A high-magnification, high-initial-efficiency biomass hard carbon composite negative electrode material, a preparation method and application thereof

Abstract

The application provides a high-rate and high-initial-efficiency biomass hard carbon composite negative electrode material and a preparation method and application thereof. The hard carbon composite negative electrode material has a core-shell structure, the core-shell structure comprises a core, an inner shell and an outer shell, the core comprises biomass hard carbon containing P and graphite crystallites, the inner shell comprises graphene, and the outer shell comprises carbon nanotubes. In the application, a graphene layer is coated on the surface of the biomass hard carbon, and carbon nanotubes are grown in situ on the surface of the graphene. During high-temperature carbonization, transition metals cooperatively induce the hard carbon to grow along the graphene layer, so that the graphene coating layer is tightly coated, and the cycle performance of the material is improved. Transition metal ions catalyze graphitization, and generate a more regular and ordered graphite-like layer crystalline structure coating layer, so that the defects of the graphene and the carbon nanotubes are reduced, and the initial coulomb efficiency is improved. In addition, phosphorus doping improves the ion transmission capacity, the graphite crystallites generated in the hard carbon by catalysis improve the electronic transmission capacity, the ion transmission and the electronic transmission are synergistically improved, and the rate performance is ensured.

Description

Technical Field

[0001] This invention relates to the field of anode materials for sodium-ion batteries, and in particular to a high-rate, high-efficiency biomass hard carbon composite anode material, its preparation method, and its application. Background Technology

[0002] With the rapid development of the electric vehicle market, the demand for lithium-ion batteries has surged. However, the reserves of lithium resources in the Earth's crust are limited, resulting in high costs for lithium-ion batteries. Meanwhile, sodium-ion batteries operate on a similar principle to lithium-ion batteries, have near-zero energy density, and are abundant, evenly distributed, and inexpensive, making them a promising candidate for large-scale energy storage devices.

[0003] Hard carbon is the most commonly used anode material in commercial sodium-ion batteries. Currently, the main raw materials for hard carbon materials fall into two categories: petroleum- and coal-based materials and biomass-based materials. Petroleum- and coal-based materials are widely available, and the hard carbon prepared from them exhibits stable performance and high carbon yield, but their structural characteristics result in a lower specific capacity compared to other materials. Biomass, as a carbon source, has become a focus of attention for hard carbon material preparation due to its environmental friendliness and low cost. However, the commonly used biomass hard carbon anode suffers from low first-cycle coulombic efficiency (<70%) and poor rate performance, hindering its commercial application. Therefore, developing hard carbon materials with high first-cycle coulombic efficiency and high rate performance is currently a key research focus and hot topic in sodium-ion battery development.

[0004] Currently, many strategies have been proposed to improve the first-cycle coulombic efficiency and rate performance of hard carbon, but achieving both goals simultaneously is difficult. Designing structures with large specific surface areas or doping with heteroatoms are common methods to improve the rate performance of hard carbon. However, hard carbon materials with large specific surface areas or heteroatom doping tend to have lower first-cycle coulombic efficiency. In contrast, hard carbon materials with low defects and low specific surface areas can achieve high first-cycle coulombic efficiency but do not improve sodium storage kinetics. Therefore, effective methods still need to be explored to simultaneously achieve high first-cycle coulombic efficiency and excellent rate performance. Summary of the Invention

[0005] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a high-rate, high-first-cycle-efficiency biomass hard carbon composite anode material that can balance excellent first-cycle coulombic efficiency, rate performance, and long cycle life. Furthermore, this invention will also provide a method for preparing the hard carbon composite anode material; in addition, this invention will also provide applications of the hard carbon composite anode material.

[0006] To achieve the above and other related objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a high-rate, high-efficiency biomass hard carbon composite anode material, comprising the following steps:

[0008] (1) After mixing biomass raw materials and phosphoric acid evenly, heat and react for a period of time, and then crush and sieve to obtain the P-doped precursor;

[0009] (2) The P-doped precursor from step (1) is pre-carbonized, then cooled to room temperature, and then pulverized to obtain the P-doped pre-carbonized precursor.

[0010] (3) Mix graphene with water and disperse it by ultrasonication to obtain a graphene dispersion;

[0011] (4) Transfer the graphene dispersion from step (3) to a constant temperature water bath, add the P-doped pre-carbonized precursor and transition metal salt to obtain a suspension, filter, wash and dry the suspension to obtain the P-doped pre-carbonized precursor coated with graphene.

[0012] (5) The graphene-coated P-doped pre-carbonized precursor from step (4) is subjected to chemical vapor deposition with a carbon source to obtain material 1.

[0013] (6) Place the material 1 from step (5) in a high-temperature furnace for high-temperature carbonization treatment, and after cooling to room temperature, obtain material 2;

[0014] (7) The material 2 obtained in step (6) is soaked in an acid solution, and finally washed with deionized water until neutral and dried to obtain the hard carbon composite anode material.

