Ternary doped hard carbon negative electrode material and preparation method and application thereof

Abstract

The invention discloses a ternary doped hard carbon negative electrode material and a preparation method and application thereof, and the preparation method of the ternary doped hard carbon negative electrode material comprises the following steps: S1, mixing and stirring litchi wood powder and ammonium hydrogen phosphate, and evaporating a solvent to dryness; s2, performing high-temperature calcination on the mixture after the solvent is evaporated to dryness in an inert gas atmosphere for carbonization; and S3, cooling the calcined product, crushing and screening to obtain the ternary co-doped hard carbon negative electrode material. According to the ternary doped hard carbon negative electrode material and the preparation method and application thereof, three doping elements of nitrogen, oxygen and phosphorus are doped into crystal lattices of the hard carbon negative electrode material through the same raw material by a one-step method, so that the conductivity of the material is improved, and the graphite layer spacing of the hard carbon negative electrode material is increased; a charge distribution structure and charge transfer dynamics in an electrochemical sodium storage process are optimized, and the structural stability of the hard carbon material is improved, so that the output voltage, the energy density, the cycle performance and the low-voltage platform capacity of the hard carbon negative electrode sodium ion battery are improved.

Description

Technical Field

[0001] The present invention relates to the field of new energy materials and devices, and particularly to a ternary-doped hard carbon anode material and its preparation method and application. Background Art

[0002] Lithium-ion batteries have always dominated the market due to their excellent performance. However, 70% of the lithium in China depends on imports, and resources such as lithium severely limit the large-scale energy storage applications of lithium-ion batteries. Therefore, there is an urgent need for alternatives to lithium-ion batteries both in the market and at the national level. Sodium and lithium are elements in the same main group and have similar physical and chemical properties. Moreover, the crust abundance of sodium (2.74%) is more than 420 times higher than that of lithium (0.0065%), and it is widely distributed and inexpensive. Therefore, compared with lithium-ion batteries, sodium-ion batteries have advantages such as low cost, high safety, and excellent performance in a wide temperature range. Therefore, sodium-ion batteries have very broad application prospects in the fields of energy storage, base stations, and low-speed electric vehicles.

[0003] However, due to the larger atomic radius of sodium than lithium, in the graphite anode that has been mature in the lithium-ion battery industry chain, its interlayer spacing is only 0.3354 nm, and the atomic radius of sodium ions is more than 35% larger than that of lithium ions, making it difficult to embed into the graphite interlayer to form an intercalation compound. Therefore, graphite is difficult to be used as the anode of sodium-ion batteries. Therefore, it is particularly important to develop high-performance anode materials for sodium-ion batteries. Hard carbon is a carbon material that is difficult to graphitize at high temperatures, with randomly oriented graphite microcrystals and a large number of voids inside the structure, which is beneficial to storing sodium ions with a relatively large ionic radius. Therefore, hard carbon is considered to be the most likely anode material for sodium-ion batteries to be industrialized first. According to the different sources of precursors, hard carbon can be divided into resin-based (phenolic resin, epoxy resin, polyfurfuryl alcohol, etc.), pitch-based (coal tar pitch, petroleum pitch, natural pitch, etc.), and biomass-based (cellulose, lignin, starch, and agricultural and forestry waste, etc.). Due to the unique microstructure and low cost of biomass, biomass hard carbon materials are considered to be a type of anode material for sodium-ion batteries with promising applications. CN117865123 A discloses a high-performance litchi wood-based hard carbon anode material for sodium-ion batteries, its preparation method, and its application in sodium-ion batteries. The method comprises the steps of: subjecting litchi wood to machine sawing to obtain a size of 1 - 50 cm 3Litchi wood blocks are washed with deionized water and then dried at 70 °C for 24 h to obtain pretreated litchi wood blocks. The obtained litchi wood blocks are subjected to one-step stepped carbonization under an inert gas atmosphere. After the sample is cooled, it is transferred to an acidic solution, stirred, washed, filtered until neutral, dried, ground, and then sieved to obtain a high-performance hard carbon anode material based on litchi wood for sodium-ion batteries. In this patented technology, directly carbonizing litchi wood blocks and using them as anode materials has the following defects: directly high-temperature carbonizing litchi wood easily leads to structural collapse, making it difficult to increase the graphite layer spacing of the hard carbon anode material, which is not conducive to the intercalation and deintercalation of sodium with a relatively large atomic radius and "bulky body" between the graphite layers, thus making it difficult to improve the reaction kinetics during sodium storage and the capacity in the low-voltage plateau region of the hard carbon anode material. + During the intercalation and deintercalation between the graphite layers, it is thus difficult to improve the reaction kinetics during sodium storage and the capacity in the low-voltage plateau region. Summary of the Invention

[0004] To solve the above problems, the present invention provides a ternary-doped hard carbon anode material, its preparation method and application. Among them, a nitrogen source, an oxygen source, and a phosphorus source are doped into the hard carbon anode material, which improves the conductivity of the material, increases the graphite layer spacing of the hard carbon anode material, optimizes the charge distribution structure and the charge transfer kinetics during the electrochemical sodium storage process, improves the structural stability, and thus improves the output voltage, energy density, cycle performance, and low-voltage plateau capacity of the sodium-ion battery.

