Method for preparing closed-cell-rich soft and hard carbon composite material from silk fibers and coal liquefied asphalt and sodium electricity application thereof

Abstract

The invention discloses a method for preparing a closed-pore-rich soft and hard carbon composite material from silk fibers and coal liquefied asphalt and sodium electricity application of the closed-pore-rich soft and hard carbon composite material, and belongs to the field of novel composite material preparation and electrochemical energy storage. The preparation method comprises the following steps: firstly, pre-carbonizing silk fibers to obtain pre-carbonized silk fibers; secondly, an open pore structure is constructed on the pre-carbonized silk fiber through chemical activation, and activated silk carbon fiber is obtained; thirdly, the surface of the activated silk carbon fiber is coated with coal liquefaction pitch through an impregnation method; and finally, co-carbonization is carried out to complete packaging of the open pores, and the composite material rich in the closed pore structure is obtained. The soft and hard carbon composite material disclosed by the invention has the characteristic of rich closed pores, and shows excellent specific capacity and low-voltage platform capacity; benefited from the core-shell synergistic effect of the inner-layer hard carbon and the outer-layer soft carbon, the conductivity and the sodium ion diffusion efficiency of the material are improved, so that the rate capability and the cycling stability of the material are enhanced; in addition, soft carbon packaging is used for replacing traditional ultrahigh-temperature carbonization to construct a closed-pore structure, and the preparation process is safe, controllable and energy-saving.

Description

Technical Field

[0001] The present invention belongs to the field of composite new material preparation and electrochemical energy storage, relates to a carbon-based sodium ion battery negative electrode material, and in particular to a preparation method of a closed-pore-rich soft and hard carbon composite material. Background Art

[0002] Due to the abundance and low cost of sodium, sodium-ion batteries (Na-ion batteries) are widely considered a potential alternative to lithium-ion batteries for large-scale energy storage. However, due to the large ionic radius of sodium ions, graphite, a suitable anode material for Li-ion batteries, is no longer suitable for Na-ion batteries. Therefore, there is an urgent need to develop a low-cost, high-performance anode material to promote the commercialization of Na-ion batteries.

[0003] Over the past decade, people have never stopped exploring negative electrode materials for sodium-ion batteries, and have made progress in materials such as metal oxides, alloys, organic compounds, and MXene. However, these negative electrode materials generally exhibit low initial coulombic efficiency (ICE), limited cycle stability, and high redox potential, which greatly limits their practical application in sodium-ion batteries. In contrast, hard carbon materials (amorphous carbon materials that cannot be graphitized even at ultra-high temperatures of 2500°C) have a high reversible capacity (about 300mAh g -1 ), low working potential (about 0.1V), long cycle life and wide resource sources have gradually become the focus of research on negative electrode materials for sodium ion batteries.

[0004] As anode materials for sodium-ion batteries, hard carbon exhibits a typical galvanostatic charge-discharge (GCD) curve consisting of two distinct regions: a high-voltage slope and a low-voltage plateau (<0.1V). In recent years, convincing experimental evidence has strongly demonstrated that the low-voltage plateau capacity stems from the filling of closed pores within the material by quasi-metallic sodium clusters; this low-voltage plateau capacity fundamentally determines the operating voltage and energy density of the full battery. Therefore, effectively constructing a closed-pore structure to enhance the material's sodium storage performance has become a major research focus. Currently, the construction of closed-cell structures always requires the use of ultra-high temperature carbonization. This is because ultra-high temperature can induce the rearrangement of carbon layers, thereby forming a closed-cell structure [H.Kim, JCHyun, DHKim, JHKwak, JBLee, JHMoon, J.Choi, HDLim, SJYang, HMJin, DJAhn, K.Kang, HJJin, HKLim, YSYun, Revisiting Lithium-and Sodium-Ion Storage in Hard Carbon Anodes, Adv Mater 35(2023).]. However, ultra-high temperature increases energy consumption, prolongs the preparation cycle, and accelerates equipment aging, resulting in excessively high production costs. Therefore, it is very necessary to develop an economical and effective method to construct closed-cell structures.

