Nitrogen-doped carbon nanosheet and preparation method and application thereof

Abstract

This invention belongs to the field of electrochemical energy storage materials and technology, and relates to a nitrogen-doped carbon nanosheet, its preparation method, and its application. The nitrogen-doped carbon nanosheet has a three-dimensional hierarchical porous nanosheet structure, which is composed of cross-linked two-dimensional nanosheets. The nitrogen-doped carbon nanosheet has the following characteristics: nitrogen doping amount of 5-20 at%, carbon interlayer spacing of 0.38-0.42 nm, and specific surface area of ​​250-350 m². 2 / g. The prepared N-CNS has an expanded carbon interlayer spacing, which is conducive to the rapid insertion and extraction of sodium ions; the high specific surface area and hierarchical porosity provide abundant active sites; the high nitrogen doping content (especially pyridine nitrogen and graphitic nitrogen) significantly enhances the adsorption capacity of the carbon surface for sodium ions and improves the conductivity of the material. When used as a negative electrode for SIBs, this material exhibits high reversible specific capacity, excellent rate performance and ultra-long cycle stability.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage materials and technology, specifically relating to a nitrogen-doped carbon nanosheet, its preparation method, and its application. Background Technology

[0002] Sodium is abundant and inexpensive, making sodium-ion batteries (SIBs) a promising next-generation energy storage technology. However, due to the availability and low cost of sodium... + The ionic radius (approximately 1.02 Å) is significantly larger than that of Li. + (Approximately 0.76 Å) The kinetics of its insertion / deintercalation process are slower and the volume effect is more pronounced, placing higher demands on the structural stability and diffusion kinetics of the anode material. Currently reported sodium-ion battery anode materials mainly include intercalated carbon materials, conversion-type metal compounds, and alloy materials. Among them, carbon materials have attracted much attention due to their low cost, good conductivity, and high structural stability; however, their theoretical specific capacity is limited, making it difficult to achieve high capacity and high rate performance while ensuring long lifespan.

[0003] To improve the sodium storage performance of carbon-based materials, researchers have proposed various modification strategies, such as heteroatom doping, pore structure modulation, and increased interlayer spacing. Among these, nitrogen doping can not only adjust the electronic structure of carbon materials and improve their conductivity, but also introduce more defects and active sites, thereby enhancing the sodium storage performance. + The adsorption and surface pseudocapacitance contribute to this. Urea is an inexpensive and commonly used nitrogen source, and it can also undergo condensation to form g-C3N4 during heat treatment. However, most existing techniques only use urea as a general nitrogen source or foaming agent, resulting in bulk or irregular porous carbon structures. They have failed to utilize g-C3N4 formed in situ during urea pyrolysis as a sacrificial template to precisely construct carbon materials with two-dimensional nanosheet morphologies, and also lack a universal method that can synergistically control morphology and N doping for various carbon sources. For example, in CN116675211A and the literature (J. HAZARD. MATER. 2020,393:121280), urea is only used as a nitrogen source, and the resulting products are irregular porous carbon, without mentioning or forming two-dimensional nanosheet structures guided by g-C3N4 templates.

[0004] On the other hand, 3D printing technology, especially direct ink writing (DIW), provides a new approach to constructing electrodes with designable macroscopic structures and three-dimensional channels. Although several 3D-printed electrodes and sodium-ion batteries have been reported in existing technologies, most focus on the printing process or conductive framework structure. The synergistic optimization of the microstructure of the active material itself and heteroatom doping is still insufficient, and there are few systematic solutions that combine commonly prepared N-doped carbon nanosheets with 3D-printed electrode systems. Summary of the Invention

[0005] To address the aforementioned problems in existing technologies, a nitrogen-doped carbon nanosheet and its preparation method are proposed. The nitrogen-doped carbon nanosheet has good raw material versatility, controllable structure, and both high specific capacity and high rate performance. Based on this material, high-performance 3D printed sodium-ion batteries can be constructed.

[0006] To achieve the above objectives, a first aspect of the present invention provides a nitrogen-doped carbon nanosheet having a three-dimensional hierarchical porous nanosheet structure, wherein the three-dimensional hierarchical porous nanosheet structure is composed of mutually cross-linked two-dimensional nanosheets. The nitrogen-doped carbon nanosheets have the following characteristics: nitrogen doping amount of 5-20 at%, carbon interlayer spacing of 0.38-0.42 nm, and specific surface area of ​​250-350 m². 2 / g.

