Negative electrode material, sodium ion battery and preparation method of negative electrode material

Abstract

The invention provides a negative electrode material, a sodium ion battery and a preparation method of the negative electrode material, and belongs to the technical field of secondary batteries, the negative electrode material comprises a hard carbon material, the hard carbon material is doped with an X element, and the hard carbon material has a C-X network structure; wherein the X element comprises at least two combinations of N, P and S. According to the negative electrode material disclosed by the invention, at least two heteroatoms are uniformly doped in the biomass hard carbon material, the uniformly doped heteroatoms can change the surface structure of the hard carbon material, so that a rich C-X network structure is formed in the hard carbon material, and the synergistic effect of the heteroatoms can effectively improve the conductivity of the material and enlarge the spacing of carbon layers in a graphite domain; more ionic reaction active sites are introduced, so that the initial coulombic efficiency and the cycle life of the hard carbon material can be greatly improved while the gram volume of the material is improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of secondary batteries, in particular to a negative electrode material, a sodium ion battery and a preparation method of the negative electrode material. BACKGROUND

[0002] The massive use of non-renewable fuels in the industrial revolution has brought serious impact on the environment. Greenhouse gases and pollutants such as carbon dioxide, nitrogen oxides and sulfides are discharged in large quantities. In order to alleviate this problem, various renewable energy sources such as wind energy, solar energy and geothermal energy are being adopted by countries. However, the production of renewable energy sources is affected by factors such as season and climate, and shows characteristics such as intermittency and instability. In order to effectively utilize this new type of energy, it is necessary to develop high-efficiency and convenient large-scale energy storage technology. Among various energy storage technologies, lithium / sodium ion batteries provide an efficient power management method. However, the limited lithium resources limit its application in large-scale energy storage systems. In view of this, researchers have turned their attention to sodium ion batteries which have similar properties to lithium ion batteries. Sodium resources are abundant, low in cost, evenly distributed around the world, and sodium ion batteries can use aluminum foil as a negative electrode current collector, further reducing costs, and showing great application prospects in the field of large-scale energy storage.

[0003] At present, the selection and design of high-performance negative electrodes are the main challenges for the development of sodium ion batteries. The negative electrode determines the discharge rate of the battery and plays a key role in the overall coulombic efficiency of the battery. Compared with lithium ions, sodium ions have a larger diameter, which makes the sodium intercalation capacity of traditional sodium ion battery negative electrode material graphite very low and the cycle performance poor. Therefore, it is necessary to develop negative electrode materials with large interlayer distance (>0.37 nm) to deintercalate sodium ions in sodium ion batteries. Hard carbon type negative electrode materials can solve the problem of volume expansion and provide high capacity due to the presence of rich graphite microcrystalline structures and a large number of active centers in the internal structure, and are one of the best electrode candidates for sodium ion batteries.

[0004] At present, hard carbon negative electrode precursor materials are complex and diverse, including biomass, resin-based, pitch and the like. However, the preparation of hard carbon using pitch, coal tar or organic polymers as raw materials has the disadvantages of non-renewable and pollution in the reaction process. Although the use of biomass materials as hard carbon precursors can solve the problem of high pollution in the hard carbon manufacturing process, hard carbon negative electrode materials using biomass materials as precursors generally have the problems of low initial coulombic efficiency and serious capacity attenuation, which limits their practical application in sodium ion batteries.

[0005] Therefore, it is necessary to design a negative electrode material, a sodium ion battery and a preparation method of the negative electrode material to solve the above problems. SUMMARY

[0006] In view of the above prior art defects, the present application provides a negative electrode material, a sodium ion battery and a preparation method of the negative electrode material, to solve the technical problems of low initial coulomb efficiency and serious capacity attenuation of the hard carbon material prepared from the biomass material as the negative electrode material of the sodium ion battery in the prior art.

[0007] To achieve the above object and other related objects, the present application provides a negative electrode material, which comprises a hard carbon material, wherein X elements are doped in the hard carbon material, and the hard carbon material has a C-X network structure; wherein the X elements comprise at least two combinations of N, P and S.

[0008] In an example of the present application, the X elements are a combination of N and P or a combination of N and S.

[0009] In an example of the present application, the hard carbon material has graphite domains, and the X elements are distributed at least between the carbon layers of the graphite domains.

[0010] The present application also provides a preparation method of the negative electrode material as described in any one of the above examples, which comprises:

[0011] using a biomass raw material as a carbon source precursor, pre-carbonizing the carbon source precursor, mixing and sintering the carbon source precursor with an X source to obtain the negative electrode material;

[0012] wherein the X elements comprise at least two combinations of N, P and S; and the X source is a compound containing the X elements.

[0013] In an example of the present application, the preparation method further comprises a pretreatment step of the biomass raw material before the pre-carbonization of the carbon source precursor, and the pretreatment step comprises:

[0014] placing the biomass raw material in an acid solution, stirring and mixing and water bath heating; wherein the water bath heating temperature is 50-85℃, the water bath heating temperature time is 8-10h, and the stirring speed is 500-1000r / min; and then taking out the biomass raw material from the acid solution and sequentially performing washing and drying.

[0015] In an example of the present application, the pre-carbonization of the carbon source precursor comprises:

[0016] placing the carbon source precursor in deionized water and performing hydrothermal reaction to complete the pre-carbonization of the carbon source precursor;

[0017] The mass of the carbon source precursor and the volume of the deionized water are (1g-3g):(20mL-60mL); the temperature of the hydrothermal reaction is 160-200 DEG C; and the time of the hydrothermal reaction is 5-10h.

[0018] In an example of the present application, the mixing and sintering of the carbon source precursor and the X source to obtain the negative electrode material comprises:

[0019] The carbon source precursor after the pre-carbonization treatment is uniformly mixed with the X source at a mass ratio of 1:(0.5-5) to obtain a mixture;

[0020] The mixture is sintered and carbonized under an inert atmosphere to obtain the negative electrode material;

[0021] The temperature of the carbonization is 600-1300 DEG C; and the time of the carbonization is 90-150min.

