Nitrogen and phosphorus co-doped porous hard carbon material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of electrochemical materials, and particularly relates to a nitrogen-phosphorus co-doped porous hard carbon material and a preparation method and application thereof. The method comprises the following steps: firstly, carrying out hydrothermal treatment on a biomass material by adopting a sodium silicate aqueous solution, and converting into a SiO2 / hydrothermal carbon composite material; then removing generated SiO2 by using a KOH solution, and then carrying out heat treatment; after redundant KOH is removed by using a phosphoric acid solution, the carbon material is subjected to secondary heat treatment, so that the hard carbon material has phosphorus doping. The nitrogen and phosphorus co-doped porous hard carbon has a controllable porous structure, and nitrogen and phosphorus in the porous hard carbon material are co-doped, so that the porous hard carbon material has rich defect active sites. The nitrogen-phosphorus co-doped porous hard carbon as a sodium-ion battery negative electrode material has good rate capability, high specific discharge capacity and excellent cycle stability.

Description

Technical Field

[0001] This invention belongs to the technical field of electrochemical materials, specifically relating to a nitrogen-phosphorus co-doped porous hard carbon material, its preparation method, and its application. Background Technology

[0002] With the rapid growth of energy demand and the continuous depletion of fossil fuel reserves, the development and use of clean energy has received widespread attention in today's society. Clean energy has advantages such as sustainability, zero pollution, and low cost; however, its large-scale application is limited by geographical location and the discontinuity of production, relying on stable and efficient energy storage devices. Among various energy storage devices, lithium-ion batteries (LIBs) are widely used in portable electronic devices and electric vehicles due to their high energy density, good cycle stability, and high safety. However, due to the limited lithium reserves and uneven geographical distribution, LIBs are gradually failing to meet the future demand for large-scale, high-performance energy storage. Sodium-ion batteries, as a new type of energy storage device, have advantages such as abundant sodium reserves and low production costs. However, due to the limited availability and uneven geographical distribution of sodium, LIBs are gradually failing to meet the future demand for large-scale, high-performance energy storage. + Its large size leads to sluggish reaction kinetics with the negative electrode material, and Na + The inability to form thermally stable intercalation compounds with traditional LIBs (graphite anodes) severely limits the large-scale application of sodium-ion batteries. Hard carbon materials, due to their advantages such as large interlayer spacing, low sodium insertion / extraction potential, and low manufacturing cost, are considered the most promising and commercially viable anode materials for sodium-ion batteries.

[0003] Biomass-derived hard carbon materials are among the most promising anode materials for sodium-ion batteries due to their wide availability of raw materials and low production costs. Currently, various types of biomass-derived hard carbon materials are being developed and used as anode materials for sodium-ion batteries, such as those derived from coconut shells, peanut shells, straw, pine nut shells, and grapefruit peels. Direct carbonization is the main method for preparing biomass-derived hard carbon materials; however, hard carbon materials obtained through direct pyrolysis typically have a low specific surface area and untunable pore structure, limiting further improvement in their sodium storage performance. To further enhance the sodium storage performance of biomass-derived hard carbon materials, existing research often uses KOH, NaOH, K2CO3, etc., to further activate the carbon materials. However, the activated carbon materials usually have an open microporous structure, which is not conducive to Na+ storage. + Storage. Furthermore, the diverse sources of biomass precursors mean that experimental protocols are often not universally applicable when preparing porous hard carbon.

[0004] CN117985692A discloses a nitrogen-phosphorus co-doped porous hard carbon material, its preparation method, and a sodium-ion battery. This patent involves low-temperature pre-carbonization of a biomass carbon source to obtain a precursor; mixing the precursor with a phosphorus source and a nitrogen source to obtain a nitrogen-phosphorus co-doped porous hard carbon precursor; and high-temperature carbonization of the nitrogen-phosphorus co-doped porous hard carbon precursor to obtain the nitrogen-phosphorus co-doped porous hard carbon material. This technical solution has the following problems: (1) it requires the introduction of an additional nitrogen source, increasing the preparation cost; (2) it requires more than 1000... o The high-temperature carbonization process of C is complex; (3) the nitrogen-phosphorus co-doped hard carbon material has a small specific surface area and poor electrochemical performance at high rates.

