Preparation method of hollow core-shell nanosilicon-carbon composite material for negative electrode of lithium-ion battery

Abstract

A preparation method of a hollow core-shell nano-silicon carbon composite material for the negative electrode of a lithium-ion battery, which belongs to the technical field of lithium-ion batteries. The present invention solves the problems of poor long-cycle stability of existing nano-scale silicon carbon negative electrode materials and easy destruction of the silicon carbon structure. Method: First, the preparation of hollow core-shell -Si@C; second, the treatment of the hollow core-shell -Si@C crude product. The present invention is used for the preparation of a hollow core-shell nano-silicon carbon composite material for the negative electrode of a lithium-ion battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries. Background Art

[0002] Currently, the world is facing severe energy and environmental challenges, and there is an urgent need to develop and effectively utilize clean energy. The combination of electrochemical energy storage technology and clean energy is regarded as an effective way to solve the limitations of clean energy in terms of time and space. As the most widely used electrochemical energy storage battery at present, the energy density of lithium-ion batteries is, however, restricted by the low capacity of electrode materials. Therefore, in order to improve the energy density of lithium-ion batteries, researchers around the world are actively carrying out research on high-capacity anode materials.

[0003] Silicon is highly regarded as a promising substitute for graphite in lithium-ion batteries due to its extensive reserves, excellent theoretical capacity, and suitable voltage. However, silicon undergoes a large volume expansion (greater than 400%) during the lithium storage process and exhibits poor electronic and ionic conductivities. These inherent characteristics lead to safety problems such as electrode cracking, battery performance degradation, and rapid capacity decline. Therefore, the limitations brought about by these inherent characteristics of silicon-based anode materials have hindered their practical applications and the rapid development in the field of lithium-ion batteries.

[0004] According to existing literature, although various nano-sized silicon-carbon anode materials are emerging in an endless stream, due to the lack of strong bonding between silicon and carbon, the silicon-carbon structure is unstable, resulting in problems such as silicon-carbon separation or shedding during the cycling process of the material, which affects the long-term cycling stability of the material. Therefore, in order to solve this problem, it is very important to design a nano-silicon-carbon composite material with a stable structure. Summary of the Invention

[0005] The present invention aims to solve the problems of poor long-term cycling stability and easy destruction of the silicon-carbon structure of existing nano-sized silicon-carbon anode materials, and further provides a preparation method for a hollow core-shell nano-silicon-carbon composite material for the anode of a lithium-ion battery.

[0006] A preparation method for a hollow core-shell nano-silicon-carbon composite material for the anode of a lithium-ion battery is carried out according to the following steps:

[0007] I. Preparation of hollow core-shell - Si@C:

[0008] ① Cobalt(II) nitrate hexahydrate and 2-methylimidazole are added to methanol and ultrasonically dispersed and mixed, then stirred for 1 h to 12 h, aged at room temperature, and finally centrifuged and washed to collect the precipitate to obtain ZIF-67;

[0009] ② Add ZIF-67 to ethanol and disperse and mix them by ultrasonic treatment. Then add distilled water, tetraethyl orthosilicate, and ammonia water and stir for 1 h to 12 h. Finally, centrifuge and wash to collect the precipitate to obtain ZIF-67 with in-situ grown SiO2.

[0010] ③ Add ZIF-67 with in-situ grown SiO2 to ethanol and disperse and mix them by ultrasonic treatment. Then add resorcinol, ammonia water, and formaldehyde and stir for 1 h to 12 h. Finally, centrifuge and wash to collect the precipitate to obtain ZIF-67@SiO2 with in-situ grown phenolic resin.

[0011] ④ Add ZIF-67@SiO2 with in-situ grown phenolic resin to ethanol and disperse and mix them by ultrasonic treatment. Then add hydrochloric acid and perform ultrasonic treatment. Finally, centrifuge and wash to collect the precipitate and dry it under vacuum to obtain hollow core-shell - SiO2@RF after etching the internal ZIF-67.

[0012] ⑤ Under the conditions of Ar atmosphere and a temperature of 600 °C to 800 °C, calcine the hollow core-shell - SiO2@RF after etching the internal ZIF-67 for 1 h to 6 h to obtain hollow core-shell - SiO2@C.

[0013] ⑥ Mix the hollow core-shell - SiO2@C, Mg powder, and NaCl, and pyrolyze them for 1 h to 6 h under the conditions of Ar / H2 atmosphere and a temperature of 600 °C to 800 °C to obtain a crude product of hollow core-shell - Si@C.