[0022] In an example of the present application, the biomass raw material comprises at least one of a plant biomass raw material and an animal biomass raw material, the plant biomass raw material is a plant material containing hemicellulose, cellulose or lignin, and the animal biomass raw material is an animal material containing chitin, protein or chitosan.

[0023] In an example of the present application, when the X element is a combination of N and P, the X source is selected from at least one of ammonium phosphate, ammonium dihydrogen phosphate and di-ammonium hydrogen phosphate; and when the X element is a combination of N and S, the X source is selected from at least one of ammonium sulfate, ammonium persulfate, thiourea, guanidine sulfate, cysteine and cystine.

[0024] The present application also provides a sodium ion battery, which comprises a negative electrode and a positive electrode, a separator and an electrolyte, the negative electrode comprises a negative electrode current collector and a negative electrode active material layer, and the negative electrode active material layer comprises the negative electrode material in any of the examples or the negative electrode material prepared by the preparation method in any of the examples.

[0025] The present application provides a negative electrode material, which is uniformly doped with at least two kinds of heteroatoms in a biomass hard carbon material, the uniformly doped heteroatoms can change the surface structure of the hard carbon material, so that a rich C-X network structure is formed in the hard carbon material, the synergistic effect of the heteroatoms can effectively improve the conductivity of the material, expand the carbon layer spacing in the graphite domain, and introduce more ion reaction active sites, thereby greatly improving the initial coulombic efficiency and cycle life of the hard carbon material while improving the material specific capacity. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to make the technical solutions of the embodiments of the present application or the prior art clearer, the accompanying drawings needed in the embodiments or prior art description will be briefly introduced. Obviously, the accompanying drawings in the following description only need to be some embodiments of the present application, and other embodiments can be obtained by those skilled in the art without creative effort on the basis of these drawings.

[0027] Figure 1 (a) is an SEM test diagram of the negative electrode material in an embodiment of the present application, Figure 1 (b) to Figure 1 (e) is an EDS test diagram of the negative electrode material in an embodiment of the present application;

[0028] Figure 2 is an XPS test diagram of the negative electrode material in an embodiment of the present application;

[0029] Figure 3 is an XRD test diagram of the negative electrode material in an embodiment of the present application;

[0030] Figure 4 is a flowchart of the preparation method of the negative electrode material in an embodiment of the present application;

[0031] Figure 5 is a flowchart of step S200 in an embodiment of the present application;

[0032] Figure 6 is a flowchart of step S300 in an embodiment of the present application. DETAILED DESCRIPTION

[0033] The embodiments of the present application will be described in detail by specific, concrete examples. Those skilled in the art can easily understand other advantages and effects of the present application from the content disclosed in the specification. The present application can also be implemented or applied by different specific embodiments, and each detail in the specification can be modified or changed based on different views and applications without departing from the spirit of the present application. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict. It should also be understood that the terms used in the embodiments of the present application are for describing specific, concrete embodiments, and are not intended to limit the protection scope of the present application. The test methods in the following embodiments are not specified, and are usually performed according to conventional conditions or according to the conditions recommended by the manufacturers.

[0034] Biomass-derived hard carbon, as a kind of hard carbon material, is mainly composed of amorphous carbon with randomly distributed small graphitized fragments and hierarchical pore structure, and has good geometric shape, hierarchical structure, electrical conductivity, stability, safety and high crystallinity, so it is widely used in the research of sodium ion battery negative electrode materials. However, the cycle performance of hard carbon material obtained by carbonizing general biomass material is not ideal, and the initial coulombic efficiency is low.

[0035] In order to improve the initial coulombic efficiency and cycle life of biomass-derived hard carbon material, the application provides a negative electrode material, which is doped with at least two kinds of X element heteroatoms in the biomass-derived hard carbon material. The X elements introduced into the hard carbon material are fixed on the carbon skeleton, so that the X elements form a uniformly distributed C-X connection structure in the hard carbon material together with the carbon skeleton, and then a uniformly distributed C-X conductive network structure is formed in the hard carbon material. In the hard carbon material, multiple X elements synergistically act, form electron cloud delocalization around carbon atoms by utilizing the differences in bond length, atomic diameter and electronegativity of multiple X elements, thereby effectively enhancing the ion transport dynamics of the hard carbon material and improving the electrical conductivity of the hard carbon material; while increasing the active sites of the electrochemical adsorption / desorption center, the occurrence of irreversible side reactions is inhibited to some extent, so as to improve the charge and discharge specific capacity and initial coulombic efficiency of the hard carbon material, and improve the cycle performance of the negative electrode material.

[0036] The above-mentioned negative electrode material includes a hard carbon material, which is obtained by carbonizing a biomass precursor material, and the hard carbon material is doped with X elements, wherein the X elements include at least two combinations of N, P and S. For example, the X elements doped in the hard carbon can be a combination of N and P elements, or a combination of N and S elements, or a combination of P and S elements, or a combination of N, P and S. In addition, when the X elements doped in the hard carbon material are multiple element combinations, the ratio of each element in the combination is not limited.

[0037] Please refer to Figure 1 and Figure 2 , the X elements are uniformly distributed in the hard carbon material and are adsorbed on the carbon skeleton of the hard carbon material through electrostatic force, covalent bond and other interactions, and then a uniformly distributed C-X network structure is formed in the hard carbon material. In the C-X network structure, multiple X elements can produce more defect sites around carbon atoms due to the differences in bond length, atomic diameter and electronegativity, thereby effectively increasing the active sites of the electrochemical adsorption / desorption center and improving the charge and discharge specific capacity, initial coulombic efficiency and cycle performance of the hard carbon material; and the introduced X elements also form electron cloud delocalization around carbon atoms, effectively improving the electrical conductivity of the hard carbon material and further improving the rate performance of the hard carbon material.