[0005] Therefore, developing a universal method for preparing biomass-derived porous hard carbon materials with excellent rate performance is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to address the aforementioned deficiencies by providing a nitrogen-phosphorus co-doped porous hard carbon material with tunable pore structure, its preparation method, and its applications. Using biomass materials as both carbon and nitrogen sources, sodium silicate as a template agent, and phosphoric acid as a phosphorus source, nitrogen-phosphorus co-doped porous hard carbon is prepared via hydrothermal and carbonization methods. The carbon source is widely available and inexpensive, and the template agent is also inexpensive. The material obtained by this invention has distinct structural characteristics: firstly, the carbon material has a well-developed pore structure; secondly, the pore structure of the carbon material is tunable; and thirdly, the prepared carbon material exhibits nitrogen and phosphorus co-doping.

[0007] This invention first uses a sodium silicate aqueous solution to perform hydrothermal treatment on biomass materials. Since biomass materials are porous and hydrophilic, after hydrothermal treatment, sodium silicate generates SiO2 within the biomass material, which acts as a template, transforming it into a SiO2 / hydrothermal carbon composite material. Then, KOH solution is used to remove the generated SiO2 before heat treatment. During this process, KOH and SiO2 synergistically etch the hydrothermal carbon, giving the carbon material a rich porous structure. After removing excess KOH with an excess phosphoric acid solution, a second heat treatment of the carbon material allows for phosphorus doping of the hard carbon material.

[0008] The biomass-derived nitrogen-phosphorus co-doped porous hard carbon prepared by this invention has a controllable porous structure, and the nitrogen-phosphorus co-doping in the porous hard carbon material gives it abundant defective active sites. This nitrogen-phosphorus co-doped porous hard carbon exhibits good rate performance, high discharge specific capacity, and excellent cycle stability as a negative electrode material for sodium-ion batteries.

[0009] The specific technical solution is as follows:

[0010] A method for preparing a nitrogen-phosphorus co-doped porous hard carbon material includes the following steps:

[0011] (1) Preparation of precursors:

[0012] First, the biomass material is washed with deionized water to remove impurities, dried, and then pulverized in a pulverizer and sieved through a 10-50 mesh to obtain biomass material powder. This biomass material can be selected from carbon-containing agricultural waste, such as straw, peanut shells, sugarcane bagasse, bark, fruit shells (coconut shells, mangosteen shells, walnut shells, etc.) and fruit peels.

[0013] Then, the obtained biomass material powder is added to an aqueous sodium silicate solution and ultrasonically dispersed to obtain a biomass material / sodium silicate mixed solution.

[0014] (2) Preparation of SiO2 / hydrothermal carbon composite material:

[0015] First, the obtained biomass material / sodium silicate mixed solution is transferred to a hydrothermal reactor, the hydrothermal temperature is controlled at 200-250℃, and the holding time is 2-4h. After hydrothermal treatment, SiO2 / hydrothermal carbon composite material is obtained.

[0016] Then, the obtained SiO2 / hydrothermal carbon composite material was washed by centrifugation with deionized water and dried.

[0017] (3) Preparation of nitrogen-doped porous hard carbon materials:

[0018] First, the SiO2 / hydrothermal carbon composite material obtained in step (2) was soaked in KOH aqueous solution for 24 hours at room temperature to remove SiO2, and then centrifuged and dried.

[0019] Then, it is placed in an alumina ceramic boat and heated to 600-800℃ under nitrogen protection for carbonization treatment. After holding at this temperature for 1-4 hours, it is cooled under nitrogen protection to obtain nitrogen-doped porous hard carbon material.

[0020] (4) Preparation of nitrogen-phosphorus co-doped porous hard carbon materials:

[0021] First, the obtained nitrogen-doped porous hard carbon material was placed in an aqueous phosphoric acid solution to remove excess KOH, and then centrifuged and dried.

[0022] Then, under nitrogen protection, it is heated to 600-800℃ for a second heat treatment, held at that temperature for 1-4 hours, cooled under nitrogen protection, washed with deionized water until neutral, and dried to obtain nitrogen-phosphorus co-doped porous hard carbon. The temperature of the second heat treatment is the same as the carbonization temperature in step (3).

[0023] Furthermore, in step (1), the mass ratio of sodium silicate to biomass powder is (1-5):1.

[0024] Furthermore, in step (1), the biomass material is selected from any one of straw, peanut shells, bamboo, sugarcane bagasse, tree bark, coconut shells, mangosteen shells, or walnut shells.

[0025] Furthermore, the concentration of the KOH aqueous solution in step (3) is 2-7 mol / L.

[0026] Furthermore, the heating rate of the carbonization process in step (3) is 2℃ / min.