[0014] II. Treatment of the crude product of hollow core-shell - Si@C:

[0015] Centrifuge the crude product of hollow core-shell - Si@C successively with absolute ethanol, hydrochloric acid, deionized water, hydrofluoric acid, and absolute ethanol, and finally dry it to obtain the hollow core-shell - Si@C composite material.

[0016] The beneficial effects of the present invention are as follows:

[0017] 1. Different from the general coating of traditional silicon-carbon materials that will separate and fall off during the lithiation process, the present invention realizes a double-layer in-situ growth method, achieving a silicon-carbon composite material with deep bonding. The composite material synthesized by this method not only improves the conductivity of the silicon-based material but also makes it difficult for silicon-carbon to separate during the lithiation process, thereby achieving the cycle stability.

[0018] 2. The present invention designs a unique hollow core-shell structured silicon-carbon composite structure. The outer carbon shell is used to enhance electron and ion transport and act as a stress buffer, and the internal silicon nanoparticles are firmly attached to the carbon shell, effectively preventing separation and pulverization during lithiation and delithiation processes. This structure can protect the internal silicon nanoparticles from direct contact with the electrolyte and provide a sealed space for the inward contraction of silicon during lithium storage, which is conducive to the formation of a stable SEI layer and effectively avoids the pulverization of silicon particles.

[0019] 3. The material prepared by the present invention has excellent cycle stability due to its innovative preparation method and unique structure. After 250 cycles at a current density of 200 mA / g, its reversible capacity is 927.1 mAh g -1 , and after 800 cycles at a current density of 1000 mA / g, its reversible capacity is 883 mAh g -1 . Moreover, the ultra-long cycle performance at high current is a very important characteristic of electrode materials. After 3000 cycles at a high current density of 5000 mA / g, the material of the present invention still maintains a reversible capacity of 713 mAh g -l , and the capacity retention rate is above 85%, demonstrating excellent battery performance.

[0020] The main innovation of the present invention is to prepare a negative electrode material with a unique hollow core-shell structured silicon-carbon composite structure through a two-step in-situ growth method, and this material exhibits excellent cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is the preparation flow chart of the hollow core-shell nano-silicon-carbon composite material for the negative electrode of the lithium-ion battery of the present invention;

[0022] Figure 2 is the XRD pattern of the hollow core-shell - Si@C composite material prepared in Step 2 of Example 1;

[0023] Figure 3 is the XPS full spectrum of the hollow core-shell - Si@C composite material prepared in Step 2 of Example 1;

[0024] Figure 4 is the SEM image of the hollow core-shell - Si@C composite material prepared in Step 2 of Example 1, with a scale bar of 500 nm;

[0025] Figure 5 is the SEM image of the hollow core-shell - Si@C composite material prepared in Step 2 of Example 1, with a scale bar of 100 nm;

[0026] Figure 6The cyclic performance graph of the button battery prepared using the hollow core-shell Si@C composite material prepared in Step 2 of Example 1. 1 represents charging, and 2 represents discharging, at a current density of 200 mA / g.

[0027] Figure 7 The cyclic performance graph of the button battery prepared using the hollow core-shell Si@C composite material prepared in Step 2 of Example 1 (activated at 200 mA / g for the first three cycles), at a current density of 1000 mA / g. 1 represents charging, and 2 represents discharging.

[0028] Figure 8 The cyclic performance graph of the button battery prepared using the hollow core-shell Si@C composite material prepared in Step 2 of Example 1 (activated at 200 mA / g for the first three cycles), at a current density of 5000 mA / g. 1 represents charging, and 2 represents discharging. Detailed implementation method

[0029] Detailed implementation method 1: A preparation method of a hollow core-shell nano-silicon carbon composite material for the negative electrode of a lithium-ion battery, which is carried out according to the following steps:

[0030] I. Preparation of hollow core-shell Si@C:

[0031] ① Add cobalt nitrate hexahydrate and 2-methylimidazole to methanol, ultrasonically disperse and mix them, then stir for 1 h to 12 h, then age at room temperature, and finally centrifuge and wash to collect the precipitate to obtain ZIF-67.

[0032] ② Add ZIF-67 to ethanol, ultrasonically disperse and mix it, then add distilled water, tetraethyl orthosilicate and ammonia water and stir for 1 h to 12 h, and finally centrifuge and wash to collect the precipitate to obtain ZIF-67 with in-situ grown SiO2.

[0033] ③ Add ZIF-67 with in-situ grown SiO2 to ethanol, ultrasonically disperse and mix it, then add resorcinol, ammonia water and formaldehyde and stir for 1 h to 12 h, and finally centrifuge and wash to collect the precipitate to obtain ZIF-67@SiO2 with in-situ grown phenolic resin.

