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

Abstract

The invention belongs to the technical field of sodium ion batteries, and relates to a nitrogen-phosphorus co-doped hard carbon negative electrode material and a preparation method and application thereof. Aiming at the technical problems of relatively low initial coulombic efficiency, low charge-discharge specific capacity and poor rate capability of a hard carbon negative electrode material in the prior art, the invention provides the preparation method of the nitrogen-phosphorus co-doped hard carbon negative electrode material, and the preparation method comprises the following steps: mixing sucrose and urea phosphate, and carrying out hydrothermal reaction to obtain a hard carbon precursor; performing high-temperature carbonization on the hard carbon precursor to obtain the nitrogen-phosphorus co-doped hard carbon negative electrode material, the mass of the urea phosphate accounts for 1-10% of the mass of the cane sugar. The method has the advantages of simple and efficient preparation process, no need of special equipment, low cost and wide application range, and the obtained nitrogen-phosphorus co-doped hard carbon negative electrode material has high initial coulombic efficiency, high charge-discharge specific capacity and high rate performance. The invention also provides application of the nitrogen-phosphorus co-doped hard carbon negative electrode material in preparation of a negative electrode plate and preparation of a sodium ion battery.

Description

Technical Field

[0001] This application belongs to the technical field of sodium-ion batteries, and specifically relates to a nitrogen and phosphorus co-doped hard carbon negative electrode material, its preparation method and application. Background Art

[0002] Energy is the foundation of human development and survival. With the continuous development of technology, the energy crisis and environmental pollution have become severe challenges faced by human society. The large consumption of traditional fossil fuels (oil, natural gas, and coal) has caused irreversible pollution and damage to the ecology and nature, and its non-renewability will also trap humanity in a predicament of resource shortage. To address the growing energy demand and the limited domestic resources, developing new energy to replace traditional fossil energy, promoting transportation electrification, and focusing on the development of large-scale energy storage industries have become effective ways for China to achieve the goal of "carbon neutrality" by 2060, ensure national energy security, and maintain sustainable social development.

[0003] Renewable energy sources (such as solar energy, wind energy, water energy, biomass energy, etc.) have become ideal choices to replace traditional fossil energy due to their clean and renewable characteristics. However, the intermittency and instability of renewable energy sources limit their large-scale application, and efficient energy storage technologies are needed to solve the problems of continuity and stability of energy supply. Therefore, developing high-performance energy storage materials is of great significance for promoting the wide application of renewable energy sources and achieving the goal of "carbon neutrality".

[0004] In energy storage technologies, sodium-ion batteries have become powerful alternatives to lithium-ion batteries due to their advantages such as rich resources, low cost, and high safety. Hard carbon materials are considered strong candidates for the negative electrode materials of sodium-ion batteries due to their unique structures and properties. Its main advantages include: abundant sources, simple preparation, relatively high sodium storage capacity, low average working potential, and excellent cycling performance. However, currently, hard carbon still faces problems such as unclear sodium storage mechanism, poor rate performance, low initial Coulomb efficiency, and high cost, and there is still room for improvement in sodium storage capacity.

[0005] In the prior art, sucrose, as a biomass raw material with a wide range of sources and low cost, is widely used in the preparation of hard carbon anode materials. For example, Chinese Patent Publication No. CN118619256A, with an application date of August 6, 2024, and an invention title of "A Sucrose-Based Hard Carbon with a High Proportion of Capacity in the Long Plateau Region and Its Preparation Method," discloses a preparation method of a sucrose-based hard carbon with a high proportion of capacity in the long plateau region, including the following steps: (1) Hydrothermally carbonize the sucrose solution, wash and dry it after hydrothermal treatment to obtain a sucrose-based hard carbon precursor M-SC; (2) After ball-milling the precursor M-SC obtained in step (1) into a powder, perform low-temperature pre-carbonization at a temperature of 400°C to 600°C, with a heating rate of 1 to 7°C / min, and introduce an inert gas for protection during the heating and insulation processes; (3) High-temperature carbonize the material that has undergone low-temperature pre-carbonization in step (2) in a tube furnace; (4) After high-temperature carbonization, wait for the material to cool down, take out the black material, wash, filter by suction, and dry it to obtain a sucrose-based hard carbon anode material with a high proportion of capacity in the long plateau region. The high-temperature carbonization temperature of this method is 1300°C, which is relatively high, with high energy consumption and high cost.

