Hard carbon material and preparation method thereof, negative pole piece and sodium ion battery

Abstract

The invention relates to a hard carbon material and a preparation method thereof, a negative pole piece and a sodium ion battery, the hard carbon material is formed by semicoke through oxidation, cross-linking and molten salt carbonization, the hard carbon material satisfies the following formula: 0.15 < = [(W1-W2) / W1] * SSA * (d002-0. 354) < = 0.50, W1 is the oxygen content, wt%, of cross-linked semicoke, W2 is the oxygen content, wt%, of cross-linked semicoke, W3 is the oxygen content, wt%, of cross-linked semicoke, and W4 is the oxygen content, wt%, of cross-linked semicoke. W2 is the oxygen content of the hard carbon material, wt%; sSA is the specific surface area of the hard carbon material, m < 2 > / g; d002 is the (002) crystal face interlayer spacing, nm, of the hard carbon material. The hard carbon material disclosed by the invention is used for the negative electrode of the sodium-ion battery and has relatively high specific capacity, initial coulombic efficiency and rate capability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a hard carbon material and its preparation method, a negative electrode sheet, and a sodium-ion battery. Background Technology

[0002] With the development of the new energy industry, lithium-ion batteries have been widely used in electric vehicles and other industries. However, their further development is constrained by a series of key issues, such as lithium resource reserves and the cost of lithium-ion batteries. Compared with lithium-ion batteries, sodium-ion batteries have the advantages of low cost and abundant and widely distributed sodium reserves. Among them, hard carbon, as a negative electrode material for sodium-ion batteries, has attracted widespread attention due to its low cost and high theoretical capacity. The disordered amorphous structure of hard carbon materials gives them more defects and micropores, providing more active sodium storage sites. At the same time, the larger interlayer spacing in hard carbon materials not only facilitates the diffusion of sodium ions but also helps maintain structural stability during the sodiumification / desodiumification process. However, the inherent disordered microstructure of traditional hard carbon materials results in poor conductivity and rate performance. Summary of the Invention

[0003] Based on this, it is necessary to provide a hard carbon material and its preparation method, a negative electrode sheet, and a sodium-ion battery to address the above problems. The hard carbon material described in this invention is used as a negative electrode in sodium-ion batteries and has high specific capacity, initial coulombic efficiency, and rate performance.

[0004] A hard carbon material is formed by oxidation, cross-linking, and molten salt carbonization of semi-coke, wherein the hard carbon material satisfies: 0.15 ≤ [(W1-W2) / W1] × SSA × (d 002 -0.354)≤0.50;

[0005] Wherein, W1 is the oxygen content of the cross-linked semi-coke, wt%; W2 is the oxygen content of the hard carbon material, wt%; and SSA is the specific surface area of ​​the hard carbon material, m². 2 / g;d 002 denoted as (002) interlayer spacing of hard carbon material, in nm.

[0006] In one embodiment, the hard carbon material satisfies at least one of the following conditions:

[0007] (1) The hard carbon material has a three-dimensional structure formed by the recombination and aggregation of carbon fragments;

[0008] (2) The oxygen content W1 of the cross-linked semi-coke is ≥30wt%;

[0009] (3) The oxygen content W2 of the hard carbon material is ≤6wt%;

[0010] (4) The specific surface area SSA of the hard carbon material is 1m². 2 / g~20m 2 / g;

[0011] (5) The interlayer spacing d of the (002) crystal planes of the hard carbon material 002 ≥0.375nm.

[0012] The hard carbon material of this invention is formed by oxidation, crosslinking, and molten salt carbonization of semi-coke. The oxygen content in the hard carbon material and the rate of change of oxygen content in the crosslinked semi-coke satisfy a specific quantitative relationship with the specific surface area and (002) crystal interlayer spacing of the hard carbon material. The -C-(O)-O- crosslinking network introduced by crosslinking generates a steric hindrance effect during carbonization, which expands the (002) crystal interlayer spacing. The thermal decomposition of oxygen-containing groups generates an in-situ pore-forming effect, forming more porous structures to improve the specific capacity of the hard carbon material. On this basis, a precursor with a larger oxygen content and specific surface area within a certain range is formed to obtain a hard carbon material with expanded (002) crystal interlayer spacing. Therefore, the hard carbon material of this invention, when used as a negative electrode in sodium-ion batteries, has high specific capacity, initial coulombic efficiency, and rate performance.