[0015] In steps (1) and (2), after the biomass raw material and phosphoric acid are mixed, they are heated and pre-carbonized at low temperature to allow phosphorus atoms to insert into the interlayer spacing of the hard carbon precursor, thus obtaining a P-doped hard carbon precursor (i.e., a P-doped pre-carbonized precursor); in steps (3) and (4), graphene that can be uniformly dispersed in water is mixed with transition metal salts and P-doped hard carbon precursors. During the mixing process, graphene can uniformly coat the surface of the P-doped hard carbon precursor, and transition metal ions can uniformly adhere to the graphene surface and be uniformly dispersed inside the P-doped hard carbon precursor; subsequently, in the chemical vapor deposition process of step (5), carbon nanotubes are grown in situ on the graphene surface under the catalysis of transition metal ions, thus forming a core-shell structure of core-inner shell-outer shell; in the high-temperature carbonization process of step (6), graphene and its surface The transition metals on the surface synergistically induce the growth of hard carbon along the graphene layers, enhancing the chemical bonding between the coating layer and the hard carbon. This results in the coating layer tightly encapsulating the hard carbon surface, ultimately forming a highly elastic and rigid three-dimensional network coating layer structure of graphene and carbon nanotubes on the hard carbon surface, improving the cycling performance of the composite material. The transition metal ions on the coating layer surface can also catalyze graphitization, generating a more regular and ordered graphite-like microcrystalline structure coating layer, reducing defects in graphene and carbon nanotubes, and improving the first coulombic efficiency of the composite material. Furthermore, during high-temperature carbonization, graphene, carbon nanotubes, and metal ions catalyze graphitization, generating graphite microcrystals within the hard carbon, enhancing electron transport kinetics. Simultaneously, phosphorus doping widens the interlayer spacing of the hard carbon, improving sodium ion insertion / extraction kinetics. The synergistic enhancement of ion and electron transport ensures excellent rate performance.

[0016] As a preferred embodiment of the present invention, in step (1), the biomass raw material includes at least one of lychee wood, apple wood, poplar wood, bamboo, fruit shell, straw, coconut shell and corn cob.

[0017] As a preferred embodiment of the present invention, in step (1), the mass ratio of phosphoric acid to biomass raw material is (5-20):100.

[0018] As a preferred embodiment of the present invention, in step (1), after the biomass raw material and phosphoric acid are mixed, they are heated to 150℃-200℃ and reacted for 1-6 hours.

[0019] As a preferred embodiment of the present invention, in step (2), the pre-carbonization treatment is: pre-carbonizing the P-doped precursor at 300℃-600℃ for 1-6h under a nitrogen atmosphere.

[0020] As a preferred embodiment of the present invention, in step (3), the graphene includes at least one of sulfonated graphene and graphene oxide. The source of the graphene is not specifically defined; it can be prepared by methods known in the art or obtained through commercial purchase.

[0021] As a preferred embodiment of the present invention, in step (3), the mass of graphene added per milliliter of water is (1-1.5) mg; for example, 1 mg, 1.1 mg, 1.2 mg, 1.3 mg, 1.4 mg or 1.5 mg.

[0022] As a preferred embodiment of the present invention, in step (3), the power and time of ultrasound are not particularly limited, as long as the graphene is uniformly dispersed. For example, graphene is mixed with water and ultrasonically dispersed for 1-10 hours to obtain a graphene dispersion.

[0023] As a preferred embodiment of the present invention, in step (4), the temperature of the constant temperature water bath is 80-95℃; for example, 80℃, 85℃, 90℃ or 95℃.

[0024] As a preferred embodiment of the present invention, in step (4), the P-doped pre-carbonized precursor and the transition metal salt are added to the graphene dispersion under stirring conditions.

[0025] As a preferred embodiment of the present invention, in step (4), the mass ratio of the graphene to the P-doped pre-carbonized precursor is (0.1-0.4):100; for example, 0.1:100, 0.2:100, 0.3:100 or 0.4:100.

[0026] As a preferred embodiment of the present invention, in step (4), the transition metal salt includes at least one of the transition metal nitrate, chloride, sulfate, acetate and oxalate; the transition metal is at least one of iron, cobalt, nickel and chromium.

[0027] Since the P-doped pre-carbonized precursor is rich in micropores, after step (4), transition metal ions are uniformly distributed in the graphene coating and the P pre-carbonized precursor.

[0028] As a preferred embodiment of the present invention, in step (4), the mass ratio of the transition metal salt to the P-doped pre-carbonized precursor is (1-5):100; for example, 1:100, 2:100, 3:100, 4:100 or 5:100.

[0029] As a preferred embodiment of the present invention, in step (4), filtration is performed using methods known in the art, such as vacuum filtration, centrifugal filtration, etc.; washing is performed using methods known in the art, such as washing with deionized water.