[0006] In the prior art, the electronic structure of hard carbon is also adjusted through chemical doping (such as doping with elements such as nitrogen and phosphorus). For example, Chinese Patent Publication No. CN118108211A, with an application date of April 29, 2024, and an invention title of "A Phosphorus-Doped Hard Carbon Anode Material and Its Preparation Method, Vehicle," discloses a preparation method of a phosphorus-doped sodium-ion battery hard carbon anode material, including: (1) Add ammonium phosphate salt to the carbon source and perform hydrothermal treatment to obtain a precursor; (2) Perform heat treatment to obtain hard carbon; (3) Obtain a phosphorus-doped sodium-ion battery hard carbon anode material through chemical vapor deposition; the carbon source is selected from at least one of sucrose, glucose, or a glucose derivative containing a thiourea group. The battery prepared with the hard carbon anode material obtained by this method only has a reversible capacity of 148 mAh / g at a current density of 500 mA / g (2C), and the rate performance is not high.

[0007] Therefore, there is an urgent need for a hard carbon material with high initial efficiency, high charge specific capacity, and high rate performance. Summary of the Invention

[0008] 1. Problems to be Solved

[0009] Aiming at the technical problems of the hard carbon anode material in the prior art, such as low initial Coulomb efficiency (referred to as initial efficiency for short), low charge-discharge specific capacity, and poor rate performance, the present application provides a preparation method of a nitrogen and phosphorus co-doped hard carbon anode material, which has low cost and simple process, and the obtained nitrogen and phosphorus co-doped hard carbon anode material has high initial efficiency, high charge-discharge specific capacity, and high rate performance.

[0010] The present application also provides a negative electrode sheet and an assembled sodium-ion battery.

[0011] Meanwhile, the present application also provides the use of a nitrogen and phosphorus co-doped hard carbon negative electrode material in the preparation of a negative electrode sheet and a sodium-ion battery.

[0012] 2. Technical solutions

[0013] To solve the above problems, the technical solutions adopted in the present application are as follows:

[0014] A preparation method of a nitrogen and phosphorus co-doped hard carbon negative electrode material, comprising the following steps:

[0015] Mix sucrose and urea phosphate, and obtain a hard carbon precursor through hydrothermal reaction;

[0016] Subject the hard carbon precursor to high-temperature carbonization to obtain a nitrogen and phosphorus co-doped hard carbon negative electrode material;

[0017] The mass of the urea phosphate accounts for 1% to 10% of the mass of the sucrose.

[0018] Preferably, the mixed solution obtained by mixing sucrose and urea phosphate is ultrasonically treated for 10 min to 20 min.

[0019] Preferably, the product obtained from the hydrothermal reaction is repeatedly washed with deionized water and absolute ethanol.

[0020] Further, the concentration of the sucrose is 0.1 mol / L to 1 mol / L.

[0021] Within the concentration range of 0.1 mol / L to 1 mol / L, the concentration of the sucrose solution is appropriate, which can effectively control the size and morphology of the hard carbon particles and avoid particle aggregation; too high a sucrose concentration will cause the solution to be too viscous, making the hard carbon particles formed during the pyrolysis process more likely to aggregate, and this aggregation phenomenon will hinder the diffusion of sodium ions and reduce the specific capacity and rate performance of the material; too low a concentration of the sucrose solution will result in a smaller specific surface area and less abundant pore structure of the formed hard carbon material, thereby reducing the specific capacity of the material.

[0022] Further, the temperature of the hydrothermal reaction is 150°C to 250°C, and the time of the hydrothermal reaction is 12 h to 24 h.

[0023] A temperature of 150°C to 250°C can enable sucrose and urea phosphate to undergo appropriate chemical reactions to form a nitrogen and phosphorus co-doped hard carbon material with the required structure and properties. If the temperature is too high, it may cause changes in the structure and properties of the product, which is not conducive to subsequent applications in sodium-ion batteries. A time of 12 h to 24 h can ensure the full progress of the reaction while minimizing the reaction time.

[0024] Further, the product obtained from the hydrothermal reaction is dried at a temperature of 80°C to 120°C for 5 h to 12 h.

[0025] Further, the specific parameters of the high-temperature carbonization are as follows: heating up to 600°C to 1000°C at a rate of 1°C / min to 8°C / min and holding for 1 h to 5 h.

[0026] Heating up at a rate of 1°C / min to 8°C / min can make the temperature distribution inside the material more uniform, avoid the non-uniformity of the material structure caused by too large a temperature gradient, and at the same time, it can shorten the carbonization time and improve the production efficiency on the premise of ensuring the material quality. Holding at 600°C to 1000°C for 1 h to 5 h can fully carbonize the hard carbon precursor to form a hard carbon material with a rich pore structure and good conductivity.

[0027] Further, an inert gas is introduced during the heating-up and holding processes, and the gas flow rate is 50 ml / min to 100 ml / min.

