A method for preparing carbon-encapsulated porous hard carbon / Si composite negative electrode material

Abstract

The present invention discloses a method for preparing a carbon-encapsulated porous hard carbon / Si composite negative electrode material, which belongs to the technical field of new energy materials. The present invention allows carbon nanospheres, hexadecyltrimethylammonium bromide, ammonia water, m-aminophenol and formaldehyde to act together on the porous hard carbon / Si composite material, which can effectively alleviate the volume expansion of the silicon-based material and shorten the transmission distance of lithium ions, thereby improving the cycle stability of the electrode plate. The use of the carbon-encapsulated porous hard carbon / Si composite negative electrode material of the present invention significantly increases the cycle stability of the lithium-ion battery, and the capacity does not significantly decay during high current charging and discharging, while also showing excellent rate performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy materials, and in particular relates to a method for preparing a carbon-encapsulated porous hard carbon / Si composite negative electrode material. Background Art

[0002] Graphite is currently the most widely used negative electrode material for lithium-ion batteries. Graphite negative electrode materials have advantages such as low lithium insertion potential and good cycle stability. However, the theoretical specific capacity of graphite is low and cannot meet the needs of current technological development. With the discovery of silicon materials, it has become one of the most promising alternatives to graphite. Silicon-based materials undergo an alloying reaction during the charge and discharge process, and the theoretical specific capacity is as high as 4200mAh·g -1 , while the lithium insertion potential is relatively low. However, silicon-based materials will expand in volume during the charge and discharge process, resulting in pulverization of the active material and poor electrical contact between the active material and the current collector, which in turn leads to instability of the solid electrolyte interface (SEI) and severe capacity decay. Improving the cycle stability of silicon-based negative electrode materials is crucial for the practical application of silicon. Researchers have used various methods to improve the electrochemical performance of silicon-based materials, including structural nano-scaling, porous silicon structure design, carbon coating, and composite with other materials.

[0003] However, problems with current silicon-based anode material production and application processes have severely limited their commercial application. The shedding and agglomeration of the current collector during charge and discharge of the composited anode material affects the performance of lithium-ion batteries. A synthesis method that is simple to operate, consumes little energy, is environmentally friendly, and can be produced on a large scale is also key to whether silicon-based anode materials can be put into commercial use. The preparation of carbon-silicon composite materials often uses high-molecular organic carbon such as asphalt, phenolic resin, and glucose as a carbon source. This process often uses organic binders during the preparation process to achieve a better composite effect. During the encapsulation or coating process, CVD (CN118610384A, CN118117082B) and metal deposition (CN118645600A) are often used to encapsulate silicon-based materials to synthesize core-shell structures and yolk-shell structures (CN115377399B). However, these processes are prone to introducing excess substances during the preparation process, and problems such as impurity removal need to be urgently addressed. Furthermore, the cumbersome preparation process and high equipment prices make them unsuitable for large-scale production.

[0004] In order to solve the above problems, relevant workers have done a lot of research. Many of them use inorganic large-skeleton porous carbon, namely wood activated carbon, modified coal-based activated carbon, porous hard carbon, etc. as carbon sources and composite with nano-silicon (patent numbers: US20230219819A1, CN117317205A, CN117476919A). Although the above-mentioned modification methods have significantly improved the cycle stability of the battery, the silicon-based negative electrode material has side reactions with the electrolyte during the cycle of lithium-ion batteries, resulting in particle pulverization, gas production, and structural collapse. The problem is still a major problem. Therefore, how to reduce the exposure of silicon particles, reduce the occurrence of side reactions, and improve structural stability is a prominent problem that urgently needs to be solved. Summary of the Invention

[0005] In view of the above shortcomings of the prior art, the present invention provides a method for preparing a carbon-encapsulated porous hard carbon / Si composite negative electrode material.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for preparing a carbon-encapsulated porous hard carbon / Si composite negative electrode material comprises the following steps:

[0008] (1) Sawdust and potassium hydroxide powder are placed in water, sealed, stirred, dried, and then calcined under protective gas. The calcined product is then soaked in hydrochloric acid, rinsed, and finally dried to obtain porous hard carbon.

[0009] (2) dissolving nano-silicon in a mixture of anhydrous ethanol and water to obtain a nano-silicon solution; dispersing porous hard carbon in a mixture of anhydrous ethanol and water to obtain a porous hard carbon dispersion solution;

[0010] (3) mixing the nano-silicon solution and the porous hard carbon dispersion solution to obtain a porous hard carbon / Si composite material mixed solution;

[0011] (4) adding carbon nanospheres, hexadecyltrimethylammonium bromide, ammonia water, m-aminophenol and formaldehyde solution to the porous hard carbon / Si composite material mixed solution continuously stirred in step (3), sealing, stirring, standing, centrifuging, washing and drying to obtain a carbon-encapsulated porous hard carbon / Si composite material precursor;

[0012] (5) calcining the carbon-encapsulated porous hard carbon / Si composite material precursor in a protective gas to obtain a carbon-encapsulated porous hard carbon / Si composite material;

[0013] (6) The carbon-encapsulated porous hard carbon / Si composite material, a conductive agent, and a binder are mixed to prepare a slurry, and then coated on a current collector and dried to obtain a negative electrode sheet for a lithium-ion battery.