[0005] To achieve the above object, the present application adopts the following technical solutions: A preparation method of a ternary-doped hard carbon anode material, comprising:

[0006] S1: Mix and stir litchi wood powder and ammonium hydrogen phosphate, and evaporate the solvent to dryness;

[0007] S2: Calcinate the mixture after evaporating the solvent to dryness at a high temperature under an inert gas atmosphere for carbonization;

[0008] S3: Cool the calcined product, crush and screen it to obtain a ternary co-doped hard carbon anode material.

[0009] Preferably, in step S1, the ammonium hydrogen phosphate is ammonium dihydrogen phosphate and / or ammonium hydrogen phosphate.

[0010] Preferably, in step S1, the mass ratio of litchi wood powder to ammonium hydrogen phosphate is (92.5 - 97.5):(2.5 - 7.5).

[0011] Preferably, the preparation method of the litchi wood powder in step S1 includes: first crush the litchi wood, wash it successively with hydrochloric acid, deionized water, and absolute ethanol, then dry it, and sieve the dried product with a 200-mesh sieve to obtain litchi wood powder.

[0012] Preferably, in step S2, the carbonization temperature is 1200 - 1400 °C.

[0013] Preferably, the high-temperature calcination in step S2 is specifically as follows: heating up to 450°C at a rate of 2°C per minute and holding for a first duration; then heating up to the carbonization temperature at a rate of 2°C per minute and holding for a second duration. -1 Preferably, the first duration is at least 1 hour, and the second duration is at least 3 hours. -1 Preferably, the crushing and screening in step S3 refers to screening the cooled calcined product through a 400-mesh sieve.

[0014] A ternary-doped hard carbon anode material is prepared by using the preparation method as described above.

[0015] An application of a ternary-doped hard carbon anode material, wherein the ternary-doped hard carbon anode material is used for the anode of a sodium-ion battery.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] The present invention first obtains litchi wood powder by crushing and acid-treating litchi wood raw materials, and impregnating and blending it with NH4H2PO4 or (NH4)2HPO4 by a liquid-phase method. Subsequently, the solvent in the blended liquid is evaporated and carbonized in an N2 atmosphere, and after cooling, it is screened through a sieve to obtain a litchi wood-derived nitrogen, oxygen, and phosphorus co-doped hard carbon material. The present invention adopts a homologous one-step doping preparation method, which is simple and convenient, the material source is rich and the cost is low, providing a new idea for the preparation of hard carbon anode materials.

[0018] The present invention uses ammonium hydrogen phosphate as the doping raw material for N, O, and P elements. Not only can the doping component ratios of N, O, and P be controlled, but also the chemical bonding between the OH groups in NH4H2PO4 or (NH4)2HPO4 and the abundant hydroxyl groups contained in the cellulose and hemicellulose in litchi wood is utilized. Compared with solid-phase physical mixing, it is more conducive to the dispersion of the doping raw materials and makes the distribution of the doping elements more uniform.

[0019] The phase characterization results of the present invention confirm that the ternary-doped hard carbon anode material belongs to amorphous carbon and presents a block structure. The nitrogen, oxygen, and phosphorus elements are evenly distributed in the amorphous carbon, with low porosity and high tap density. The charge and discharge results of the battery prepared from the ternary-doped hard carbon anode material show that the reversible discharge specific capacity is as high as 493.4 mAh g-1 at 70 mA g-1, and it still retains 92.9% of the initial capacity after 250 cycles at 200 mA g-1, and the capacity ratio in the low-voltage region of the discharge curve is as high as 95%.

[0020]

[0021]

[0022] ​​With the increase in the dosage of NH4H2PO4 in the present invention, nitrogen, oxygen, and phosphorus elements in NH4H2PO4 are introduced as heteroatom defects into the carbonized litchi wood, restricting the growth of the graphite microcrystals in the litchi wood. At the same time, the relatively large atomic radii and interatomic repulsive forces of N, O, and P will also cause the interlayer spacing of the graphite microcrystals in the litchi wood to further increase, facilitating the + intercalation and storage of Na between the graphite layers, thereby improving the electrochemical performance of the ternary-doped hard carbon anode material.

[0023] After nitrogen, oxygen, and phosphorus elements in NH4H2PO4 of the present invention are doped into the graphite microcrystals of litchi wood, the abundant functional groups in the litchi wood bond with the heteroatoms to form various chemical bonds including C, N, P, and O, which can further improve the sodium storage electrochemical performance of the ternary-doped hard carbon anode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings herein are incorporated into the specification and form a part of the specification, showing embodiments consistent with the present invention and used together with the specification to explain the principles of the present invention.