[0034] ④ Add ZIF-67@SiO2 with in-situ grown phenolic resin to ethanol, ultrasonically disperse and mix it, then add hydrochloric acid and ultrasonicate, and finally centrifuge and wash to collect the precipitate and dry it under vacuum to obtain hollow core-shell SiO2@RF after etching the internal ZIF-67.

[0035] ⑤ Under the condition of Ar atmosphere and a temperature of 600 °C to 800 °C, calcine the hollow core-shell SiO2@RF after etching the internal ZIF-67 for 1 h to 6 h to obtain hollow core-shell SiO2@C.

[0036] ⑥ Mix the hollow core-shell SiO₂@C, Mg powder and NaCl, and pyrolyze them for 1 h to 6 h under the conditions of an Ar / H₂ atmosphere and a temperature of 600 °C to 800 °C to obtain a crude hollow core-shell Si@C product;

[0037] II. Treatment of the crude hollow core-shell Si@C product:

[0038] Centrifuge the crude hollow core-shell Si@C product successively with absolute ethanol, hydrochloric acid, deionized water, hydrofluoric acid and absolute ethanol, and finally dry it to obtain a hollow core-shell Si@C composite material.

[0039] Figure 1 This is the preparation flow chart of the hollow core-shell nano-silicon carbon composite material for the negative electrode of the lithium-ion battery of the present invention; First, stir a certain amount of cobalt nitrate hexahydrate and 2-methylimidazole to synthesize size-controllable ZIF-67 as the inner core, endowing each nanoparticle with a dodecahedron structure. Then, In situ generate a silica (SiO₂) layer on ZIF-67 through a process, and in situ generate a phenolic resin (RF) layer on the silica layer through the polycondensation reaction between resorcinol and formaldehyde to form ZIF-67@SiO₂@RF. Then etch the internal ZIF-67 core with hydrochloric acid, and carbonize the phenolic resin layer under argon protection to obtain a hollow core-shell SiO₂@C with a hollow dodecahedron structure; Finally, through the magnesiothermic reaction:

[0040] 2Mg(g) + SiO₂(s) = Si(s) + 2MgO(s);

[0041] Reduce the silica layer inside the structure, and then centrifuge the reduced black powder successively with absolute ethanol, hydrochloric acid, deionized water, hydrofluoric acid and absolute ethanol, and dry it to obtain the final product, a silicon carbon composite material with a hollow core-shell structure.

[0042] The beneficial effects of this embodiment are:

[0043] 1. Different from the general coating of traditional silicon-carbon materials that will separate and fall off during the lithiation process, this embodiment realizes a double-layer in-situ growth method, realizing a silicon-carbon composite material with deep binding. The composite material synthesized by this method not only improves the conductivity of the silicon-based material, but also makes it difficult for silicon and carbon to separate during the lithiation process, thereby achieving cycle stability.

[0044] 2. This embodiment designs a unique hollow core-shell structured silicon-carbon composite structure. The outer carbon shell is used to enhance electron and ion transport and act as a stress buffer, and the internal silicon nanoparticles are firmly attached to the carbon shell, effectively preventing separation and pulverization during lithiation and delithiation processes. This structure can protect the internal silicon nanodots from direct contact with the electrolyte and provide a sealed space for the inward contraction of silicon during lithium storage, which is beneficial to the formation of a stable SEI layer and effectively avoids the pulverization of silicon particles.

[0045] 3. The material prepared in this embodiment has excellent cycle stability due to its innovative preparation method and unique structure. After 250 cycles at a current density of 200 mA / g, its reversible capacity is 927.1 mAh g -1 , and after 800 cycles at a current density of 1000 mA / g, its reversible capacity is 883 mAh g -1 , and the ultra-long cycle performance at high current is a very important characteristic of the electrode material. After 3000 cycles at a high current density of 5000 mA / g, the material in this embodiment still maintains a reversible capacity of 713 mAh g -l , and the capacity retention rate is over 85%, proving excellent battery performance.

[0046] The main innovation of this embodiment is to prepare a negative electrode material with a unique hollow core-shell structured silicon-carbon composite structure through a two-layer in-situ growth method, and this material exhibits excellent cycle stability.