[0030] As a preferred embodiment of the present invention, in step (4), the drying temperature is 80-100°C.

[0031] As a preferred embodiment of the present invention, in step (5), the carbon source includes at least one of acetylene, ethylene, methane, ethane, propane and n-butane.

[0032] As a preferred embodiment of the present invention, in step (5), the chemical vapor deposition is carried out under an inert atmosphere, which is selected from a nitrogen atmosphere or an argon atmosphere.

[0033] As a preferred embodiment of the present invention, in step (5), the flow rate of the carbon source is not limited and can be adjusted according to the actual situation, so as to satisfy the volume ratio of the carbon source to the inert atmosphere as (1-5):(50-100).

[0034] As a preferred embodiment of the present invention, in step (5), the temperature of the chemical vapor deposition is 500-800°C; for example, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, or 800°C. The chemical vapor deposition time is 1-6 hours; for example, 2-5 hours; preferably 4 hours or 6 hours. Further, after the chemical vapor deposition process is completed, the obtained product is subjected to natural cooling treatment.

[0035] As a preferred embodiment of the present invention, in step (6), the high-temperature carbonization process is as follows: material 1 is placed in a high-temperature furnace and carbonized at 900℃-1400℃ for 1-6 hours under a nitrogen atmosphere.

[0036] As a preferred embodiment of the present invention, in step (7), the acid in the acid solution is selected from at least one of hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid.

[0037] As a preferred embodiment of the present invention, in step (7), the pH of the acid solution is 3-4.

[0038] In a second aspect, the present invention provides a high-rate, high-efficiency biomass hard carbon composite anode material prepared by the above-described preparation method. The hard carbon composite anode material has a core-shell structure, which includes a core, an inner shell, and an outer shell. The inner shell covers the surface of the core, and the outer shell covers the surface of the inner shell. The core includes biomass hard carbon containing phosphorus and graphite microcrystals, the inner shell includes graphene, and the outer shell includes carbon nanotubes.

[0039] Furthermore, the thickness of the outer shell is 5nm-10nm; for example, 5nm, 6nm, 7nm, 8nm, 9nm or 10nm.

[0040] Furthermore, the thickness of the inner shell is 10nm-25nm; for example, 10nm, 15nm, 20nm or 25nm.

[0041] A third aspect of the present invention provides an application of the above-described high-rate, high-efficiency biomass hard carbon composite anode material for use in sodium-ion batteries; preferably, it is used to prepare the anode of sodium-ion batteries.

[0042] In a fourth aspect, the present invention provides a negative electrode for a sodium-ion battery, the negative electrode comprising the above-described high-rate, high-efficiency biomass hard carbon composite negative electrode material.

[0043] As described above, the high-rate, high-efficiency biomass hard carbon composite anode material, its preparation method, and its application of the present invention have the following beneficial effects:

[0044] This invention involves mixing biomass raw materials and phosphoric acid, followed by heating and low-temperature pre-carbonization to insert phosphorus atoms into the interlayer spacing of a hard carbon precursor, resulting in a phosphorus-doped hard carbon precursor. Graphene, which can be uniformly dispersed in water, is then mixed with a transition metal salt and the phosphorus-doped hard carbon precursor. During mixing, the graphene uniformly coats the surface of the phosphorus-doped hard carbon precursor, while the transition metal ions are uniformly attached to the graphene surface and uniformly dispersed within the precursor. Subsequently, during chemical vapor deposition, carbon nanotubes are grown in situ on the graphene surface under the catalysis of the transition metal ions. During the high-temperature carbonization process…

[0045] On the one hand, the graphene coating and the transition metals on its surface synergistically induce the hard carbon to grow along the graphene layer, enhancing the chemical bonding between the graphene coating and the hard carbon, so that the graphene coating is tightly wrapped on the surface of the hard carbon. Finally, graphene and carbon nanotubes form a three-dimensional network coating structure with high elasticity and high rigidity on the surface of hard carbon, which effectively improves the cycle performance of the hard carbon composite anode material.

[0046] On the other hand, the transition metal ions on the surface of the graphene coating can catalyze graphitization, generating a more regular and ordered graphene-like microcrystalline structure coating, reducing defects in graphene and carbon nanotubes, improving the first coulombic efficiency of the composite anode material, and solving the problem of reduced first-cycle coulombic efficiency caused by P atom doping while increasing the rate capability.

[0047] On the other hand, phosphorus doping can widen the interlayer spacing of hard carbon and improve the sodium ion insertion / extraction kinetics; graphene, carbon nanotubes and metal ions catalyze graphitization, generating graphite microcrystals inside hard carbon, which improves electron transport kinetics; the synergistic improvement of ion transport and electron transport ensures good rate performance. Detailed Implementation

[0048] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0049] Example 1

[0050] A high-rate, high-first-efficiency biomass hard carbon composite anode material is prepared by the following steps:

[0051] (1) Mix 100g of fruit shell and 15g of phosphoric acid evenly, heat to 160℃ and react for 4h, then crush and sieve to obtain the P-doped precursor;

[0052] (2) The P-doped precursor obtained in step (1) was pre-carbonized at 500°C for 2 hours under a nitrogen atmosphere, then cooled to room temperature and pulverized to obtain the P-doped pre-carbonized precursor.