[0014] As a preferred embodiment of the present invention, the mass ratio of the carbon nanospheres, hexadecyltrimethylammonium bromide, ammonia water, m-aminophenol, formaldehyde solution and nano-silicon is (1-10):(1-6):(6-15):(2-7):(4-15):1.

[0015] As a preferred embodiment of the present invention, the diameter of the carbon nanospheres is 20-100 nm, the mass percentage of ammonia in the ammonia water is 38%, and the mass percentage of formaldehyde in the formaldehyde solution is 40%; the pore size of the porous hard carbon is 500 nm-1 μm.

[0016] The carbon nanospheres have a diameter of 20 to 100 nm, which is conducive to entering the porous hard carbon pores and interlacing with the nano-silicon, promoting the ions and electrons in the battery to better combine with the silicon base of the negative electrode material during the charge and discharge process. If the diameter of the carbon nanospheres is larger than this range, the carbon nanospheres cannot enter the porous hard carbon pores and can only stay outside the porous hard carbon and cannot play a conductive role. If the diameter of the carbon nanospheres is smaller than this range, although they can enter the porous hard carbon pores, the effect of improving the battery's rate performance is limited.

[0017] As a preferred embodiment of the present invention, the carbon nanospheres, hexadecyltrimethylammonium bromide, ammonia, m-aminophenol, and formaldehyde solution are added to the porous hard carbon / Si composite material mixed solution in the following order: carbon nanospheres, hexadecyltrimethylammonium bromide, ammonia, m-aminophenol, and formaldehyde solution. This ensures that all materials are fully dispersed or dissolved. Otherwise, the conductivity of the negative electrode material and the rate performance of the battery are not significantly improved, and only silicon volume expansion is slightly alleviated. The interval between material additions is 10 to 180 minutes, and the reaction is continued for 4 to 12 hours after the materials are added.

[0018] The present invention uses cetyltrimethylammonium bromide, ammonia water, m-aminophenol and formaldehyde solution to collaboratively perform encapsulation pretreatment on nano-silicon and carbon nanospheres that enter the pores of porous hard carbon. The ammonia water provides conditions for the polycondensation reaction of m-aminophenol and formaldehyde, and cetyltrimethylammonium bromide can form micelles to encapsulate m-aminophenol and formaldehyde into a whole during the polycondensation process. This prevents the agglomeration of Si nanoparticles while also preventing the polycondensation reaction from proceeding only on the surface of the silicon particles, thereby reducing the repulsive force between particles with the same charge. If cetyltrimethylammonium bromide is not added, the coating layer will be directly on the surface of the Si particles, with each Si particle acting as a core and the carbon layer as an outer shell, thus failing to perform an encapsulation function, alleviate silicon volume expansion, and significantly improve the electrochemical properties of the battery.

[0019] As a preferred embodiment of the present invention, the mass ratio of the sawdust to the potassium hydroxide powder is (1-2):(1-3); and the concentration of potassium hydroxide after the sawdust and the potassium hydroxide powder are added to water is 0.4-2.0 mol / L.

[0020] As a preferred embodiment of the present invention, in step (1), the calcination specifically includes: heating from room temperature to 300-500°C at a heating rate of 5°C / min and keeping warm for 0.2-1h, then heating to 700-900°C and keeping warm for 0.5-3h.

[0021] As a preferred embodiment of the present invention, in step (1), the hydrochloric acid soaking time is 12 to 24 hours, and the concentration of hydrochloric acid is 0.5 to 2 mol / L.

[0022] The pore diameter of the porous hard carbon is 500 nm to 1 μm.

[0023] As a preferred embodiment of the present invention, in step (2), the volume ratio of anhydrous ethanol to water is (1-5):1.

[0024] The present invention utilizes a mixture of anhydrous ethanol and water to facilitate the dispersion or dissolution of porous hard carbon or nano-silicon. Using only water can result in poor dispersion of the nano-silicon on the surface or within the pores of the porous hard carbon, ultimately significantly degrading the electrochemical properties of the lithium-ion battery. Alternatively, using only ethanol is detrimental to the successful preparation of negative electrode materials.

[0025] As a preferred embodiment of the present invention, in step (3), the mass ratio of nano-silicon to porous hard carbon is (1-3):1.

[0026] As a preferred embodiment of the present invention, in step (5), the calcination specifically includes: heating from room temperature to 300-500°C at a heating rate of 5°C / min and keeping warm for 0.1-1h, then heating to 700-900°C and keeping warm for 1-3h.

[0027] As a preferred embodiment of the present invention, the conductive agent is conductive carbon black; the binder is sodium alginate; and the mass ratio of the carbon-encapsulated porous hard carbon / Si composite material, the conductive agent, and the binder is 6:2:2.

[0028] As a preferred embodiment of the present invention, the thickness of the slurry on the current collector is 15 microns.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) The present invention uses sawdust as a carbon source, which has a higher theoretical specific capacity than traditional graphite. At the same time, the hard carbon material has a stable structure, good cyclability, and can work at a lower temperature; the porous hard carbon can make the nanoparticles evenly embedded in the pore channels of the porous hard carbon, buffering the volume expansion of silicon particles during the charging and discharging process.