[0013] A method for preparing the hard carbon material as described above includes the following steps:

[0014] Oxidized semi-coke is prepared by mixing semi-coke with modified alkaline solution and then carrying out a hydrothermal reaction.

[0015] Crosslinked semi-coke is obtained by adding acid solution to oxidized semi-coke and performing acid washing and crosslinking, wherein the acid solution includes at least organic acid;

[0016] The hard carbon material is prepared by mixing and melting cross-linked semi-coke with a metal salt and then carbonizing it.

[0017] In one embodiment, the oxygen content of the crosslinked semi-coke is ≥30 wt%;

[0018] And / or, the specific surface area of ​​the oxidized semi-coke is 500 m². 2 / g~600m 2 / g.

[0019] In one embodiment, the step of mixing the semi-coke with the modified alkaline solution and then carrying out a hydrothermal reaction satisfies at least one of the following conditions:

[0020] (1) The mass ratio of the semi-coke to the modified alkaline solution is 1:(0.4~0.6);

[0021] (2) The modified alkaline solution is selected from a mixture of sodium hydroxide, hydrogen peroxide, and sodium dodecylbenzenesulfonate;

[0022] (3) The temperature of the hydrothermal reaction is 180℃~200℃ and the time is 6h~8h.

[0023] In one embodiment, the step of adding acid to the oxidized semi-coke for acid washing and crosslinking satisfies at least one of the following conditions:

[0024] (1) The mass ratio of the oxidized semi-coke to the acid solution is 1:(0.4~1);

[0025] (2) The organic acid is selected from at least one of citric acid, malic acid, tartaric acid, acetic acid, succinic acid and oxalic acid;

[0026] (3) The acid solution also includes inorganic acids;

[0027] (4) The cross-linking temperature is 70℃~100℃ and the time is 4h~10h.

[0028] In one embodiment, the step of mixing and melting the crosslinked semi-coke with a metal salt and then carbonizing it satisfies at least one of the following conditions:

[0029] (1) The mass ratio of the cross-linked semi-coke to the metal salt is 1:(5~10);

[0030] (2) The metal salt is selected from at least one of potassium chloride, zinc chloride, calcium chloride, and sodium chloride;

[0031] (3) The melting temperature is 500℃~800℃ and the melting time is 1h~4h;

[0032] (4) The carbonization temperature is 1200℃~1400℃ and the time is 2h~4h.

[0033] In one embodiment, after the crosslinked semi-coke and metal salt are mixed and melted, and before carbonization, the mixture is ball-milled at a speed of 200 r / s to 600 r / s for 20 min to 40 min.

[0034] A negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer comprising the hard carbon material as described above.

[0035] A sodium-ion battery, comprising the negative electrode sheet as described above.

[0036] The method for preparing hard carbon material described in this invention involves first subjecting semi-coke to alkaline-oxygen oxidation, followed by a hydrothermal reaction to form a rich, interpenetrating porous structure on the surface. This facilitates electrolyte permeation and, in conjunction with acid washing and crosslinking with organic acids, creates uniform and abundant oxygen-containing functional groups on the semi-coke surface, effectively preventing the rearrangement of carbon microcrystals during carbonization. Based on this, a molten salt evaporation method is used to transform the crosslinked semi-coke with a dense carbon layer into a thin carbon layer, thereby forming a rich, three-dimensional nanostructure with large interlayer spacing and sheet-like structure. This increases the interlayer gap, enhances conductivity, reduces ion diffusion pathways, and the amorphous carbon with surface defects is more conducive to sodium storage. Ultimately, this improves the specific capacity, initial coulombic efficiency, and rate performance of the hard carbon material when used as a negative electrode in sodium-ion batteries. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a scanning electron microscope image of the cross-linked semi-coke prepared in Example 1 of the present invention, with a magnification of 50,000.