[0007] A second aspect of the present invention provides a method for preparing the nitrogen-doped carbon nanosheets, comprising the following steps: (1) Obtain a mixture of carbon-containing organic precursor and urea; (2) Under an inert atmosphere, the mixture of the carbon-containing organic precursor and urea is subjected to a first-stage heat treatment to obtain an intermediate product; (3) Under the protection of an inert atmosphere, the intermediate product is subjected to a second-stage carbonization treatment to obtain the nitrogen-doped carbon nanosheets.

[0008] A third aspect of the present invention provides a 3D printing electrode ink for sodium-ion batteries, comprising: an active material, a conductive agent, a binder, and a solvent; The active material is the nitrogen-doped carbon nanosheet; Based on the total mass of the 3D printed electrode ink, the content of the active material is 60-75 wt%, the content of the conductive agent is 10-20 wt%, the content of the binder is 5-15 wt%, and the balance is solvent.

[0009] A fourth aspect of the present invention provides a 3D printed electrode, characterized in that the 3D printed electrode is obtained by printing the 3D printed electrode ink for sodium-ion batteries using a 3D printer.

[0010] The fifth aspect of the present invention provides the application of the 3D-printed electrode in a sodium-ion battery.

[0011] A sixth aspect of the present invention provides a sodium-ion battery, wherein the negative electrode is the 3D-printed electrode described above.

[0012] The present invention has the following beneficial effects: (1) The prepared N-CNS has an expanded carbon interlayer spacing, which is conducive to the rapid insertion and extraction of sodium ions; the high specific surface area and hierarchical porosity provide abundant active sites; the high nitrogen doping content (especially pyridine nitrogen and graphitic nitrogen) significantly enhances the adsorption capacity of the carbon surface for sodium ions and improves the electrical conductivity of the material. DFT calculations confirmed that nitrogen doping can reduce Na+ adsorption capacity. + Adsorption energy and diffusion energy barrier.

[0013] (2) When used as the anode of SIBs, this material exhibits a high reversible specific capacity (at 0.1 Ag). -1 The following is 244.6 mAhg -1 Excellent rate performance (at 5 A g) -1 The following is 118.6 mAh g -1 ) and extremely long cycling stability (at 5 Ag) -1 The capacity does not decay after 3000 cycles.

[0014] (3) This material is compatible with conductive agents and binders to form inks suitable for direct-write printing, thereby enabling the fabrication of electrodes with customized three-dimensional structures. This structure is beneficial for electrolyte wetting and ion transport, especially under high areal loading conditions.

[0015] (4) The 3D-printed N-CNS anode and the 3D-printed NVP cathode were successfully integrated to assemble a fully 3D-printed sodium-ion pouch cell, which exhibited good full-cell performance (0.1 Ag). -1 The lower capacity is 118.3 mAh g. -1 This verifies the practical potential of the material and technology approach.

[0016] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0017] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of the invention.

[0018] Figure 1 The SEM image of N-CNS-1 prepared according to Embodiment 1 of the present invention is shown.

[0019] Figure 2 The XRD pattern of N-CNS-1 prepared according to Embodiment 1 of the present invention is shown.

[0020] Figure 3 The diagram shows the specific capacity and rate performance of the half-cell with N-CNS-1 as the negative electrode prepared in Example 1 of the present invention.

[0021] Figure 4 The cycling performance diagram of a half-cell with N-CNS-1 as the negative electrode prepared in Example 1 of the present invention is shown.

[0022] Figure 5 The SEM image of the N-CNS prepared by the present invention using PVP as a carbon source is shown.

[0023] Figure 6 The diagram shows the capacity performance of the 3D-printed full battery of N-CNS-1 prepared according to Embodiment 1 of the present invention.

[0024] Figure 7 The diagram shows the cycle performance of the 3D-printed full battery of N-CNS-1 prepared in Embodiment 1 of the present invention. Detailed Implementation

[0025] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0026] A first aspect of the present invention provides a nitrogen-doped carbon nanosheet having a three-dimensional hierarchical porous nanosheet structure, wherein the three-dimensional hierarchical porous nanosheet structure is composed of mutually cross-linked two-dimensional nanosheets. The nitrogen-doped carbon nanosheets have the following characteristics: nitrogen doping amount of 5-20 at%, carbon interlayer spacing of 0.38-0.42 nm, and specific surface area of ​​250-350 m². 2 / g.