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0026] In the drawings:

[0027] Figure 1 is the TG result of the litchi wood precursor under a nitrogen atmosphere at an NH4H2PO4 (5wt%) dosage;

[0028] Figure 2 is the yield of the LWHC hard carbon material prepared with different dosages of NH4H2PO4;

[0029] Figure 3 is the XRD pattern of the litchi wood-derived hard carbon material obtained at different carbonization temperatures;

[0030] Figure 4 is the XRD pattern of the litchi wood-derived hard carbon material obtained at different carbonization temperatures;

[0031] Figure 5 are the SEM photos and element distributions of the litchi wood-derived hard carbon material: (a) LWHC-1300-0.025, (b) LWHC-1300-0.05, and (c) LWHC-1300-0.075;

[0032] Figure 6It is the HRTEM image of LWHC-1300-0.025;

[0033] Figure 7 It is the HRTEM image of LWHC-1300-0.05;

[0034] Figure 8 It is the HRTEM image of LWHC-1300-0.075;

[0035] Figure 9 It is the Raman spectrum result of the hard carbon material from litchi wood;

[0036] Figure 10 It is the high-resolution XPS spectrum of the LWHC-1300-0.05 hard carbon material: (a) C element, (b) N element, (c) O element, (d) P element;

[0037] Figure 11 It is the specific surface area of the hard carbon material from litchi wood;

[0038] Figure 12 It is the tap density result graph of the hard carbon material from litchi wood;

[0039] Figure 13 It is the particle size distribution curve graph of the LWHC-1300-0.05 hard carbon material;

[0040] Figure 14 It is the long-term cycling performance of the hard carbon anode material from litchi wood obtained under different dosages of NH4H2PO4 at 70 mA g- 1 ;

[0041] Figure 15 It is the long-term cycling performance of the hard carbon anode material from litchi wood obtained under different dosages of NH4H2PO4 at 200 mA g- 1 ;

[0042] Figure 16 It is the long-term cycling performance of the hard carbon anode from litchi wood obtained at different carbonization temperatures at 70 mA g- 1 ;

[0043] Figure 17 It is the charge-discharge curves of different hard carbon anode materials from litchi wood at 70 mA g- 1 ;

[0044] Figure 18 It is the rate performance of the hard carbon anode material from litchi wood. Specific embodiments

[0045] The following examples are used to illustrate the present invention, but not to limit the scope of the present invention. Without departing from the spirit and essence of the present invention, any modification or replacement of the methods, steps or conditions of the present invention shall fall within the protection scope of the present invention. Unless otherwise specified, the experimental materials, reagents, instruments, etc. used in the embodiments of the present invention can be obtained commercially; unless otherwise specifically specified, all technical means in the embodiments of the present invention are conventional means well known to those skilled in the art.

[0046] A preparation method of a ternary-doped hard carbon anode material provided by the present application includes:

[0047] S1: Mix and stir litchi wood powder and ammonium hydrogen phosphate, and evaporate the solvent to dryness. Among them, the mass ratio of litchi wood powder to ammonium hydrogen phosphate is (92.5 - 97.5):(2.5 - 7.5); the mixing and stirring time is 11 - 13 hours.

[0048] The preparation method of litchi wood powder includes: first crush litchi wood, and wash it successively with 1 mol / L -1 hydrochloric acid, deionized water, and absolute ethanol, then dry it overnight at 80 °C. The dried product is sieved through a 200-mesh sieve to obtain litchi wood powder.

[0049] S2: Calcinate the mixture after evaporating the solvent to dryness at high temperature under an inert gas atmosphere for carbonization; the carbonization temperature is 1200 - 1400 °C.

[0050] The high-temperature calcination is specifically: heat it up to 450 °C at a rate of 2 °C / min and keep it warm for the first duration; then heat it up to the carbonization temperature at a rate of 2 °C / min -1 and keep it warm for the second duration. The first duration is at least 1 hour, and the second duration is at least 3 hours. -1

[0051] S3: Cool the calcined product and then crush and sieve it to obtain a ternary co-doped hard carbon anode material. Among them, crushing and sieving means sieving the cooled calcined product through a 400-mesh sieve.

[0052] The present application also provides a ternary-doped hard carbon anode material prepared by the above method, which is used as the anode of a sodium-ion battery.