[0039] Figure 2 The image shown is a scanning electron microscope (SEM) image of the hard carbon material prepared in Example 1 of this invention, with a magnification of 100,000. Detailed Implementation

[0040] To facilitate understanding of the present invention, it will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. Rather, these embodiments or examples are provided to make the disclosure of the present invention more thorough and complete.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments or examples only and is not intended to limit the invention. The optional scope herein includes any one of two or more of the related listed items, as well as any and all combinations of the related listed items, including any two related listed items, any more related listed items, or a combination of all related listed items. In this invention, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, and every value between such minimum and maximum values. Further, when a range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Furthermore, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0042] This invention provides a hard carbon material, which is formed by oxidation, cross-linking, and molten salt carbonization of semi-coke. The hard carbon material satisfies the following condition: 0.15 ≤ [(W1-W2) / W1] × SSA × (d 002 -0.354)≤0.50, where W1 is the oxygen content of the cross-linked semi-coke, wt%; W2 is the oxygen content of the hard carbon material, wt%; and SSA is the specific surface area of ​​the hard carbon material, m². 2 / g;d 002 denoted as (002) interlayer spacing of hard carbon material, in nm.

[0043] The hard carbon material of the present invention is formed by oxidation, cross-linking and molten salt carbonization of semi-coke. The change rate of oxygen content and cross-linked semi-coke oxygen content in the hard carbon material satisfies a specific quantitative relationship with the specific surface area and (002) crystal interlayer spacing of the hard carbon material. The -C-(O)-O- cross-linking network introduced by cross-linking generates a steric hindrance effect during carbonization, which expands the (002) crystal interlayer spacing. The thermal decomposition of oxygen-containing groups generates an in-situ pore-forming effect, forming more porous structures to improve the specific capacity of the hard carbon material. On this basis, a precursor with a larger oxygen content and specific surface area within a certain range is formed to obtain a hard carbon material with expanded (002) crystal interlayer spacing.

[0044] It should be noted that the relationship between the hard carbon materials in this invention only represents quantitative relationships and does not involve unit conversion. Semi-coke is a solid product obtained by low-temperature (500℃~700℃) dry distillation of peat, lignite, and high-volatile bituminous coal. This invention does not limit the specific type of semi-coke and conventional products can be used.

[0045] It is understandable that the relation [(W1-W2) / W1]×SSA×(d 002 The value of -0.354) includes, but is not limited to, any one of 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 or any range between two of them, preferably 0.2 to 0.4.

[0046] In one embodiment of the present invention, the oxygen content W1 of the crosslinked semi-coke is ≥30wt%, including but not limited to any one of 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, or any range between two, preferably 40wt%~50wt%.

[0047] In one embodiment of the present invention, the oxygen content W2 of the hard carbon material is ≤6wt%, including but not limited to any one of 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, or any range between two, preferably 2wt%~5wt%.

[0048] In one embodiment of the present invention, the specific surface area SSA of the hard carbon material is 1m². 2 / g~20m 2 / g, including but not limited to 1m 2 / g、5m 2 / g, 10m 2 / g, 15m 2 / g、20m 2 Any point value in / g or any range between the two, preferably 5m 2 / g~15m 2 / g.

[0049] In one embodiment of the present invention, the (002) interlayer spacing d of the hard carbon material 002 ≥0.375nm, including but not limited to any one of 0.375nm, 0.376nm, 0.377nm, 0.378nm, 0.379nm, 0.380nm, 0.381nm or any range between two of them, preferably 0.378nm~0.381nm.

[0050] In one embodiment of the present invention, the hard carbon material has a three-dimensional structure formed by the recombination and aggregation of carbon fragments. The robust carbon skeleton in this structure helps to reduce structural stress during sodium insertion and extraction.

[0051] This invention provides a method for preparing the hard carbon material as described above, comprising the following steps:

[0052] S1, oxidized semi-coke is prepared by mixing semi-coke with modified alkaline solution and then carrying out a hydrothermal reaction.

[0053] S2 adds acid to oxidized semi-coke for acid washing and crosslinking to obtain crosslinked semi-coke, wherein the acid solution includes at least organic acids;

[0054] S3, cross-linked semi-coke is mixed and melted with metal salt and then carbonized to obtain the hard carbon material.