[0005] Silk fiber, a natural biomass material with a long history, possesses inherent advantages such as biocompatibility, environmental friendliness, renewability, and resource abundance. Furthermore, silk fibers are primarily composed of fibroin, whose intramolecular β-sheet crystallites exhibit remarkable local order similarities to the short-range ordered graphitic domains of hard carbon materials. Therefore, silk fibers are an ideal biomass precursor for preparing standard hard carbon materials. Furthermore, the nitrogen and oxygen elements in fibroin can be retained in various forms after carbonization, resulting in in situ nitrogen- and oxygen-doped hard carbon materials. The presence of nitrogen and oxygen not only increases the sodium ion diffusion coefficient by expanding the interlayer spacing of the carbon layers but also provides abundant active sites for interfacial chemical crosslinking, facilitating further modification. Furthermore, coal liquefaction pitch, a major organic byproduct of direct coal liquefaction technology, is primarily composed of condensed aromatic hydrocarbons and heterocyclic compounds. The soft carbon materials formed after carbonization possess a continuous graphitic crystallite network, endowing the materials with excellent electrical conductivity. While soft carbon materials lack the sodium storage performance of hard carbon materials, the rational construction of a soft-hard carbon composite structure can impart high electrical conductivity, which hard carbon materials lack on their own. Therefore, by synergistically designing a structure combining silk fiber-based hard carbon and coal liquefaction pitch-based soft carbon, a closed-pore structure can be constructed at relatively low temperatures to enhance the sodium storage performance of electrode materials. Summary of the Invention

[0006] The present invention provides a method for preparing closed-pore soft and hard carbon composite materials from silk fibers and coal liquefaction pitch, and its sodium electricity application. The soft and hard carbon composite materials prepared by the present invention are characterized by rich closed pores, showing excellent specific capacity and low-voltage platform capacity; at the same time, thanks to the core-shell synergistic effect of the inner hard carbon layer and the outer soft carbon layer, the electrical conductivity and sodium ion diffusion efficiency of the material are improved, thereby enhancing the material's rate performance and cycle stability. In addition, the present invention uses soft carbon encapsulation to replace traditional ultra-high temperature carbonization to construct a closed-pore structure, and the preparation process is safe, controllable and energy-saving.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A method for preparing a closed-pore soft and hard carbon composite material from silk fibers and coal liquefaction pitch. The method includes: first, pre-carbonizing the silk fibers to obtain pre-carbonized silk fibers; then, chemically activating the pre-carbonized silk fibers to construct an open-pore structure to obtain activated silk carbon fibers; then, coating the activated silk carbon fibers with coal liquefaction pitch by impregnation; and finally, co-carbonizing the activated silk carbon fibers to encapsulate the open pores, thereby obtaining a closed-pore soft and hard carbon composite material with a soft carbon shell and a hard carbon core. The method specifically includes the following steps:

[0009] Step 1, cutting a piece of silk fiber, pre-carbonizing it at 500-600° C. for 1-3 hours under a nitrogen or argon atmosphere to pyrolyze the silk fiber to obtain pre-carbonized silk fiber, and further grinding the pre-carbonized silk fiber into 300-800 mesh pre-carbonized silk fiber powder;

[0010] Step 2, grinding and mixing the pre-carbonized silk fiber powder obtained in step 1 and potassium hydroxide, calcining the mixture at 700-800° C. for 1-3 hours under a nitrogen or argon atmosphere, washing the product with hydrochloric acid and deionized water, and drying to obtain activated silk carbon fibers; wherein the mass ratio of potassium hydroxide to pre-carbonized silk fiber powder is 1:(1.0-5.0);

[0011] Step 3, dispersing the activated silk carbon fiber obtained in step 2 in an organic solvent dissolved with coal liquefaction pitch, coating the surface of the activated silk carbon fiber with coal liquefaction pitch by an impregnation method, and evaporating the organic solvent to obtain a composite structure of activated silk carbon fiber coated with coal liquefaction pitch; wherein the mass ratio of coal liquefaction pitch to activated silk carbon fiber is 1:(4.0-19.0);

[0012] Step 4: Under a nitrogen or argon atmosphere, the composite structure of coal liquefaction pitch coated activated silk carbon fiber obtained in step 3 is calcined at 350-450°C for 1-3 hours, and then the temperature is raised to 1000°C for high-temperature carbonization for 1-3 hours to obtain a soft and hard carbon composite material rich in closed pores.

