Nitrogen and phosphorus co-doped hard carbon material, and preparation method and application thereof

Abstract

The application discloses a nitrogen and phosphorus co-doped hard carbon material and a preparation method and application thereof, and belongs to the technical field of sodium ion battery materials. The nitrogen and phosphorus co-doped hard carbon material is a micro-sized porous sieve pipe structure, Canada goldenrod is used as a precursor, and a dense nanoscale pore is formed on the surface of the sieve pipe through chemical activation, high-temperature carbonization and heteroatom co-doping, nitrogen and phosphorus co-doped heteroatoms are introduced into the derived hard carbon material, the preparation process is simple and efficient, and the method is suitable for large-scale industrial production. When the nitrogen and phosphorus co-doped hard carbon material is applied to a sodium ion battery negative electrode material, the material is rich in pseudo-capacitance active sites, has excellent electrochemical performance, the first coulombic efficiency reaches 67.2%, and excellent rate characteristics and cycle performance are exhibited.

Description

Technical Field

[0001] This invention relates to a hard carbon material, particularly a nitrogen-phosphorus co-doped hard carbon material, and also to the preparation method and application of the above-mentioned hard carbon material. Background Technology

[0002] As a component of the energy sector, energy storage is increasingly becoming a key factor restricting the development of energy technology. Lithium-ion batteries, with their high energy density, low self-discharge rate, rapid charging and discharging, and excellent adaptability, are widely used in large-scale energy storage systems. Sodium-ion batteries, with similar energy storage mechanisms to lithium-ion batteries, have also become a research hotspot in large-scale energy storage due to their abundant reserves and lower cost. The negative electrode material of a battery greatly affects its energy density. Therefore, it is necessary to develop a high-performance, low-cost negative electrode material for rechargeable batteries and its preparation method. Hard carbon, as one of many carbon-based materials, provides a large number of sodium storage sites, including disordered pores, wide graphite interlayer gaps, and graphene planar defects, due to its unique disordered porous structure, resulting in high theoretical capacity and making it stand out among negative electrode materials.

[0003] However, hard carbon materials still suffer from low initial coulombic efficiency and voltage polarization, which greatly limits their commercialization. Therefore, improving the selection of raw materials and the preparation process of hard carbon materials to enhance their sodium storage performance is a key research focus in this field. Currently, precursors for preparing hard carbon include biomass precursors, carbohydrates, and resins.

[0004] Canadian goldenrod is an invasive species. Native to North America and belonging to the Asteraceae family, it was introduced in the 1930s, initially cultivated as an ornamental plant. However, after escaping into the wild, it first appeared in the areas surrounding Shanghai and southern Jiangsu, and gradually spread to other regions. Canadian goldenrod has a wide temperature tolerance, making it highly adaptable to different environments. It can grow in poor, arid lands, severely reducing and damaging local biodiversity and threatening the ecological balance of its habitats to some extent. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a nitrogen-phosphorus co-doped hard carbon material that uses the invasive species Goldenrod as a biomass precursor, has low cost, high initial coulombic efficiency, and high rate performance. The invention also provides a method for preparing the above-mentioned nitrogen-phosphorus co-doped hard carbon material and its applications.

[0006] Technical solution: The present invention provides a nitrogen-phosphorus co-doped hard carbon material with a micron-sized porous sieve structure. Using Canadian goldenrod as a precursor, nitrogen and phosphorus co-doped heteroatoms are introduced through chemical activation, high-temperature carbonization and heteroatom co-doping to form dense nanoscale pores on the sieve surface.

[0007] Preferably, the size of the nanoscale pores is 100-800 nm.

[0008] The method for preparing the above-mentioned hard carbon material includes the following steps:

[0009] (1) Take Canadian goldenrod plant, clean and dry it, cut off the stem of the plant, crush and grind it to obtain precursor particles.

[0010] (2) The precursor particles were added to a potassium hydroxide solution for chemical activation. After activation, they were separated and dried to obtain activated precursor particles.