[0053] (3) Mix 1.2g of sulfonated graphene with 1 liter of water and ultrasonically disperse for 4 hours to obtain a sulfonated graphene dispersion;

[0054] (4) Transfer the sulfonated graphene dispersion from step (3) to an 85°C constant temperature water bath. Add 1000g of P-doped pre-carbonized precursor during stirring. After stirring for 6 hours, add 30g of ferric nitrate and continue stirring for 2 hours to obtain a suspension. Filter the suspension, wash it with pure water, and dry it to obtain the graphene-coated P-doped pre-carbonized precursor.

[0055] (5) The graphene-coated P-doped pre-carbonized precursor from step (4) was placed in a rotary kiln and acetylene was introduced at a flow rate of 1 L / min under a nitrogen atmosphere of 100 L / min and an environment of 600°C for 6 hours to generate carbon nanotubes. Then, the nitrogen atmosphere was switched to be introduced and the mixture was naturally cooled to room temperature to obtain material 1.

[0056] (6) Place material 1 in a high-temperature furnace and carbonize it at 1200°C for 5 hours under a nitrogen atmosphere. After cooling to room temperature, material 2 is obtained.

[0057] (7) Soak material 2 in hydrochloric acid solution with pH 3 for 1 hour, then wash with deionized water until neutral and dry to obtain the hard carbon composite anode material.

[0058] The prepared hard carbon composite anode material has a core-shell structure, which includes a core, an inner shell, and an outer shell. The inner shell covers the surface of the core, and the outer shell covers the surface of the inner shell. The core is hard carbon containing phosphorus and graphite microcrystals, the inner shell is graphene, and the outer shell is carbon nanotubes. The thickness of the outer shell is 6 nm, and the thickness of the inner shell is 22 nm.

[0059] Example 2

[0060] A high-rate, high-first-efficiency biomass hard carbon composite anode material is prepared by the following steps:

[0061] (1) Mix 100g of coconut shell and 10g of phosphoric acid evenly, heat to 180℃ and react for 3h, then crush and sieve to obtain the P-doped precursor;

[0062] (2) The P-doped precursor product obtained in step (1) was pre-carbonized at 400°C for 2 hours under a nitrogen atmosphere, then cooled to room temperature and pulverized to obtain the P-doped pre-carbonized precursor.

[0063] (3) Mix 1g of graphene oxide with 1 liter of water and ultrasonically disperse for 5 hours to obtain a graphene oxide dispersion.

[0064] (4) Transfer the graphene oxide dispersion from step (3) to an 85°C constant temperature water bath. Add 1000g of P-doped pre-carbonized precursor during stirring. After stirring for 6 hours, add 15g of ferric nitrate and continue stirring for 1 hour to obtain a suspension. Filter the suspension, wash it with pure water, and dry it to obtain the graphene-coated P-doped pre-carbonized precursor.

[0065] (5) The graphene-coated P-doped pre-carbonized precursor from step (4) was placed in a rotary kiln and ethane was introduced at a flow rate of 2 L / min under a nitrogen atmosphere of 100 L / min and an environment of 800 °C for 1 hour to generate carbon nanotubes. Then, nitrogen was introduced and the mixture was naturally cooled to room temperature to obtain material 1.

[0066] (6) Place material 1 in a high-temperature furnace and carbonize it at 1000°C for 6 hours under a nitrogen atmosphere. After cooling to room temperature, material 2 is obtained.

[0067] (7) Soak material 2 in hydrochloric acid solution with pH 3 for 1 hour, then wash with deionized water until neutral and dry to obtain the hard carbon composite anode material.

[0068] The prepared hard carbon composite anode material has a core-shell structure, which includes a core, an inner shell, and an outer shell. The inner shell covers the surface of the core, and the outer shell covers the surface of the inner shell. The core is hard carbon containing phosphorus and graphite microcrystals, the inner shell is graphene, and the outer shell is carbon nanotubes. The thickness of the outer shell is 7 nm, and the thickness of the inner shell is 20 nm.

[0069] Example 3

[0070] A high-rate, high-first-efficiency biomass hard carbon composite anode material is prepared by the following steps:

[0071] (1) Mix 100g coconut shell and 20g phosphoric acid evenly, heat to 180℃ and react for 4h, then crush and sieve to obtain the P-doped precursor;

[0072] (2) The P-doped precursor product obtained in step (1) was pre-carbonized at 500°C for 2 hours under a nitrogen atmosphere, cooled to room temperature, and then pulverized to obtain the P-doped pre-carbonized precursor.