[0047] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that: in step ① of step one, the mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 1:(6 - 12); in step ① of step one, the mass of cobalt nitrate hexahydrate to the volume of methanol is 1 g:(90 - 100) mL; in step ① of step one, under the condition of a rotation speed of 200 rpm - 600 rpm, stir for 1 h - 12 h; in step ① of step one, age at room temperature for 12 h - 24 h. Others are the same as Specific Embodiment 1.

[0048] Specific Embodiment 3: The difference between this embodiment and one of Specific Embodiments 1 or 2 is as follows: In step 1②, the mass-to-volume ratio of ZIF-67 to ethanol is 1 g:(40 - 60) mL; the mass-to-volume ratio of ZIF-67 to distilled water is 1 g:(1 - 5) mL; the mass-to-volume ratio of ZIF-67 to tetraethyl orthosilicate is 1 g:(1 - 5) mL; the mass-to-volume ratio of ZIF-67 to ammonia water is 1 g:(1 - 5) mL; the mass percentage of ammonia water in step 1② is 20% - 40%; in step 1②, under the condition of a rotation speed of 200 rpm - 600 rpm, stirring is carried out for 1 h - 12 h. Others are the same as Specific Embodiment 1 or 2.

[0049] Specific Embodiment 4: The difference between this embodiment and one of Specific Embodiments 1 to 3 is as follows: In step 1③, the mass-to-volume ratio of ZIF-67@SiO₂ with in-situ grown SiO₂ to ethanol is 1 g:(40 - 60) mL; the mass ratio of ZIF-67@SiO₂ with in-situ grown SiO₂ to resorcinol is 1:(0.02 - 0.08); the mass-to-volume ratio of ZIF-67@SiO₂ with in-situ grown SiO₂ to ammonia water is 1 g:(0.1 - 0.4) mL; the mass-to-volume ratio of ZIF-67@SiO₂ with in-situ grown SiO₂ to formaldehyde is 1 g:(0.02 - 0.08) mL; the mass percentage of ammonia water in step 1③ is 20% - 40%; in step 1③, under the condition of a rotation speed of 200 rpm - 600 rpm, stirring is carried out for 1 h - 12 h. Others are the same as Specific Embodiments 1 to 3.

[0050] Specific Embodiment 5: The difference between this embodiment and one of Specific Embodiments 1 to 4 is as follows: In step 1④, the mass-to-volume ratio of ZIF-67@SiO₂@Phenolic resin with in-situ grown phenolic resin to ethanol is 1 g:(40 - 60) mL; the mass-to-volume ratio of ZIF-67@SiO₂@Phenolic resin with in-situ grown phenolic resin to hydrochloric acid is 1 g:(1 - 2) mL; the concentration of hydrochloric acid in step 1④ is 0.1 M - 0.5 M; in step 1④, hydrochloric acid is added, and under the condition of an ultrasonic power density of 10 W / L - 20 W / L, ultrasonic treatment is carried out for 5 min - 10 min; the vacuum drying in step 1④ is specifically carried out under the condition of a temperature of 40 °C - 80 °C for 1 h - 12 h. Others are the same as Specific Embodiments 1 to 4.

[0051] Specific Embodiment 6: The difference between this embodiment and one of Specific Embodiments 1 to 5 is as follows: The ultrasonic dispersion and mixing described in steps 1① to 1④ are specifically carried out under the condition of an ultrasonic power density of 10 W / L - 20 W / L for 5 min - 10 min. Others are the same as Specific Embodiments 1 to 5.

[0052] Embodiment VII: The difference between this embodiment and any one of Embodiments I - VI is as follows: The centrifugal washing and collecting of the precipitate described in Steps ① - ④ is specifically carried out under the condition of a rotational speed of 4000 rpm to 6000 rpm, using ethanol for centrifugal washing for 4 min to 10 min, and repeating the centrifugal washing 1 to 6 times. Others are the same as those in Embodiments I - VI.

[0053] Embodiment VIII: The difference between this embodiment and any one of Embodiments I - VII is as follows: In Step ⑤, under an Ar atmosphere, the temperature is raised to 600 °C to 800 °C at a heating rate of 2 °C / min to 10 °C / min. Others are the same as those in Embodiments I - VII.

[0054] Embodiment IX: The difference between this embodiment and any one of Embodiments I - VIII is as follows: In Step ⑥, the mass ratio of the hollow core - shell - SiO₂@C to the Mg powder is 1:(1 - 3); the mass ratio of the hollow core - shell - SiO₂@C to the NaCl is 1:(1 - 15); the volume percentage of Ar gas in the Ar / H₂ atmosphere is 90% - 95%; in the Ar / H₂ atmosphere, the temperature is raised to 600 °C to 800 °C at a heating rate of 2 °C / min to 10 °C / min. Others are the same as those in Embodiments I - VIII.