[0038] Specifically, taking the hard carbon material doped with N and P elements in Example 1 below as an example, from the perspective of the differences in bond length, atomic diameter and electronegativity of N and P elements, the X elements in the hard carbon material can form electron cloud delocalization around carbon atoms, thereby effectively increasing the ion transport dynamics of the hard carbon material and improving the electrical conductivity of the hard carbon material.Figure 1 As can be seen from the SEM and EDS test images of the hard carbon material in Example 1 below ( Figure 1 Figure (a) shows the SEM image of hard carbon material. Figure 1 Four EDS images (b) to (e) show the distribution areas of C, O, P, and N elements in the same region of the hard carbon material. In image (b), the green area represents the C element distribution area; in image (c), the blue area represents the O element distribution area; in image (d), the red area represents the P element distribution area; and in image (e), the purple area represents the N element distribution area. The hard carbon material mainly contains C, O, N, and P elements, with the co-doped N and P elements evenly distributed in positions close to the C element. Furthermore... Figure 2 XPS tests on this hard carbon material show that nitrogen (N) with a binding energy of around 400 eV has an N1s state and can be fixed to the carbon framework via NC bonds. P (P) with binding energies of around 200 eV and 130 eV has P2s and P2p states and can be fixed to the carbon framework via P-(CO) or P-(CH) bonds. A small amount of P can also be connected to the carbon framework via PO bonds. Therefore, the doped X elements are at least chemically bonded to the hard carbon material, forming a uniformly distributed CX network structure.

[0039] Furthermore, hard carbon materials contain graphite domains, which are randomly distributed graphite-like layers within the material. Element X is at least partially distributed between the carbon layers within these graphite domains. Due to its larger atomic size, element X located between carbon layers effectively expands the interlayer spacing, further enhancing the ion transport rate of the hard carbon material and improving its charge / discharge capacity and rate performance.

[0040] Taking the hard carbon materials doped with N and P elements in Comparative Example 1 and Example 1 as examples, such as Figure 3 The XRD test results show that the XRD images of the undoped and N and P-doped hard carbon materials in Comparative Example 1 and Example 1 both have characteristic peaks representing the typical (002) and (100) crystal planes of hard carbon materials. Compared with the two, the characteristic peak of the (002) crystal plane of the hard carbon material in Example 1 shifts to a smaller angle, indicating that N and P doping can expand the interlayer spacing of carbon materials. A larger interlayer spacing is beneficial to the rapid insertion / extraction of ions.

[0041] In some embodiments, the X element doped in the hard carbon material is a combination of N and P or a combination of N and S. Among them, the N, P and S elements are uniformly adsorbed around the carbon skeleton of the hard carbon material, and the N element doping can form nitrogen substances such as pyridine nitrogen (N-6), pyrrole nitrogen (N-5), graphite nitrogen (N-Q) and oxidized nitrogen (N-O) on the carbon skeleton, wherein the pyridine nitrogen and the pyrrole nitrogen have been proved to have high electrochemical activity, can provide active point positions for adsorption / desorption of sodium ions, and the graphite nitrogen can improve the conductivity of the material, thereby having good kinetic performance. And P or S element doping can form P-C bond, P-O bond or S-C bond, S-O bond structure on the carbon skeleton, on the one hand, the above bond structure can change the morphology and composition of the solid electrolyte interface (SEI) layer, which is beneficial to the formation of a thin and dense SEI layer, and the P-O bond can also prevent the decomposition of the solvated PF 6- On the other hand, the above bond structure can also provide electrochemically active sites for the deintercalation of sodium ions, improve the charge and discharge capacity, initial coulombic efficiency of the hard carbon material; at the same time, the larger atomic size of P and S elements can increase the interlayer spacing of the carbon layer in the hard carbon material, improve the ion transmission rate of the hard carbon material, and improve the rate performance of the hard carbon material.

[0042] In some embodiments, the molar fraction x of the X element in the hard carbon material satisfies 0% < x < 14%, for example, it can be 1%, 2%, 3%, 5%, 7%, 10%, 12% or 14%.

[0043] On the other hand, in order to be able to prepare a biomass-derived hard carbon material uniformly doped with multiple X elements, the present application also provides a preparation method of a negative electrode material. As shown in the figure, the preparation method of the negative electrode material comprises the following steps: Figure 4

[0044] S100, taking biomass raw material as carbon source precursor, and pre-carbonizing the carbon source precursor;

[0045] S200, mixing and sintering the carbon source precursor and X source, so that the X element is uniformly doped into the formed hard carbon material to obtain a negative electrode material;

[0046] Among them, the X element includes at least two combinations of N, P and S; the X source is a compound containing all X elements.

[0047] ​This preparation method first pre-carbonizes the carbon source precursor to reduce some irreversible defects. Then, a one-step doping method using high-temperature sintering is employed to sinter the X source containing multiple X elements together with the carbon source precursor, thereby uniformly doping multiple X elements into the formed hard carbon material and obtaining a hard carbon material with a rich CX network structure. The introduction of uniformly distributed X elements into this hard carbon material improves the surface properties, expands the carbon interlayer spacing, and introduces more ionic reactive sites. This, in turn, increases the charge / discharge specific capacity of the hard carbon material while significantly improving its initial coulombic efficiency and cycle life.

[0048] In some embodiments, in step S100, the carbon source precursor is pre-carbonized using a hydrothermal reaction. The hydrothermal reaction allows glucose molecules in the biomass raw material to gradually dehydrate and form 5-hydroxymethylfurfural. 5-hydroxymethylfurfural polymerizes and nucleates in the aqueous phase, and the subsequent carbonization further introduces a stable morphology, thereby removing irreversible structural defects in the carbon source precursor and releasing more reactive sites in the synthesis of hard carbon materials, thus improving the initial coulombic efficiency of hard carbon material preparation.