[0073] (3) Mix 1.5g of sulfonated graphene with 1 liter of water and ultrasonically disperse for 4 hours to obtain a sulfonated graphene dispersion;

[0074] (4) Transfer the sulfonated graphene dispersion from step (3) to an 85°C constant temperature water bath, add 1000g of P-doped pre-carbonized precursor while stirring, add 45g of ferric nitrate after stirring for 6 hours, and continue stirring for 2 hours to obtain a suspension. Filter the suspension, wash it with pure water, and dry it to obtain the graphene-coated P-doped pre-carbonized precursor.

[0075] (5) The graphene-coated P-doped pre-carbonized precursor from step (4) was placed in a rotary kiln and ethane was introduced at a flow rate of 1 L / min under a nitrogen atmosphere of 80 L / min and an environment of 700°C for 2 hours to generate carbon nanotubes. Then, nitrogen was introduced and the mixture was naturally cooled to room temperature to obtain material 1.

[0076] (6) Place material 1 in a high-temperature furnace and carbonize it at 1400°C for 5 hours under a nitrogen atmosphere. After cooling to room temperature, material 2 is obtained.

[0077] (7) Soak material 2 in hydrochloric acid solution with pH 4 for 1 hour, then wash with deionized water until neutral and dry to obtain the hard carbon composite anode material.

[0078] The prepared hard carbon composite anode material has a core-shell structure, which includes a core, an inner shell, and an outer shell. The inner shell covers the surface of the core, and the outer shell covers the surface of the inner shell. The core is hard carbon containing phosphorus and graphite microcrystals, the inner shell is graphene, and the outer shell is carbon nanotubes. The thickness of the outer shell is 10 nm, and the thickness of the inner shell is 25 nm.

[0079] Example 4

[0080] A high-rate, high-first-efficiency biomass hard carbon composite anode material is prepared by the following steps:

[0081] (1) Mix 100g of coconut shell and 6g of phosphoric acid evenly, heat to 160℃ and react for 4h, then crush and sieve to obtain the P-doped precursor;

[0082] (2) The P-doped precursor product obtained in step (1) was pre-carbonized at 600°C for 2 hours under a nitrogen atmosphere, cooled to room temperature, and then pulverized to obtain the P-doped pre-carbonized precursor.

[0083] (3) Mix 1g of sulfonated graphene with 1 liter of water and ultrasonically disperse for 2 hours to obtain a sulfonated graphene dispersion;

[0084] (4) Transfer the sulfonated graphene dispersion from step (3) to an 85°C constant temperature water bath, add 1000g of P-doped pre-carbonized precursor while stirring, add 12g of ferric nitrate after stirring for 6 hours, and continue stirring for 2 hours to obtain a suspension. Filter the suspension, wash it with pure water, and dry it to obtain the graphene-coated P-doped pre-carbonized precursor.

[0085] (5) The graphene-coated P-doped pre-carbonized precursor from step (4) is placed in a rotary kiln and ethane is introduced at a flow rate of 1 L / min under a nitrogen atmosphere of 70 L / min and an environment of 800°C for 1 h to generate carbon nanotubes. Then, the nitrogen atmosphere is switched to be introduced and the mixture is naturally cooled to room temperature to obtain material 1.

[0086] (6) Place material 1 in a high-temperature furnace and carbonize it at 1200°C for 5 hours under a nitrogen atmosphere. After cooling to room temperature, material 2 is obtained.

[0087] (7) Soak material 2 in hydrochloric acid solution with pH 4 for 1 hour, then wash with deionized water until neutral and dry to obtain the hard carbon composite anode material.

[0088] The prepared hard carbon composite anode material has a core-shell structure, which includes a core, an inner shell, and an outer shell. The inner shell covers the surface of the core, and the outer shell covers the surface of the inner shell. The core is hard carbon containing phosphorus and graphite microcrystals, the inner shell is graphene, and the outer shell is carbon nanotubes. The thickness of the outer shell is 5 nm, and the thickness of the inner shell is 12 nm.

[0089] Comparative Example 1

[0090] A hard carbon composite anode material, the preparation method of which includes the following steps:

[0091] (1) Mix 100g of fruit shell and 15g of phosphoric acid evenly, heat to 160℃ and react for 4h, then crush and sieve to obtain the P-doped precursor;

[0092] (2) The P-doped precursor product obtained in step (1) was pre-carbonized at 500°C for 2 hours under a nitrogen atmosphere, cooled to room temperature, and then pulverized to obtain the P-doped pre-carbonized precursor.

[0093] (3) Mix 1.2g of sulfonated graphene with 1 liter of water and ultrasonically disperse for 4 hours to obtain a sulfonated graphene dispersion;

[0094] (4) Transfer the sulfonated graphene dispersion from step (3) to an 85°C constant temperature water bath, add 1000g of P-doped pre-carbonized precursor while stirring, add 30g of ferric nitrate after stirring for 6 hours, and continue stirring for 2 hours to obtain a suspension. Filter the suspension, wash it with pure water, and dry it to obtain the graphene-coated P-doped pre-carbonized precursor.