[0053] The following further explains the present application through specific examples and experimental examples. The test and analysis methods used in the examples:

[0054] Example 1

[0055] A preparation method of a ternary-doped hard carbon anode material provided by this example includes:

[0056] S1: First crush litchi wood and wash it with 1 mol / L -1 ​Wash in hydrochloric acid (HCl) for more than 8 h, then wash with deionized water and absolute ethanol, and dry overnight at 80 °C. The dried product is sieved through a 200-mesh sieve to obtain litchi wood powder. Subsequently, mix the litchi wood powder and ammonium dihydrogen phosphate (NH4H2PO4) at a ratio of 95:5 in a solution (turbid solution), stir for 12 h, and evaporate the solvent to dryness;

[0057] S2: Under an inert gas (N2) atmosphere, heat the obtained mixture at a rate of 2 °C / min -1 to 450 °C, hold for 1 h, and then heat at a rate of 2 °C / min -1 to 1300 °C and hold for 3 h.

[0058] S3: After sufficient cooling, crush the calcined product again and sieve it through a 400-mesh sieve to obtain the N, O, P co-doped hard carbon anode material, named LWHC-1300-0.05.

[0059] Example 2

[0060] A preparation method of a ternary-doped hard carbon anode material provided in this example includes:

[0061] S1: First, crush the litchi wood and wash it in 1 mol / L -1 hydrochloric acid (HCl) for more than 8 h, then wash with deionized water and absolute ethanol, and dry overnight at 80 °C. The dried product is sieved through a 200-mesh sieve to obtain litchi wood powder. Subsequently, mix the litchi wood powder and ammonium dihydrogen phosphate (NH4H2PO4) at a ratio of 97.5:2.5 in a solution (turbid solution), stir for 12 h, and evaporate the solvent to dryness;

[0062] S2: Under an inert gas (N2) atmosphere, heat the obtained mixture at a rate of 2 °C / min -1 to 450 °C, hold for 1 h, and then heat at a rate of 2 °C / min -1 to 1300 °C and hold for 3 h.

[0063] S3: After sufficient cooling, crush the calcined product again and sieve it through a 400-mesh sieve to obtain the N, O, P co-doped hard carbon anode material, named LWHC-1300-0.075.

[0064] Example 3

[0065] A preparation method of a ternary-doped hard carbon anode material provided in this example includes:

[0066] S1: First, crush the litchi wood and wash it in 1 mol / L -1Wash in hydrochloric acid (HCl) for more than 8 h, then wash with deionized water and absolute ethanol, and dry overnight at 80 °C. Screen the dried product through a 200-mesh sieve to obtain litchi wood powder. Subsequently, mix the litchi wood powder and ammonium dihydrogen phosphate (NH4H2PO4) at a ratio of 92.5:7.5 in a solution (turbid solution), stir for 12 h, and evaporate the solvent to dryness;

[0067] S2: Under an inert gas (N2) atmosphere, heat the obtained mixture at a rate of 2 °C / min -1 to 450 °C, hold for 1 h, and then heat at a rate of 2 °C / min -1 to 1300 °C and hold for 3 h.

[0068] S3: After sufficient cooling, crush the calcined product again and screen it through a 400-mesh sieve to obtain the N, O, P co-doped hard carbon anode material, named LWHC-1300-0.025.

[0069] Example 4

[0070] A preparation method of a ternary-doped hard carbon anode material provided in this example includes:

[0071] S1: First, crush litchi wood and wash it in 1 mol / L -1 hydrochloric acid (HCl) for more than 8 h, then wash with deionized water and absolute ethanol, and dry overnight at 80 °C. Screen the dried product through a 200-mesh sieve to obtain litchi wood powder. Subsequently, mix the litchi wood powder and ammonium dihydrogen phosphate (NH4H2PO4) at a ratio of 95:5 in a solution (turbid solution), stir for 12 h, and evaporate the solvent to dryness;

[0072] S2: Under an inert gas (N2) atmosphere, heat the obtained mixture at a rate of 2 °C / min -1 to 450 °C, hold for 1 h, and then heat at a rate of 2 °C / min -1 to 1200 °C and hold for 3 h.

[0073] S3: After sufficient cooling, crush the calcined product again and screen it through a 400-mesh sieve to obtain the N, O, P co-doped hard carbon anode material, named LWHC-1200-0.05.

[0074] Example 5

[0075] A preparation method of a ternary-doped hard carbon anode material provided in this example includes:

[0076] S1: First, crush litchi wood and wash it in 1 mol / L -1Wash in hydrochloric acid (HCl) for more than 8 h, then wash with deionized water and absolute ethanol, and dry overnight at 80 °C. Screen the dried product through a 200-mesh sieve to obtain litchi wood powder. Subsequently, mix the litchi wood powder and ammonium dihydrogen phosphate (NH4H2PO4) at a ratio of 95:5 in a solution (turbid solution) and stir for 12 h, and then evaporate the solvent to dryness;

[0077] S2: Under an inert gas (N2) atmosphere, heat the obtained mixture at a rate of 2 °C / min -1 to 450 °C, hold for 1 h, and then heat at a rate of 2 °C / min -1 to 1400 °C and hold for 3 h.