[0049] Specifically, step S100 includes placing the carbon source precursor in a high-temperature reactor, adding deionized water to the high-temperature reactor, then placing the high-temperature reactor in an oven for high-temperature heating to carry out a hydrothermal reaction, and finally cooling to room temperature to complete the pre-carbonization treatment of the carbon source precursor. In the hydrothermal reaction, the mass ratio of the carbon source precursor to the volume of deionized water is (1g~3g):(20mL~60mL), for example, 1g:20mL, 1g:40mL, 1g:60mL, 2g:20mL, 2g:40mL, 2g:60mL, 2.5g:60mL, 3g:20mL, 3g:40mL, or 3g:60mL; the high temperature of the hydrothermal reaction is 160℃~200℃, for example, 160℃, 170℃, 180℃, 190℃, or 200℃; the hydrothermal reaction time is 5h~10h, for example, 5h, 6h, 7h, 8h, 9h, or 10h.

[0050] In some embodiments, in step S100, the biomass raw material includes at least one of plant biomass raw material and animal biomass raw material.

[0051] Among them, plant biomass raw materials are plant materials containing hemicellulose, cellulose, or lignin. For example, plant biomass raw materials are selected from at least one of almond shells, apricot shells, peanut shells, coconut shells, walnut shells, pine nut shells, pistachio shells, bamboo, and straw. That is, plant biomass raw materials can be any one of the types of plant biomass raw materials listed above, such as almond shells, apricot shells, peanut shells, coconut shells, walnut shells, pine nut shells, pistachio shells, bamboo, or straw, etc. Plant biomass raw materials can also be any combination of two or more of the types of plant biomass raw materials listed above. For example, plant biomass raw materials can be a combination of almond shells and apricot shells, or a combination of peanut shells and coconut shells, or a combination of walnut shells and pine nut shells, or a combination of pistachio shells and bamboo, or a combination of straw and walnut shells, or a combination of almond shells, apricot shells, and peanut shells, or a combination of coconut shells, pine nut shells, and pistachio shells, or a combination of bamboo, straw, and any other type of shell.

[0052] Animal biomass raw materials are animal materials containing chitin, protein, or chitosan, or carbon source extracts of animal materials such as chitin. For example, animal biomass raw materials are selected from at least one of crab shells, shrimp shells, and hair. That is, animal biomass raw materials can be any of the types of animal biomass raw materials listed above, such as crab shells, shrimp shells, or hair. Animal biomass raw materials can also be any combination of two or more of the types of animal biomass raw materials listed above. For example, animal biomass raw materials can be a combination of crab shells and shrimp shells, or a combination of crab shells and hair, or a combination of shrimp shells and hair, or a combination of crab shells, shrimp shells, and hair.

[0053] Furthermore, when the biomass feedstock is a combination of two or more of the above-mentioned materials, the proportion of each material within the combination is not limited. In other embodiments, the biomass feedstock may also be material types not listed above.

[0054] like Figure 5 As shown, in some embodiments, step S200 includes the following steps:

[0055] S201. The carbon source precursor after pre-carbonization treatment is uniformly mixed with the X source at a mass ratio of 1:(0.5~5) to obtain a mixture; wherein, the X source is a compound containing multiple X elements.

[0056] S202. The mixture is sintered and carbonized under an inert atmosphere to obtain a negative electrode material.

[0057] In step S201, the pre-carbonized carbon source precursor is mixed with the X source and then thoroughly ground to obtain a mixture. The mass ratio of the carbon source precursor to the X source is 1:(0.5–5), for example, it can be 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, or 1:5. The grinding time is 60 min–150 min, for example, it can be 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, 120 min, 130 min, 140 min, or 150 min.

[0058] In step S202, the mixture is placed in a furnace filled with an inert atmosphere, and the furnace temperature is raised to the carbonization temperature at a heating rate of 3℃ / min to 7℃ / min. The mixture is then held at the carbonization temperature for the specified sintering time, after which heating is stopped. The furnace is allowed to cool naturally to room temperature before the sintered product is removed, yielding the negative electrode material. The sintering and carbonization temperature is any temperature within the range of 600℃ to 1300℃, for example, 600℃, 700℃, 800℃, 900℃, 1000℃, 1100℃, 1200℃, or 1300℃; the sintering and carbonization time is any time within the range of 90min to 150min, for example, 90min, 100min, 110min, 120min, 130min, 140min, or 150min.

[0059] The X source used in step S200 includes multiple X elements to be doped, and the X source can uniformly dope multiple X elements into the formed hard carbon material during the sintering process in step S202.

[0060] In some embodiments, when the X element co-doped into the hard carbon material is a combination of N and P, the X source is selected from at least one of ammonium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. That is, the X source can be any one of the compound types listed above, such as ammonium phosphate, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate, etc.; the X source can also be any combination of two or more of the compound types listed above, for example, the X source can be a combination of ammonium phosphate and ammonium dihydrogen phosphate, or a combination of ammonium dihydrogen phosphate and diammonium hydrogen phosphate, or a combination of ammonium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0061] In other embodiments, if the X element co-doped into the hard carbon material is a combination of N and S, the X source is selected from at least one of ammonium sulfate, ammonium persulfate, thiourea, guanidine sulfate, cysteine, and cystine. That is, the X source can be any one of the compound types listed above, such as ammonium sulfate, ammonium persulfate, thiourea, guanidine sulfate, cysteine, or cystine, etc.; the X source can also be any two or more combinations of the compound types listed above, for example, the X source can be a combination of ammonium sulfate and ammonium persulfate, or a combination of thiourea and guanidine sulfate, or a combination of cysteine ​​and cystine, or a combination of ammonium sulfate and thiourea, or a combination of ammonium persulfate and cysteine, or a combination of guanidine sulfate and cystine, or a combination of ammonium sulfate, ammonium persulfate, and thiourea, or a combination of guanidine sulfate, cysteine, and cystine, or a combination of ammonium sulfate, guanidine sulfate, and ammonium persulfate.