[0095] (5) The graphene-coated P-doped pre-carbonized precursor from step (4) was placed in a high-temperature furnace and carbonized at 1200°C for 5 hours under a nitrogen atmosphere. After cooling to room temperature, material 2 was obtained.

[0096] (6) Soak the material 2 from step (5) in a hydrochloric acid solution with pH 3 for 1 hour, and finally wash it with deionized water until neutral and dry it to obtain the hard carbon composite anode material.

[0097] The prepared hard carbon composite anode material has a core-shell structure, which includes a core and an outer shell. The core is hard carbon containing P and graphite microcrystals, and the outer shell is graphene with a thickness of 22 nm.

[0098] Comparative Example 2

[0099] A hard carbon composite anode material, the preparation method of which includes the following steps:

[0100] (1) Mix 100g of fruit shell and 15g of phosphoric acid evenly, heat to 160℃ and react for 4h, then crush and sieve to obtain the P-doped precursor;

[0101] (2) The P-doped precursor product of step (1) was pre-carbonized at 500°C for 2 hours under a nitrogen atmosphere, cooled to room temperature, and then pulverized to obtain the P-doped pre-carbonized precursor.

[0102] (3) Mix 1.2g of sulfonated graphene with 1 liter of water and ultrasonically disperse for 4 hours to obtain a sulfonated graphene dispersion;

[0103] (4) Transfer the sulfonated graphene dispersion from step (3) to an 85°C constant temperature water bath, add 1000g of P-doped precarbonized precursor while stirring, and stir for 6 hours to obtain a suspension. Filter the suspension, wash it with pure water, and dry it to obtain the graphene-coated P-doped precarbonized precursor.

[0104] (5) The graphene-coated P-doped pre-carbonized precursor from step (4) is placed in a high-temperature furnace and carbonized at 1200°C for 5 hours under a nitrogen atmosphere. After cooling to room temperature, the hard carbon composite anode material is obtained.

[0105] The prepared hard carbon composite anode material has a core-shell structure, which includes a core and an outer shell. The core is P-containing hard carbon, and the outer shell is graphene with a thickness of 22 nm.

[0106] Comparative Example 3

[0107] A hard carbon composite anode material, the preparation method of which includes the following steps:

[0108] (1) Mix 100g of fruit shell and 15g of phosphoric acid evenly, heat to 160℃ and react for 4h, then crush and sieve to obtain the P-doped precursor;

[0109] (2) The P-doped precursor product obtained in step (1) was pre-carbonized at 500°C for 2 hours under a nitrogen atmosphere, cooled to room temperature, and then pulverized to obtain the P-doped pre-carbonized precursor.

[0110] (3) Add 1000g of P-doped precarbonized precursor and 30g of ferric nitrate from step (2) to 1L of water, stir for 6 hours, add 30g of ferric nitrate to obtain a suspension, filter the suspension, wash with pure water and dry it to obtain the P-doped precarbonized precursor.

[0111] (4) The P-doped pre-carbonized precursor from step (3) was placed in a rotary kiln and acetylene was introduced at a flow rate of 1 L / min for 6 hours under a nitrogen atmosphere of 100 L / min and an environment of 600°C to generate carbon nanotubes. Then, nitrogen was introduced and the material was naturally cooled to room temperature to obtain material 1. Material 1 was placed in a high-temperature furnace and carbonized at 1200°C for 5 hours under a nitrogen atmosphere. After cooling to room temperature, material 2 was obtained.

[0112] (5) The material 2 from step (4) is soaked in a hydrochloric acid solution with a pH of 3 for 1 hour, and then washed with deionized water until neutral and dried to obtain the hard carbon composite anode material.

[0113] The prepared hard carbon composite anode material has a core-shell structure, which includes a core and an outer shell. The core is hard carbon containing P and graphite microcrystals, and the outer shell is carbon nanotubes with a thickness of 6 nm.

[0114] Comparative Example 4

[0115] A hard carbon composite anode material, the preparation method of which includes the following steps:

[0116] (1) Mix 100g of fruit shell and 15g of phosphoric acid evenly, heat to 160℃ and react for 4h, then crush and sieve to obtain the P-doped precursor;

[0117] (2) The P-doped precursor product of step (1) was pre-carbonized at 500°C for 2 hours under a nitrogen atmosphere, cooled to room temperature, and then pulverized to obtain the P-doped pre-carbonized precursor.

[0118] (3) Add 1000g of P-doped precarbonized precursor and 30g of ferric nitrate from step (2) to 1L of water, stir for 6 hours, add 30g of ferric nitrate to obtain a suspension, filter the suspension, wash with pure water and dry it to obtain the P-doped precarbonized precursor.