[0027] According to the present invention, preferably, the nitrogen-doped carbon nanosheets are obtained by a two-step heat treatment of a mixture of a carbon-containing organic precursor and urea; the mass ratio of the carbon-containing organic precursor to urea is 1:(2~8). Preferably, the carbon-containing organic precursor is selected from at least one of citric acid, sucrose, polyvinylpyrrolidone (PVP), chitosan, and starch.

[0028] A second aspect of the present invention provides a method for preparing the nitrogen-doped carbon nanosheets, comprising the following steps: (1) Obtain a mixture of carbon-containing organic precursor and urea; (2) Under an inert atmosphere, the mixture of the carbon-containing organic precursor and urea is subjected to a first-stage heat treatment to obtain an intermediate product; (3) Under the protection of an inert atmosphere, the intermediate product is subjected to a second-stage carbonization treatment to obtain the nitrogen-doped carbon nanosheets.

[0029] According to the present invention, preferably, the conditions for the first stage heat treatment include: a temperature of 250-400℃, a time of 0.5-4h, and a heating rate of 1-10℃ / min; The conditions for the second stage carbonization treatment include: a temperature of 600-1000℃, a time of 0.5-4h, and a heating rate of 2-10℃ / min; In steps (2) and (3), the inert gas is each independently at least one of argon, nitrogen and helium.

[0030] In this invention, the first stage of heat treatment causes urea to polymerize and generate a graphitic carbon nitride (g-C3N4) template.

[0031] A third aspect of the present invention provides a 3D printing electrode ink for sodium-ion batteries, comprising: an active material, a conductive agent, a binder, and a solvent; The active material is the nitrogen-doped carbon nanosheet; Based on the total mass of the 3D printed electrode ink, the content of the active material is 60-75 wt%, the content of the conductive agent is 10-20 wt%, the content of the binder is 5-15 wt%, and the balance is solvent.

[0032] According to the present invention, preferably, the conductive agent is selected from at least one of carbon nanotubes, conductive carbon black and graphene; The adhesive is selected from at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride and polyvinyl alcohol; The solvent is selected from at least one of deionized water, N-methylpyrrolidone, and ethanol.

[0033] According to the present invention, preferably, the 3D printing electrode ink is heated to 25°C and has a shear rate of 1 s. -1 It has a viscosity of 5000-20000 mPa·s and exhibits shear-thinning properties.

[0034] A fourth aspect of the present invention provides a 3D printed electrode, characterized in that the 3D printed electrode is obtained by printing the 3D printed electrode ink for sodium-ion batteries using a 3D printer.

[0035] In this invention, the 3D printing electrode ink is used to print an electrode preform with a preset three-dimensional structure on a current collector using direct writing 3D printing technology with a nozzle of 200-600 μm in diameter, at a pressure of 0.2-0.6 MPa and a printing speed of 200-800 mm / min. The electrode preform is freeze-dried or heat-treated to obtain the 3D printed electrode.

[0036] The fifth aspect of the present invention provides the application of the 3D-printed electrode in a sodium-ion battery.

[0037] A sixth aspect of the present invention provides a sodium-ion battery, wherein the negative electrode is the 3D-printed electrode described above.

[0038] Example 1

[0039] Weigh 2.0 g of citric acid and 6.0 g of urea, add them to 20 mL of deionized water, and stir at 80 °C until a uniform, transparent gel is formed. Transfer the resulting gel to a quartz boat and place it in a tube furnace. Under an argon atmosphere, heat the furnace to 350 °C at a rate of 5 °C / min and hold for 2 hours, then allow it to cool naturally to room temperature to obtain a pale yellow solid intermediate product. Then, carbonize the intermediate product under an argon atmosphere by heating it to 800 °C at a rate of 5 °C / min and holding for 2 hours. After natural cooling, a black powdery product is obtained, which is nitrogen-doped carbon nanosheets, denoted as N-CNS-1. The specific surface area is approximately 305 m². 2 / g.

