon nanosheets with phosphorus doping to regulate nitrogen activity; In S1, the amine compound includes hexamethylenetetramine.

2. The preparation method according to claim 1, characterized in that, In S1, the metal source includes metal nitrates; Preferably, the metal nitrate is Cd(NO3)2·4H2O.

3. The preparation method according to claim 1, characterized in that, In S1, the mass concentration of the metal source solution is 2-10%; the mass concentration of the amine compound solution is 2-8%.

4. The preparation method according to claim 1, characterized in that, In S1, the molar ratio of the metal source to the amine compound is 1 to 1.5:1; The stirring temperature is 25–45°C, and the stirring time is 18–30 h.

5. The preparation method according to claim 1, characterized in that, In step S2, the drying temperature is 60–80°C and the drying time is 8–12 hours.

6. The preparation method according to claim 1, characterized in that, In S3, the temperature is increased to 850-1000℃ at a rate of 2-4℃ / min; The heat preservation time is 1 to 3 hours.

7. The preparation method according to claim 1, characterized in that, In S4, the method for phosphating the two-dimensional N / C nanosheets is as follows: the two-dimensional N / C nanosheets and the phosphorus-containing compound are placed in a calcination device, a vacuum is drawn, argon gas is introduced, and the temperature is kept constant for 0.5 to 1 hour. Then, the temperature is raised to the calcination temperature and kept at that temperature.

8. The preparation method according to claim 7, characterized in that, The phosphorus-containing compound includes NaH2PO2·H2O; the isothermal treatment temperature is 25–35°C; the heating rate is 2–5°C / min; the calcination temperature is 350–450°C; and the holding time is 2–5 h.

9. A nitrogen-rich porous carbon nanosheet with phosphorus doping and nitrogen activity regulated by the preparation method according to any one of claims 1-8, characterized in that, The nitrogen-rich porous carbon nanosheets with phosphorus doping to regulate nitrogen activity contain 6.52–8.52% N and 3.52–5.52% P; and / or, The phosphorus-doped nitrogen-rich porous carbon nanosheets with nitrogen-modified activity have a specific surface area of ​​1242–1343 m². 2 / g.

10. The application of a phosphorus-doped nitrogen-rich porous carbon nanosheet with nitrogen-modified activity prepared by the preparation method according to any one of claims 1 to 8, or the phosphorus-doped nitrogen-rich porous carbon nanosheet with nitrogen-modified activity according to claim 9, characterized in that, The nanosheets are used as anode materials for sodium-ion batteries.

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