[0027] Furthermore, the concentration of the phosphoric acid aqueous solution in step (4) is 1-7 mol / L.

[0028] Furthermore, the heating rate of the secondary heat treatment in step (4) is 2℃ / min.

[0029] The nitrogen-phosphorus co-doped porous hard carbon material prepared by the above method has a specific surface area of ​​500-1500 m². 2 / g; pore size is 1-300nm.

[0030] The application of the nitrogen-phosphorus co-doped porous hard carbon material prepared by the above method, or the application of the above-mentioned nitrogen-phosphorus co-doped porous hard carbon material as a negative electrode material in sodium-ion batteries. The prepared nitrogen-phosphorus co-doped porous hard carbon material and binder are mixed in a 93:7 ratio to prepare a slurry, which is then uniformly coated onto copper foil. After drying in a vacuum oven, the working electrode is cut into pieces. A coin cell is assembled using metallic sodium as the counter electrode.

[0031] A sodium-ion battery uses a nitrogen-phosphorus co-doped porous hard carbon material prepared by the above-mentioned preparation method or the above-mentioned nitrogen-phosphorus co-doped porous hard carbon material as the negative electrode material; the sodium-ion battery has a discharge specific capacity of >200mAh / g at a current density of 20A / g and a reversible discharge specific capacity of >200mAh / g after 500 cycles at a current density of 2A / g.

[0032] The beneficial effects of this invention are as follows:

[0033] 1. This invention proposes the use of sodium silicate as a template agent for biomass-derived hard carbon. Sodium silicate hydrolyzes during the hydrothermal process to generate a template. The pore structure of the generated carbon material can be adjusted by regulating the ratio of sodium silicate to biomaterial and the heat treatment temperature.

[0034] Using sodium silicate solution for hydrothermal pretreatment of biomass materials gives this method good versatility, making it applicable to various biomass precursors. First, the biomass material is hydrothermally treated with sodium silicate. During the hydrothermal process, sodium silicate hydrolyzes to generate SiO2, which serves as a template. Then, KOH is used for activation, giving the material a well-developed porous structure, such as... Figure 1As shown. Biomass is porous and hydrophilic, and will be converted into hydrothermal carbon after hydrothermal treatment. Therefore, the method described in this invention makes the selection of biomass materials more universal. Even biomass materials selected from agricultural residues can be converted, and sodium silicate will generate SiO2 inside as a template.

[0035] 2. Using biomass materials as nitrogen and carbon sources can reduce environmental pollution and lower costs. While removing excess KOH with phosphoric acid, a secondary heat treatment can be used to dope carbon materials with phosphorus, improving their sodium storage capacity. Attached Figure Description

[0036] Figure 1 This is a scanning electron microscope (SEM) image of the nitrogen-phosphorus co-doped porous hard carbon material obtained in Example 1.

[0037] Figure 2 The image shows the X-ray diffraction pattern of the nitrogen-phosphorus co-doped porous hard carbon material obtained in Example 1.

[0038] Figure 3 The image shows the X-ray photoelectron spectrum of the nitrogen-phosphorus co-doped porous hard carbon material obtained in Example 1.

[0039] Figure 4 The nitrogen isothermal adsorption-desorption curve and pore size distribution diagram of the nitrogen-phosphorus co-doped porous hard carbon material obtained in Example 1 are shown.

[0040] Figure 5 The rate performance of the nitrogen-phosphorus co-doped porous hard carbon obtained in Example 4 as a sodium-ion battery anode material is shown.

[0041] Figure 6 The cycling stability of the nitrogen-phosphorus co-doped porous hard carbon obtained in Example 4 as a sodium-ion battery anode material is evaluated. Detailed Implementation

[0042] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0043] The biomass materials described in this invention are mainly agricultural residues, including straw, rice husks, coconut shells, sugarcane bagasse, peanut shells, tree bark, etc.

[0044] Example 1

[0045] The specific steps for preparing the nitrogen-phosphorus co-doped porous hard carbon material are as follows:

[0046] (1) Preparation of precursors:

[0047] First, wash the peanut shells with deionized water to remove impurities, dry them, and then grind them in a grinder. Sieve them through a 10-mesh sieve to obtain peanut shell powder.

[0048] Then, 4g of the obtained peanut shell powder was weighed and added to 40mL of an aqueous solution containing 4g of sodium silicate, and ultrasonically dispersed to obtain a peanut shell powder / sodium silicate mixed solution; wherein, the mass ratio of sodium silicate to peanut shell powder was 1:1.