[0028] During the high-temperature carbonization process, the surface of the material is prone to react with oxygen, resulting in oxidation. The inert gas can displace the oxygen in the furnace to form an anaerobic environment, thereby preventing the oxidation of the material surface and at the same time preventing unnecessary chemical reactions to prepare a high-performance hard carbon negative electrode material. The gas flow rate of 50 ml / min to 100 ml / min can ensure the replacement efficiency of the inert gas.

[0029] A nitrogen and phosphorus co-doped hard carbon negative electrode material is prepared by the preparation method of the nitrogen and phosphorus co-doped hard carbon negative electrode material.

[0030] A negative electrode plate includes the nitrogen and phosphorus co-doped hard carbon negative electrode material.

[0031] A sodium-ion battery includes the negative electrode plate.

[0032] An application of the nitrogen and phosphorus co-doped hard carbon negative electrode material is to apply the negative electrode material to the preparation of a negative electrode plate and / or a sodium-ion battery.

[0033] 3. Beneficial effects

[0034] Compared with the prior art, the beneficial effects of this application are as follows:

[0035] 1. A preparation method of a nitrogen and phosphorus co-doped hard carbon anode material of the present invention involves mixing sucrose and urea phosphate, followed by a hydrothermal reaction to obtain a hard carbon precursor; and then subjecting the hard carbon precursor to high-temperature carbonization to obtain the nitrogen and phosphorus co-doped hard carbon anode material. The advantage of choosing sucrose as the hard carbon precursor is that sucrose is a very common biomass resource, which has the advantages of wide source, low price, short production cycle, and no environmental pollution. Moreover, sucrose itself is a granular material, and most of the grains are ellipsoidal, making it an ideal raw material for preparing hard carbon. Introducing heteroatoms (N, P) into hard carbon is the best strategy to improve sodium storage performance by constructing pore structures, generating abundant active sites / defects, adjusting the interlayer spacing, and improving electronic conductivity by adjusting the electronic structure. In this method, the mass of urea phosphate accounts for 1% - 10% of the mass of sucrose. Compared with other raw materials, urea phosphate can achieve a good nitrogen and phosphorus co-doping effect, and has higher specific capacity, initial efficiency, and rate performance compared with single nitrogen doping and phosphorus doping. The preparation process of this method is simple and efficient, does not require special equipment, has low cost, and has a wide application range.

[0036] 2. A nitrogen and phosphorus co-doped hard carbon anode material of the present invention has improved charge-discharge specific capacity, improved initial Coulomb efficiency, and also enhanced rate performance. By introducing more active sites and increasing the storage sites for sodium ions, the charge specific capacity reaches 310 mAh / g; by improving the surface properties of the hard carbon material and reducing irreversible side reactions, the initial Coulomb efficiency reaches 79%; by increasing the interlayer spacing, increasing ion diffusion channels, and improving electronic conductivity, it can still maintain a reversible capacity of 195 mAh / g at a high rate of 15C, showing excellent rate performance.

[0037] 3. An application of a nitrogen and phosphorus co-doped hard carbon anode material of the present invention in the preparation of a negative electrode plate and / or a sodium-ion battery improves the specific capacity, initial Coulomb efficiency, rate performance, and cycle stability of the sodium-ion battery, while simplifying the preparation process and reducing the production cost. Description of the Drawings

[0038] Figure 1 is the SEM image of the nitrogen and phosphorus co-doped hard carbon anode material prepared in Example 3;

[0039] Figure 2 is the first charge-discharge curves of the nitrogen and phosphorus co-doped hard carbon anode materials prepared in Example 1, Example 2, Example 3, and Example 4 at different nitrogen and phosphorus ratios;

[0040] Figure 3 is the first charge-discharge curves of the nitrogen and phosphorus co-doped hard carbon anode materials prepared in Example 3, Example 5, Example 6, and Example 7 at different heating rates;

[0041] Figure 4XRD patterns of the nitrogen and phosphorus co-doped hard carbon anode material prepared in Example 3 and the blank sample;

[0042] Figure 5 Rate performance graphs of the nitrogen and phosphorus co-doped hard carbon anode material prepared in Example 3 and the blank sample;

[0043] Figure 6 Initial charge-discharge curves of the sucrose-based hard carbon anode material at different carbonization temperatures in Comparative Example 1;

[0044] Figure 7 Initial charge-discharge curves of the sucrose-based hard carbon anode material at different heating rates in Comparative Example 1;

[0045] Figure 8 Initial charge-discharge curves of the nitrogen and phosphorus co-doped hard carbon anode material prepared in Example 3 and the phosphorus-doped hard carbon anode material prepared in Comparative Example 3;

[0046] Figure 9 Initial charge-discharge curves of the nitrogen and phosphorus co-doped hard carbon anode material prepared in Example 3 and the nitrogen-doped hard carbon anode material prepared in Comparative Example 4. Detailed implementation manners

[0047] The present application will be further described below in conjunction with specific embodiments.