[0078] S3: After sufficient cooling, crush the calcined product again and screen it through a 400-mesh sieve to obtain the N, O, P co-doped hard carbon anode material, named LWHC-1400-0.05.

[0079] Example 6

[0080] A preparation method of a ternary-doped hard carbon anode material provided in this example includes:

[0081] S1: First, crush litchi wood and wash it in 1 mol / L -1 hydrochloric acid (HCl) for more than 8 h, then wash with deionized water and absolute ethanol, and dry overnight at 80 °C. Screen the dried product through a 200-mesh sieve to obtain litchi wood powder. Subsequently, mix the litchi wood powder and ammonium hydrogen phosphate at a ratio of 95:5 in a solution (turbid solution) and stir for 12 h, and then evaporate the solvent to dryness;

[0082] S2: Under an inert gas (N2) atmosphere, heat the obtained mixture at a rate of 2 °C / min -1 to 450 °C, hold for 1 h, and then heat at a rate of 2 °C / min -1 to 1300 °C and hold for 3 h.

[0083] S3: After sufficient cooling, crush the calcined product again and screen it through a 400-mesh sieve to obtain the N, O, P co-doped hard carbon anode material.

[0084] Experimental Example 1: TG result analysis

[0085] TG results of the NH4H2PO4 (5 wt%) and litchi wood co-blended precursor are as Figure 1As shown in the figure, the carbonization process of the litchi wood precursor is divided into three stages. In the first stage (room temperature to 300 °C), the material only has a slight weight loss, mainly due to the removal of adsorbed water on the biomass surface. In the second stage (300 - 450 °C), the material undergoes a drastic weight loss, corresponding to drastic structural changes within the material, including the pyrolysis of cellulose, hemicellulose, and lignin, as well as the cross-linking rearrangement of the carbonaceous skeleton. At 377.5 °C, the instantaneous weight loss (DTG) reaches its peak. In the third stage (≥450 °C), as the temperature further rises, the weight loss rate of the material gradually slows down, indicating that the main thermal decomposition process has ended. At this time, the weight loss mainly corresponds to the growth of the graphitic layered structure at high temperatures and the pyrolysis and volatilization of some inorganic salts within the material (including NH4H2PO4 doping). Therefore, the carbonization temperature and time of litchi wood and doped elements need to be maintained within an appropriate range.

[0086] In addition, from Figure 1 the mass retention rate at the end of the TG curve in Figure 2 and the yield of the material in

[0087] Experimental Example 2: XRD Result Analysis

[0088] Figure 3 and Figure 4 the XRD spectra of

[0089] show that the hard carbon material derived from litchi wood exhibits a peak envelope near 2θ = 22° and 43°, corresponding to the (002) and (100) diffraction peaks of the carbonaceous material, respectively, indicating that the obtained material is amorphous carbon. Compared with the relatively sharp and strong XRD diffraction peaks of inorganic compounds and traditional graphite materials, the diffraction peaks of the hard carbon material tend to be broader and flatter "bread-like peaks" or "peak envelopes", fully indicating that the crystallinity of the material is relatively low, conforming to the amorphous characteristics. In addition, no characteristic peaks of NH4H2PO4 are observed in the XRD pattern of the material, proving that the doped NH4H2PO4 has been fully decomposed and successfully doped into the carbonaceous material.

[0089] It should be noted that the (002) peak in amorphous carbon is usually related to the graphitization structure of the material. From the Bragg equation in Equation 3.1:

[0090] 2dsinθ = nλ (3.1) where d is the interplanar spacing to be measured (nm), and θ (usually written as Theta according to convention) is the diffraction angle ( °), where λ is the wavelength of the incident ray (0.154 nm). As Figure 3 shown, under the condition of the same amount of NH4H2PO4 used, when the carbonization temperature gradually rises from 1200 °C to 1400 °C, the graphite layer spacing of the material gradually decreases from 0.390 nm to 0.373 nm, indicating that with the increase of the carbonization temperature, the graphite crystallites in the material gradually grow and the degree of graphitization increases.

[0091] As Figure 4 shown, when the carbonization temperature is the same (1300 °C), with the increase of the amount of NH4H2PO4 used, the graphite layer spacing of the material gradually increases from 0.386 nm to 0.390 nm, proving that NH4H2PO4 can inhibit the growth of graphite crystallites in the material to a certain extent by introducing heteroatom defects, and the relatively large atomic radius and interatomic repulsive force of N, O, and P will also cause the graphite layer spacing to further increase, facilitating the + intercalation / deintercalation and storage of Na.

[0092] Experimental Example 3: Analysis of SEM and HRTEM Results

[0093] The SEM results of the hard carbon materials prepared with different amounts of NH4H2PO4 are as Figure 5 shown. It can be observed from the figure that the morphologies of the obtained hard carbon materials are similar, all of which are irregular block structures, and the block diameters are about 5 - 15 μm. In addition, discontinuous and irregular layered structures can also be observed on the surfaces of some blocks, which are graphite crystallite fragments of the hard carbon materials. This dense graphitized structure is beneficial to reducing the irreversible capacity of the hard carbon anode material, thereby improving its initial Coulomb efficiency and the energy density of the battery.