[0011] (3) Take the activated precursor particles, pyrolyze and carbonize them, cool them to obtain pyrolyzed hard carbon material, add dilute hydrochloric acid to react, wash, centrifuge and filter to obtain hard carbon material.

[0012] (4) Take hard carbon material and melamine, add them to water and stir to react to obtain a mixed solution. Then add phosphoric acid and stir to react. After centrifugation, filtration, drying and grinding, pyrolysis and carbonization are carried out to obtain nitrogen and phosphorus co-doped hard carbon material.

[0013] Preferably, in step (1), the cleaning of Canadian goldenrod includes coarse cleaning and fine cleaning. Coarse cleaning involves soaking in clean water for 1-2 hours and then washing for 30 minutes. Fine cleaning involves washing with deionized water for 10 minutes and then ultrasonic cleaning for 20-30 minutes. The stem of the plant is cut to 0.5-1 cm and crushed mechanically using a ball mill or crusher.

[0014] Preferably, in step (2), the concentration of the potassium hydroxide solution is 0.5-1.5M, the chemical activation time is 4-12h, and the drying is specifically drying at 60-80℃ for 12-20h.

[0015] Preferably, in step (3), the pyrolysis carbonization specifically involves heating to 800℃-1400℃ and holding for 1-10 hours in an inert gas environment before cooling.

[0016] Preferably, in step (3), the concentration of the dilute hydrochloric acid is 0.5-1.5M, the reaction time is 1-3h, the washing is done with water until the solution is neutral, and the centrifugation speed is 8000-12000r / min.

[0017] Preferably, in step (4), the mass ratio of the hard carbon material to melamine is 3 to 5:1, and the reaction time in water is 15 to 30 hours.

[0018] Preferably, in step (4), the pyrolysis carbonization specifically involves heating to 500-700°C and holding for 1-3 hours in an inert gas environment before cooling.

[0019] Preferably, the inert gas is one or a mixture of nitrogen and argon, and the heating and cooling rate is 4-5°C / min.

[0020] The above-mentioned nitrogen-phosphorus co-doped hard carbon materials are used in sodium-ion battery anode materials.

[0021] Invention Principle: This invention uses readily available stem organs of Canadian goldenrod as biomass raw material. During its growth, the stem organs of Canadian goldenrod possess sieve-like long channels for transporting nutrients and water, allowing the derived hard carbon material to inherit the original porous structure. Furthermore, the potassium hydroxide activation treatment destroys the original oxygen-containing functional groups in the precursor, ultimately forming micro- and nano-sized vacancies of varying sizes after high-temperature pyrolysis and carbonization. This provides numerous active sites for the sodium storage process, enhancing electron transport capacity and thus improving electrochemical performance. In addition, by introducing nitrogen and phosphorus atom co-doping, the specific surface area of ​​the hard carbon material is increased, effectively altering the interlayer spacing and increasing pseudocapacitive adsorption sites, thereby further improving the sodium storage capacity and achieving high initial coulombic efficiency and high rate performance.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The stem of Canadian goldenrod is used as raw material, and the preparation process of the derived hard carbon material is simple, the raw materials are readily available and the cost is low; (2) Melamine and phosphoric acid are used as nitrogen and phosphorus atom donors to introduce nitrogen and phosphorus co-doped heteroatoms into the derived hard carbon material. The preparation process is simple and efficient and suitable for large-scale industrial production; (3) When the nitrogen and phosphorus co-doped hard carbon material is applied to the negative electrode material of sodium-ion battery, it is rich in pseudocapacitive active sites, has excellent electrochemical performance, and the first coulombic efficiency reaches 67.2%, showing excellent rate characteristics and cycle performance. Attached Figure Description

[0023] Figure 1 This is a SEM image of the nitrogen-phosphorus co-doped hard carbon material in Example 1 of the present invention;

[0024] Figure 2 The cycling curve of the hard carbon material derived from Canadian goldenrod obtained in Comparative Example 1 of this invention at 20mA in sodium-ion battery testing.