[0013] The specific steps of the preparation method are as follows:

[0014] Furthermore, in step 2: the specific steps of washing and drying are to disperse the silk carbon fibers activated by potassium hydroxide in a 1-2 mol / L hydrochloric acid solution and stir for 1-2 hours, then transfer them to a filtration device, and use deionized water to repeatedly filter and wash until the filtrate is neutral. After drying for 12-24 hours, a solid product of activated silk carbon fibers is obtained, and the drying temperature is 80-100°C.

[0015] Furthermore, in step 3, the organic solvent includes N-methylpyrrolidone, xylene, N,N-dimethylformamide and tetrahydrofuran, preferably N-methylpyrrolidone.

[0016] Furthermore, step 3 specifically comprises: first, adding solid coal liquefaction pitch to an organic solvent, stirring for 1-2 hours, and then filtering to obtain an organic solvent containing the coal liquefaction pitch. Subsequently, dispersing activated silk carbon fibers in the organic solvent containing the coal liquefaction pitch, stirring for 8-12 hours, transferring the mixture to an oil bath, and evaporating the organic solvent at a temperature of 180-200°C to obtain a composite structure of activated silk carbon fibers coated with coal liquefaction pitch.

[0017] A closed-pore soft and hard carbon composite material prepared from silk fiber and coal liquefaction pitch is obtained by the above-mentioned preparation method. It uses hard carbon derived from silk fiber as a matrix and is coated with soft carbon derived from coal liquefaction pitch as an outer layer to construct a rich closed-pore structure and a composite structure of soft carbon and hard carbon, showing excellent sodium storage performance.

[0018] The invention discloses an application of preparing closed-pore soft and hard carbon composite materials by silk fibers and coal liquefaction pitch, which is applied in the field of negative electrode materials for sodium ion batteries.

[0019] The principle and innovation of this invention lies in: pre-carbonization and chemical activation to produce open-pore hard carbon derived from silk fibers. This hard carbon retains abundant nitrogen- and oxygen-containing groups, which can cross-link with coal liquefaction pitch, facilitating the coating of the coal liquefaction pitch and the formation of a soft carbon layer (formed by high-temperature calcination of the coal liquefaction pitch). Furthermore, the soft carbon layer is used to encapsulate the open pores in the activated hard carbon, creating a closed-pore structure, replacing the conventional ultra-high-temperature carbonization method.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) The present invention uses silk fiber and coal liquefaction pitch as precursors of hard carbon and soft carbon, respectively. The amino acid peptide chain polymer molecular structure and β-pleated microcrystalline structure of silk fiber give its derived carbon typical nitrogen self-doped hard carbon characteristics, and the polycyclic aromatic structure of coal liquefaction pitch molecules gives its derived carbon typical soft carbon characteristics, thus achieving a close combination of soft carbon and hard carbon;

[0022] (2) The present invention prepares a soft and hard carbon composite material rich in closed-pore structure, and the process route is reasonable. First, an activator is used to construct open pores, and then a soft carbon layer is used to encapsulate the open pores, thereby constructing closed pores. Finally, carbonization treatment is performed to obtain a soft and hard carbon composite material rich in closed-pore structure.

[0023] (3) The material prepared by the present invention has the characteristics of being rich in closed pores, exhibiting excellent specific capacity and low-voltage platform capacity, and having a composite core-shell structure with an inner hard carbon layer and an outer soft carbon layer. The electrical conductivity and sodium ion diffusion efficiency of the material are improved, thereby enhancing the material's rate performance and cycle stability;

[0024] (4) The present invention utilizes soft carbon layer encapsulation instead of conventional ultra-high temperature carbonization to construct a closed-cell structure, and the preparation process is safe, controllable, and energy-saving;

[0025] (5) The present invention realizes the high value-added utilization of coal liquefaction pitch in the field of sodium electricity, which is of great significance to the direct coal liquefaction technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The scanning and transmission electron micrographs of the closed-pore-rich soft and hard carbon composite material obtained in Example 1 of the present invention are as follows: Figure 1 (a) is a scanning electron microscope image; Figure 1 (b) is a transmission electron microscope image.