[0094] Furthermore, through the corresponding energy spectrum analysis, it is found that the material contains C, O, N, and P elements and their distributions are uniform. Using ammonium hydrogen phosphate as the doping raw material for N, O, and P three elements, not only can the doping component ratios of N, O, and P be controlled, but also due to the chemical bonding between the OH groups in NH4H2PO4 or (NH4)2HPO4 and the abundant hydroxyl groups contained in the cellulose and hemicellulose in litchi wood, compared with solid-phase physical mixing, it is more conducive to the dispersion of the doping raw materials and makes the distribution of the doping elements more uniform.

[0095] As Figures 6 - 8As shown in the figure, the HRTEM result analysis shows that the prepared hard carbon material exhibits a curved graphitic interlayer structure, and these interlayer structures are discontinuous, irregular and evenly distributed, indicating that the doped litchi wood-derived hard carbon anode material is amorphous carbon containing locally graphitized microcrystals. The graphitic layer spacings of LWHC-1300-0.025, LWHC-1300-0.05 and LWHC-1300-0.075 are 0.385, 0.388 and 0.390 nm respectively, indicating that the internal structure and graphitic layer spacing of the hard carbon anode material can be effectively regulated by multi-element doping of N, O and P.

[0096] Experimental Example 5: Raman Spectrum Result Analysis

[0097] The Raman spectrum results of the hard carbon material are as Figure 9 shown. The two peaks of the material at 1350 cm -1 and 1580 cm -1 respectively represent the internal disordered defect structure (D peak) and the ordered graphitized structure (G peak), and the integral intensity ratio (I D / I G ) between the two peaks represents the graphitization degree of the material.

[0098] Calculations show that the I D / I G values of the hard carbon materials LWHC-1200-0.05, LWHC-1300-0.025, LWHC-1300-0.05, LWHC-1300-0.075, LWHC-1400-0.05 are 1.21, 1.11, 1.12, 1.20, 1.08 respectively. The results show that at the same dosage of NH4H2PO4, with the further increase of the carbonization temperature, the graphitic microcrystalline structure of the material further grows, resulting in an increase in the graphitization degree of the material; in contrast, keeping the carbonization temperature unchanged, with the increase of the addition amount of NH4H2PO4, more impurity atoms are introduced into the material, and the above impurity atoms replace the original carbon atoms in the carbonaceous skeleton, introducing more defects into the material, thereby leading to a decrease in the graphitization degree of the material.

[0099] Experimental Example 6: Raman Spectrum Result Analysis

[0100] XPS analysis was carried out on the LWHC-1300-0.05 material to verify the elements and their states contained in the material. As Figure 10 shown, the LWHC-1300-0.05 material contains abundant C and O elements and a certain amount of N and P elements, corresponding to the SEM energy spectrum results of the material.

[0101] Furthermore, the high-resolution XPS spectrum of the material was analyzed. The results show that the C element in the material ( Figure 10The existing forms of a) mainly include C-C, C-N, C-P, C-O, C=O, etc.; the N element ( Figure 10 The existing forms of b) mainly include pyrrole-N, graphitized-N, oxidized-N, etc.; the O element ( Figure 10 The existing forms of c) mainly include C-O, O=C-O, and adsorbed-O, etc.; while the P element ( Figure 10 The existing forms of d) mainly include P=O, P-O, P-C, etc.). The above results further prove that through the chemical reaction of dehydration to form ether between the OH groups in the doped raw material phosphate such as NH4H2PO4 and the abundant hydroxyl groups contained in cellulose and hemicellulose in litchi wood, it is more conducive to the formation of doping bonds such as C-N, C-P, and C-O, thereby increasing the defects and conductivity of the hard carbon material, increasing the layer spacing of graphite microcrystals, being beneficial to the sodium ion transmission and deintercalation kinetics process, and thus improving its sodium storage electrochemical performance.

[0102] Experimental Example 7: Analysis of Specific Surface Area and Tap Density

[0103] Further analyze the specific surface area and tap density of the materials. The results are as Figure 11 and Figure 12 shown. The specific surface areas of the hard carbon materials LWHC-1300-0.025, LWHC-1300-0.05, and LWHC-1300-0.075 are 4.46, 3.15, and 22.25 m 2 g -1 , respectively, and the corresponding tap densities are 0.887, 0.882, and 0.872 g cm -3 ( Figure 12 ). The above results show that the ternary-doped hard carbon anode materials all have low porosity and high tap density, which is beneficial to reducing the irreversible capacity of the hard carbon anode material, thereby improving the first Coulomb efficiency and volumetric specific energy density of the sodium-ion battery. However, with the gradual increase in the dosage of NH4H2PO4, more defective pores will be introduced into the carbonaceous material, leading to the surface loosening of the material, that is, the specific surface area increases and the tap density decreases.