[0040] Characterization of N-CNS-1: Scanning electron microscopy (SEM) observations revealed that the material consists of a three-dimensional porous network structure composed of interconnected ultrathin two-dimensional nanosheets (see [link]). Figure 1 X-ray diffraction (XRD) patterns show (see...) Figure 2 The interlayer spacing corresponding to its (002) crystal plane is calculated to be approximately 0.40 nm. X-ray photoelectron spectroscopy (XPS) analysis shows that the atomic percentage content of nitrogen on the material surface is approximately 14.4%.

[0041] N-CNS-1 active material, conductive carbon black, and sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 7:2:1, and deionized water was added to grind them into a uniform slurry. This slurry was then coated onto copper foil, dried, and die-cut into electrode sheets. Using a sodium metal sheet as the counter electrode, a glass fiber membrane as the separator, and a 1M NaClO4 EC / PC (volume ratio 1:1) solution as the electrolyte, a CR2032 coin cell was assembled in an argon glove box.

[0042] Tests were conducted using an electrochemical workstation: at 0.1 mV s -1 At the scan rate, its cyclic voltammetry curves showed obvious redox peaks and high overlap. Charge-discharge tests were performed on the Blue Electric test system: at 0.1 A g... -1 At a current density of approximately 244.6 mAh g, the reversible specific capacity is approximately 244.6 mAh g. -1 5 A g -1 The discharge specific capacity at the current density is 118.6 mAh g. -1 This electrode exhibits excellent rate performance (see...). Figure 3). In 5 A g -1 At high current densities, N-CNS-1 exhibits long-term cycling stability (see [reference]). Figure 4 Its capacity remains at 143.3 mAh g after 3000 cycles. -1 It has a high level of performance and excellent capacity retention.

[0043] Examples 2-5

[0044] Citric acid in Example 1 was replaced with sucrose, PVP, chitosan, and starch, respectively, while maintaining a mass ratio of 1:3 with urea, and other steps remained unchanged. The resulting materials were designated N-CNS-3 to N-CNS-6, respectively. SEM showed that they all formed similar nanosheet structures, with PVP being a typical example (see [link to SEM]). Figure 5 Furthermore, the ability to obtain high-performance sodium-ion anode materials from different carbon sources further demonstrates the versatility of this method, as shown in Table 1.

[0045] Table 1 Electrochemical performance of sodium-based anode materials with different carbon sources

[0046] Example 6

[0047] Weigh 1.4 g of N-CNS-1 powder, 0.4 g of multi-walled carbon nanotubes (CNTs), and 0.2 g of sodium carboxymethyl cellulose (CMC) prepared in Example 1, add 3.5 g of deionized water, and stir in a planetary mixer at 2000 rpm for 2 hours to obtain a uniform black ink with certain viscoelasticity. Using a rotational rheometer, the ink exhibits significant shear thinning behavior with increasing shear rate, making it suitable for direct-write 3D printing.

[0048] The ink was loaded into a syringe equipped with a 410 μm diameter flat nozzle and placed on a pneumatic extrusion 3D printer. At a constant pressure of 0.35 MPa and a printing speed of 500 mm / min, an electrode pattern with a size of 8 mm × 8 mm and a double-layer mesh structure was continuously printed on an aluminum foil current collector. The printed electrode preform was immediately placed in a -50°C freeze dryer for 24 hours to obtain a 3D-printed N-CNS negative electrode.

[0049] Following a similar method, a 3D-printed cathode with Na3V2(PO4)3@C as the positive electrode active material was prepared.

[0050] Using a glass fiber membrane as the separator and a 1M NaClO4 EC / PC solution as the electrolyte, the above-mentioned 3D printed positive electrode, separator, and 3D printed negative electrode are stacked in sequence and packaged into a soft-pack full cell.

[0051] The full cell is at 0.1 A g -1 Charge-discharge tests were conducted at current density, and the initial discharge specific capacity was 118.3 mAhg. -1 (See) Figure 6 Furthermore, this full cell exhibits excellent long-term stability, maintaining a capacitance of 0.1 Ag after more than 500 cycles. -1 Under these conditions, it maintains 113.6 mAh g. -1 capacity (see) Figure 7 The pouch battery successfully powered an LCD calculator.