[0049] (2) Preparation of SiO2 / hydrothermal carbon composite material:

[0050] First, the obtained peanut shell powder / sodium silicate mixed solution was transferred to a hydrothermal reactor, the hydrothermal temperature was controlled at 230℃ and the holding time was 2h, and SiO2 / hydrothermal carbon composite material was obtained after hydrothermal treatment.

[0051] Then, the obtained SiO2 / hydrothermal carbon composite material was washed by centrifugation with deionized water and dried.

[0052] (3) Preparation of nitrogen-doped porous hard carbon materials:

[0053] First, weigh 1g of the SiO2 / hydrothermal carbon composite material obtained in step (2) and soak it in a KOH aqueous solution with a concentration of 4mol / L for 24h at room temperature to remove SiO2, and then centrifuge and dry it.

[0054] Then, it was placed in an alumina ceramic boat and heated to 600℃ at 2℃ / min under nitrogen protection for carbonization treatment. After holding at this temperature for 2 hours, it was cooled under nitrogen protection to obtain nitrogen-doped porous hard carbon material.

[0055] (4) Preparation of nitrogen-phosphorus co-doped porous hard carbon materials:

[0056] First, the obtained nitrogen-doped porous hard carbon material was placed in 100 mL of a 1 mol / L phosphoric acid aqueous solution to remove excess KOH, and then centrifuged and dried.

[0057] Then, under nitrogen protection, it was heated to 600℃ at 2℃ / min for a second heat treatment, held at that temperature for 2h, cooled under nitrogen protection, washed with deionized water until neutral, and dried to obtain nitrogen-phosphorus co-doped porous hard carbon. The temperature of the second heat treatment was the same as the carbonization temperature in step (3).

[0058] Depend on Figure 1 It can be seen that the prepared carbon material has a well-developed pore structure, and there are a large number of interconnected pores, which can provide a basis for Na… + Storage provides multiple sites and effectively accelerates Na... + The transmission.

[0059] Depend on Figure 2 It can be seen that the obtained carbon material only at 24 o The presence of a broad, domed peak on either side indicates that the prepared carbon material is amorphous carbon. The amorphous structure of the carbon material can accelerate the reaction of Na+.+ Embedding / exiting.

[0060] Depend on Figure 3 It can be seen that the porous hard carbon material is composed of three elements: C, N, and P. This confirms the successful doping of nitrogen and phosphorus elements, which can increase the sodium storage active sites in the carbon material and improve its conductivity.

[0061] XPS quantitative analysis results showed that the nitrogen doping content in the obtained carbon material was 1.8%, and the phosphorus doping content was 2.1%.

[0062] Depend on Figure 4 The nitrogen isotherm adsorption-desorption curves for a show that the specific surface area of ​​the prepared carbon material is 1099 m². 2 / g. From Figure 4 As can be seen from the pore size distribution diagram of b, the mesopore size of the obtained porous hard carbon material ranges from 1 to 20 nm.

[0063] The nitrogen-phosphorus co-doped hard carbon material prepared in this example, when used in a sodium-ion battery, exhibits a discharge specific capacity of 202 mAh / g at a current density of 20 A / g. After 500 cycles at a current density of 2 A / g, it exhibits a discharge specific capacity of 220 mAh / g.

[0064] Example 2

[0065] The difference from Example 1 is that the biomass material used in this example is coconut shell.

[0066] The others are the same as in Example 1.

[0067] The nitrogen-phosphorus co-doped hard carbon material prepared in this embodiment, when used in a sodium-ion battery, exhibits a discharge specific capacity of 210 mAh / g at a current density of 20 A / g. After 500 cycles at a current density of 2 A / g, it exhibits a discharge specific capacity of 243 mAh / g.

[0068] Example 3

[0069] The difference from Example 1 is that the biomass material used in this example is straw.

[0070] The others are the same as in Example 1.

[0071] The nitrogen-phosphorus co-doped hard carbon material prepared in this embodiment, when used in a sodium-ion battery, exhibits a discharge specific capacity of 225 mAh / g at a current density of 20 A / g. After 500 cycles at a current density of 2 A / g, it exhibits a discharge specific capacity of 235 mAh / g.

[0072] Example 4

[0073] The difference from Example 1 is that the biomass material used in this example is bamboo.

[0074] The others are the same as in Example 1.

[0075] The nitrogen-phosphorus co-doped hard carbon material prepared in this embodiment, when used in a sodium-ion battery, exhibits a discharge specific capacity of 230 mAh / g at a current density of 20 A / g. After 500 cycles at a current density of 2 A / g, it exhibits a discharge specific capacity of 226 mAh / g.