[0055] In steps S1 and S2, low-cost, single-component semi-coke is used as raw material. The semi-coke is first subjected to alkaline-oxygen oxidation, and a rich, interpenetrating porous structure is formed on the surface by hydrothermal reaction. This facilitates the penetration of electrolytes and, together with the acid washing and crosslinking of organic acids, can effectively reduce ash content. This results in the formation of uniform and abundant carbon-oxygen groups (including COC and -O=CO) on the surface of the semi-coke. Among them, oxygen-containing functional groups (-O-, -C=O) can effectively prevent the rearrangement of carbon microcrystals in the semi-coke during carbonization, which is beneficial to improving the electrical properties of hard carbon materials.

[0056] In one embodiment of the present invention, the mass ratio of the semi-coke to the modified alkali solution is 1:(0.4~0.6), including but not limited to any one of 1:0.4, 1:0.45, 1:0.5, 1:0.55, 1:0.6 or any range between two of them, preferably 1:0.45~1:0.55.

[0057] In one embodiment of the present invention, the modified alkaline solution is selected from a mixture of sodium hydroxide, hydrogen peroxide, and sodium dodecylbenzenesulfonate. By modifying the conventional alkaline solution to construct surface carboxyl group positioning sites, it is beneficial to form a stable precursor containing -C-(O)-O- groups through subsequent cross-linking reactions induced by organic acids.

[0058] In one embodiment of the present invention, the temperature of the hydrothermal reaction is 180℃~200℃, including but not limited to any one of 180℃, 185℃, 190℃, 195℃, and 200℃ or any range between two of them.

[0059] In one embodiment of the present invention, the hydrothermal reaction time is 6h to 8h, including but not limited to any one of 6h, 6.5h, 7h, 7.5h, 8h or any range between two of them.

[0060] In one embodiment of the present invention, the specific surface area of ​​the oxidized semi-coke is 500 m². 2 / g~600m 2 / g, including but not limited to 500m 2 / g、520m 2 / g、550m 2 / g、580m 2 / g、600m 2The value at any point in / g or any range between the two is preferably 525m. 2 / g~575m 2 / g.

[0061] In one embodiment of the present invention, the oxygen content of the crosslinked semi-coke is ≥30wt%, including but not limited to any one of 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, or any range between two, preferably 40wt%~50wt%.

[0062] In one embodiment of the present invention, the organic acid includes, but is not limited to, at least one of citric acid, malic acid, tartaric acid, acetic acid, succinic acid, and oxalic acid.

[0063] In another embodiment of the present invention, in addition to including at least organic acids in the acid solution, inorganic acids are also included, including but not limited to at least one of hydrochloric acid, nitric acid, and sulfuric acid.

[0064] It is understood that the acid solution can be citric acid, malic acid, or a mixture of citric acid and malic acid, or a mixture of citric acid and inorganic acid, malic acid and inorganic acid, or a mixture of citric acid, malic acid and inorganic acid.

[0065] In one embodiment of the present invention, the mass ratio of the oxidized semi-coke to the acid solution is 1:(0.4~1), including but not limited to any one of 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1 or any range between two, preferably 1:0.5~1:0.8.

[0066] In one embodiment of the present invention, the crosslinking temperature is 70℃~100℃, including but not limited to any one of 70℃, 80℃, 90℃, 100℃ or any range between two of them; the crosslinking time is 4h~10h, including but not limited to any one of 4h, 5h, 6h, 7h, 8h, 9h and 10h or any range between two of them.

[0067] In step S3, cross-linked semi-coke with a dense carbon layer is transformed into a thin carbon layer by molten salt evaporation, thereby forming a rich three-dimensional nanostructure with a large carbon layer spacing and sheet-like structure. The catalytic effect of metal salt helps to expand the distance between carbon layers, increase the interlayer gap, enhance conductivity, and reduce ion diffusion pathways. Furthermore, the amorphous carbon with surface defects is more conducive to sodium storage, thereby improving the specific capacity, initial coulombic efficiency, and rate performance of hard carbon materials when used as a negative electrode in sodium-ion batteries, which is beneficial to comprehensively improving the performance of the finished battery.

[0068] In one embodiment of the present invention, the mass ratio of the crosslinked semi-coke to the metal salt is 1:(5~10), including but not limited to any one of 1:5, 1:6, 1:7, 1:8, 1:9, 1:10 or any range between the two, preferably 1:6~1:8.

[0069] In one embodiment of the present invention, the metal salt is selected from at least one of potassium chloride, zinc chloride, calcium chloride, and sodium chloride. Using such a metal salt as a template is more conducive to the formation of a thin sheet-like three-dimensional nanostructure of hard carbon material, thereby further enhancing conductivity.