[0048] It should be noted that the terms such as "upper", "lower", "left", "right", "middle" cited in this specification are only for the convenience of narration and are not used to limit the scope of implementation. The change or adjustment of their relative relationship, without substantial change in the technical content, should also be regarded as the scope that the present application can implement.

[0049] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs; the term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0050] For those conditions not specified in the examples, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0051] As used herein, the term "about" is used to provide flexibility and imprecision associated with a given term, measurement, or value. Those skilled in the art can easily determine the degree of flexibility of a specific variable.

[0052] As used herein, the term "at least one of..." is intended to be synonymous with "one or more of...". For example, "at least one of A, B, and C" clearly includes only A, only B, only C, and their respective combinations.

[0053] Concentration, amount and other numerical data can be presented in range format herein.It should be understood that such range format is only used for convenience and simplicity, and should be flexibly interpreted as not only including the numerical value clearly described as range limit, but also including all single numerical values ​​or sub-ranges contained in the range, just as each numerical value and sub-range are clearly described.For example, the numerical range of about 1 to about 4.5 should be interpreted as not only including the limit value of 1 to about 4.5 clearly described, but also including single numerals (such as 2,3,4) and sub-ranges (such as 1 to 3, 2 to 4, etc.).The same principle is applicable to the scope of only narrating a numerical value, such as "less than about 4.5," which should be interpreted as including all the above-mentioned values ​​and ranges.In addition, no matter how the breadth of the described range or feature is, this explanation should be applicable.

[0054] Example 1

[0055] A method for preparing a nitrogen-phosphorus co-doped hard carbon negative electrode material according to this embodiment comprises the following steps:

[0056] S1. Prepare a sucrose solution with a concentration of 0.5 M, add urea phosphate to the sucrose solution, the mass of urea phosphate is 1% of the mass of sucrose, obtain a mixed aqueous solution, and perform ultrasonic treatment for 20 minutes to obtain a mixed aqueous solution after ultrasonic treatment;

[0057] 70 ml of the ultrasonic mixed aqueous solution was measured and placed in a 100 ml polytetrafluoroethylene hydrothermal kettle, and placed in a 200° C. vacuum oven for 18 h. The hydrothermal reaction was completed to obtain a hydrothermal reaction product.

[0058] The obtained hydrothermal reaction product was repeatedly washed with deionized water and anhydrous ethanol, and then dried at 100°C for 12 h to obtain a black block-like sucrose-based hard carbon precursor.

[0059] S2. The obtained black block sucrose-based hard carbon precursor is carbonized in a tubular furnace with the following specific parameters: heating to 1000°C at a rate of 3°C / min, keeping warm for 2 hours, introducing high-purity argon gas for protection during the heating and keeping warm process, and controlling the gas flow rate at 50ml / min~100ml / min to obtain a black material; after the black material cools to room temperature, it is taken out from the tubular furnace and passed through a 300-mesh sieve to obtain a nitrogen-phosphorus co-doped hard carbon negative electrode material.

[0060] Example 2

[0061] The preparation method of a nitrogen-phosphorus co-doped hard carbon negative electrode material in this embodiment is basically the same as that in Embodiment 1, except that:

[0062] In step S1, the mass of urea phosphate is 3% of the mass of sucrose.

[0063] Example 3

[0064] The preparation method of a nitrogen-phosphorus co-doped hard carbon negative electrode material in this embodiment is basically the same as that in Embodiment 1, except that:

[0065] In step S1, the mass of urea phosphate is 4% of the mass of sucrose.

[0066] Example 4

[0067] The preparation method of a nitrogen-phosphorus co-doped hard carbon negative electrode material in this embodiment is basically the same as that in Embodiment 1, except that:

[0068] In step S1, the mass of urea phosphate is 5% of the mass of sucrose.

[0069] Example 5

[0070] A method for preparing a nitrogen-phosphorus co-doped hard carbon negative electrode material according to this embodiment comprises the following steps:

[0071] S1, preparing a 0.5M sucrose solution, adding urea phosphate to the sucrose solution, wherein the mass percentage of urea phosphate to sucrose is 4%, obtaining a mixed aqueous solution, and ultrasonicating for 20 minutes to obtain a post-ultrasonication mixed aqueous solution;

[0072] 70 ml of the ultrasonic mixed aqueous solution was measured and placed in a 100 ml polytetrafluoroethylene hydrothermal kettle, and placed in a 200° C. vacuum oven for 18 h. The hydrothermal reaction was completed to obtain a hydrothermal reaction product.