[0031] (2) The present invention uses carbon nanospheres, hexadecyltrimethylammonium bromide, ammonia, m-aminophenol, and formaldehyde to introduce carbon nanospheres into the pores of porous hard carbon, thereby confining silicon within the hard carbon. This effectively mitigates silicon volume expansion and thus failure, and shortens the transmission distance of lithium ions. Simultaneously, the hard carbon also improves the conductivity of the electrode sheet. This significantly improves the electrochemical properties of lithium-ion batteries and has great development prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 These are SEM images of the porous hard carbon / Si composite material prepared in Example 1, (a) is a low-magnification SEM image; (b) is a medium-magnification SEM image; and (c) is a high-magnification SEM image.

[0033] Figure 2 These are SEM images of the carbon-encapsulated porous hard carbon / Si composite material prepared in Example 1, (a) is a low-magnification SEM image; (b) is a medium-magnification SEM image; and (c) is a high-magnification SEM image.

[0034] Figure 3 These are the XRD curves of nano-silicon, porous hard carbon, porous hard carbon / Si composite material and carbon-encapsulated porous hard carbon / Si composite material prepared in Example 1.

[0035] Figure 4 The lithium ion battery assembled with the additional materials prepared in Example 1 and Comparative Examples 1-4 was tested at 200 mA g -1 Cycling performance curves at different current densities.

[0036] Figure 5 Cycling rate performance curves of lithium-ion batteries assembled with the additional materials prepared in Example 1 and Comparative Example 3 at different current densities.

[0037] Figure 6 The lithium ion battery assembled with the materials prepared in Example 1 and Comparative Example 3 was tested at 1000 mAg -1 Long cycle performance curve under different current densities. DETAILED DESCRIPTION

[0038] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0039] Example 1

[0040] A method for preparing a lithium-ion battery negative electrode material by carbon-encapsulated porous hard carbon / Si composite comprises the following steps:

[0041] (1) Weigh 10 g of birch sawdust and place it in 250 mL of a 1 mol / L potassium hydroxide solution. Cover the beaker with plastic wrap, control the temperature to 80 °C, and stir on a magnetic stirrer for 8 h. After the reaction is completed, pour off the upper solution and add deionized water to rinse repeatedly 2-3 times. Dry the sawdust in an 80 °C drying oven for 12 h and place it in a tubular furnace. Heat it to 300 °C at a rate of 5 °C / min under an argon atmosphere and keep it warm for 0.5 h. Then continue to heat it to 800 °C and start calcining. Calcinate for 1 h, and soak the intermediate product in 1 mol / L hydrochloric acid for 24 hours and then rinse with deionized water. Finally, dry it to obtain a porous hard carbon material with a pore size of 500 nm to 1 μm.

[0042] (2) 0.3 g of nano-silicon was placed in a mixed solution of 40 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a nano-silicon dispersion solution; at the same time, 0.2 g of porous hard carbon was placed in a mixed solution of 40 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a porous hard carbon dispersion solution.

[0043] (3) The porous hard carbon dispersion solution was stirred in a magnetic stirrer, and the nano-silicon dispersion solution was slowly added and stirred for 2 h to obtain a porous hard carbon / Si composite material mixed solution, which was stirred continuously for the subsequent steps.

[0044] (4) Add 0.9 g of carbon nanospheres to the continuously stirred porous hard carbon / Si composite material mixed solution, stir for 2 h, then add 0.4 g of hexadecyltrimethylammonium bromide (CTAB); continue stirring for 20 min, then add 4 mL of ammonia water and 1.2 g of m-aminophenol; continue stirring for 30 min, then add 2.4 mL of formaldehyde solution, seal, continue stirring at a reaction temperature of 40°C for 8 h, let it stand for 4 h, centrifuge, wash the intermediate product twice with anhydrous ethanol, and dry it in a drying oven at 80°C for 12 h to obtain a carbon-encapsulated porous hard carbon / Si composite material precursor, the mass percentage of ammonia in the ammonia water is 38%, and the mass percentage of formaldehyde in the formaldehyde solution is 40%.

[0045] (5) The carbon-encapsulated porous hard carbon / Si composite material precursor was placed in a tubular furnace, heated to 300°C at a heating rate of 5°C / min and kept warm for 1 hour, then heated to 800°C at the same heating rate and kept warm for 3 hours, and the product was collected to obtain a carbon-encapsulated porous hard carbon / Si composite material.

[0046] (6) Weigh 0.3 g of the carbon-encapsulated porous hard carbon / Si composite material obtained in step (5) as an active material, 0.1 g of conductive carbon black as a conductive agent, and 0.1 g of sodium alginate as a binder, place them in a mortar and grind them thoroughly, then add an appropriate amount of deionized water to a beaker with a dropper and continue grinding to form a negative electrode material.

[0047] (7) The negative electrode material obtained in step (6) is coated on the current collector at a thickness of 15 μm, and then placed in a vacuum drying oven and dried at 80° C. for 12 hours to obtain a negative electrode sheet for a lithium-ion battery.