[0027] Figure 2 This is the XRD pattern of the closed-pore-rich soft and hard carbon composite material obtained in Example 1 of the present invention.

[0028] Figure 3 This is the full XPS spectrum of the closed-pore-rich soft and hard carbon composite material obtained in Example 1 of the present invention.

[0029] Figure 4 The closed-pore soft and hard carbon composite material obtained in Example 1 of the present invention is used as the negative electrode of the sodium ion battery at 0.1mV s -1 CV curve diagram of the test under scanning speed.

[0030] Figure 5 The closed-pore soft and hard carbon composite material obtained in Example 1 of the present invention is used as the negative electrode of a sodium ion battery at 20 mAg -1 Constant current charge and discharge curves under current density.

[0031] Figure 6 The closed-pore soft and hard carbon composite material, hard carbon material and porous hard carbon material obtained in Example 1 of the present invention are used as the negative electrode of sodium ion battery at 20mAg -1 Comparison of platform capacity and total capacity under different current densities.

[0032] Figure 7 The closed-pore soft and hard carbon composite material, hard carbon material and porous hard carbon material obtained in Example 1 of the present invention are used as the negative electrode of sodium ion battery at 500mAg -1 Cycling performance diagram at different current densities.

[0033] Figure 8 This is a rate performance diagram of the closed-pore soft and hard carbon composite material, hard carbon material and porous hard carbon material obtained in Example 1 of the present invention used as the negative electrode of sodium ion battery at different current densities. DETAILED DESCRIPTION

[0034] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to the following embodiments.

[0035] Example 1:

[0036] (1) A piece of silk fiber of 20×4 cm was cut and pre-carbonized at 600°C for 1 hour under an argon atmosphere. The product was ground into a powder of 300-800 mesh.

[0037] (2) Weigh 300 mg of the powder prepared in step (1) and 100 mg of potassium hydroxide and mix them evenly by grinding. The mixture is calcined under an argon atmosphere at a temperature of 800° C. for 1 hour. The calcined product is dispersed in a 2 mol / L hydrochloric acid solution and stirred for 2 hours. The product is then filtered and washed with deionized water until the filtrate is neutral, and dried at 80° C. for 24 hours.

[0038] (3) Weigh 10 mg of coal liquefaction pitch and add it to 4 ml of N-methylpyrrolidone (NMP). Stir for 1 hour and filter to obtain an NMP solution of coal liquefaction pitch. Weigh 90 mg of the product obtained in step (2) and disperse it in the prepared NMP solution of coal liquefaction pitch. Stir and immerse for 8 hours, and then remove the NMP solution at 180°C.

[0039] (4) calcining the product obtained in step (3) under an argon atmosphere, first calcining at 400° C. for 1 hour, and then continuing to heat up to 1000° C. for 1 hour to obtain a closed-pore-rich soft and hard carbon composite material.

[0040] The scanning electron microscopy of the closed-pore soft and hard carbon composite material prepared in this embodiment is as follows: Figure 1As shown in (a), the surface of the hard carbon fiber is covered by a soft carbon layer, proving the successful coating of the soft carbon. Figure 1 As shown in (b), the disordered hard carbon region and the ordered soft carbon layer lattice stripes are clearly distinguished, and the thickness of the soft carbon layer is about 5.8nm. Figure 2 As shown, it can be seen that the obtained closed-pore-rich soft and hard carbon composite material is a typical amorphous carbon material. Figure 3 This is its full XPS spectrum, proving the presence of three elements: carbon (C), nitrogen (N), and oxygen (O) in closed-pore soft and hard carbon composite materials.