[0104] Experimental Example 8: Laser Particle Size Analysis

[0105] In view of the relatively inhomogeneous and irregular internal structure of the biomass-derived hard carbon material, which is likely to lead to problems such as poor consistency and repeatability of the electrochemical performance of the assembled sodium-ion battery, therefore, the particle size distribution of the material is crucial. Therefore, before the next electrochemical performance test, all hard carbon materials are crushed and screened again, and the representative material LWHC-1300-0.05 is subjected to particle size testing to exclude the influence of particle size on the performance of the corresponding sodium-ion battery.

[0106] Figure 13The results show that the material particle diameters are approximately normally distributed, with the median D50 of the particle size being about 10.1 μm, and 90% of the particle sizes being less than 29.1 μm. The above results mean that the particle distribution of the material is uniform and the proportion of large particles is low.

[0107] Experimental Example 9: Charge-discharge performance test

[0108] The charge-discharge performance of the litchi wood-derived hard carbon anode material is as Figure 14 、 Figure 15 and Figure 16 shown. When the dosage of NH4H2PO4 is 5% and the carbonization temperature is 1300 °C, the obtained LWHC-1300-0.05 material exhibits satisfactory electrochemical performance: the corresponding sodium-ion battery has reversible (second cycle) discharge specific capacities of 493.4 and 329.3 mAh g -1 at 70 and 200 mA g -1 , the initial Coulombic efficiency is 77.0%, and it still retains 92.9% of the initial capacity after 250 cycles at 200 mA g -1 . This performance exceeds that of the batteries in the prior art.

[0109] In contrast, the LWHC-1300-0.025 and LWHC-1300-0.075 sodium-ion batteries obtained by changing the dosage of NH4H2PO4 have reversible discharge specific capacities of 426.7 and 402.8 mAh g -1 at 70 mA g -1 , and the first-cycle Coulombic efficiencies are reduced to 76% and 67% respectively. The above results show that doping with a certain amount of N, O, and P heteroatoms can increase the graphite layer spacing of the hard carbon anode, improve the ability to adsorb Na + , thereby improving the sodium storage ability of the hard carbon material by enhancing the pseudocapacitance of the battery, making the hard carbon anode have excellent sodium storage electrochemical performance. However, if the dosages of N, O, and P elements are too large, more unnecessary defects and pores will be introduced into the material, resulting in irreversible insertion of Na + during the first discharge and the generation of more unnecessary solid-electrolyte interfaces (SEI), thus affecting the first-cycle Coulombic efficiency and discharge specific capacity of the corresponding sodium-ion battery.

[0110] In addition, as Figure 16 is known, the discharge specific capacity of the hard carbon material is also related to the carbonization temperature during the preparation process. Among them, the sodium-ion batteries assembled with LWHC-1200-0.05 and LWHC-1400-0.05 materials have reversible discharge specific capacities of 436.9 and 358.0 mAh g -1 at 70 mA g -1 , respectively, both of which are inferior to the materials obtained at 1300 °C. CombiningFigure 3 and Figure 4 in the XRD pattern, when the carbonization temperature is low, the graphite layer spacing of the material is large (~0.39 nm), which makes part of the Na + more inclined to be stored between the graphite layers in the form of pseudo-adsorption instead of the ideal insertion-extraction form, thus affecting the discharge capacity of the material; conversely, when the carbonization temperature is too high, the graphite layer spacing is too narrow (~0.373 nm), which will also affect the insertion and extraction kinetics and discharge capacity of Na + .

[0111] Figure 17 is the charge-discharge curve of the litchi wood-derived hard carbon anode at 70 mA g -1 during cycling. It is observed that the curve shows a slope in the high voltage region, corresponding to the surface capacitance reaction of Na + . There is a long plateau near 0 V, corresponding to the redox process (Sun et al., 2022) of the insertion and extraction of in the hard carbon graphite layer:

[0112]

[0113] Based on the energy storage principle of the hard carbon material composite, the above charge-discharge curve is divided into a low voltage region and a high voltage region with 0.8 V as the boundary. The low voltage region capacity ratios of the charge curves of LWHC-1200-0.05, LWHC-1300-0.025, LWHC-1300-0.05, LWHC-1300-0.075, and LWHC-1400-0.05 materials reach 77%, 77%, 78%, 76%, and 73% respectively, and the low voltage region capacity ratios of the discharge curves reach 93%, 94%, 95%, 91%, and 91% respectively. Among them, the highest low voltage region capacity ratio of the LWHC-1300-0.05 material lays a foundation for effectively improving the energy density and output voltage of the corresponding sodium-ion full battery. In addition, the charge-discharge curves of the above materials at the 2nd, 5th, and 20th cycles basically coincide, proving their excellent cycling performance.