[0076] Example 5

[0077] The difference from Example 1 is that the biomass material used in this example is sugarcane bagasse.

[0078] The others are the same as in Example 1.

[0079] The nitrogen-phosphorus co-doped porous hard carbon material prepared in this embodiment, when used in a sodium-ion battery, exhibits a discharge specific capacity of 210 mAh / g at a current density of 20 A / g. After 500 cycles at a current density of 2 A / g, it exhibits a discharge specific capacity of 236 mAh / g.

[0080] Example 6

[0081] The difference from Example 1 is that the carbonization temperature in step (3) of this example is 700°C, and the temperature of the secondary heat treatment in step (4) is also 700°C.

[0082] The prepared nitrogen-phosphorus co-doped porous hard carbon material has a density of 1226 μm. 2 / g high specific surface area.

[0083] The others are the same as in Example 1.

[0084] The hard carbon material prepared in this example, when used in a sodium-ion battery, exhibits a discharge specific capacity of 235 mAh / g at a current density of 20 A / g. After 500 cycles at a current density of 2 A / g, it displays a discharge specific capacity of 242 mAh / g.

[0085] Example 7

[0086] The difference from Example 1 is that the concentration of the phosphoric acid aqueous solution in step (4) of this example is 5 mol / L.

[0087] The phosphorus content in the prepared nitrogen-phosphorus co-doped porous hard carbon material is 4.5%.

[0088] The others are the same as in Example 1.

[0089] The hard carbon material prepared in this embodiment is used in sodium-ion batteries and exhibits a discharge specific capacity of 233 mAh / g at a current density of 20 A / g. After 500 cycles at a current density of 2 A / g, it exhibits a discharge specific capacity of 265 mAh / g.

[0090] Example 8

[0091] The difference from Example 1 is that in step (1) of this example, 4g of the obtained peanut shell powder is added to 40mL of an aqueous solution containing 8g of sodium silicate, wherein the mass ratio of sodium silicate to peanut shell powder is 2:1.

[0092] The others are the same as in Example 1.

[0093] The mesopore size of the prepared nitrogen-phosphorus co-doped porous hard carbon material ranges from 5 to 20 nm.

[0094] Depend on Figure 5 It can be seen that the battery capacity does not decrease significantly with increasing current density. Even at a current density of 20 A / g, the electrode material still has a discharge specific capacity of 205 mAh / g.

[0095] Depend on Figure 6 It can be seen that the material still has a reversible discharge specific capacity of 260 mAh / g after 500 cycles at a current density of 2 A / g, demonstrating excellent sodium storage performance.

[0096] Comparative Example 1

[0097] The difference from Example 1 is that in step (1) of this comparative example, the mass ratio of sodium silicate to peanut shell powder is 1:2.

[0098] The others are the same as in Example 1.

[0099] Comparative Example 2

[0100] The difference from Example 1 is that the concentration of the KOH aqueous solution in step (3) of this comparative example is 9 mol / L.

[0101] The others are the same as in Example 1.

[0102] Comparative Example 3

[0103] The difference from Example 1 is that the carbonization temperature in step (3) and the secondary heat treatment temperature in step (4) of this comparative example are both 900°C.

[0104] The others are the same as in Example 1.

[0105] Comparative Example 4

[0106] The difference from Example 1 is that the carbonization temperature in step (3) of this comparative example is 600°C, while the temperature of the secondary heat treatment in step (4) is 900°C.

[0107] The others are the same as in Example 1.

[0108] The nitrogen-phosphorus co-doped porous hard carbon materials obtained in the various embodiments and comparative examples were mixed with a binder at a ratio of 93:7 to prepare slurries, which were then uniformly coated onto copper foil. After drying in a vacuum oven, working electrodes were cut into pieces. Button-type sodium-ion batteries were assembled using metallic sodium as the counter electrode. The performance of each assembled sodium-ion battery was tested, and the results are shown in Table 1.

[0109] Table 1 Performance Test Results

[0110] project Discharge specific capacity (mAh / g) at a current density of 20 A / g Discharge specific capacity (mAh / g) after 500 cycles at a current density of 2A / g. Example 1 202 220 Example 2 210 243 Example 3 225 235 Example 4 230 226 Example 5 209 236 Example 6 235 242 Example 7 233 265 Example 8 205 260 Comparative Example 1 125 134 Comparative Example 2 86 132 Comparative Example 3 69 88 Comparative Example 4 73 115

[0111] As can be seen from the data analysis in Table 1, the nitrogen-phosphorus co-doped porous hard carbon obtained in this invention has good rate performance, high discharge specific capacity and excellent cycle stability as a negative electrode material for sodium-ion batteries.