[0048] from Figure 1 It can be clearly seen that the nano-silicon particles have entered the pore channels of the porous hard carbon, and the pore channels of the porous hard carbon can alleviate the volume expansion of silicon. Figure 2 It shows that the porous hard carbon morphology can no longer be seen in the carbon encapsulated porous hard carbon / Si composite material. A large number of silicon particles are encapsulated in the coating layer. The carbon encapsulation layer can alleviate the volume expansion of silicon. At the same time, it can also reduce the direct contact between silicon and the electrolyte, reduce the formation of the solid electrolyte (SEI) film, and reduce the consumption of lithium ions, thereby increasing the cycle stability of lithium-ion batteries. Figure 3 It can be seen that the Si in the porous hard carbon / Si composite material and the carbon-encapsulated porous hard carbon / Si composite material maintains a crystalline structure, and the porous hard carbon is an amorphous structure, indicating that during the preparation of the Si / porous HC (hard carbon abbreviated as HC) composite material, the crystal structure of the nano-silicon particles was well preserved and did not change.

[0049] Example 2

[0050] A method for preparing a lithium-ion battery negative electrode material by carbon-encapsulated porous hard carbon / Si composite comprises the following steps:

[0051] (1) Weigh 10 g of birch sawdust and place it in 250 mL of a 0.8 mol / L potassium hydroxide solution. Cover the beaker with plastic wrap, control the temperature to 80 °C, and stir on a magnetic stirrer for 8 h. After the reaction is completed, pour off the upper solution and add deionized water to rinse repeatedly 2-3 times. Dry the sawdust in an 80 °C drying oven for 12 h and place it in a tubular furnace. Under an argon atmosphere, heat it to 300 °C at a rate of 5 °C / min and keep it warm for 1 h. Then continue to heat it to 900 °C and start calcining. Calcinate for 0.5 h, and soak the intermediate product in 2 mol / L hydrochloric acid for 24 hours and then rinse with deionized water. Finally, dry it to obtain a porous hard carbon material with a pore size of 500 nm to 1 μm.

[0052] (2) 0.6 g of nano-silicon was placed in a mixed solution of 50 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a nano-silicon dispersion solution; at the same time, 0.6 g of porous hard carbon was placed in a mixed solution of 50 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a porous hard carbon dispersion solution.

[0053] (3) The porous hard carbon dispersion solution was stirred in a magnetic stirrer, and the nano-silicon dispersion solution was slowly added and stirred for 2 h to obtain a porous hard carbon / Si composite material mixed solution, which was stirred continuously for the subsequent steps.

[0054] (4) 6 g of carbon nanospheres were added to the continuously stirred porous hard carbon / Si composite material mixed solution, and 3.6 g of hexadecyltrimethylammonium bromide (CTAB) was added after stirring for 2 h; 10 mL of ammonia water and 4.2 g of m-aminophenol were added after stirring for 20 min; 9 mL of formaldehyde solution was added after stirring for 30 min, and the mixture was sealed. After stirring for 4 h at a reaction temperature of 40 ° C, the mixture was allowed to stand for 4 h, centrifuged, and the intermediate product was washed twice with anhydrous ethanol and dried in a drying oven at 80 ° C for 12 h to obtain a carbon-encapsulated porous hard carbon / Si composite material precursor. The mass percentage of ammonia in the ammonia water was 38%, and the mass percentage of formaldehyde in the formaldehyde solution was 40%.

[0055] (5) The carbon-encapsulated porous hard carbon / Si composite material precursor was placed in a tubular furnace, heated to 400°C at a heating rate of 5°C / min and kept warm for 0.2 h, then heated to 900°C at the same heating rate and kept warm for 1 h, and the product was collected to obtain a carbon-encapsulated porous hard carbon / Si composite material.

[0056] (6) Weigh 0.3 g of the carbon-encapsulated porous hard carbon / Si composite material obtained in step (5) as an active material, 0.1 g of conductive carbon black as a conductive agent, and 0.1 g of sodium alginate as a binder, place them in a mortar and grind them thoroughly, then add an appropriate amount of deionized water to a beaker with a dropper and continue grinding to form a negative electrode material.

[0057] (7) The negative electrode material obtained in step (6) is coated on the current collector at a thickness of 15 μm, and then placed in a vacuum drying oven and dried at 80° C. for 12 hours to obtain a negative electrode sheet for a lithium-ion battery.

[0058] The performance of the lithium-ion battery negative electrode sheet prepared in this embodiment is similar to that of embodiment 1.

[0059] Example 3

[0060] A method for preparing a lithium-ion battery negative electrode material by carbon-encapsulated porous hard carbon / Si composite comprises the following steps:

[0061] (1) Weigh 10 g of birch sawdust and place it in 250 mL of a 2 mol / L potassium hydroxide solution. Cover the beaker with plastic wrap, control the temperature to 70 °C, and stir on a magnetic stirrer for 4 h. After the reaction is completed, pour off the upper solution and add deionized water to rinse repeatedly 2-3 times. Dry the sawdust in an 80 °C drying oven for 12 h and place it in a tubular furnace. Under an argon atmosphere, heat it to 500 °C at a rate of 5 °C / min and keep it warm for 0.2 h. Then continue to heat it to 700 °C and start calcining. Calcinate for 3 h, and soak the intermediate product in 2 mol / L hydrochloric acid for 12 hours and then rinse with deionized water. Finally, dry it to obtain a porous hard carbon material with a pore size of 500 nm to 1 μm.