[0114] As Figure 18 shown, even at a large current density of 2.0 and 3.0 A g -1 , compared with the control group whose capacity has almost decayed, the LWHC-1300-0.05 hard carbon anode still maintains reversible specific capacities of 131.0 and 71.1 mAh g -1 respectively, and when the current density is restored to 0.05 A g -1 , the capacity recovery rate exceeds 100%, fully verifying the practical application potential of the material.

[0115] Preparation of Litchi Wood-derived Hard Carbon Anode Materials and Their Performance Study for Sodium-ion Batteries. Using litchi wood as raw material and NH4H2PO4 as the nitrogen and phosphorus doping source, N, O, P ternary co-doped litchi wood-derived hard carbon anode materials (LWHC) were prepared by one-step high-temperature carbonization method, and the effects of carbonization temperature and NH4H2PO4 dosage on the electrochemical performance of the hard carbon anode materials were optimized. The results of phase characterization confirmed that the litchi wood-derived hard carbon materials all belonged to amorphous carbon and presented a blocky structure, with uniform distribution of each element, low porosity and high tap density. The charge-discharge results showed that the optimal group LWHC-1300-0.05 hard carbon anode material had a reversible discharge specific capacity of 493.4 mAh g -1 at 70 mA g -1 , the initial Coulombic efficiency reached 77.0%, and still retained 92.9% of the initial capacity after 250 cycles at 200 mA g -1 , and the capacity ratio in the low voltage region of the discharge curve was as high as 95%. It was found that the interlayer spacing of the graphitization layer of the LWHC hard carbon anode material gradually increased with the decrease of the carbonization temperature and the increase of the heteroatom content, and the LWHC-1300-0.05 material presented a composite Na + energy storage process of surface adsorption-desorption and internal insertion-extraction. Through the optimization of the preparation process in this work, the regulation of the apparent porosity and the internal graphite layer spacing of the hard carbon materials was realized, providing a useful reference for the preparation and screening of high-performance hard carbon materials.

[0116] It can be understood that the above embodiments only express the preferred embodiments of the present invention, and the description is relatively specific and detailed, but it cannot be understood as a limitation to the scope of the patent of the present invention; it should be pointed out that for those of ordinary skill in the art, without departing from the concept of the present invention, the above technical features can be freely combined, and several deformations and improvements can also be made, which all belong to the protection scope of the present invention; therefore, all equivalent transformations and modifications made to the scope of the claims of the present invention shall fall within the scope covered by the claims of the present invention.

Claims

1. A method for preparing a ternary doped hard carbon negative electrode material, characterized in that: include: S1: mixing litchi wood powder and ammonium hydrogen phosphate, and evaporating the solvent to dryness; S2: calcining the mixture after evaporating the solvent at high temperature in an inert gas atmosphere for carbonization; S3: Cool the calcined product, crush it and sieve it to obtain a ternary co-doped hard carbon negative electrode material.

2. The method for preparing a ternary doped hard carbon negative electrode material according to claim 1, characterized in that: The ammonium hydrogen phosphate in step S1 is diammonium hydrogen phosphate and / or monoammonium hydrogen phosphate.

3. The method for preparing a ternary doped hard carbon negative electrode material according to claim 1, characterized in that: The mass ratio of litchi wood powder to ammonium hydrogen phosphate in step S1 is (92.5-97.5):(2.5-7.5).

4. The method for preparing a ternary doped hard carbon negative electrode material according to claim 1, characterized in that: The preparation method of litchi wood powder in step S1 includes: firstly crushing litchi wood, washing with hydrochloric acid, deionized water and anhydrous ethanol in sequence, and then drying, and sieving the dried product with a 200-mesh sieve to obtain litchi wood powder.

5. The method for preparing a ternary doped hard carbon negative electrode material according to claim 1, characterized in that: The carbonization temperature in step S2 is 1200-1400°C.

6. The method for preparing a ternary doped hard carbon negative electrode material according to claim 5, characterized in that: The high temperature calcination in step S2 is specifically: 2°C min -1 Heat to 450℃ and keep warm for the first time; then heat at 2℃min -1 Raise the temperature to the carbonization temperature and keep it for a second time.

7. The method for preparing a ternary doped hard carbon negative electrode material according to claim 6, characterized in that: The first duration is at least 1 hour and the second duration is at least 3 hours.

8. The method for preparing a ternary doped hard carbon negative electrode material according to claim 1, characterized in that: The crushing and screening in step S3 refers to screening the cooled calcined product through a 400-mesh sieve.

9. A ternary doped hard carbon negative electrode material, characterized in that: The method is prepared by any one of claims 1 to 8.

10. An application of a ternary doped hard carbon negative electrode material, characterized in that: The ternary doped hard carbon negative electrode material described in claim 9 is used for the negative electrode of a sodium ion battery.

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