[0062] Furthermore, when the X source is a combination of two or more of the above-mentioned materials, the proportion of each material within the combination is not limited. In other embodiments, the X source may also be a type of material not listed above.

[0063] In some embodiments, the inert gas used in step S202 may be nitrogen, argon, helium or neon.

[0064] In some embodiments, step S202 further includes washing and drying the sintered product after sintering and carbonization. Specifically, deionized water is used as the cleaning agent, and the sintered product is filtered and washed 3 to 5 times using a vacuum filtration device. After washing, the sintered product is heated and dried to obtain the negative electrode material. The drying temperature is 80℃ to 100℃, for example, 80℃, 85℃, 90℃, 95℃, or 100℃; the drying time is 9h to 14h, for example, 9h, 10h, 11h, 12h, 13h, or 14h.

[0065] like Figure 6 As shown, in some embodiments, before performing step S100 to pre-carbonize the carbon source precursor, the preparation method further includes a pretreatment step S300 of the biomass raw material, which includes the following steps:

[0066] S301. The biomass raw material is placed in an acid solution, stirred and mixed, and heated in a water bath to remove impurities from the biomass raw material, improve the purity of the prepared hard carbon material, and reduce irreversible defects in the hard carbon material.

[0067] S302. The biomass raw material is removed from the acid solution and washed and dried in sequence.

[0068] In the pickling process of step S301, the acid solution used can be hydrochloric acid with a concentration of 1-2 mol / L or sulfuric acid with a concentration of 0.5-1 mol / L; the mass ratio of biomass raw material to acid solution volume is (2.0g-8.0g):(20mL-80mL), for example, it can be 2g:20ml, 2g:40ml, 2g:60ml, 2g:80ml, 4g:20ml, 4g:40ml, 4g:60ml, 4g:80ml, 6g:20ml, 6g:40ml, 6g:60ml, 6g:80ml, 8g:20ml, 8g:40ml, 8g:60ml or 8g:80ml.

[0069] In the water bath heating process of step S301, the water bath heating temperature of the biomass raw material in the acid solution is 50℃~85℃, for example, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃ or 85℃; the water bath heating time is 8h~10h, for example, 8h, 9h or 10h; the stirring speed of the biomass raw material and the acid solution is 500r / min~1000r / min, for example, 500r / min, 600r / min, 700r / min, 800r / min, 900r / min or 1000r / min.

[0070] In step S302, the biomass raw material is washed with deionized water; after washing, the biomass raw material is heated and dried. The drying temperature of the biomass raw material is 70℃~90℃, for example, 70℃, 75℃, 80℃, 85℃ or 90℃; the drying time of the biomass raw material is 10h~14h, for example, 10h, 11h, 12h, 13h or 14h.

[0071] This invention also provides a sodium-ion battery, which can be a solid-state secondary battery, a semi-solid-state secondary battery, or a liquid secondary battery. Taking a liquid secondary battery as an example, the sodium-ion battery includes a positive electrode, a negative electrode, a separator disposed between the positive and negative electrode, and an electrolyte. The negative electrode includes a negative current collector and a negative active material layer disposed on the negative current collector. The negative active material layer includes a conductive agent, a thickener, a binder, and the negative electrode material described in any of the above embodiments, or the negative electrode material prepared by the preparation method described in any of the above embodiments. The positive electrode includes a positive current collector and a positive active material layer disposed on the positive current collector. The positive active material layer includes a positive electrode material, a conductive agent, and a binder. The positive and negative electrode materials can intercalate and deintercalate sodium ions to achieve energy storage and release. The electrolyte is the carrier for sodium ion transport between the positive and negative electrodes. The separator is permeable to sodium ions but non-conductive, thus separating the positive and negative electrodes to prevent short circuits.

[0072] It should be noted that the preparation of the positive electrode, negative electrode, separator, and electrolyte in a sodium-ion battery, as well as the assembly process of the sodium-ion battery, can be carried out using conventional methods in the art. The following description uses a pouch cell as a specific example to illustrate the preparation method of the electrochemical device:

[0073] (1) Preparation of positive electrode sheet: The positive electrode material, conductive agent, and binder are mixed in a weight ratio of (90 to 99):(1 to 10):(1 to 10), optionally 93:3:4. N-methylpyrrolidone (NMP) solvent is added and the mixture is stirred thoroughly to obtain a positive electrode slurry. The mixture is stirred under vacuum until it becomes uniform and transparent to obtain the positive electrode slurry. The positive electrode slurry is uniformly coated onto the positive electrode current collector aluminum foil, and then the positive electrode current collector aluminum foil is dried at room temperature and transferred to an oven for drying. After cold pressing and slitting, the positive electrode sheet is obtained. The conductive agent can be selected from at least one of the following conductive materials: carbon black (Super P), acetylene black, carbon nanotubes (CNTs), graphene, and carbon nanofibers (VGCF). For example, the conductive agent can be SP and CNTs, with a mass ratio of SP to CNTs of 2:1. The binder can be selected from at least one of the following: polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). For example, PVDF can be used as the binder. The positive electrode material includes at least one material that can insert and extract sodium. Further, the positive electrode material includes sodium-containing compounds, specifically including NaFeO2, NaCoO2, NaCrO2, NaMnO2, NaNiO2, and NaNi 1 / 2 Ti 1 / 2 O2, NaNi 1 / 2 Mn 1 / 2 O2, Na 2 / 3 Fe 1 / 3 Mn 2 / 3 O2, NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, NaMn2O4, NaNi 1 / 2Mn 3 / 2 The cathode material can be any one or at least two of the following: O2, NaFePO4, NaMnPO4, NaCoPO4, Na2FePO4F, Na2MnPO4F, and Na2CoPO4F. In other words, the cathode material can be any one of the materials listed above, such as NaFeO2 or Na... 2 / 3 Fe 1 / 3 Mn 2 / 3O2, or Na2FePO4F, etc.; or any combination of two or more of the materials listed above, such as a combination of NaFeO2 and NaCoO2, or a combination of NaCrO2, NaMnO2 and NaNiO2, or a combination of Na2FePO4F and Na2MnPO4F, or NaNi 1 / 2 Ti 1 / 2 O2, NaNi 1 / 2 Mn 1 / 2 O2 and Na 2 / 3 Fe 1 / 3 Mn 2 / 3 Compositions of O2, etc., will not be listed here. It should be noted that the positive electrode material can also be any positive electrode active material not listed above. When the positive material is a combination of two or more, there is no restriction on the ratio between the components in the composition, and they can be mixed in any proportion.