[0062] (2) 0.3 g of nano-silicon was placed in a mixed solution of 25 mL of anhydrous ethanol and 25 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a nano-silicon dispersion solution; at the same time, 0.1 g of porous hard carbon was placed in a mixed solution of 25 mL of anhydrous ethanol and 25 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a porous hard carbon dispersion solution.

[0063] (3) The porous hard carbon dispersion solution was stirred in a magnetic stirrer, and the nano-silicon dispersion solution was slowly added and stirred for 2 h to obtain a porous hard carbon / Si composite material mixed solution, which was stirred continuously for the subsequent steps.

[0064] (4) Add 0.3 g of carbon nanospheres to the continuously stirred porous hard carbon / Si composite material mixed solution, stir for 2 h, then add 0.3 g of hexadecyltrimethylammonium bromide (CTAB); continue stirring for 20 min, then add 2 mL of ammonia water and 0.6 g of m-aminophenol; continue stirring for 30 min, then add 1.2 mL of formaldehyde solution, seal, continue stirring at a reaction temperature of 40°C for 12 h, let it stand for 1 h, centrifuge and wash the intermediate product twice with anhydrous ethanol, and dry it in a drying oven at 80°C for 12 h to obtain a carbon-encapsulated porous hard carbon / Si composite material precursor, the mass percentage of ammonia in the ammonia water is 38%, and the mass percentage of formaldehyde in the formaldehyde solution is 40%.

[0065] (5) The carbon-encapsulated porous hard carbon / Si composite material precursor was placed in a tubular furnace, heated to 500°C at a heating rate of 5°C / min and kept warm for 0.1 h, then heated to 700°C at the same heating rate and kept warm for 3 h, and the product was collected to obtain a carbon-encapsulated porous hard carbon / Si composite material.

[0066] (6) Weigh 0.3 g of the carbon-encapsulated porous hard carbon / Si composite material obtained in step (5) as an active material, 0.1 g of conductive carbon black as a conductive agent, and 0.1 g of sodium alginate as a binder, place them in a mortar and grind them thoroughly, then add an appropriate amount of deionized water to a beaker with a dropper and continue grinding to form a negative electrode material.

[0067] (7) The negative electrode material obtained in step (6) is coated on the current collector at a thickness of 15 μm, and then placed in a vacuum drying oven and dried at 80° C. for 24 hours to obtain a negative electrode sheet for a lithium-ion battery.

[0068] The performance of the lithium-ion battery negative electrode sheet prepared in this embodiment is similar to that of embodiment 1.

[0069] Comparative Example 1

[0070] The preparation method of a lithium ion battery negative electrode material in this comparative example is different from the preparation method of the carbon-encapsulated porous hard carbon / Si composite lithium ion battery negative electrode material in Example 1 in that porous hard carbon is not added. The specific steps are as follows:

[0071] (1) 0.3 g of nano-silicon was placed in a mixed solution of 40 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 h to obtain a nano-silicon dispersion solution.

[0072] (2) 0.9 g of carbon nanospheres were added to the continuously stirred nano-silicon dispersion solution, and after stirring for 2 h, 0.4 g of hexadecyltrimethylammonium bromide (CTAB) was added; after continuing to stir for 20 min, 4 mL of ammonia water and 1.2 g of m-aminophenol were added; after continuing to stir for 30 min, 1.6 mL of formaldehyde solution was added, sealed, and stirred for 8 h at a reaction temperature of 40 ° C., and then allowed to stand for 4 h. After centrifugation, the intermediate product was washed twice with anhydrous ethanol and dried in a drying oven at 80 ° C. for 12 h to obtain a carbon-encapsulated Si composite material precursor, wherein the mass percentage of ammonia in the ammonia water was 38%, and the mass percentage of formaldehyde in the formaldehyde solution was 40%.

[0073] (3) The carbon-encapsulated Si composite material precursor was placed in a tubular furnace, heated to 300°C at a heating rate of 5°C / min and kept warm for 1 hour, then heated to 800°C at the same heating rate and kept warm for 3 hours, and the product was collected to obtain a carbon-encapsulated Si composite material.

[0074] (4) Weigh 0.3 g of the carbon-encapsulated Si composite material obtained in step (3) as an active material, 0.1 g of conductive carbon black as a conductive agent, and 0.1 g of sodium alginate as a binder, place them in a mortar and grind them thoroughly. Then, use a dropper to add an appropriate amount of deionized water to a beaker and continue grinding to form a negative electrode material.

[0075] (5) The negative electrode material obtained in step (4) is coated on the current collector at a thickness of 15 μm, and then placed in a vacuum drying oven and dried at 80° C. for 12 hours to obtain a negative electrode sheet for a lithium-ion battery.

[0076] Comparative Example 2

[0077] The only difference between the preparation method of a lithium ion battery negative electrode material in this comparative example and the preparation method of a carbon-encapsulated porous hard carbon / Si composite lithium ion battery negative electrode material in Example 1 is that CTAB is not added in step (4). The specific steps are as follows:

[0078] (1) Weigh 10 g of birch sawdust and place it in 250 mL of a 1 mol / L potassium hydroxide solution. Cover the beaker with plastic wrap, control the temperature to 80 °C, and stir on a magnetic stirrer for 8 h. After the reaction is completed, pour off the upper solution and add deionized water to rinse repeatedly 2-3 times. Dry the sawdust in an 80 °C drying oven for 12 h and place it in a tubular furnace. Heat it to 300 °C at a rate of 5 °C / min under an argon atmosphere and keep it warm for 0.5 h. Then continue to heat it to 800 °C and start calcining. Calcinate for 1 h, and soak the intermediate product in 1 mol / L hydrochloric acid for 24 hours and then rinse with deionized water. Finally, dry it to obtain a porous hard carbon material with a pore size of 500 nm to 1 μm.