[0070] In one embodiment of the invention, the melting temperature is 500°C to 800°C. The metal salt melts and transforms into a solution, effectively decomposing the macromolecules present in the semi-coke in the liquid phase. This depolymerization process forms small carbon fragments. Due to their interaction with the metal salt ions, these carbon fragments undergo a coupling reaction. As the temperature continues to rise, these carbon fragments undergo a reorganization and aggregation process, forming a three-dimensional stacked, connected, and entangled nanosheet structure. Furthermore, the chloride ions present during the pyrolysis process can also etch the carbon sheets, using the salt as a pore template to generate high porosity.

[0071] Specifically, the melting temperature includes, but is not limited to, any one of 500°C, 600°C, 700°C, and 800°C, or any range between two of them; the melting time is 1h to 4h, including, but not limited to, any one of 1h, 2h, 3h, and 4h, or any range between two of them.

[0072] In one embodiment of the present invention, the carbonization temperature is 1200℃~1400℃, including but not limited to any one of 1200℃, 1250℃, 1300℃, 1350℃, 1400℃ or any range between two; the carbonization time is 2h~4h, including but not limited to any one of 2h, 2.5h, 3h, 3.5h, 4h or any range between two.

[0073] In one embodiment of the present invention, after the crosslinked semi-coke and the metal salt are mixed and melted, and before carbonization, the mixture is ball-milled, which helps to promote the full mixing of the crosslinked semi-coke and the metal salt and is more conducive to uniform melting.

[0074] Preferably, the ball mill rotation speed is 200 r / s to 600 r / s, including but not limited to any one of 200 r / s, 300 r / s, 400 r / s, 500 r / s, 600 r / s or any range between two; the ball milling time is 20 min to 40 min, including but not limited to any one of 20 min, 25 min, 30 min, 35 min, 40 min or any range between two.

[0075] This invention provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer comprises the hard carbon material as described above. It is understood that the negative electrode active material layer may also include materials such as binders, and this invention does not limit this.

[0076] The present invention also provides a sodium-ion battery, including the negative electrode as described above. It is understood that the sodium-ion battery also includes a positive electrode, a separator, and an electrolyte; however, the present invention does not limit the positive electrode, separator, and electrolyte.

[0077] The hard carbon material described in this invention is used as the anode of sodium-ion batteries, which can achieve high specific capacity, initial coulombic efficiency and rate performance. Specifically, the reversible specific capacity is as high as about 307 mAh / g and the initial coulombic efficiency is greater than 88%.

[0078] The following specific embodiments will further illustrate the hard carbon material, its preparation method, the negative electrode sheet, and the sodium-ion battery. However, those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified, specific conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0079] Example 1

[0080] Sodium hydroxide, sodium dodecylbenzenesulfonate, and hydrogen peroxide were mixed to form a modified alkaline solution. The semi-coke was then mixed with the modified alkaline solution at a mass ratio of 5:3 and subjected to a hydrothermal reaction at 180℃ for 8 hours to obtain oxidized semi-coke through alkaline-oxygen oxidation.

[0081] Citric acid was added to oxidized semi-coke at a mass ratio of 2:1, and the mixture was reacted at 90℃ for 2 hours for acid washing and crosslinking to obtain crosslinked semi-coke. The scanning electron microscope image of this crosslinked semi-coke is shown below. Figure 1 As shown.

[0082] Crosslinked semi-coke, potassium chloride, and calcium chloride were mixed in a mass ratio of 1:2:3, ball-milled at 400 r / s for 40 min, then reacted at 700℃ for 2 h for low-temperature melting, followed by high-temperature carbonization at 1300℃ for 2 h to obtain a hard carbon material. The scanning electron microscope image of this hard carbon material is shown below. Figure 2 As shown.

[0083] Example 2

[0084] Sodium hydroxide, sodium dodecylbenzenesulfonate, and hydrogen peroxide were mixed to form a modified alkaline solution. The semi-coke was then mixed with the modified alkaline solution at a mass ratio of 2:1 and subjected to a hydrothermal reaction at 200℃ for 6 hours to obtain oxidized semi-coke through alkaline-oxygen oxidation.