[0041] The closed-pore soft and hard carbon composite material prepared in this example was mixed with super p and carboxymethyl cellulose in a mass ratio of 7:2:1 to prepare the negative electrode material. The current collector was copper foil, and 1.0 M sodium hexafluorophosphate (100% ethylene glycol dimethyl ether) was used as the electrolyte. Sodium ion half-cells were assembled in a glove box (H2O <0.1ppm, O2 <0.1ppm) for electrochemical performance testing. Figure 4 is its 0.1mV s -1 CV curve tested at the scanning rate; Figure 5 Is its 20mAg -1 The constant current charge and discharge curves of the first two cycles at a current density of 20 mA g show that the curve has a significantly extended platform area, proving the successful construction of closed pores. -1 The comparison of platform capacity and total capacity under current density is shown in the figure below. Figure 6 As shown in Figure 2, the total capacity and platform capacity of the closed-pore soft and hard carbon composites are 392.8 mAh g -1 and 234.1mAh g -1 , which are significantly higher than those of single-component hard carbon materials and activated porous hard carbon materials. In terms of cycle performance, the closed-pore soft and hard carbon composites are -1 The current density is 235.5 mAh g -1 The discharge specific capacity of the closed-cell soft and hard carbon composite material is significantly better than that of the single-component hard carbon material and the activated porous hard carbon material; the specific capacity of the closed-cell soft and hard carbon composite material remains at 229.3 mAh g after 500 cycles. -1 , with excellent cyclic stability (such as Figure 7 ). At the same time, in terms of rate performance, the closed-cell soft and hard carbon composite materials have the highest rate performance at 20, 50, 100, 500, 1000, and 2000 mA g -1 At current densities of 1, 2, 3, 4, 5, 7, 8, 10, 15, 20, 30, 27, 24, 31, 20, 18, 12, 25, 30, 31, 25 ... -1 , which is also significantly better than single-component hard carbon materials and activated porous hard carbon materials (such as Figure 8 ).

[0042] Comparative Example 1:

[0043] (1) A piece of silk fiber of 20×4 cm was cut and pre-carbonized at 600°C for 1 hour under an argon atmosphere. The product was ground into a powder of 300-800 mesh.

[0044] (2) Weigh 300 mg of the powder prepared in step (1) and calcine it under an argon atmosphere at a temperature of 800° C. for 1 hour.

[0045] (4) Weigh 100 mg of the product obtained in step (2) and calcine it under an argon atmosphere, first at 400° C. for 1 hour, then continue to heat it to 1000° C. and calcine it for 1 hour to obtain a hard carbon material.

[0046] Comparative Example 2:

[0047] (1) A piece of silk fiber of 20×4 cm was cut and pre-carbonized at 600°C for 1 hour under an argon atmosphere. The product was ground into a powder of 300-800 mesh.

[0048] (2) Weigh 300 mg of the powder prepared in step (1) and 100 mg of potassium hydroxide and mix them evenly by grinding. The mixture is calcined under an argon atmosphere at a temperature of 800° C. for 1 hour. The calcined product is dispersed in a 2 mol / L hydrochloric acid solution and stirred for 2 hours. The product is then filtered and washed with deionized water until the filtrate is neutral, and dried at 80° C. for 24 hours.

[0049] (3) Weigh 100 mg of the product obtained in step (2) and calcine it under an argon atmosphere, first at 400°C for 1 hour, then continue to raise the temperature to 1000°C and calcine for 1 hour to obtain a porous hard carbon material.

[0050] The materials obtained in Example 1, Comparative Example 1 and Comparative Example 2 were respectively mixed with super p and carboxymethyl cellulose in a mass ratio of 7:2:1 to prepare negative electrode materials. The current collector was copper foil. 1.0M sodium hexafluorophosphate (100% ethylene glycol dimethyl ether) was used as the electrolyte. Sodium ion half-cells were assembled in a glove box (H2O<0.1ppm, O2<0.1ppm) for electrochemical performance testing. In terms of cycle performance, the closed-pore soft and hard carbon composite material of Example 1 had a high cycling performance at 500mAg -1 At this current density, the specific capacity remains at 229.3 mA h g after 500 cycles. -1 As a comparison, the hard carbon material of Example 1 has a specific capacity of only 135.5 mAh g after 500 cycles. -1The porous hard carbon material of comparative example 2 has a specific capacity of only 93.2 mA hg after 500 cycles. -1 In terms of rate performance, at 2000mA g -1 At a high current density, the specific capacity of the closed-pore soft and hard carbon composite material remains at 188.1 mAh g -1 The specific capacity of the hard carbon material in Comparative Example 1 is only 112.1 mAh g -1 The specific capacity of the porous hard carbon material of Comparative Example 2 is only 65.9 mA h g -1 Therefore, the cycle performance and rate performance of the material in Example 1 when used as the negative electrode material for sodium ion batteries are much better than those of the two comparative examples, proving that the closed-pore soft and hard carbon composite material exhibits excellent electrochemical properties.