[0055] Embodiment X: The difference between this embodiment and any one of Embodiments I - IX is as follows: The concentration of the hydrochloric acid described in Step ② is 0.1 M to 0.5 M; the concentration of the hydrofluoric acid described in Step ② is 0.1 M to 0.5 M; the centrifugation treatment described in Step ② is specifically carried out under the condition of a rotational speed of 4000 rpm to 6000 rpm for 4 min to 10 min respectively. Others are the same as those in Embodiments I - IX.

[0056] The following examples are used to verify the beneficial effects of the present invention:

[0057] Example 1:

[0058] A preparation method of a hollow core - shell nanosilicon - carbon composite material for the negative electrode of a lithium - ion battery, which is carried out according to the following steps:

[0059] I. Preparation of hollow core - shell - Si@C:

[0060] ① Cobalt(II) nitrate hexahydrate and 2 - methylimidazole are added to methanol, and under the condition of an ultrasonic power density of 15 W / L, ultrasonic dispersion is carried out for 10 min, then under the condition of a rotational speed of 250 rpm, stirring is carried out for 1 h, then aging is carried out at room temperature for 24 h, and finally under the condition of a rotational speed of 4000 rpm, centrifugal washing is carried out with ethanol for 5 min, and the centrifugal washing is repeated 3 times to collect the precipitate, obtaining ZIF - 67;

[0061] The mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 1:11; the mass ratio of cobalt nitrate hexahydrate to the volume of methanol is 1 g:92 mL;

[0062] ② Add ZIF-67 to ethanol, and under the condition of an ultrasonic power density of 15 W / L, ultrasonically disperse for 10 min. Then add distilled water, tetraethyl orthosilicate, and ammonia water, and under the condition of a rotation speed of 250 rpm, stir for 6 h. Finally, under the condition of a rotation speed of 4000 rpm, use ethanol to centrifugally wash for 5 min, and repeat the centrifugal washing 3 times. Collect the precipitate to obtain ZIF-67 with in-situ grown SiO2;

[0063] The mass ratio of ZIF-67 to the volume of ethanol is 1 g:50 mL; the mass ratio of ZIF-67 to the volume of distilled water is 1 g:1 mL; the mass ratio of ZIF-67 to the volume of tetraethyl orthosilicate is 1 g:1 mL; the mass ratio of ZIF-67 to the volume of ammonia water is 1 g:2 mL; the mass percentage of ammonia water is 27%;

[0064] ③ Add ZIF-67 with in-situ grown SiO2 to ethanol, and under the condition of an ultrasonic power of 15 W / L, ultrasonically disperse for 10 min. Then add resorcinol, ammonia water, and formaldehyde, and under the condition of a rotation speed of 250 rpm, stir for 12 h. Finally, under the condition of a rotation speed of 4000 rpm, use ethanol to centrifugally wash for 5 min, and repeat the centrifugal washing 3 times. Collect the precipitate to obtain ZIF-67@SiO2 with in-situ grown phenolic resin;

[0065] The mass ratio of ZIF-67 with in-situ grown SiO2 to the volume of ethanol is 1 g:50 mL; the mass ratio of ZIF-67 with in-situ grown SiO2 to resorcinol is 1:0.05; the mass ratio of ZIF-67 with in-situ grown SiO2 to the volume of ammonia water is 1 g:0.2 mL; the mass ratio of ZIF-67 with in-situ grown SiO2 to the volume of formaldehyde is 1 g:0.05 mL; the mass percentage of ammonia water is 27%;

[0066] ④ Add ZIF-67@SiO2 with in-situ grown phenolic resin to ethanol, and under the condition of an ultrasonic power of 15 W / L, ultrasonically disperse for 10 min. Then add hydrochloric acid, and under the condition of an ultrasonic power of 15 W / L, ultrasonically treat for 10 min. Then, under the condition of a rotation speed of 4000 rpm, use ethanol to centrifugally wash for 5 min, and repeat the centrifugal washing 3 times. Collect the precipitate, and finally, under the condition of a temperature of 60 °C, vacuum dry for 12 h to obtain hollow core-shell - SiO2@RF after etching the internal ZIF-67;

[0067] The mass-to-volume ratio of the in-situ grown phenolic resin-coated ZIF-67@SiO2 to ethanol is 1 g:50 mL; the mass-to-volume ratio of the in-situ grown phenolic resin-coated ZIF-67@SiO2 to hydrochloric acid is 1 g:1 mL; the concentration of the hydrochloric acid is 0.1 M;