[0085] Malic acid was added to oxidized semi-coke at a mass ratio of 3:2 and reacted at 90℃ for 2 hours for acid washing and crosslinking to obtain crosslinked semi-coke.

[0086] Crosslinked semi-coke, calcium chloride, and sodium chloride were mixed in a mass ratio of 1:3:4, ball-milled at 400 r / s for 40 min, then reacted at 600℃ for 3 h for low-temperature melting, and then carbonized at 1200℃ for 2 h to obtain hard carbon material.

[0087] Example 3

[0088] Sodium hydroxide, sodium dodecylbenzenesulfonate, and hydrogen peroxide were mixed to form a modified alkaline solution. The semi-coke was then mixed with the modified alkaline solution at a mass ratio of 5:3 and subjected to a hydrothermal reaction at 200℃ for 6 hours to obtain oxidized semi-coke through alkaline-oxygen oxidation.

[0089] Oxalic acid was added to oxidized semi-coke at a mass ratio of 3:2 and reacted at 90℃ for 2 hours for acid washing and crosslinking to obtain crosslinked semi-coke.

[0090] Crosslinked semi-coke, sodium chloride, and potassium chloride were mixed in a mass ratio of 1:3:3, ball-milled at 400 r / s for 40 min, then reacted at 700℃ for 2 h for low-temperature melting, and then carbonized at 1300℃ for 2 h to obtain hard carbon material.

[0091] Example 4

[0092] Sodium hydroxide, sodium dodecylbenzenesulfonate, and hydrogen peroxide were mixed to form a modified alkaline solution. The semi-coke was then mixed with the modified alkaline solution at a mass ratio of 2:1 and subjected to a hydrothermal reaction at 180℃ for 8 hours to obtain oxidized semi-coke through alkaline-oxygen oxidation.

[0093] Acetic acid was added to oxidized semi-coke at a mass ratio of 2:1 and reacted at 90℃ for 2 hours to perform acid washing and crosslinking, thus obtaining crosslinked semi-coke.

[0094] Crosslinked semi-coke, zinc chloride, and potassium chloride were mixed in a mass ratio of 1:2:5, ball-milled at 400 r / s for 40 min, then reacted at 800℃ for 2 h for low-temperature melting, and then carbonized at 1300℃ for 3 h to obtain hard carbon material.

[0095] Example 5

[0096] Sodium hydroxide, sodium dodecylbenzenesulfonate, and hydrogen peroxide were mixed to form a modified alkaline solution. The semi-coke was then mixed with the modified alkaline solution at a mass ratio of 5:2, and the mixture was subjected to a hydrothermal reaction at 180℃ for 7 hours to obtain oxidized semi-coke through alkaline-oxygen oxidation.

[0097] Tartaric acid was added to oxidized semi-coke at a mass ratio of 2:1 and reacted at 90℃ for 2 hours for acid washing and crosslinking to obtain crosslinked semi-coke.

[0098] Crosslinked semi-coke, potassium chloride, and calcium chloride were mixed in a mass ratio of 1:3:4, ball-milled at 400 r / s for 40 min, then reacted at 800℃ for 2 h for low-temperature melting, and then carbonized at 1300℃ for 3 h to obtain hard carbon material.

[0099] Comparative Example 1

[0100] The only difference between Comparative Example 1 and Example 1 is that the semi-coke was directly mixed with sodium hydroxide solution at a mass ratio of 5:3 and then subjected to a hydrothermal reaction at 180°C for 8 hours to obtain oxidized semi-coke.

[0101] Comparative Example 2

[0102] The only difference between Comparative Example 2 and Example 1 is that the semi-coke and modified alkali solution were mixed in a mass ratio of 5:3 and then heated and stirred at 100°C for 6 hours to obtain a semi-coke intermediate.

[0103] Comparative Example 3

[0104] The only difference between Comparative Example 3 and Example 1 is that, instead of acid washing and crosslinking, oxidized semi-coke, potassium chloride, and calcium chloride were directly mixed in the same mass ratio and then processed into hard carbon material using the same method.

[0105] Comparative Example 4

[0106] The only difference between Comparative Example 4 and Example 1 is that nitric acid was added to the oxidized semi-coke at a mass ratio of 2:1 and reacted at 90°C for 2 hours to obtain the semi-coke intermediate.