[0051] Example 2:

[0052] (1) A piece of silk fiber of 20 × 4 cm was cut and pre-carbonized at 600 °C for 1 hour under an argon atmosphere. The product was ground into powder.

[0053] (2) Weigh 300 mg of the powder prepared in step (1) and 60 mg of potassium hydroxide and mix them evenly by grinding. The mixture is calcined under an argon atmosphere at a calcination temperature of 700°C for 2 hours. The calcined product is dispersed in a 2 mol / L hydrochloric acid solution and stirred for 1 hour. The product is then filtered and washed with deionized water until the filtrate is neutral, and dried at 90°C for 18 hours.

[0054] (3) Weigh 10 mg of coal liquefaction pitch and add it to 4 ml of NMP. Stir for 1 hour and then filter to obtain an NMP solution of coal liquefaction pitch. Weigh 90 mg of the product obtained in step (2) and disperse it in the prepared NMP solution of coal liquefaction pitch. Stir and soak for 8 hours, and then evaporate the NMP solution at 180°C.

[0055] (4) calcining the product obtained in step (3) under an argon atmosphere, first calcining at 400° C. for 1 hour, and then continuing to heat up to 1000° C. for 1 hour to obtain a closed-pore-rich soft and hard carbon composite material.

[0056] The closed-pore hard and soft carbon composite material prepared in this embodiment was mixed with super P and carboxymethyl cellulose in a mass ratio of 7:2:1 to prepare the negative electrode material. The current collector was copper foil. 1.0M sodium hexafluorophosphate (100% ethylene glycol dimethyl ether) was used as the electrolyte. A sodium ion half-cell was assembled in a glove box (H2O<0.1ppm, O2<0.1ppm) for electrochemical performance testing. The closed-pore hard and soft carbon composite material was measured to have a negative electrode performance of 20mAg -1 The current density is 321.7 mAh g -1reversible specific capacity.

[0057] Example 3:

[0058] (1) A piece of silk fiber of 20 × 4 cm was cut and pre-carbonized at 600 °C for 1 hour under an argon atmosphere. The product was ground into powder.

[0059] (2) Weigh 300 mg of the powder prepared in step (1) and 300 mg of potassium hydroxide and mix them evenly by grinding. The mixture is calcined under an argon atmosphere at a calcination temperature of 750° C. for 3 hours. The calcined product is dispersed in a 2 mol / L hydrochloric acid solution and stirred for 1.5 hours. The product is then filtered and washed with deionized water until the filtrate is neutral, and dried at 100° C. for 12 hours.

[0060] (3) Weigh 10 mg of coal liquefaction pitch and add it to 4 ml of NMP. After stirring for 1 hour, filter and obtain the coal liquefaction pitch NMP solution. Weigh 90 mg of the product obtained in step (2), disperse it in the prepared coal liquefaction pitch NMP solution, stir and soak for 8 hours, and then evaporate the NMP solution at 200°C.

[0061] (4) calcining the product obtained in step (3) under an argon atmosphere, first calcining at 450°C for 1 hour, and then continuously heating to 1000°C for calcining for 3 hours to obtain a closed-pore-rich soft and hard carbon composite material.

[0062] The closed-pore hard and soft carbon composite material prepared in this example was mixed with super P and carboxymethyl cellulose in a mass ratio of 7:2:1 to prepare the negative electrode material. The current collector was copper foil. 1.0M sodium hexafluorophosphate (100% ethylene glycol dimethyl ether) was used as the electrolyte. A sodium ion half-cell was assembled in a glove box (H2O<0.1ppm, O2<0.1ppm) for electrochemical performance testing. The closed-pore hard and soft carbon composite material was measured to have a capacitance of 20mA g -1 The current density is 225.3 mAh g -1 reversible specific capacity.

[0063] Example 4:

[0064] (1) A piece of silk fiber of 20 × 4 cm was cut and pre-carbonized at 550 °C for 2 h in an argon atmosphere. The product was ground into powder.