[0073] The obtained hydrothermal reaction product was repeatedly washed with deionized water and anhydrous ethanol, and then dried at 100°C for 12 h to obtain a black block-like sucrose-based hard carbon precursor.

[0074] S2. The obtained black block sucrose-based hard carbon precursor is carbonized in a tubular furnace with the following specific parameters: heating to 1000°C at a rate of 2°C / min and keeping warm for 2 hours. During the heating and keeping warm process, high-purity argon gas is introduced for protection and the gas flow rate is controlled at 50ml / min~100ml / min to obtain a black material. After the black material cools to room temperature, it is taken out from the tubular furnace and passed through a 300-mesh sieve to obtain a nitrogen-phosphorus co-doped hard carbon negative electrode material.

[0075] Example 6

[0076] The preparation method of a nitrogen-phosphorus co-doped hard carbon negative electrode material in this embodiment is basically the same as that in Embodiment 5, except that:

[0077] In step S2, the temperature is increased to 1000°C at a rate of 4°C / min.

[0078] Example 7

[0079] The preparation method of a nitrogen-phosphorus co-doped hard carbon negative electrode material in this embodiment is basically the same as that in Embodiment 5, except that:

[0080] In step S2, the temperature is increased to 1000°C at a rate of 5°C / min.

[0081] Comparative Example 1

[0082] The preparation method of a sucrose-based hard carbon negative electrode material in this comparative example comprises the following steps:

[0083] S1. Prepare a 0.5M sucrose solution, measure 70 ml of the sucrose solution into a 100 ml polytetrafluoroethylene hydrothermal kettle, put it into a 200°C vacuum oven and keep it warm for 18 hours. The hydrothermal reaction is completed to obtain a hydrothermal reaction product;

[0084] The obtained hydrothermal reaction product was repeatedly washed with deionized water and anhydrous ethanol, and then dried at a drying temperature of 100° C. for 12 h to obtain a reddish-brown blocky sucrose-based hard carbon precursor.

[0085] S2. The obtained reddish-brown block-like sucrose-based hard carbon precursor is carbonized in a tubular furnace. The specific parameters are: heating to 800°C, 900°C, and 1000°C at a rate of 3°C / min, respectively, and keeping warm for 2 hours. During the heating and keeping warm process, high-purity argon gas is introduced for protection, and the gas flow rate is controlled at 50ml / min~100ml / min to obtain a black material; after the black material cools to room temperature, it is taken out from the tubular furnace and passed through a 300-mesh sieve to obtain a sucrose-based hard carbon negative electrode material.

[0086] The blank sample is a sucrose-based hard carbon negative electrode material obtained by heating the sample to 1000°C in step S2, i.e., the carbonization temperature is 1000°C.

[0087] Comparative Example 2

[0088] The preparation method of a sucrose-based hard carbon negative electrode material in this comparative example comprises the following steps:

[0089] S1. Prepare a 0.5M sucrose solution, measure 70 ml of the sucrose solution into a 100 ml polytetrafluoroethylene hydrothermal kettle, put it into a 200°C vacuum oven and keep it warm for 18 hours. The hydrothermal reaction is completed to obtain a hydrothermal reaction product;

[0090] The obtained hydrothermal reaction product was repeatedly washed with deionized water and anhydrous ethanol, and then dried at a drying temperature of 100° C. for 12 h to obtain a reddish-brown blocky sucrose-based hard carbon precursor.

[0091] S2. Carbonize the obtained reddish-brown blocky sucrose-based hard carbon precursor in a tubular furnace with the following specific parameters: Heat it to 1000 °C at rates of 1 °C / min, 3 °C / min, and 5 °C / min respectively, hold for 2 hours, and introduce high-purity argon for protection during the heating and holding processes. Control the gas flow rate at 50 ml / min - 100 ml / min to obtain a black material. After the black material cools down to room temperature, take it out of the tubular furnace, and obtain the sucrose-based hard carbon anode material after passing through a 300-mesh sieve.

[0092] Comparative Example 3

[0093] A preparation method of a phosphorus-doped hard carbon anode material in this comparative example includes the following steps:

[0094] S1. Prepare a 0.5 M sucrose solution, add sodium dihydrogen phosphate to the sucrose solution, where the mass of sodium dihydrogen phosphate accounts for 4% of the mass of sucrose, to obtain a mixed aqueous solution, and ultrasonicate for 20 min to obtain the ultrasonically treated mixed aqueous solution.