[0079] (2) 0.3 g of nano-silicon was placed in a mixed solution of 40 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a nano-silicon dispersion solution; at the same time, 0.2 g of porous hard carbon was placed in a mixed solution of 40 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a porous hard carbon dispersion solution.

[0080] (3) The porous hard carbon dispersion solution was stirred in a magnetic stirrer, and the nano-silicon dispersion solution was slowly added and stirred for 2 h to obtain a porous hard carbon / Si composite material mixed solution, which was stirred continuously for the subsequent steps.

[0081] (4) Add 0.9 g of carbon nanospheres to the continuously stirred porous hard carbon / Si composite material mixed solution, stir for 2 h, add 4 mL of ammonia water and 1.2 g of m-aminophenol; continue stirring for 30 min, add 1.6 mL of formaldehyde solution, seal, continue stirring at a reaction temperature of 40°C for 8 h, let it stand for 4 h, centrifuge and wash the intermediate product twice with anhydrous ethanol, and dry it in a drying oven at 80°C for 12 h to obtain a carbon-encapsulated porous hard carbon / Si composite material precursor, the mass percentage of ammonia in the ammonia water is 38%, and the mass percentage of formaldehyde in the formaldehyde solution is 40%.

[0082] (5) The carbon-encapsulated porous hard carbon / Si composite material precursor was placed in a tubular furnace, heated to 300°C at a heating rate of 5°C / min and kept warm for 1 hour, then heated to 800°C at the same heating rate and kept warm for 3 hours, and the product was collected to obtain a carbon-encapsulated porous hard carbon / Si composite material.

[0083] (6) Weigh 0.3 g of the carbon-encapsulated porous hard carbon / Si composite material obtained in step (5) as an active material, 0.1 g of conductive carbon black as a conductive agent, and 0.1 g of sodium alginate as a binder, place them in a mortar and grind them thoroughly, then add an appropriate amount of deionized water to a beaker with a dropper and continue grinding to form a negative electrode material.

[0084] (7) The negative electrode material obtained in step (6) is coated on the current collector at a thickness of 15 μm, and then placed in a vacuum drying oven and dried at 80° C. for 12 hours to obtain a negative electrode sheet for a lithium-ion battery.

[0085] Comparative Example 3

[0086] The only difference between the preparation method of a lithium ion battery negative electrode material in this comparative example and the preparation method of a carbon-encapsulated porous hard carbon / Si composite lithium ion battery negative electrode material in Example 1 is that carbon nanospheres are not added in step (4). The specific steps are as follows:

[0087] (1) Weigh 10 g of birch sawdust and place it in 250 mL of a 1 mol / L potassium hydroxide solution. Cover the beaker with plastic wrap, control the temperature to 80 °C, and stir on a magnetic stirrer for 8 h. After the reaction is completed, pour off the upper solution and add deionized water to rinse repeatedly 2-3 times. Dry the sawdust in an 80 °C drying oven for 12 h and place it in a tubular furnace. Heat it to 800 °C at a rate of 5 °C / min under an argon atmosphere and start calcining. Calcinate for 1 h, and soak the intermediate product in 1 mol / L hydrochloric acid for 24 hours and then rinse with deionized water. Finally, dry it to obtain a porous hard carbon material with a pore size of 500 nm to 1 μm.

[0088] (2) 0.3 g of nano-silicon was placed in a mixed solution of 40 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a nano-silicon dispersion solution; at the same time, 0.2 g of porous hard carbon was placed in a mixed solution of 40 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a porous hard carbon dispersion solution.

[0089] (3) The porous hard carbon dispersion solution was stirred in a magnetic stirrer, and the nano-silicon dispersion solution was slowly added and stirred for 2 h to obtain a porous hard carbon / Si composite material mixed solution, which was stirred continuously for the subsequent steps.

[0090] (4) Add 0.4 g of hexadecyltrimethylammonium bromide (CTAB) to the continuously stirred porous hard carbon / Si composite material mixed solution, stir for 20 minutes, add 4 mL of ammonia water and 1.2 g of m-aminophenol; continue stirring for 30 minutes, add 2.4 mL of formaldehyde solution, seal, continue stirring at a reaction temperature of 40°C for 8 hours, let it stand for 4 hours, centrifuge and wash the intermediate product twice with anhydrous ethanol, and dry it in a drying oven at 80°C for 12 hours to obtain a carbon-encapsulated porous hard carbon / Si composite material precursor, the mass percentage of ammonia in the ammonia water is 38%, and the mass percentage of formaldehyde in the formaldehyde solution is 40%.

[0091] (5) The carbon-encapsulated porous hard carbon / Si composite material precursor was placed in a tubular furnace, heated to 300°C at a heating rate of 5°C / min and kept warm for 1 hour, then heated to 800°C at the same heating rate and kept warm for 3 hours, and the product was collected to obtain a carbon-encapsulated porous hard carbon / Si composite material.