Claims

1. A method for preparing a nitrogen-phosphorus co-doped porous hard carbon material, characterized in that, Includes the following steps: (1) Preparation of precursors: First, the biomass material is washed, dried, pulverized, and sieved through a 10-50 mesh to obtain biomass material powder. Then, the obtained biomass material powder is added to an aqueous sodium silicate solution and ultrasonically dispersed to obtain a biomass material / sodium silicate mixed solution; (2) Preparation of SiO2 / hydrothermal carbon composite material: First, the obtained biomass material / sodium silicate mixed solution is transferred to a hydrothermal reactor, the hydrothermal temperature is controlled at 200-250℃, and the holding time is 2-4h. After hydrothermal treatment, SiO2 / hydrothermal carbon composite material is obtained. Then, the obtained SiO2 / hydrothermal carbon composite material was centrifuged, washed, and dried. (3) Preparation of nitrogen-doped porous hard carbon materials: First, the SiO2 / hydrothermal carbon composite material obtained in step (2) was soaked in KOH aqueous solution for 24 hours at room temperature to remove SiO2, and then centrifuged and dried. Then, under nitrogen protection, it is heated to 600-800℃ for carbonization treatment, held at that temperature for 1-4 hours, and then cooled under nitrogen protection. Nitrogen-doped porous hard carbon materials were obtained; (4) Preparation of nitrogen-phosphorus co-doped porous hard carbon materials: First, the obtained nitrogen-doped porous hard carbon material was placed in an aqueous phosphoric acid solution to remove excess KOH, and then centrifuged and dried. Then, under nitrogen protection, it is heated to 600-800℃ for secondary heat treatment, held at the temperature for 1-4 hours, cooled and cleaned to neutral under nitrogen protection, and dried to obtain nitrogen-phosphorus co-doped porous hard carbon. The temperature of the secondary heat treatment is the same as the temperature of the carbonization treatment in step (3).

2. The method for preparing nitrogen-phosphorus co-doped porous hard carbon material according to claim 1, characterized in that, In step (1), the mass ratio of sodium silicate to biomass powder is (1-5):

1.

3. The method for preparing nitrogen-phosphorus co-doped porous hard carbon material according to claim 1, characterized in that, In step (1), the biomass material is selected from any one of straw, peanut shells, bamboo, sugarcane bagasse, tree bark, coconut shells, mangosteen shells, or walnut shells.

4. The method for preparing nitrogen-phosphorus co-doped porous hard carbon material according to claim 1, characterized in that, The concentration of the KOH aqueous solution in step (3) is 2-7 mol / L.

5. The method for preparing nitrogen-phosphorus co-doped porous hard carbon material according to claim 1, characterized in that, The heating rate of the carbonization process in step (3) is 2℃ / min.

6. The method for preparing nitrogen-phosphorus co-doped porous hard carbon material according to claim 1, characterized in that, The concentration of the phosphoric acid aqueous solution in step (4) is 1-7 mol / L.

7. The method for preparing nitrogen-phosphorus co-doped porous hard carbon material according to claim 1, characterized in that, The heating rate of the secondary heat treatment in step (4) is 2℃ / min.

8. The nitrogen-phosphorus co-doped porous hard carbon material prepared by the preparation method according to any one of claims 1-7, characterized in that, The specific surface area of ​​the nitrogen-phosphorus co-doped porous hard carbon material is 500-1500 m². 2 / g; pore size is 1-300nm.

9. The application of the nitrogen-phosphorus co-doped porous hard carbon material prepared by the preparation method according to any one of claims 1-7 or the nitrogen-phosphorus co-doped porous hard carbon material according to claim 8 as a negative electrode material in sodium-ion batteries.

10. A sodium-ion battery, characterized in that, The nitrogen-phosphorus co-doped porous hard carbon material prepared by any one of the preparation methods described in claims 1-7 or the nitrogen-phosphorus co-doped porous hard carbon material described in claim 8 is used as the negative electrode material; the sodium-ion battery has a discharge specific capacity of >200mAh / g at a current density of 20A / g, and a reversible discharge specific capacity of >200mAh / g after 500 cycles at a current density of 2A / g.

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