[0095] Measure 70 ml of the ultrasonically treated mixed aqueous solution into a 100 ml polytetrafluoroethylene hydrothermal autoclave, place it in a 200 °C vacuum oven and hold for 18 h. After the hydrothermal reaction ends, obtain the hydrothermal reaction product.

[0096] Wash the obtained hydrothermal reaction product repeatedly with deionized water and absolute ethanol, and then dry it at a drying temperature of 100 °C for 12 h to obtain a black blocky sucrose-based hard carbon precursor.

[0097] S2. Carbonize the obtained black blocky sucrose-based hard carbon precursor in a tubular furnace with the following specific parameters: Heat it to 1000 °C at a rate of 3 °C / min, hold for 2 hours, and introduce high-purity argon for protection during the heating and holding processes. Control the gas flow rate at 50 ml / min - 100 ml / min to obtain a black material. After the black material cools down to room temperature, take it out of the tubular furnace, and obtain the phosphorus-doped hard carbon anode material after passing through a 300-mesh sieve.

[0098] Comparative Example 4

[0099] A preparation method of a nitrogen-doped hard carbon anode material in this comparative example includes the following steps:

[0100] S1. Prepare a 0.5 M sucrose solution, add urea to the sucrose solution, where the mass of urea accounts for 4% of the mass of sucrose, to obtain a mixed aqueous solution, and ultrasonicate for 20 min to obtain the ultrasonically treated mixed aqueous solution.

[0101] Measure 70 ml of the ultrasonically treated mixed aqueous solution into a 100 ml polytetrafluoroethylene hydrothermal autoclave, place it in a 200 °C vacuum oven and hold for 18 h. After the hydrothermal reaction ends, obtain the hydrothermal reaction product.

[0102] The obtained hydrothermal reaction product was repeatedly washed with deionized water and anhydrous ethanol, and then dried at 100°C for 12 h to obtain a black block-like sucrose-based hard carbon precursor.

[0103] S2. The obtained black block sucrose-based hard carbon precursor is carbonized in a tubular furnace. The specific parameters are: heating to 1000°C at a rate of 3°C / min, keeping warm for 2 hours, introducing high-purity argon gas for protection during the heating and keeping warm process, and controlling the gas flow rate at 50ml / min~100ml / min to obtain a black material; after the black material cools to room temperature, it is taken out from the tubular furnace and passed through a 300-mesh sieve to obtain a nitrogen-doped hard carbon negative electrode material.

[0104] Performance Test:

[0105] (1) Figure 1 Shown is a scanning electron microscope image (SEM image) of the nitrogen-phosphorus co-doped hard carbon negative electrode material in Example 3. The figure reflects that after the introduction of urea phosphate, the particle diameter of the obtained nitrogen-phosphorus co-doped hard carbon negative electrode material increases to 5μm, the surface presents evenly distributed nanoscale protrusions, and the dispersibility is significantly improved. This morphological difference is due to the fact that the nitrogen-containing groups (such as pyridinic nitrogen and pyrrolic nitrogen) generated by the pyrolysis of urea phosphate enhance the cross-linking degree of the carbon skeleton through CN bonding, resulting in an increase in particle size; the local electron enrichment generated by nitrogen doping and the PO polar groups induced by phosphorus doping synergistically inhibit the agglomeration of particles during high-temperature carbonization by enhancing the electrostatic repulsion.

[0106] (2) XRD patterns of the nitrogen-phosphorus co-doped hard carbon negative electrode material prepared in Example 3 and the blank sample prepared in Comparative Example 1 were measured using an X-ray diffractometer.

[0107] like Figure 4 As shown, compared with the undoped sucrose-based hard carbon material, after nitrogen and phosphorus co-doping, the XRD diffraction peak shifts to the left and the peak becomes wider, indicating that nitrogen and phosphorus doping increases the interlayer spacing. The increase in the interlayer spacing is conducive to the embedding and extraction of sodium ions, thereby improving the specific capacity and rate performance of the material.

[0108] (3) First charge and discharge test

[0109] The hard carbon negative electrode material obtained in the above examples and comparative examples is prepared into a negative electrode sheet, comprising the following steps:

[0110] Using the hard carbon negative electrode material, conductive agent carbon black (SP), and binder polyvinylidene fluoride (PVDF) obtained from the above-mentioned examples and comparative examples as raw materials, a slurry was prepared according to a mass ratio of 8:1:1. The hard carbon negative electrode material (0.24 g) was fully mixed with conductive agent carbon black (0.03 g), ground for 15 min, placed in a drying oven to dry, and after cooling to room temperature, the prepared 5% binder (PVDF) was added and ground for 5 min, and then coated on the current collector aluminum foil. It was placed in a vacuum drying oven and dried at 120 °C for 12 h. After drying, the cut pieces were removed to obtain the negative electrode sheet.