[0074] (2) Preparation of negative electrode sheet: The negative electrode material, conductive agent, thickener and binder are mixed in a mass ratio of 8:1:1, and deionized water is added to adjust the slurry solid content to 55%. Then, the mixture is thoroughly stirred under vacuum to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on both sides of an 8μm negative electrode current collector copper foil. After drying at room temperature, it is transferred to an oven for drying, and then the negative electrode sheet is obtained through cold pressing, slitting and other processes. The conductive agent can be selected from at least one of conductive materials such as carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, and carbon nanofibers (VGCF). The binder is selected from at least one of the binder materials such as polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), and polyvinylidene fluoride (PVDF).

[0075] (3) Electrolyte preparation: In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC) and dimethyl carbonate (DMC) are uniformly mixed at a volume ratio of 1:1 to obtain an organic solvent. Then, fluoroethylene carbonate (FEC) additives accounting for 5% of the mass fraction of ethylene carbonate (EC) and dimethyl carbonate (DMC) are added to the organic solvent. Next, fully dried sodium salt NaClO4 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1mol / L.

[0076] (4) Separator preparation: The separator is a conventional separator material in the art. For example, the separator includes a polyethylene base membrane and a nano-alumina coating coated on the polyethylene base membrane, wherein the polyethylene base membrane has a thickness of 9 μm and the nano-alumina coating has a thickness of 3 μm.

[0077] (5) Battery assembly: Battery assembly is carried out according to conventional methods. For example, after preparation, the negative electrode, separator, and positive electrode are stacked in sequence and placed in an aluminum-plastic film to obtain a dry cell (a soft-pack cell without electrolyte filling). The dry cell is then baked to remove water. The prepared electrolyte is injected into the dry cell and sealed to obtain the finished sodium-ion soft-pack battery.

[0078] The technical solution of the present invention will be described in detail below through several specific embodiments and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following embodiments are commercially available products or can be prepared by conventional methods in the art.

[0079] Example 1

[0080] This embodiment provides a negative electrode material, which includes a hard carbon material derived from biomass feedstock. The hard carbon material is co-doped with multiple X elements through a one-step sintering process, where the X elements are a combination of N and P. The preparation method of this negative electrode material is as follows:

[0081] S100. Using almond shells as biomass raw material, the biomass raw material was ground and pulverized, and then heated and stirred in a 1 mol / L hydrochloric acid solution at a temperature of 70°C for 9 hours at a stirring speed of 700 r / min. The sample was then repeatedly washed with deionized water until the pH value reached neutral. Finally, the biomass raw material was dried at an ambient temperature of 80°C for 12 hours to obtain a carbon source precursor that can be used to prepare hard carbon.

[0082] Weigh 2.5g of carbon source precursor and place it into a 100ml polytetrafluoroethylene (PTFE) liner. Then add 60mL of deionized water to the PTFE liner. Place the PTFE liner in a high-temperature reactor and seal it. Then place the high-temperature reactor in an oven and heat it at 180℃ for 8 hours. After the hydrothermal reaction, filter and wash the carbon source precursor in the PTFE liner three times with deionized water and anhydrous ethanol. Finally, place it in an oven and dry it at 80℃ for 12 hours.

[0083] S200. Using ammonium phosphate as the X source, 1.2g of carbonized carbon source precursor and 0.6g of ammonium phosphate were weighed and thoroughly mixed and ground to obtain a mixture. The mixture was packaged into a ceramic boat and placed in a tube furnace. Under an argon atmosphere, the temperature was raised to 700℃ at a rate of 5℃ / min and then held for 2 hours for carbonization. During the heating process, argon gas with a flow rate of 200sccm was continuously introduced into the furnace. After the pyrolysis was completed, the temperature was lowered to room temperature and the sintered product was taken out. It was repeatedly filtered and washed three times with deionized water and then dried in an 80℃ oven for 12 hours to obtain a biomass-derived hard carbon material uniformly doped with N and P elements.

[0084] Example 2

[0085] This embodiment prepares a negative electrode material with the same system as in Example 1. This negative electrode material includes hard carbon material derived from biomass feedstock. The hard carbon material is co-doped with multiple X elements through one-step sintering; the X elements are a combination of N and P. The difference between this embodiment and Example 1 is that in step S200, 1.2g of carbonized carbon source precursor and 1.2g of ammonium phosphate are weighed, thoroughly mixed, and ground to obtain a mixture.

[0086] Example 3

[0087] This embodiment prepares a negative electrode material with the same system as in Example 1. This negative electrode material includes hard carbon material derived from biomass feedstock. The hard carbon material is co-doped with multiple X elements through a one-step sintering process; the X elements are a combination of N and P. The difference between this embodiment and Example 1 is that in step S200, 1.2g of carbonized carbon source precursor and 2.4g of ammonium phosphate are weighed, thoroughly mixed, and ground to obtain a mixture.

[0088] Example 4

[0089] This embodiment prepares a negative electrode material with the same system as in Example 1. This negative electrode material includes hard carbon material derived from biomass feedstock. The hard carbon material is co-doped with multiple X elements through one-step sintering; the X elements are a combination of N and P. The difference between this embodiment and Example 1 is that in step S200, 1.2g of carbonized carbon source precursor is weighed and thoroughly mixed and ground with 6.0g of ammonium phosphate to obtain a mixture.