[0068] ⑤ Under an Ar atmosphere, heat up to 700 °C at a heating rate of 5 °C / min. Under the conditions of an Ar atmosphere and a temperature of 700 °C, calcine the hollow core-shell SiO2@RF after etching the internal ZIF-67 for 3 h to obtain hollow core-shell SiO2@C;

[0069] ⑥ Mix the hollow core-shell SiO2@C, Mg powder, and NaCl. Under an Ar / H2 atmosphere, heat up to 680 °C at a heating rate of 5 °C / min, and then pyrolyze for 6 h under the conditions of an Ar / H2 atmosphere and a temperature of 680 °C to obtain a crude hollow core-shell Si@C product;

[0070] The mass ratio of the described hollow core-shell SiO2@C to Mg powder is 1:1.2; the mass ratio of the hollow core-shell SiO2@C to NaCl is 1:14; the volume percentage of Ar gas in the Ar / H2 atmosphere is 95%;

[0071] II. Treatment of the crude hollow core-shell Si@C product:

[0072] Centrifuge the crude hollow core-shell Si@C product successively with absolute ethanol, hydrochloric acid, deionized water, hydrofluoric acid, and absolute ethanol, and finally dry it to obtain a hollow core-shell Si@C composite material;

[0073] The concentration of the hydrochloric acid is 0.1 M; the concentration of the hydrofluoric acid is 0.1 M; the specific centrifugation treatment is carried out at a rotation speed of 4000 rpm for 5 min respectively.

[0074] Figure 2 XRD pattern of the hollow core-shell Si@C composite material prepared in Step 2 of Example 1; According to the XRD pattern, all Si characteristic diffraction peaks at 28.3°, 47.3°, and 56.2° can be observed, corresponding to the (111), (220), and (311) planes of Si respectively, which indicates the successful synthesis of silicon with good crystallinity.

[0075] Figure 3 XPS full spectrum of the hollow core-shell Si@C composite material prepared in Step 2 of Example 1; According to the XPS pattern, obvious peaks corresponding to C1s, Si 2s, and Si 2p are shown, proving the successful synthesis of a silicon-carbon composite by this method.

[0076] Figure 4 SEM image of the hollow core-shell Si@C composite material prepared in Step 2 of Example 1. The scale bar is 500 nm. As can be seen from the figure, the hollow core-shell Si@C composite material maintains the integrity and uniform dispersion of the dodecahedron.

[0077] Figure 5 SEM image of the hollow core-shell Si@C composite material prepared in Step 2 of Example 1. The scale bar is 100 nm. As can be seen from the figure, a distinct hollow core-shell structure was prepared by the double-layer in-situ growth method, with the silicon layer as the core and the amorphous carbon layer as the shell.

[0078] Electrochemical tests (the instrument for electrochemical tests is the LANHE CT2001A battery test system produced by Wuhan Blue Electric Co., Ltd., and the test voltage range is 0.005 V to 3 V):

[0079] First, active material at a mass fraction of 60%, conductive agent at 20%, and binder at 20% are placed in a ceramic crucible. Among them, the active material is the hollow core-shell Si@C composite material prepared in Step 2 of Example 1, acetylene black is used as the conductive agent, and sodium carboxymethyl cellulose (CMC) is the binder. After adding a few drops of water (H2O) to obtain a viscous liquid, it is stirred with a rotor for 40 min to obtain the battery negative electrode active material slurry; the mass percentage of the active material in the battery negative electrode active material slurry is 60%. Subsequently, the battery negative electrode active material slurry is evenly coated on a copper foil that has been wiped clean and flat with anhydrous ethanol using a scraper, and dried in a vacuum drying oven at 60 °C for 3 h to obtain a dried electrode plate. The dried electrode plate is cut into circular plates with a diameter of 12 mm by a cutting machine to obtain negative electrode plates.

[0080] The negative electrode plate is sent into an argon glove box, and in sequence, a battery negative electrode shell, a lithium metal sheet (as the counter electrode), a separator (microporous polypropylene membrane), the dried electrode plate, a gasket, and a spring sheet are placed. Subsequently, the positive electrode battery shell is covered (2 - 3 drops of electrolyte need to be dropped between the two electrodes and the separator), and after assembly, it is sealed with a hydraulic sealer to obtain a sealed CR2032 type button battery. The electrolyte is prepared by fully compounding secondary electrolyte (LB-002) at a volume percentage of 95% and 5% of FEC (fluoroethylene carbonate). The sealed CR2032 type button battery is taken out of the argon glove box, left to stand for 12 h, and then the subsequent electrochemical performance tests are carried out at room temperature.