[0107] Comparative Example 5

[0108] The only difference between Comparative Example 5 and Example 1 is that the ball-milled products of cross-linked semi-coke, potassium chloride, and calcium chloride were directly placed in a high-temperature carbonization reaction at 1300°C for 2 hours to obtain hard carbon material.

[0109] The nitrogen adsorption-desorption specific surface area of ​​the oxidized semi-coke prepared in Examples 1-5 and Comparative Examples 1-5 was detected, and the C and O content in the crosslinked semi-coke was determined using an organic elemental analyzer (EA). The results are shown in Table 1.

[0110] Table 1

[0111]

[0112] By measuring the C and O elements and the specific surface area of ​​cross-linked semi-coke and oxidized semi-coke, it was found that cross-linking the oxidized semi-coke after alkaline-oxygen oxidation with subsequent acid washing with organic acids can effectively reduce the ash content and control the specific surface pore structure. The cross-linked semi-coke has a higher oxygen content and is rich in more oxygen-containing groups.

[0113] The specific surface area of ​​the hard carbon materials prepared in Examples 1-5 and Comparative Examples 1-5 was measured by nitrogen adsorption-desorption. The interlayer spacing of the (002) crystal planes of the hard carbon materials was calculated by XRD (incident light wavelength was 1.54056 Å). The C and O content in the hard carbon materials was determined by an organic elemental analyzer (EA). The results are shown in Table 2.

[0114] Table 2

[0115]

[0116] The C, O, and specific surface area of ​​the hard carbon material were measured. The oxidized semi-coke produced by alkali-oxygen oxidation had a higher oxygen content and was rich in oxygen-containing functional groups. Subsequent acid washing and crosslinking with organic acids effectively reduced ash content and controlled the specific surface area and porosity. This, in turn, allowed for the reduction of specific surface area and expansion of interlayer spacing during the subsequent carbonization stage through the ablation of oxygen-containing functional groups. In contrast, the crosslinked semi-coke produced by conventional alkali-oxygen oxidation had a smaller specific surface area. Subsequent carbonization using metal salt mixtures significantly improved the specific surface area and interlayer spacing of the prepared semi-coke-based hard carbon.

[0117] The hard carbon materials prepared in Examples 1-5 and Comparative Examples 1-5 were used to fabricate button-type sodium-ion batteries and their performance was tested according to the following methods:

[0118] (1) According to the ratio of active material:SP:CMC:SBR=92:2:2:4, weigh the negative electrode material, SP, CMC and SBR respectively and mix them evenly in deionized water to prepare a slurry; coat the evenly mixed slurry onto the aluminum foil current collector, bake it in an oven at 80℃ for 1 hour, and then take it out and cool it to room temperature.

[0119] (2) Adjust the roller spacing and roll the electrode sheet. Cut the rolled electrode sheet into small round pieces with a diameter of 14 mm and weigh them as m1. Similarly, cut the aluminum foil current collector into aluminum foil round pieces with a diameter of 14 mm and weigh them as m2. Wherein (m1-m2)×0.94 is the mass of the active material, recorded as m3. Place the weighed small round pieces into an 80℃ oven and vacuum dry for 12 hours;

[0120] (3) Transfer the vacuum-dried small discs to a glove box, use sodium discs as counter and auxiliary electrodes, the electrolyte formula is 1M NaPF6 / EC:DMC:DEC=2:2:1, and use glass fiber membrane as separator, and assemble sodium ion button cells in a glove box where the oxygen and water content are both less than 0.01ppm.

[0121] (4) Let the assembled button sodium-ion battery stand for 12 hours, and then test its electrochemical performance at a constant current on the Wuhan Landian Battery Testing System.

[0122] The test results are shown in Table 3.

[0123] Table 3

[0124]

[0125] As shown in Table 3, conventional alkaline treatment alone has a very low modification effect on the semi-coke, resulting in fewer oxygen-containing functional groups that can participate in subsequent cross-linking, thus affecting the electrochemical performance of the prepared hard carbon. Hydrothermal synthesis using modified alkaline solutions can effectively increase the oxygen-containing functional groups in the semi-coke, which is beneficial for cross-linking with organic acids. Furthermore, the ash content of the oxidized semi-coke is significantly reduced, effectively improving the efficiency of acid washing and impurity removal. Acid washing with inorganic acids can only reduce the ash content and cannot achieve effective cross-linking. After metal salt template doping, ineffective melting prevents structure formation, resulting in poor electrochemical performance of the prepared hard carbon.