[0092] (6) Weigh 0.3 g of the carbon-encapsulated porous hard carbon / Si composite material obtained in step (5) as an active material, 0.1 g of conductive carbon black as a conductive agent, and 0.1 g of sodium alginate as a binder, place them in a mortar and grind them thoroughly, then add an appropriate amount of deionized water to a beaker with a dropper and continue grinding to form a negative electrode material.

[0093] (7) The negative electrode material obtained in step (6) is coated on the current collector at a thickness of 15 μm, and then placed in a vacuum drying oven and dried at 80° C. for 12 hours to obtain a negative electrode sheet for a lithium-ion battery.

[0094] Comparative Example 4

[0095] A method for preparing a lithium-ion battery negative electrode material by carbon-encapsulated porous hard carbon / Si composite comprises the following steps:

[0096] (1) Weigh 10 g of birch sawdust and place it in 250 mL of a 1 mol / L potassium hydroxide solution. Cover the beaker with plastic wrap, control the temperature to 80 °C, and stir on a magnetic stirrer for 8 h. After the reaction is completed, pour off the upper solution and add deionized water to rinse repeatedly 2-3 times. Dry the sawdust in an 80 °C drying oven for 12 h and place it in a tubular furnace. Heat it to 300 °C at a rate of 5 °C / min under an argon atmosphere and keep it warm for 0.5 h. Then continue to heat it to 800 °C and start calcining. Calcinate for 1 h, and soak the intermediate product in 1 mol / L hydrochloric acid for 24 hours and then rinse with deionized water. Finally, dry it to obtain a porous hard carbon material with a pore size of 500 nm to 1 μm.

[0097] (2) 0.3 g of nano-silicon was placed in a mixed solution of 40 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a nano-silicon dispersion solution; at the same time, 0.2 g of porous hard carbon was placed in a mixed solution of 40 mL of anhydrous ethanol and 10 mL of deionized water, and ultrasonically vibrated for 1 hour to obtain a porous hard carbon dispersion solution.

[0098] (3) The porous hard carbon dispersion solution was stirred in a magnetic stirrer, and the nano-silicon dispersion solution was slowly added and stirred for 2 hours to obtain a porous hard carbon / Si composite material mixed solution. After continuous stirring, the solution was dried in an 80°C drying oven for 12 hours to obtain a porous hard carbon / Si composite material.

[0099] (4) Weigh 0.3 g of the carbon-encapsulated porous hard carbon / Si composite material obtained in step (3) as an active material, 0.1 g of conductive carbon black as a conductive agent, and 0.1 g of sodium alginate as a binder, place them in a mortar and grind them thoroughly, then add an appropriate amount of deionized water to a beaker with a dropper and continue grinding to form a negative electrode material.

[0100] (5) The negative electrode material obtained in step (4) is coated on the current collector at a thickness of 15 μm, and then placed in a vacuum drying oven and dried at 80° C. for 12 hours to obtain a negative electrode sheet for a lithium-ion battery.

[0101] Effect Examples

[0102] The negative electrode sheets of the lithium-ion batteries in Examples 1-3 and Comparative Examples 1-4 were assembled into batteries, CR2032 button batteries were selected, glass fiber separators were selected as battery diaphragms, the electrolyte was 1 mol / L LiPF6 dissolved in a mixed solution of EC / DEC with a volume ratio of 1:1, and 1.0 mol / L LiPF6 dissolved in a mixed solution of EC / DEC / DMC with a volume ratio of 1:1:1 was added as an additive; after the batteries were packaged, they were allowed to stand for 24 hours and then their performance was tested.