[0065] (2) Weigh 300 mg of the powder prepared in step (1) and 100 mg of potassium hydroxide and mix them evenly by grinding. The mixture is calcined under an argon atmosphere at a temperature of 800° C. for 1 hour. The calcined product is dispersed in a 1 mol / L hydrochloric acid solution and stirred for 2 hours. The product is then filtered and washed with deionized water until the filtrate is neutral, and dried at 80° C. for 12 hours.

[0066] (3) Weigh 5 mg of coal liquefaction pitch and add it to 4 ml of NMP. After stirring for 1.5 hours, filter and obtain an NMP solution of coal liquefaction pitch. Weigh 95 mg of the product obtained in step (2), disperse it in the prepared NMP solution of coal liquefaction pitch, stir and soak for 12 hours, and then evaporate the NMP solution at 180°C.

[0067] (4) calcining the product obtained in step (3) under an argon atmosphere, first at 350° C. for 3 hours, then continuously heating to 1000° C. for 1 hour, to obtain a closed-pore-rich soft and hard carbon composite material.

[0068] The closed-pore hard and soft carbon composite material prepared in this example was mixed with super P and carboxymethyl cellulose in a mass ratio of 7:2:1 to prepare the negative electrode material. The current collector was copper foil. 1.0M sodium hexafluorophosphate (100% ethylene glycol dimethyl ether) was used as the electrolyte. A sodium ion half-cell was assembled in a glove box (H2O<0.1ppm, O2<0.1ppm) for electrochemical performance testing. The closed-pore hard and soft carbon composite material was measured to have a capacitance of 20mA g -1 The current density is 263.1mA hg -1 reversible specific capacity.

[0069] Example 5:

[0070] (1) A piece of silk fiber of 20 × 4 cm was cut and pre-carbonized at 500 °C for 3 h in an argon atmosphere. The product was ground into powder.

[0071] (2) Weigh 300 mg of the powder prepared in step (1) and 100 mg of potassium hydroxide and mix them evenly by grinding. The mixture is calcined under an argon atmosphere at a temperature of 800° C. for 1 hour. The calcined product is dispersed in a 1.5 mol / L hydrochloric acid solution and stirred for 2 hours. The product is then filtered and washed with deionized water until the filtrate is neutral, and dried at 100° C. for 12 hours.

[0072] (3) Weigh 20 mg of coal liquefaction pitch and add it to 4 ml of NMP. After stirring for 2 hours, filter and obtain an NMP solution of coal liquefaction pitch. Weigh 80 mg of the product obtained in step (2), disperse it in the prepared NMP solution of coal liquefaction pitch, stir and soak for 10 hours, and then evaporate the NMP solution at 190°C.

[0073] (4) calcining the product obtained in step (3) under an argon atmosphere, first at 400° C. for 2 hours, and then continuing to heat up to 1000° C. for 2 hours to obtain a closed-pore soft and hard carbon composite material.

[0074] The closed-pore hard and soft carbon composite material prepared in this example was mixed with super P and carboxymethyl cellulose in a mass ratio of 7:2:1 to prepare the negative electrode material. The current collector was copper foil. 1.0M sodium hexafluorophosphate (100% ethylene glycol dimethyl ether) was used as the electrolyte. A sodium ion half-cell was assembled in a glove box (H2O<0.1ppm, O2<0.1ppm) for electrochemical performance testing. The closed-pore hard and soft carbon composite material was measured to have a capacitance of 20mA g -1 The current density is 343.9mA hg -1 reversible specific capacity.

[0075] The above-described embodiments merely express the implementation methods of the present invention, but should not be understood as limiting the scope of the patent of the present invention. It should be pointed out that for those skilled in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a closed-cell soft and hard carbon composite material from silk fiber and coal liquefaction pitch, characterized in that: The method first pre-carbonizes silk fibers to obtain pre-carbonized silk fibers; then, an open pore structure is constructed on the pre-carbonized silk fibers through chemical activation to obtain activated silk carbon fibers; then, coal liquefaction pitch is coated on the surface of the activated silk carbon fibers through an impregnation method; finally, co-carbonization is performed to complete the encapsulation of the open pores, thereby obtaining a closed-pore soft and hard carbon composite material with a soft carbon shell and a hard carbon core.