[0126] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0127] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A hard carbon material, characterized in that, The hard carbon material is formed by oxidation, cross-linking, and molten salt carbonization of semi-coke, and the hard carbon material satisfies: 0.15≤[(W1-W2) / W1]×SSA×(d 002 -0.354)≤0.50; Wherein, W1 is the oxygen content of the cross-linked semi-coke, wt%; W2 is the oxygen content of the hard carbon material, wt%; and SSA is the specific surface area of ​​the hard carbon material, m². 2 / g;d 002 denoted as (002) interlayer spacing of hard carbon material, in nm.

2. The hard carbon material according to claim 1, characterized in that, The hard carbon material satisfies at least one of the following conditions: (1) The hard carbon material has a three-dimensional structure formed by the recombination and aggregation of carbon fragments; (2) The oxygen content W1 of the cross-linked semi-coke is ≥30wt%; (3) The oxygen content W2 of the hard carbon material is ≤6wt%; (4) The specific surface area SSA of the hard carbon material is 1m². 2 / g~20m 2 / g; (5) The interlayer spacing d of the (002) crystal planes of the hard carbon material 002 ≥0.375nm.

3. A method for preparing the hard carbon material as described in claim 1 or 2, characterized in that, Includes the following steps: Oxidized semi-coke is prepared by mixing semi-coke with modified alkaline solution and then carrying out a hydrothermal reaction. Crosslinked semi-coke is obtained by adding acid solution to oxidized semi-coke and performing acid washing and crosslinking, wherein the acid solution includes at least organic acid; The hard carbon material is prepared by mixing and melting cross-linked semi-coke with a metal salt and then carbonizing it.

4. The method for preparing hard carbon material according to claim 3, characterized in that, The oxygen content of the cross-linked semi-coke is ≥30wt%; And / or, the specific surface area of ​​the oxidized semi-coke is 500 m². 2 / g~600m 2 / g.

5. The method for preparing hard carbon material according to claim 3, characterized in that, The step of mixing semi-coke with modified alkaline solution and then carrying out a hydrothermal reaction satisfies at least one of the following conditions: (1) The mass ratio of the semi-coke to the modified alkaline solution is 1:(0.4~0.6); (2) The modified alkaline solution is selected from a mixture of sodium hydroxide, hydrogen peroxide, and sodium dodecylbenzenesulfonate; (3) The temperature of the hydrothermal reaction is 180℃~200℃ and the time is 6h~8h.

6. The method for preparing hard carbon material according to claim 3, characterized in that, The step of adding acid to oxidized semi-coke for acid washing and crosslinking satisfies at least one of the following conditions: (1) The mass ratio of the oxidized semi-coke to the acid solution is 1:(0.4~1); (2) The organic acid is selected from at least one of citric acid, malic acid, tartaric acid, acetic acid, succinic acid and oxalic acid; (3) The acid solution also includes inorganic acids; (4) The cross-linking temperature is 70℃~100℃ and the time is 4h~10h.

7. The method for preparing hard carbon material according to claim 3, characterized in that, The step of mixing and melting cross-linked semi-coke with a metal salt and then carbonizing it satisfies at least one of the following conditions: (1) The mass ratio of the cross-linked semi-coke to the metal salt is 1:(5~10); (2) The metal salt is selected from at least one of potassium chloride, zinc chloride, calcium chloride, and sodium chloride; (3) The melting temperature is 500℃~800℃ and the melting time is 1h~4h; (4) The carbonization temperature is 1200℃~1400℃ and the time is 2h~4h.

8. The method for preparing hard carbon material according to claim 3 or 7, characterized in that, After the cross-linked semi-coke and metal salt are mixed and melted, and before carbonization, the mixture is ball-milled at a speed of 200 r / s to 600 r / s for 20 min to 40 min.

9. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative current collector and a negative active material layer, wherein the negative active material layer contains the hard carbon material as described in claim 1 or 2.

10. A sodium-ion battery, characterized in that, Includes the negative electrode sheet as described in claim 9.

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