[0103] By testing, such as Figure 4 As shown, at a current density of 0.2 A / g, the battery capacity retention rates of Example 1 and Comparative Example 3 after 150 cycles are 54.41% and 44.06%, respectively, compared with 15.5%, 19.17% and 22.74% of Comparative Examples 1, 2 and 4, respectively, showing good cycle stability and capacity retention. Figure 5Comparing Example 1 and Comparative Example 3, which exhibit excellent cycling stability, the figure shows that Comparative Example 3 exhibits relatively excellent discharge specific capacity at low current densities, but suffers from severe capacity degradation at high current densities, particularly at a current density of 2 A / g, leading to battery failure. Example 1 maintains capacities exceeding 490 mAh / g and 400 mAh / g at current densities of 1 A / g and 2 A / g, respectively. Even after the current density returns to 0.1 A / g, capacity degradation is minimal, demonstrating excellent rate performance. Figure 6 The long cycle discharge performance curves of Example 1 and Comparative Example 3 at a current density of 1A / g are shown. The overall cycle curve of Comparative Example 3 is relatively stable. After 65 cycles, the capacity reaches a peak of 420mAh / g. The capacity continues to decay during the subsequent cycles, and the capacity retention rate is 35.02% after 500 cycles. The battery capacity of Example 1 gradually rises and stabilizes at 520mAh / g. After 500 cycles at high current density, the capacity remains at 45.93%. Compared with Comparative Example 3, the addition of carbon nanospheres enables Example 1 to have good electronic conductivity and ionic conductivity, so that nano-silicon can be fully utilized, and the high capacity characteristics of silicon can be brought into play at high current density, and excellent cycle stability is shown. The capacity of Comparative Examples 1, 2, and 4 is low and decays severely during the high current cycle process. Since Comparative Example 1 does not have a porous hard carbon skeleton as support, the volume expansion of silicon during the cycle causes the battery capacity to be low and continuously decline. Since no hexadecyltrimethylammonium bromide was added to Comparative Example 2, the resulting composite structure was a thin carbon layer in which the carbon encapsulation layer was only coated on the surface of the nano-silicon, and the porous hard carbon and nano-silicon were not encapsulated as a whole. During the high current cycle, the outer carbon layer was easily broken, resulting in capacity decay. In Comparative Example 4, the electrolyte was in direct contact with silicon, resulting in irreversible loss of capacity. During the high current cycle, the simple composite structure failed to improve the conductivity of the material itself, so that the charge could not pass through the silicon material and the charging and discharging was terminated, so the capacity could not be displayed. In summary, the carbon-encapsulated porous hard carbon / Si composite material described in the present invention has good cycle stability, battery capacity, and rate performance. Compared with the comparative example, the cycle stability, battery capacity, and rate performance are all improved, it has good conductivity, and has great development prospects.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a carbon-encapsulated porous hard carbon / Si composite negative electrode material, characterized in that: The steps include: (1) Sawdust and potassium hydroxide powder are placed in water, sealed, stirred, dried, and then calcined under protective gas. The calcined product is then soaked in hydrochloric acid, rinsed, and finally dried to obtain porous hard carbon. (2) dissolving nano-silicon in a mixture of anhydrous ethanol and water to obtain a nano-silicon solution; dispersing porous hard carbon in a mixture of anhydrous ethanol and water to obtain a porous hard carbon dispersion solution; (3) mixing the nano-silicon solution and the porous hard carbon dispersion solution to obtain a porous hard carbon / Si composite material mixed solution; (4) adding carbon nanospheres with a diameter of 20 to 100 nm, hexadecyltrimethylammonium bromide, ammonia water, m-aminophenol, and formaldehyde solution to the porous hard carbon / Si composite material mixed solution continuously stirred in step (3) in the order of sealing, stirring, standing, centrifuging, washing, and drying to obtain a carbon-encapsulated porous hard carbon / Si composite material precursor; wherein the interval time for adding the materials is 10 to 180 minutes; (5) calcining the carbon-encapsulated porous hard carbon / Si composite material precursor in a protective gas to obtain a carbon-encapsulated porous hard carbon / Si composite material; (6) The carbon-encapsulated porous hard carbon / Si composite material, the conductive agent, and the binder are mixed to form a slurry, which is then coated on a current collector and dried to obtain a negative electrode sheet for a lithium-ion battery.

2. The method for preparing the carbon-encapsulated porous hard carbon / Si composite negative electrode material according to claim 1, characterized in that: The mass ratio of the carbon nanospheres, hexadecyltrimethylammonium bromide, ammonia water, m-aminophenol, formaldehyde and nano-silicon is (1-10):(1-6):(6-15):(2-7):(4-15):

1.

3. The method for preparing the carbon-encapsulated porous hard carbon / Si composite negative electrode material according to claim 1, wherein: The mass percentage of ammonia in ammonia water is 38%, and the mass percentage of formaldehyde in formaldehyde solution is 40%.

4. The method for preparing the carbon-encapsulated porous hard carbon / Si composite negative electrode material according to claim 1, wherein: In step (4), the reaction is continued for 4 to 12 hours after the carbon nanospheres, hexadecyltrimethylammonium bromide, ammonia water, m-aminophenol and formaldehyde solution are added.

5. The method for preparing the carbon-encapsulated porous hard carbon / Si composite negative electrode material according to claim 1, wherein: The mass ratio of the sawdust and the potassium hydroxide powder is (1-2):(1-3); and after the sawdust and the potassium hydroxide powder are put into water, the concentration of potassium hydroxide is 0.8-2.0 mol / L.

6. The method for preparing the carbon-encapsulated porous hard carbon / Si composite negative electrode material according to claim 1, wherein: In the step (1), the calcination specifically includes: heating from room temperature to 300-500°C at a heating rate of 5°C / min and keeping the temperature for 0.2-1h, and then heating to 700-900°C and keeping the temperature for 0.5-3h.

7. The method for preparing the carbon-encapsulated porous hard carbon / Si composite negative electrode material according to claim 1, wherein: In the step (2), the volume ratio of anhydrous ethanol to water is (1-5):

1.

8. The method for preparing the carbon-encapsulated porous hard carbon / Si composite negative electrode material according to claim 1, wherein: In the step (3), the mass ratio of nano-silicon to porous hard carbon is (1-3):

1.

9. The method for preparing the carbon-encapsulated porous hard carbon / Si composite negative electrode material according to claim 1, wherein: In the step (5), the calcination specifically includes: heating from room temperature to 300-500°C at a heating rate of 5°C / min and keeping the temperature for 0.1-1h, and then heating to 700-900°C and keeping the temperature for 1-3h.

10. The method for preparing the carbon-encapsulated porous hard carbon / Si composite negative electrode material according to claim 1, wherein: The conductive agent is conductive carbon black; the binder is sodium alginate; and the mass ratio of the carbon-encapsulated porous hard carbon / Si composite material, the conductive agent, and the binder is 6:2:2.

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