[0025] Figure 3 This is a graph showing the first three charge-discharge curves of the nitrogen-phosphorus co-doped hard carbon material in Example 1 of the present invention at a current density of 20mA in a sodium-ion battery test.

[0026] Figure 4 This is a cycling curve of the nitrogen-phosphorus co-doped hard carbon material in Example 1 of the present invention at a current density of 20mA in a sodium-ion battery test.

[0027] Figure 5 The images show the XRD patterns of SCHC-800-NP in the embodiments of the present invention and SCHC-800 in the comparative examples;

[0028] Figure 6 This is a comparison chart of the first charge-discharge curves of SCHC-800-NP in the embodiment of the present invention and SCHC-800 in the comparative example in sodium-ion battery testing at a current density of 20mA.

[0029] Figure 7 The Nyquist plots are obtained from EIS tests conducted on SCHC-800-NP and SCHC-800 in the sodium-ion battery test in the embodiments and comparative examples of the present invention, from frequencies ranging from 0.01 Hz to 100 kHz. Detailed Implementation

[0030] The technical solution of the present invention will be further described below with reference to the embodiments and accompanying drawings.

[0031] Example 1

[0032] (1) First, soak 120g of collected Canadian goldenrod in 2L of clean water for 1 hour and wash it clean. Then rinse it with deionized water for 10 minutes until no impurities flow out. Clean it with ultrasound for 30 minutes and place it in a 60℃ oven to dry for 20 hours. Use scissors to separate the cleaned and dried Canadian goldenrod stems and cut the length of the separated Canadian goldenrod stems to 0.5-1cm. Then crush the cut material into precursor particles using a ball mill to obtain activated precursor particles;

[0033] (2) First, prepare 500 ml of 1 mol / L sodium hydroxide solution with deionized water. Then, add the precursor particles to the prepared solution for chemical activation and stir at 25°C for 12 h. After centrifugation and filtration, pour the activated solution into a waste liquid tank and freeze-dry the filter residue using a vacuum freeze dryer to remove most of the moisture from the activated precursor particles, thus obtaining activated precursor particles.

[0034] (3) The precursor particles were placed into a tube furnace according to standard operation and heated to 800°C at a heating rate of 5°C / min under an argon atmosphere and held for 3 hours. After the holding period, the temperature was cooled to room temperature at a cooling rate of 5°C / min to obtain 2.75g of hard carbon material derived from Canadian goldenrod.

[0035] (4) 6.25g of hard carbon material derived from Canadian goldenrod and 1.25g of melamine were added to 500ml of deionized water and stirred thoroughly in a water bath at 45℃ for 20h. Then, 5ml of phosphoric acid was added dropwise to the solution and stirred at 65℃ for 8h. After centrifugation and washing, the resulting insoluble solid was dried in an oven at 80℃ for 12h and then ball-milled for 30min. The milled material was then placed in a tube furnace and heated to 800℃ at a heating rate of 5℃ / min under an argon atmosphere and held for 3h. After the holding period, it was cooled to room temperature at a cooling rate of 5℃ / min to obtain 3.7g of nitrogen-phosphorus co-doped hard carbon material (SCHC-800-NP).

[0036] Figure 1 The image shows a scanning electron microscope (SEM) of hard carbon material (SCHC-800-NP). As can be seen from the image, the obtained sample retains the original micron-scale sieve-like pore structure, and after activation and pyrolysis, a large number of micro- and nano-scale pores are formed on the surface of the material, which theoretically provides more sites for ion storage during the charging and discharging process.