[0111] The obtained negative electrode sheets were assembled into sodium-ion batteries, and the sodium-ion batteries were subjected to the first charge-discharge test. The test conditions were as follows: constant current charge-discharge test at 0.05 C, and the test range was 0 V to 2.5 V.

[0112] As Figure 2 shown, in Example 1, Example 2, Example 3, and Example 4, the masses of urea phosphate were 1%, 3%, 4%, and 5% of the mass of sucrose in turn, the first charge-discharge curves of the nitrogen and phosphorus co-doped hard carbon negative electrode materials prepared, and the first charge-discharge curve of the blank hard carbon negative electrode material prepared in Comparative Example 1;

[0113] As Figure 3 shown, in Example 5, Example 3, Example 6, and Example 7, the heating rates were 2 °C / min, 3 °C / min, 4 °C / min, and 5 °C / min in turn. At different heating rates, the first charge-discharge curves of the nitrogen and phosphorus co-doped hard carbon negative electrode materials were obtained;

[0114] The first efficiency of Example 3 was the highest at 79%, and the charge specific capacity was also the highest at 310 mAh / g. The mass of urea phosphate was 4% of the mass of sucrose, and it was heated to 1000 °C at a rate of 3 °C / min, which were the optimal parameters. The blank sample prepared in Comparative Example 1, that is, without adding urea phosphate, had a first efficiency of 58% and a charge specific capacity of 211 mAh / g.

[0115] As Figure 6 shown, in Comparative Example 1, without adding urea phosphate, it was heated to 800 °C, 900 °C, and 1000 °C at a rate of 3 °C / min respectively, and carbonized at different temperatures to obtain the first charge-discharge curves of the hard carbon negative electrode materials;

[0116] As Figure 7 shown, in Comparative Example 2, without adding urea phosphate either, it was heated to 1000 °C at rates of 1 °C / min, 3 °C / min, and 5 °C / min, and at different heating rates, the first charge-discharge curves of the hard carbon negative electrode materials were obtained;

[0117] In Comparative Example 1 and Comparative Example 2, the temperature was increased to 1000 °C at a rate of 3 °C / min to prepare a blank sample. The initial efficiency was 58%, and the charge specific capacity was 211 mAh / g. Although it was higher than the hard carbon negative electrode materials prepared under other parameters in Comparative Examples 1 and 2, it was less than the initial efficiency and charge specific capacity of the nitrogen and phosphorus co-doped hard carbon negative electrode materials prepared in Examples 1-7.

[0118] As Figure 8 shown, the first charge-discharge curves of the nitrogen and phosphorus co-doped hard carbon negative electrode material (urea phosphate - 4% 1000 °C) prepared in Example 3 and the phosphorus-doped hard carbon negative electrode material (sodium dihydrogen phosphate - 4% 1000 °C) prepared in Comparative Example 3 were compared. The initial efficiency and charge specific capacity of urea phosphate - 4% were both higher than those of sodium dihydrogen phosphate - 4%.

[0119] As Figure 9 shown, the first charge-discharge curves of the nitrogen and phosphorus co-doped hard carbon negative electrode material (urea phosphate - 4% 1000 °C) prepared in Example 3 and the nitrogen-doped hard carbon negative electrode material (urea - 4% 1000 °C) prepared in Comparative Example 4 were compared. The initial efficiency and charge specific capacity of urea phosphate - 4% were both higher than those of urea - 4%.

[0120] (4) Rate performance test

[0121] The sodium-ion battery assembled in the first charge-discharge test in (3) was subjected to a rate performance test. The test conditions were as follows: at rates of 0.1C (about 25 mA / g), 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 15C, constant current test, and the test range was 0 V to 2.5 V.

[0122] As Figure 5 shown, the nitrogen and phosphorus co-doped hard carbon negative electrode material prepared in Example 3 was denoted as HC-NP-4%, and the blank sample prepared in Comparative Example 1 was denoted as HC-NP-0%. The rate performance of the nitrogen and phosphorus co-doped hard carbon negative electrode material was significantly better than that of the undoped blank sample. HC-NP-4% could still maintain a reversible capacity of 195 mAh / g at a high rate of 15C, and the capacity only decayed by 7.7% (254 mAh / g → 275 mAh / g) when restored to 0.1C.