[0119] (4) The P-doped pre-carbonized precursor from step (3) was placed in a high-temperature furnace and carbonized at 1200°C for 5 hours under a nitrogen atmosphere. After cooling to room temperature, it was soaked in a hydrochloric acid solution with pH 3 for 1 hour. Finally, it was washed with deionized water until neutral and dried to obtain hard carbon containing P and graphite microcrystals.

[0120] Performance testing

[0121] The hard carbon composite anode materials prepared in Examples 1-4 and Comparative Examples 1-4 were used as anode materials for sodium-ion batteries, and their electrochemical performance was tested using the following methods:

[0122] The hard carbon composite negative electrode material, conductive agent Super P, binder sodium carboxymethyl cellulose (CMC), and aqueous binder (SBR) of the above examples were weighed in a mass ratio of 95:2:1.5:1.5. After thorough grinding in an agate mortar, a small amount of deionized water was added to form a uniform black paste. This paste was coated onto a copper foil current collector as a test electrode, and a sodium metal sheet was used as a control electrode to assemble a coin cell. The electrolyte was a 1M sodium hexafluorophosphate solution dissolved in a 1:1 volume ratio mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), with 5 wt.% fluoroethylene carbonate (FEC) added as an additive. A glass fiber separator and a CR2032 stainless steel casing were used to assemble the coin cell. The capacity and initial coulombic efficiency of the obtained coin cells were tested at a constant rate of 0.1C within a voltage range of 0.01–3.0 V.

[0123] Using the hard carbon composite anode materials prepared in Examples 1-4 and Comparative Examples 1-4 as anodes, sodium nickel iron manganese oxide as cathode, and 1M NaPF6+EC:DEC:DMC (volume ratio 1:1:1) solution as electrolyte, the cells were assembled into full cells in a stacked manner. The cells were tested at room temperature with a 1C rate, and the voltage range was 1.5-3.9V.

[0124] Rate performance testing: Under normal temperature conditions, the prepared batteries were tested on the LAND battery testing system, and the specific capacity of the batteries was tested at current densities of 0.1 A / g, 0.2 A / g, 0.5 A / g, 1 A / g, 2 A / g, and 5 A / g. The test results are shown in Table 1.

[0125] Table 1. Electrochemical performance of hard carbon composite anode materials in Examples 1-4 and Comparative Examples 1-4

[0126]

[0127] As can be seen from the test results in Table 1, compared with Comparative Examples 1-4, the hard carbon composite anode material prepared by this invention has high initial charge-discharge efficiency, cycle performance and rate performance.

[0128] Comparing Comparative Example 1 and Example 1, Comparative Example 1 did not undergo chemical vapor deposition, and the resulting hard carbon composite anode material had a core-shell structure, with the core being hard carbon containing phosphorus and graphite microcrystals, and the outer shell being graphene. During the high-temperature carbonization process of Comparative Example 1, graphene and its surface transition metals synergistically induced the hard carbon to grow along the graphene layers. Chemical bonds existed between the graphene coating and the hard carbon, allowing the graphene coating to tightly adhere to the hard carbon surface. However, because the graphene coating of Comparative Example 1 lacked carbon nanotubes, the graphene tended to accumulate during charge and discharge, resulting in a slight decrease in the anode's cycle performance.

[0129] Comparing Comparative Example 2 with Examples 1 and 1, Comparative Example 2 did not involve the addition of transition metal ions or chemical vapor deposition. During the high-temperature carbonization process in Comparative Example 2, graphene induced the growth of hard carbon along the graphene layers, but without the synergistic effect of transition metal ions, the chemical bonds between the coating layer and the hard carbon weakened. Graphene easily detached during charge and discharge, resulting in a significant reduction in the anode cycle performance. Furthermore, without transition metal ions catalyzing graphitization on the coating layer surface, the resulting graphene coating layer had an irregular atomic arrangement and numerous defects, reducing the initial coulombic efficiency of the composite anode material. Additionally, phosphorus doping widened the interlayer spacing of the hard carbon, improving sodium ion insertion / extraction kinetics; however, without the catalytic graphitization of transition metal ions, no graphite crystals were formed inside the hard carbon material. Relying solely on graphene to improve electron transport kinetics led to a decrease in the rate performance of the hard carbon material.

[0130] Comparing Comparative Example 3 with Example 1, no graphene was added during the preparation process of Comparative Example 3. During high-temperature carbonization, although transition metals and carbon nanotubes can induce hard carbon to grow along the carbon nanotube layers, creating chemical bonds between the carbon nanotube coating and the hard carbon, the carbon nanotubes are cylindrical, resulting in poor bonding between the coating and the hard carbon. This makes the coating prone to detachment during charge and discharge, easily leading to a decrease in the cycle performance of the negative electrode material.