[0037] Comparative Example 1

[0038] (1) First, soak 120g of collected Canadian goldenrod in 2L of clean water for 1 hour and wash it clean. Then rinse it with deionized water for 10 minutes until no impurities flow out. Clean it with ultrasound for 30 minutes and place it in a 60℃ oven to dry for 20 hours. Use scissors to separate the cleaned and dried Canadian goldenrod stems and cut the length of the separated Canadian goldenrod stems to 0.5-1cm. Then crush the cut material into precursor particles using a ball mill.

[0039] (2) First, prepare 500 ml of 1 mol / L sodium hydroxide solution with deionized water. Then, add the precursor particles to the prepared solution for chemical activation and stir at 25°C for 12 h. After centrifugation and filtration, pour the activated solution into a waste liquid tank and freeze-dry the filter residue using a vacuum freeze dryer to remove most of the moisture from the activated precursor particles, thus obtaining activated precursor particles.

[0040] (3) The precursor particles were placed into a tube furnace according to standard operation and heated to 800°C at a heating rate of 5°C / min under an argon atmosphere and held for 3 hours. After the holding period, the temperature was cooled to room temperature at a cooling rate of 5°C / min to obtain 2.75g of hard carbon material derived from Canadian goldenrod (SCHC-800).

[0041] Figure 2The hard carbon material derived from *Solidago canadensis* obtained in Comparative Example 1 was used to prepare an electrode sheet, which was then used as the negative electrode material for electrochemical testing of a sodium-ion battery. After 50 cycles at a current density of 20 mA, it could stably maintain a capacity of 281.38 mAh / g without capacity decay.

[0042] Figure 3 To prepare an electrode sheet using nitrogen and phosphorus co-doped hard carbon material derived from Canadian goldenrod (SCHC-800-NP) as a negative electrode material for sodium-ion batteries for electrochemical testing, the first three charge-discharge curves (GCD) at a current density of 20 mA clearly showed an initial discharge capacity of 438.82 mAh / g and an initial Kunlun efficiency of 67.2%. In the second and third reversible charge-discharge cycles, the capacity rapidly stabilized at 327.33 mAh / g.

[0043] Figure 4 To fabricate an electrode sheet using nitrogen and phosphorus co-doped hard carbon material derived from Canadian goldenrod (SCHC-800-NP) as a negative electrode material for sodium-ion batteries for electrochemical testing, the material maintained a stable capacity of 303.64 mAh / g after 50 cycles at a current density of 20 mA without capacity decay.

[0044] Figure 5 X-ray powder diffraction (XRD) of hard carbon material (SCHC-800) and nitrogen- and phosphorus co-doped hard carbon material derived from Canadian goldenrod (SCHC-800-NP) showed obvious diffraction peaks at 2θ = 23.5° and 2θ = 43.5°, corresponding to the (002) and (100) crystal planes of hard carbon material, respectively, showing typical hard carbon diffraction patterns, indicating that the obtained hard carbon has high purity and low impurity content; the (002) diffraction peak of SCHC-800-NP shifted to the left and broadened, changing the peak position and reducing crystallinity, indicating that heteroatom doping is effective and the interplanar spacing increases.

[0045] Figure 6 Electrode sheets were fabricated from hard carbon material (SCHC-800) and nitrogen- and phosphorus-doped hard carbon material derived from Canadian goldenrod (SCHC-800-NP) and used as negative electrode materials for sodium-ion batteries for electrochemical testing. The initial charge-discharge curves are compared. The comparison shows that by co-doping the hard carbon material with nitrogen and phosphorus heteroatoms, the initial discharge specific capacity increased from 392.02 mAh / g to 438.82 mAh / g, and the initial coulombic efficiency increased from 52.9% to 67.2%.

[0046] Figure 7Electrode sheets were fabricated from hard carbon material (SCHC-800) and nitrogen- and phosphorus-doped hard carbon material derived from goldenrod (SCHC-800-NP) to serve as negative electrode materials for sodium-ion batteries in electrochemical testing.