[0081] Figure 6The cyclic performance graph of the button battery prepared from the hollow core-shell Si@C composite material prepared in Step 2 of Example 1. 1 represents charging and 2 represents discharging. As can be seen from the figure, at a current density of 200 mA / g, the initial discharge capacity and charging capacity are 2776 mAh g -1 and 1498 mAh g -1 respectively. After 250 cycles, its reversible capacity is 927.1 mAh g -1 . During the lithiation process, traditional silicon materials will experience drastic volume changes, and silicon and carbon will fall off or separate, resulting in electrode fracture. This continuous mechanical decomposition damages the integrity of the electrode structure and ultimately manifests as a gradual decrease in battery capacity. However, the material prepared by the double-layer in-situ growth method not only improves the conductivity of Si but also makes it difficult for silicon and carbon to separate during the lithiation process. The hollow core-shell structure adjusts the expansion of silicon to inward contraction, effectively reducing the impact of repeated expansion of nanosilicon on the physical integrity of the electrode and ensuring its stability.

[0082] Figure 7 The cyclic performance graph of the button battery prepared from the hollow core-shell Si@C composite material prepared in Step 2 of Example 1 at a current density of 1000 mA / g (the first three cycles are activated at 200 mA / g). 1 represents charging and 2 represents discharging. As can be seen from the figure, after 800 cycles at a current density of 1000 mA / g, its reversible capacity is 883 mAh g -1 . Compared with the reversible capacity of 1142 mAh g in the fourth cycle -1 , the capacity retention rate is 77%.

[0083] Figure 8 The cyclic performance graph of the button battery prepared from the hollow core-shell Si@C composite material prepared in Step 2 of Example 1 at a current density of 5000 mA / g (the first three cycles are activated at 200 mA / g). 1 represents charging and 2 represents discharging. As can be seen from the figure, after 3000 cycles of operation at a high current density of 5000 mA / g, a reversible capacity of 713 mAh g -l is still maintained. Except for the first few cycles, the Coulomb efficiency of almost all cycles is close to 99%. In addition, the hollow core-shell Si@C also has a high capacity retention rate. After 3000 cycles, compared with the reversible capacity of 837 mAh g in the fourth cycle -1 , it can still be stable above 85%, proving excellent cyclic stability. This is because the hollow core-shell dodecahedron structure of the hollow core-shell Si@C formed by double-layer in-situ growth significantly improves the conductivity and structural stability and is an effective conductive scaffold. This configuration not only expands the availability of active sites for lithium-ion interaction but also significantly reduces the volume strain associated with the lithiation / delithiation process, thus achieving high capacity and excellent stability.

Claims

1. A method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery, characterized in that It is carried out in the following steps:

1. Preparation of hollow core-shell-Si@C: ① Add cobalt nitrate hexahydrate and 2-methylimidazole to methanol and mix by ultrasonic dispersion, then stir for 1 h to 12 h, age at room temperature, and finally collect the precipitate by centrifugation and washing to obtain ZIF-67; ② Add ZIF-67 to ethanol and mix by ultrasonic dispersion, then add distilled water, tetraethyl orthosilicate and ammonia water and stir for 1h to 12h, finally collect the precipitate by centrifugation and washing to obtain ZIF-67 with in-situ growth of SiO2; ③ Add the in-situ grown SiO2 ZIF-67 into ethanol and mix by ultrasonic dispersion, then add resorcinol, ammonia and formaldehyde and stir for 1h to 12h, finally collect the precipitate by centrifugation and washing to obtain the in-situ grown phenolic resin ZIF-67@SiO2; ④ Add the ZIF-67@SiO2 grown in situ on phenolic resin into ethanol and disperse and mix by ultrasonication, then add hydrochloric acid and ultrasonicate, finally collect the precipitate by centrifugation and vacuum drying to obtain the hollow core-shell-SiO2@RF after etching the internal ZIF-67; ⑤ In an Ar atmosphere at a temperature of 600°C to 800°C, the hollow core-shell-SiO2@RF after etching the internal ZIF-67 was calcined for 1h to 6h to obtain the hollow core-shell-SiO2@C; ⑥ Mix the hollow core-shell-SiO2@C, Mg powder and NaCl, and pyrolyze them for 1h to 6h in an Ar / H2 atmosphere at a temperature of 600°C to 800°C to obtain a crude hollow core-shell-Si@C product; 2. Treatment of Hollow Core-Shell-Si@C Crude Product: The hollow core-shell-Si@C crude product was centrifuged with anhydrous ethanol, hydrochloric acid, deionized water, hydrofluoric acid and anhydrous ethanol in sequence, and finally dried to obtain a hollow core-shell-Si@C composite material.