[0090] Example 5

[0091] This embodiment prepares a negative electrode material with the same system as in Example 1. This negative electrode material includes a hard carbon material derived from biomass feedstock. The hard carbon material is co-doped with multiple X elements through a one-step sintering process; the X elements are a combination of N and P. The difference between this embodiment and Example 1 is that in step S200, the carbonization temperature for sintering the mixture is 800°C.

[0092] Example 6

[0093] This embodiment prepares a negative electrode material with the same system as in Example 1. This negative electrode material includes hard carbon material derived from biomass feedstock. The hard carbon material is co-doped with multiple X elements through a one-step sintering process; the X elements are a combination of N and P. The difference between this embodiment and Example 1 is that, in step S200, diammonium hydrogen phosphate is used as the X source.

[0094] Example 7

[0095] This embodiment prepares a negative electrode material with the same system as in Example 1. This negative electrode material includes hard carbon material derived from biomass feedstock. The hard carbon material is co-doped with multiple X elements through a one-step sintering process; the X elements are a combination of N and P. The difference between this embodiment and Example 1 is that, in step S100, shrimp shells, a biomass feedstock, are used as the carbon source precursor.

[0096] Example 8

[0097] This embodiment prepares a negative electrode material with the same system as in Example 1. This negative electrode material includes hard carbon material derived from biomass feedstock. The hard carbon material is co-doped with multiple X elements through a one-step sintering process; the X elements are a combination of N and P. The difference between this embodiment and Example 1 is that in step S100, chitin, a biomass feedstock, is used as the carbon source precursor.

[0098] Example 9

[0099] This embodiment prepares a negative electrode material with the same system as in Example 1. This negative electrode material includes hard carbon material derived from biomass feedstock. The hard carbon material is co-doped with multiple X elements through a one-step sintering process; the X elements are a combination of N and S. The difference between this embodiment and Example 1 is that, in step S200, ammonium sulfate is used as the X source.

[0100] Comparative Example 1

[0101] This comparative example provides a negative electrode material with the same system as Example 1. This negative electrode material is a hard carbon material derived from biomass raw materials, and this hard carbon material is not doped with element X. The difference between this comparative example and Example 1 is that in step S200, 1.5g of carbon source precursor is directly weighed and loaded into a ceramic boat and placed in a tube furnace, instead of adding the X source to the mixture to be sintered.

[0102] Comparative Example 2

[0103] This comparative example prepares an anode material with the same system as Comparative Example 1. This anode material includes a hard carbon material derived from biomass feedstock, which is not doped with element X. The difference between this comparative example and Example 1 is that the hydrothermal pre-carbonization treatment step of the carbon source precursor is omitted. This comparative example provides a method for preparing the anode material including:

[0104] S100. Using almond shells as biomass raw material, the biomass raw material is ground and pulverized, and then heated and stirred in a 1 mol / L hydrochloric acid solution at a temperature of 60℃ for 6 hours at a stirring speed of 800 r / min. Then, the sample is repeatedly washed with deionized water until the pH value reaches neutral. Finally, the biomass raw material is dried at an ambient temperature of 80℃ for 12 hours to obtain a carbon source precursor that can be used to prepare hard carbon.

[0105] S200: Weigh 1.5g of carbon source precursor and encapsulate it in a ceramic boat. Place the ceramic boat in a tube furnace and heat it to a carbonization temperature of 700℃ at a rate of 5℃ / min under an argon atmosphere. Then, hold the temperature and heat for 2 hours. During the heating process, continuously introduce argon gas at a flow rate of 200sccm into the furnace. After the pyrolysis is completed, cool the furnace to room temperature and take out the sintered product. Filter and wash it three times with deionized water and dry it in an 80℃ oven for 12 hours to obtain biomass-derived hard carbon material.

[0106] To further verify the efficacy of the present invention, the negative electrode materials prepared in Examples 1 to 8 and Comparative Examples 1 to 2 were assembled into coin cells, and the charge-discharge capacity and cycle capacity retention of the negative electrode materials prepared in Examples 1 to 8 and Comparative Examples 1 to 2 were tested using coin cells to verify the improvement effect of the negative electrode materials on their own charge-discharge capacity and cycle performance. The test results are shown in Table 1.

[0107] The preparation process of the button cell (CR2025 type) is as follows: The negative electrode material prepared above, the conductive agent carbon black (Super P), and the binder polyacrylic acid dissolved in N-methylpyrrolidone are mixed in a weight ratio of 8:1:1 to prepare a negative electrode slurry; the negative electrode slurry is coated on copper foil, dried under vacuum conditions of 80℃~110℃ for 9h~14h, and then rolled into a negative electrode sheet of Φ12mm. Sodium metal was used as the positive electrode. Specifically, the sodium block was removed from kerosene, its surface was dried, the oxide layer was removed, and it was rolled into a uniform thin sheet. The sheet was then stamped to form a sodium sheet with a diameter slightly smaller than that of the separator. A Celgard 2400 polypropylene (PP) membrane was used as the separator. The electrolyte was NaClO4 / EC+DMC+FEC with a concentration of 1 mol / L. NaClO4 was used as the solute, and a 1:1 volume ratio mixture of EC and DMC was used as the organic solvent. 5% FEC was also added to the organic solvent. The positive electrode shell, positive electrode, separator, negative electrode, and negative electrode shell were assembled into a half-cell in a high-purity argon glove box in sequence. 35 μL of electrolyte was dropped onto each side of the separator, and the cells were left at room temperature for 24 hours to allow the electrolyte to fully wet the electrodes before subsequent battery testing.