[0123] As can be seen from Examples 1-7, temperature, heating rate, calcination method, etc. during the formation of hard carbon have a great influence on its final electrochemical performance. In the present invention, the calcination process is regulated to affect the carbon layer arrangement, pore distribution, interlayer distance, and degree of disorder of hard carbon. This method is simple, economical, and effective; introducing heteroatoms (N, P) into hard carbon is the best strategy to improve sodium storage performance by constructing pore structures, generating abundant active sites / defects, adjusting interlayer spacing, and improving electronic conductivity by regulating the electronic structure, which helps to improve high-rate performance and reversible capacity. In the present invention, by adding urea phosphate to the sucrose solution, heteroatoms (N, P) are introduced, achieving a good effect of co-doping of nitrogen and phosphorus. Compared with non-nitrogen and phosphorus doping (Comparative Examples 1 and 2), single nitrogen doping (Comparative Example 4), and phosphorus doping (Comparative Example 3), it has higher specific capacity, initial efficiency, and rate performance, and at the same time provides relevant theoretical support for subsequent modification of hard carbon.

[0124] Hard carbon does not have a specific structural model. Its microstructure is related to the heat treatment temperature, heat treatment environment, and the source of the material itself. Therefore, it is difficult to describe and define the structure and properties of hard carbon. Most experimental results show that the hard carbon obtained below 800 °C or even 1000 °C is mainly in an amorphous structure, contains more heteroatoms, has poor material crystallinity, and has more pores. In terms of sodium storage performance, the first discharge shows a long sloping region and a short plateau region, the first charge capacity is low, and the first Coulombic efficiency is low. This is because too many heteroatoms make the storage of sodium ions mainly adsorption-based, with poor reversibility. The preparation method of the nitrogen and phosphorus co-doped hard carbon negative electrode material of the present invention mainly selects appropriate nitrogen and phosphorus doping raw materials, controls the heating rate, and positively intervenes in the growth process of sucrose-based hard carbon to achieve precise control of the microstructure of hard carbon. The present invention can also be coated and modified with other materials (soft carbon, inert materials) to further improve the initial efficiency and cycle capacity.

[0125] The above-described embodiments merely represent the preferred embodiments of the present invention, and their descriptions are relatively specific and detailed, but should not be construed as limiting the scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations, improvements, and substitutions can be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the appended claims.

Claims

1. A method for preparing a nitrogen-phosphorus co-doped hard carbon negative electrode material, characterized in that: The following steps are involved: Sucrose and urea phosphate are mixed and subjected to a hydrothermal reaction to obtain a hard carbon precursor; Carbonizing the hard carbon precursor at high temperature to obtain a nitrogen-phosphorus co-doped hard carbon negative electrode material; The mass of the urea phosphate accounts for 1% to 10% of the mass of sucrose.

2. The method for preparing a nitrogen-phosphorus co-doped hard carbon negative electrode material according to claim 1, characterized in that: The concentration of the sucrose is 0.1 mol / L to 1 mol / L.

3. The method for preparing a nitrogen-phosphorus co-doped hard carbon negative electrode material according to claim 2, characterized in that: The temperature of the hydrothermal reaction is 150° C. to 250° C., and the time of the hydrothermal reaction is 12 h to 24 h.

4. The method for preparing a nitrogen-phosphorus co-doped hard carbon negative electrode material according to claim 3, characterized in that: The product obtained by the hydrothermal reaction is dried at a temperature of 80° C. to 120° C. for a drying time of 5 h to 12 h.

5. The method for preparing a nitrogen-phosphorus co-doped hard carbon negative electrode material according to claim 1, characterized in that: The specific parameters of the high temperature carbonization are: heating to 600° C. to 1000° C. at a rate of 1° C. / min to 8° C. / min, and keeping the temperature for 1 h to 5 h.

6. The method for preparing a nitrogen-phosphorus co-doped hard carbon negative electrode material according to claim 5, characterized in that: During the heating and the heat preservation, an inert gas is introduced, and the gas flow rate is 50 ml / min to 100 ml / min.

7. A nitrogen-phosphorus co-doped hard carbon negative electrode material, characterized in that: Prepared by the method according to any one of claims 1 to 6.

8. A negative electrode plate, characterized in that: It includes the nitrogen and phosphorus co-doped hard carbon negative electrode material as described in claim 7.

9. A sodium ion battery, characterized in that: Including the negative electrode sheet as described in claim 8.

10. Application of a nitrogen-phosphorus co-doped hard carbon negative electrode material, characterized in that: The negative electrode material according to claim 7 is applied to the preparation of negative electrode sheets and / or sodium ion batteries.

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