2. The method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery according to claim 1, characterized in that The mass ratio of the cobalt nitrate hexahydrate to 2-methylimidazole described in step 1① is 1:(6-12); the volume ratio of the cobalt nitrate hexahydrate described in step 1① to methanol is 1g:(90-100)mL; in step 1①, stirring is performed at a rotation speed of 200rpm-600rpm for 1h-12h; in step 1①, aging is performed at room temperature for 12h-24h.

3. The method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery according to claim 1, characterized in that The volume ratio of the ZIF-67 described in step 1② to ethanol is 1g:(40-60)mL; the volume ratio of the ZIF-67 described in step 1② to distilled water is 1g:(1-5)mL; the volume ratio of the ZIF-67 described in step 1② to tetraethyl orthosilicate is 1g:(1-5)mL; the volume ratio of the ZIF-67 described in step 1② to ammonia water is 1g:(1-5)mL; the mass percentage of ammonia water described in step 1② is 20%-40%; in step 1②, stirring is carried out at a rotation speed of 200rpm-600rpm for 1h-12h.

4. The method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery according to claim 1, characterized in that The volume ratio of the mass of the ZIF-67 in situ grown SiO2 described in step 1③ to ethanol is 1g:(40~60)mL; the mass ratio of the ZIF-67 in situ grown SiO2 described in step 1③ to resorcinol is 1:(0.02~0.08); the volume ratio of the mass of the ZIF-67 in situ grown SiO2 described in step 1③ to ammonia water is 1g:(0.1~0.4)mL; the volume ratio of the ZIF-67 in situ grown SiO2 described in step 1③ to formaldehyde is 1g:(0.02~0.08)mL; the mass percentage of ammonia water described in step 1③ is 20%~40%; in step 1③, stirring is carried out at a rotation speed of 200rpm~600rpm for 1h~12h.

5. The method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery according to claim 1, characterized in that The volume ratio of the mass of the ZIF-67@SiO2 in situ grown phenolic resin described in step 1④ to ethanol is 1g:(40~60)mL; the volume ratio of the mass of the ZIF-67@SiO2 in situ grown phenolic resin described in step 1④ to hydrochloric acid is 1g:(1~2)mL; the concentration of the hydrochloric acid described in step 1④ is 0.1M~0.5M; hydrochloric acid is added to step 1④, and ultrasonic power density is 10W / L~20W / L, and ultrasonic is carried out for 5min~10min; vacuum drying described in step 1④ is specifically vacuum drying at a temperature of 40℃~80℃ for 1h~12h.

6. The method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery according to claim 1, characterized in that The ultrasonic dispersion mixing described in step 1 ① to ④ is specifically carried out under the condition of ultrasonic power density of 10W / L to 20W / L, and ultrasonic dispersion is carried out for 5min to 10min.

7. The method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery according to claim 1, characterized in that The centrifugal washing and collection of the precipitate described in steps 1 to 4 is specifically carried out at a rotation speed of 4000 rpm to 6000 rpm, using ethanol for centrifugal washing for 4 min to 10 min, and repeating the centrifugal washing 1 to 6 times.

8. The method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery according to claim 1, characterized in that In step 1⑤, under Ar atmosphere, the temperature is increased to 600°C to 800°C at a heating rate of 2°C / min to 10°C / min.

9. The method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery according to claim 1, characterized in that The mass ratio of the hollow core-shell-SiO2@C described in step 1⑥ to Mg powder is 1:(1~3); the mass ratio of the hollow core-shell-SiO2@C described in step 1⑥ to NaCl is 1:(1~15); the volume percentage of Ar gas in the Ar / H2 atmosphere described in step 1⑥ is 90%~95%; in step 1⑥, under Ar / H2 atmosphere, the temperature is increased to 600℃~800℃ at a heating rate of 2℃ / min~10℃ / min.

10. The method for preparing a hollow core-shell nano silicon-carbon composite material for a negative electrode of a lithium ion battery according to claim 1, characterized in that The concentration of the hydrochloric acid in step 2 is 0.1M to 0.5M; the concentration of the hydrofluoric acid in step 2 is 0.1M to 0.5M; the centrifugal treatment in step 2 is specifically carried out at a rotation speed of 4000rpm to 6000rpm for 4min to 10min respectively.

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