[0108] Charge and discharge capacity test: At room temperature of 25℃, within the test voltage range of 0V (discharge cutoff voltage) to 3.0V (charge cutoff voltage), the coin cell half-cell is subjected to one cycle of charge and discharge test at a current rate of 50mA / g. The first charge capacity and the first discharge capacity of the battery are recorded. The first charge capacity is obtained by dividing the first charge capacity by the mass of the negative electrode, the first discharge capacity is obtained by dividing the first discharge capacity by the mass of the negative electrode, and the initial coulombic efficiency is obtained by dividing the first discharge capacity by the first charge capacity.

[0109] Reversible charge-discharge capacity test: At room temperature of 25℃, within the test voltage range of 0V (discharge cutoff voltage) to 3.0V (charge cutoff voltage), the coin cell was subjected to one cycle of charge-discharge test at current rates of 1A / g and 5A / g, respectively, and the reversible discharge capacity of the negative electrode material under the current rate conditions of 1A / g and 5A / g was measured.

[0110] Battery cycle capacity retention density test: At room temperature of 25℃, within the test voltage range of 0V (discharge cutoff voltage) to 3.0V (charge cutoff voltage), the coin cell half-cell was first activated by charging and discharging for 3 cycles at a current rate of 25mA / g; then, the coin cell half-cell was subjected to 100 cycles of charge and discharge test at a current rate of 100mA / g. The discharge capacity of the battery in the first cycle and the discharge capacity in the 100th cycle were recorded, and the capacity retention rate of the battery after 100cls was calculated (capacity retention rate in 100cls = discharge capacity in 100cls / discharge capacity in 1cls).

[0111] Table 1: Battery performance test results of anode materials prepared in Examples 1 to 8 and Comparative Examples 1 to 2

[0112]

[0113] Comparing the test results of Examples 1 to 6, 9 and Comparative Examples 1, 2, it can be seen that, compared with unmodified biomass-derived hard carbon materials, the initial charge-discharge capacity, initial coulombic efficiency, reversible discharge capacity at different current rates, and cycle capacity retention of biomass-derived hard carbon materials uniformly doped with N and P elements or N and S elements are significantly improved. This demonstrates that the uniformly doped biomass-derived hard carbon materials overcome their inherent performance defects, and their initial coulombic efficiency, rate performance, and cycle performance are all significantly improved.

[0114] In summary, this invention provides a negative electrode material and its preparation method. During the preparation of this negative electrode material, the irreversible defects in the carbon source precursor are reduced through pre-carbonization treatment. Furthermore, an X source containing all X elements is used to form a uniformly co-doped X-element hard carbon material through one-step sintering with the carbon source precursor. This allows the X elements to adhere to the framework, forming a uniformly distributed CX network structure within the hard carbon material. This simultaneously improves the conductivity of the hard carbon material, increases the active sites of electrochemical adsorption / desorption centers, and enhances the interlayer spacing of the carbon layers. Consequently, it significantly improves the discharge capacity, rate performance, and cycle performance of the corresponding bio-derived hard carbon material.

[0115] 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 negative electrode material, characterized in that, include: Hard carbon material, wherein the hard carbon material is doped with element X, and the hard carbon has a CX network structure; The X element includes at least two combinations of N, P, and S.

2. The negative electrode material according to claim 1, characterized in that, The X element is a combination of N and P or a combination of N and S.

3. The negative electrode material according to claim 1, characterized in that, The hard carbon material contains graphite domains, and the X element is distributed at least between the carbon layers of the graphite domains.

4. A method for preparing a negative electrode material, characterized in that, include: Using biomass raw materials as carbon source precursors, the carbon source precursors are pre-carbonized. The carbon source precursor is mixed with the X source and sintered to obtain the negative electrode material; The element X includes at least two combinations of N, P, and S; the source of X is a compound containing the element X.

5. The preparation method according to claim 4, characterized in that, The preparation method further includes: A pretreatment step for the biomass raw material prior to the precarbonization treatment of the carbon source precursor, the pretreatment step including: The biomass raw material is placed in an acid solution, stirred and mixed, and then heated in a water bath; wherein the water bath heating temperature is 50℃~85℃, the water bath heating time is 8h~10h, and the stirring speed is 500r / min~1000r / min; The biomass raw material is removed from the acid solution and then washed and dried in sequence.

6. The preparation method according to claim 4, characterized in that, The biomass raw materials include at least one of plant biomass raw materials and animal biomass raw materials. The plant biomass raw materials are plant materials containing hemicellulose, cellulose or lignin, and the animal biomass raw materials are animal materials containing chitin, protein or chitosan.

7. The preparation method according to claim 4, characterized in that, The pre-carbonization treatment of the carbon source precursor includes: The carbon source precursor is placed in deionized water and subjected to a hydrothermal reaction to complete the pre-carbonization treatment of the carbon source precursor. The mass ratio of the carbon source precursor to the volume of the deionized water is (1g~3g):(20mL~60mL); the temperature of the hydrothermal reaction is 160℃~200℃; and the time of the hydrothermal reaction is 5h~10h.

8. The preparation method according to claim 4, characterized in that, The step of mixing and sintering the carbon source precursor with the X source to obtain the negative electrode material includes: The pre-carbonized carbon source precursor and the X source are uniformly mixed at a mass ratio of 1:(0.5-5) to obtain a mixture. The mixture is sintered and carbonized under an inert atmosphere to obtain a negative electrode material; The carbonization temperature is 600℃~1300℃; the carbonization time is 90min~150min.

9. The preparation method according to claim 4, characterized in that, When the X element is a combination of N and P, the X source is selected from at least one of ammonium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; when the X element is a combination of N and S, the X source is selected from at least one of ammonium sulfate, ammonium persulfate, thiourea, guanidine sulfate, cysteine, and cystine.

10. A sodium-ion battery, characterized in that, The invention includes a negative electrode sheet, which comprises a negative current collector and a negative active material layer, wherein the negative active material layer comprises the negative electrode material according to any one of claims 1 to 3, or the negative electrode material prepared by the preparation method according to any one of claims 4 to 9.

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