[0047] Table 1. EIS test fitting data

[0048]

[0049] Table 1 shows the EIS tests conducted on the hard carbon materials derived from *Solidago canadensis* and the nitrogen- and phosphorus-doped hard carbon materials derived from *Solidago canadensis* in the embodiments of this invention, from frequencies ranging from 0.01 Hz to 100 kHz in sodium-ion battery testing. A simplified equivalent circuit was used for fitting, and the impedance data of each part after fitting are summarized. It can be seen that the electrolyte impedance of the electrode prepared with SCHC-800-NP is 2.516 Ω and the charge transfer impedance is 60.49 Ω in the impedance test; while the electrolyte impedance of the electrode prepared with SCHC-800 is 18.91 Ω and the charge transfer impedance is 160.1 Ω in the impedance test. It is clearly observed that both the electrolyte impedance Rs and the charge transfer impedance Rct of SCHC-800-NP are smaller than those of SCHC-800, indicating that the electrode prepared using SCHC-800-NP is more conducive to charge transport.

[0050] A negative electrode was fabricated using hard carbon material (SCHC-800) and nitrogen- and phosphorus-doped hard carbon material derived from Canadian goldenrod (SCHC-800-NP) as negative electrode materials for sodium-ion batteries in electrochemical testing. The obtained Canadian goldenrod-derived hard carbon material (SCHC-800) and the nitrogen- and phosphorus-doped Canadian goldenrod-derived hard carbon material (SCHC-800-NP) exhibited high reversible specific capacity and good cycle stability, demonstrating excellent sodium storage performance. Furthermore, nitrogen and phosphorus co-doping of the hard carbon material effectively improved the reversible specific capacity and initial coulombic efficiency, significantly enhancing the electrochemical performance of the material.

Claims

1. A nitrogen-phosphorus co-doped hard carbon material for use as a negative electrode material in sodium-ion batteries, characterized in that, The hard carbon material has a micron-sized porous sieve structure. Using Canadian goldenrod as a precursor, nitrogen and phosphorus co-doped heteroatoms are introduced through chemical activation, high-temperature carbonization, and co-doping, forming dense nanoscale pores on the sieve surface. The size of these nanoscale pores is 100-800 nm. The hard carbon material is prepared by the following steps: (1) Take Canadian goldenrod plant, clean and dry it, cut off the stem of the plant, crush and grind it to obtain precursor particles. (2) The precursor particles were added to a potassium hydroxide solution for chemical activation treatment. The concentration of the potassium hydroxide solution was 0.5-1.5M, and the chemical activation time was 4h-12h. After activation, the precursor particles were separated and dried to obtain activated precursor particles. (3) Take the activated precursor particles and pyrolyze them. In an inert gas environment, heat the temperature to 800℃-1400℃ at 4-5℃ / min and keep it for 1-10h. After cooling, pyrolyzed hard carbon material is obtained. Add 0.5-1.5M dilute hydrochloric acid to react, wash, centrifuge and filter to obtain hard carbon material. (4) Take hard carbon material and melamine, add them to water and stir for 15-30h to obtain a mixed solution. Then add phosphoric acid and stir for reaction. After centrifugation, filtration, drying and grinding, carry out pyrolysis carbonization. In an inert gas environment, heat the material to 500℃-700℃ at 4-5℃ / min and keep it at 1-3h before cooling to obtain nitrogen and phosphorus co-doped hard carbon material.

2. The nitrogen-phosphorus co-doped hard carbon material according to claim 1, characterized in that, In step (2), the drying process specifically involves drying at 60-80℃ for 12-20 hours.

3. The nitrogen-phosphorus co-doped hard carbon material according to claim 1, characterized in that, In step (3), the reaction time for adding dilute hydrochloric acid is 1-3 hours, the washing is done with water until the solution is neutral, and the centrifugation is performed at a speed of 8000-12000 r / min.

4. The nitrogen-phosphorus co-doped hard carbon material according to claim 1, characterized in that, In step (4), the mass ratio of the hard carbon material to melamine is 3~5:1.

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