[0052] Comparative Example 1

[0053] Weigh 2.0 g of citric acid and 6.0 g of urea, add them to 20 mL of deionized water, and stir at 80 °C until a uniform, transparent gel is formed. Transfer the resulting gel to a quartz boat and place it in a tube furnace. Under an argon atmosphere, directly heat the furnace to 800 °C at a rate of 5 °C / min and hold at that temperature for 2 hours for carbonization. After natural cooling, a black powdery product is obtained, designated as control product-1.

[0054] The N-CNS-1 prepared in Example 1 was compared and analyzed with the comparative product-1 prepared in Comparative Example 1: ① Structural differences: SEM observation showed that the comparative product-1 exhibited an irregular blocky porous structure and did not form the two-dimensional nanosheet network structure described in this invention. This proves that the two-step heat treatment method in this invention is essential and effective for constructing the nanosheet morphology.

[0055] ② Performance differences: Under the same electrochemical testing conditions, the comparative product-1 at 0.1 A g -1 The reversible specific capacity is only 146.4 mAh g. -1 This is far lower than the 244.6 mAh g of N-CNS-1. -1 ; in 5 A g -1 Below this point, its capacity decays severely, reaching only 32 mAh g. -1 This is far lower than the 118.6 mAh g of N-CNS-1. -1 .

[0056] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A nitrogen-doped carbon nanosheet, characterized in that, The nitrogen-doped carbon nanosheets have a three-dimensional hierarchical porous nanosheet structure, which is composed of mutually cross-linked two-dimensional nanosheets. The nitrogen-doped carbon nanosheets have the following characteristics: nitrogen doping amount of 5-20 at%, carbon interlayer spacing of 0.38-0.42 nm, and specific surface area of ​​250-350 m². 2 / g.

2. The nitrogen-doped carbon nanosheets according to claim 1, wherein, The nitrogen-doped carbon nanosheets are obtained by two-step heat treatment of a mixture of carbon-containing organic precursor and urea; the mass ratio of the carbon-containing organic precursor to urea is 1:(2~8). Preferably, the carbon-containing organic precursor is selected from at least one of citric acid, sucrose, polyvinylpyrrolidone, chitosan, and starch.

3. The method for preparing nitrogen-doped carbon nanosheets according to claim 1 or 2, characterized in that, Includes the following steps: (1) Obtain a mixture of carbon-containing organic precursor and urea; (2) Under an inert atmosphere, the mixture of the carbon-containing organic precursor and urea is subjected to a first-stage heat treatment to obtain an intermediate product; (3) Under the protection of an inert atmosphere, the intermediate product is subjected to a second-stage carbonization treatment to obtain the nitrogen-doped carbon nanosheets.

4. The preparation method according to claim 3, wherein, The conditions for the first stage of heat treatment include: a temperature of 250-400℃, a time of 0.5-4h, and a heating rate of 1-10℃ / min; The conditions for the second stage carbonization treatment include: a temperature of 600-1000℃, a time of 0.5-4h, and a heating rate of 2-10℃ / min; In steps (2) and (3), the inert gas is each independently at least one of argon, nitrogen and helium.

5. A 3D printing electrode ink for sodium-ion batteries, characterized in that, include: Active substances, conductive agents, binders, and solvents; Wherein, the active material is the nitrogen-doped carbon nanosheet as described in claim 1 or 2; Based on the total mass of the 3D printed electrode ink, the content of the active material is 60-75 wt%, the content of the conductive agent is 10-20 wt%, the content of the binder is 5-15 wt%, and the balance is solvent.

6. The 3D printing electrode ink for sodium-ion batteries according to claim 5, wherein, The conductive agent is selected from at least one of carbon nanotubes, conductive carbon black, and graphene; The adhesive is selected from at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride and polyvinyl alcohol; The solvent is selected from at least one of deionized water, N-methylpyrrolidone, and ethanol.

7. The 3D printing electrode ink for sodium-ion batteries according to claim 5, wherein, The 3D printed electrode ink is heated at 25°C and a shear rate of 1 s. -1 It has a viscosity of 5000-20000 mPa·s and exhibits shear-thinning properties.

8. A 3D printed electrode, characterized in that, The 3D printed electrode is obtained by printing with a 3D printer using the 3D printed electrode ink for sodium-ion batteries as described in claim 6 or 7.

9. The application of the 3D printed electrode according to claim 8 in a sodium-ion battery.

10. A sodium-ion battery, characterized in that, The negative electrode is the 3D printed electrode as described in claim 8.

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