2. The method for preparing a closed-cell soft and hard carbon composite material from silk fiber and coal liquefaction pitch according to claim 1, characterized in that: The method specifically comprises the following steps: Step 1, cutting silk fibers, pre-carbonizing them at 500-600° C. for 1-3 hours under a nitrogen or argon atmosphere to obtain pre-carbonized silk fibers, and finally grinding the pre-carbonized silk fibers into pre-carbonized silk fiber powder; Step 2: Grind and mix the pre-carbonized silk fiber powder obtained in step 1 and potassium hydroxide, and calcine the mixture at 700-800° C. for 1-3 hours under a nitrogen or argon atmosphere to obtain activated silk carbon fibers; the mass ratio of potassium hydroxide to pre-carbonized silk fiber powder is 1:(1.0-5.0); Step 3, dispersing the activated silk carbon fiber obtained in step 2 in an organic solvent dissolved in coal liquefaction pitch, coating the surface of the activated silk carbon fiber with coal liquefaction pitch by an impregnation method, and evaporating the organic solvent to obtain a composite structure of activated silk carbon fiber coated with coal liquefaction pitch; the mass ratio of the coal liquefaction pitch to the activated silk carbon fiber is 1:(4.0-19.0); Step 4: Under a nitrogen or argon atmosphere, the composite structure of coal liquefaction pitch coated activated silk carbon fiber obtained in step 3 is calcined at 350-450°C for 1-3 hours, and then the temperature is raised to 1000°C for high-temperature carbonization for 1-3 hours to obtain a soft and hard carbon composite material rich in closed pores.

3. The method for preparing a closed-cell soft and hard carbon composite material from silk fibers and coal liquefaction pitch according to claim 2, characterized in that: In the step 1, the pre-carbonized silk fibers are ground into 300-800 mesh pre-carbonized silk fiber powder.

4. The method for preparing a closed-cell soft and hard carbon composite material from silk fibers and coal liquefaction pitch according to claim 2, characterized in that: In the step 2, after high-temperature calcination, the product is washed with hydrochloric acid and deionized water, and then dried to obtain activated silk carbon fibers.

5. The method for preparing a closed-cell soft and hard carbon composite material from silk fibers and coal liquefaction pitch according to claim 4, characterized in that: In step 2: the specific steps of washing and drying are to disperse the silk carbon fibers activated by potassium hydroxide in a 1-2 mol / L hydrochloric acid solution and stir for 1-2 hours, then repeatedly filter and wash with deionized water until the filtrate is neutral, and dry for 12-24 hours to obtain a solid product of activated silk carbon fibers, and the drying temperature is 80-100°C.

6. The method for preparing a closed-cell soft and hard carbon composite material from silk fibers and coal liquefaction pitch according to claim 2, characterized in that: The organic solvent in step 3 includes N-methylpyrrolidone, xylene, N,N-dimethylformamide and tetrahydrofuran.

7. The method for preparing a closed-cell soft and hard carbon composite material from silk fibers and coal liquefaction pitch according to claim 2, characterized in that: The organic solvent in step 3 is preferably N-methylpyrrolidone solvent.

8. The method for preparing a closed-cell soft and hard carbon composite material from silk fibers and coal liquefaction pitch according to claim 2, characterized in that: The specific steps of step 3 are as follows: first, solid coal liquefaction pitch is added to an organic solvent, stirred for 1-2 hours, and then filtered to obtain an organic solvent containing coal liquefaction pitch; then, activated silk carbon fibers are dispersed in the organic solvent containing coal liquefaction pitch, stirred for 8-12 hours, and then transferred to an oil bath, and the organic solvent is evaporated at a temperature of 180-200°C to obtain a composite structure of coal liquefaction pitch-coated activated silk carbon fibers.

9. A closed-cell soft and hard carbon composite material prepared from silk fiber and coal liquefaction pitch, characterized in that: The closed-pore-rich soft and hard carbon composite material is obtained by the preparation method described in any one of claims 1-8, with hard carbon derived from silk fibers as the matrix and soft carbon derived from coal liquefaction pitch as the outer layer coating to construct a rich closed-pore structure and a composite structure of soft carbon and hard carbon.

10. An application of the closed-cell soft and hard carbon composite material prepared by silk fiber and coal liquefaction pitch according to claim 8, characterized in that: The closed-pore-rich soft and hard carbon composite material is applied in the field of negative electrode materials for sodium ion batteries.

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