[0131] Comparing Comparative Example 4 with Example 1, Comparative Example 4 only added a transition metal. During the high-temperature carbonization process, the transition metal ions catalyzed graphitization, causing graphite microcrystals to form inside the hard carbon material. However, without graphene and carbon nanotube coating layers, the initial coulombic efficiency, cycle performance, and rate performance of the anode material were all lower.

[0132] In summary, this invention coats a graphene layer onto the surface of biomass hard carbon and grows carbon nanotubes in situ on the graphene surface. During high-temperature carbonization, transition metals synergistically induce the hard carbon to grow along the graphene layer, enhancing the chemical bonding between the coating layer and the hard carbon, resulting in a tight graphene coating and improved material cycling performance. Transition metal ions catalyze graphitization, generating a more regular and ordered graphite-like microcrystalline structure coating layer, reducing defects in graphene and carbon nanotubes and improving the material's first coulombic efficiency. Furthermore, phosphorus doping enhances ion transport capability, and the graphite microcrystals catalyzed within the hard carbon enhance electron transport capability. This synergistic enhancement of ion and electron transport ensures the material's rate performance. Therefore, this invention effectively overcomes the various shortcomings of existing technologies and possesses high industrial applicability.

[0133] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for preparing a high-rate, high-efficiency biomass hard carbon composite anode material, characterized in that, Includes the following steps: (1) After mixing biomass raw materials and phosphoric acid evenly, heat and react for a period of time, and then crush and sieve to obtain the P-doped precursor; (2) The P-doped precursor obtained in step (1) is pre-carbonized, then cooled to room temperature, and then pulverized to obtain the P-doped pre-carbonized precursor. (3) Mix graphene with water and disperse it by ultrasonication to obtain a graphene dispersion; (4) Transfer the graphene dispersion from step (3) to a constant temperature water bath, add a P-doped pre-carbonized precursor and a transition metal salt in a mass ratio of 100:(1-5), wherein the mass ratio of graphene to P-doped pre-carbonized precursor is (0.1-0.4):100, and obtain a suspension. Filter, wash and dry the suspension to obtain a graphene-coated P-doped pre-carbonized precursor, wherein the transition metal ions are uniformly dispersed inside the P-doped pre-carbonized precursor and uniformly attached to the graphene surface. (5) The graphene-coated P-doped pre-carbonized precursor from step (4) is subjected to chemical vapor deposition with a carbon source. Under the catalysis of transition metal ions, carbon nanotubes are grown in situ on the graphene surface, thereby forming a core-shell structure of core-inner shell-outer shell, and material 1 is obtained. (6) The material 1 from step (5) is placed in a high-temperature furnace for high-temperature carbonization treatment. After cooling to room temperature, material 2 is obtained. During the high-temperature carbonization process, graphene and the transition metal on its surface synergistically induce hard carbon to grow along the graphene layer. At the same time, graphene, carbon nanotubes and transition metal ions catalyze graphitization, generating graphite microcrystals inside the hard carbon. (7) The material 2 obtained in step (6) is soaked in an acid solution, and finally washed with deionized water until neutral and dried to obtain the hard carbon composite anode material.

2. The preparation method according to claim 1, characterized in that, In step (1), the biomass raw materials include at least one of lychee wood, apple wood, poplar wood, bamboo, fruit shells, straw, coconut shells and corn cobs.

3. The preparation method according to claim 1, characterized in that, In step (1), after the biomass raw material and phosphoric acid are mixed, they are heated to 150℃-200℃ and reacted for 1-6 hours.

4. The preparation method according to claim 1, characterized in that, In step (2), the pre-carbonization treatment is as follows: the P-doped precursor is pre-carbonized at 300℃-600℃ for 1-6 hours under a nitrogen atmosphere.

5. The preparation method according to claim 1, characterized in that, The graphene includes at least one of sulfonated graphene and graphene oxide.

6. The preparation method according to claim 1, characterized in that, In step (4), the transition metal salt includes at least one of the transition metal nitrate, chloride, sulfate, acetate, and oxalate; the transition metal is at least one of iron, cobalt, nickel, and chromium.

7. The preparation method according to claim 1, characterized in that, In step (6), the high-temperature carbonization process is as follows: material 1 is placed in a high-temperature furnace and carbonized at 900℃-1400℃ for 1-6 hours under a nitrogen atmosphere.

8. A hard carbon composite anode material prepared by the preparation method according to any one of claims 1-7, wherein the hard carbon composite anode material has a core-shell structure, the core-shell structure comprising a core, an inner shell and an outer shell, the inner shell covering the surface of the core, and the outer shell covering the surface of the inner shell; the core comprises biomass hard carbon containing P and graphite microcrystals, the inner shell comprises graphene, and the outer shell comprises carbon nanotubes.

9. The application of a hard carbon composite anode material prepared by the preparation method according to any one of claims 1-7, characterized in that, The hard carbon composite anode material is used as the anode material for sodium-ion batteries.

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