Core-shell silicon-carbon anode materials and their preparation methods

Abstract

This invention relates to a core-shell silicon-carbon anode material and its preparation method. The core-shell silicon-carbon anode material comprises, from the inside out, a silicon core, a TiO2 transition buffer layer, and a ZIF-8 derived carbon shell. The silicon core consists of micron-sized silicon particles with porous surfaces. The TiO2 transition buffer layer is formed by atomic layer deposition (ALD). The ZIF-8 derived carbon shell is a nitrogen-doped porous carbon shell formed by carbonizing ZIF-8. This invention's core-shell silicon-carbon anode material, through the synergistic effect of its three-layer structure, controls the volume expansion rate to within 60%, solving the problem of expansion >100% in existing technologies. The ZIF-8 derived carbon has a conductivity ≥200 S / cm, eliminating the need for excessive conductive agents, increasing the volumetric energy density by 20%. The TiO2 layer isolates side reactions, and the nitrogen-doped carbon induces stable SEI film, resulting in an initial coulombic efficiency of over 85% with no significant decay after 1000 cycles. The ALD and MOFs carbonization processes can be integrated into existing production lines in stages, and the absence of highly toxic reagents (such as HF) makes it suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of carbon anode material technology, specifically to core-shell silicon-carbon anode materials and their preparation methods. Background Technology

[0002] Lithium-ion batteries typically consist of four main parts: a positive electrode, a negative electrode, a separator, and an electrolyte. The positive and negative electrodes are where the charge carriers store and release energy in a lithium-ion battery. The separator physically separates the two electrodes to prevent them from connecting and causing an internal short circuit, while allowing lithium to flow through the separator to the electrodes.

[0003] Lithium-ion battery anode materials can be mainly divided into three categories: intercalation materials, conversion materials, and alloys. In intercalation materials (such as graphite), lithiation occurs by inserting lithium ions into interstitial sites in the anode material's crystal lattice; in conversion materials (transition metal compounds), the conversion reaction is based on the substitution of transition metals by lithium ions during lithiation; in the third type of anode material, lithiation occurs through alloying between lithium and the electrode material.

[0004] Graphite is currently the most widely used anode material in lithium-ion batteries, but its theoretical capacity is limited to 372 mAh / g, hindering its application in the emerging industry of high-energy lithium-ion batteries. Replacing traditional graphite with high-capacity materials is the most promising way to improve the energy density of lithium-ion batteries. Among them, silicon-lithium alloys can achieve a specific capacity of 4200 mAh / g, significantly higher than other anode materials. In addition, silicon also has a relatively low operating voltage (<0.4V vs. Li / Li). + Silicon, with its abundant and readily available raw materials, is considered the most promising anode material for applications. However, when silicon is used in lithium-ion batteries, the alloying reaction with lithium produces significant volume expansion, exceeding 300%. This expansion causes silicon particles to break down, losing electrical contact with the electrode. When silicon particles break down or even pulverize, the existing solid-electrolyte interface (SEI) is destroyed, and a new SEI film continuously forms. This process not only consumes electrolyte, wasting lithium ions, but also increases the thickness of the SEI film, affecting the transport of lithium ions and electrons. Therefore, the electrochemical performance of silicon anode batteries degrades rapidly during use. To address these issues, carbon materials, due to their excellent conductivity (~10² S·cm), are considered a promising alternative. -1The high mechanical strength and properties of carbon perfectly compensate for the shortcomings of silicon materials in lithium-ion batteries, making carbon materials the ideal matrix for improving these defects. In silicon / carbon (Si / C) composite anode materials, nanoscale silicon particles are uniformly dispersed in a carbon matrix. This structural design not only effectively inhibits the excessive growth of the SEI film and reduces capacity loss caused by repeated SEI film formation and rupture, but also uses the carbon matrix as a buffer layer to absorb the mechanical stress generated by silicon during lithiation, suppressing the volume expansion of silicon particles and preventing particle breakage and pulverization. The continuous conductive network formed by the carbon material provides a fast channel for the transport of electrons and lithium ions, ensuring the smooth progress of electrochemical reactions. The carbon matrix also effectively prevents the agglomeration of silicon particles during cycling, maintaining the structural stability and cycling performance of the material. By combining the advantages of silicon and carbon, Si / C composite anode materials effectively solve the key problems faced by silicon anode materials, exhibiting excellent electrochemical performance and broad application prospects.

[0005] Using silicon-carbon composite materials as the anode in lithium-ion batteries has shown in the laboratory that it can suppress the negative effects of silicon expansion to some extent. However, currently used silicon-carbon anode lithium-ion batteries still face significant technological barriers, high manufacturing costs, unsatisfactory expansion rates, and short battery lifespans compared to commercially available graphite anode lithium-ion batteries. These issues prevent the large-scale application of silicon-carbon anode lithium-ion batteries in practical engineering. From a fundamental research perspective, structural design that simultaneously mitigates volume changes, addresses the unstable SEI problem, and improves conductivity remains a significant challenge. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention aims to overcome the deficiencies of insufficient volume expansion control, discontinuous conductive network, and weak interfacial bonding in silicon-based anodes. It provides a core-shell material consisting of a silicon core, a TiO2 transition buffer layer, and a ZIF-8 derived carbon shell, along with its preparation method, thereby achieving simultaneous improvements in high capacity, long cycle life, and high rate performance.

[0007] The present invention solves the technical problem by adopting the following technical solution:

[0008] This invention provides a core-shell silicon-carbon anode material, which consists of a silicon core, a TiO2 transition buffer layer, and a ZIF-8 derived carbon shell layer, from the inside out.

[0009] The silicon core is a micron-sized silicon particle with a porous surface;

[0010] The TiO2 transition buffer layer is formed by atomic layer deposition (ALD);

[0011] The ZIF-8 derived carbon shell is a nitrogen-doped porous carbon shell formed by carbonizing ZIF-8.

[0012] Preferably, the silicon core has a particle size of 1-3 μm, a surface nanopore size of 50-100 nm, and a porosity of 20-30%.

[0013] Preferably, the thickness of the TiO2 transition buffer layer is 20-30 nm, and the density is ≥95%.

[0014] Preferably, the ZIF-8 derived carbon shell contains 5-8 wt% ZnO nanoparticles, has a thickness of 30-50 nm, a pore size of 2-5 nm, and is nitrogen-doped by 4-6 at.

[0015] In addition, the present invention also provides a method for preparing the above-mentioned core-shell silicon-carbon anode material, comprising the following steps:

[0016] S1: Preparation of silicon cores

[0017] S101: After dispersing microsilicon powder in NaOH solution and stirring for alkaline etching, wash with deionized water until neutral, and then vacuum dry to obtain a porous silicon core on the surface;

[0018] S2: Preparation of TiO2 transition buffer layer

[0019] S201: Using tetraisopropoxy titanium as a precursor and deionized water as an oxidant, a TiO2 layer is formed on the silicon core surface by alternating pulses of tetraisopropoxy titanium and deionized water through atomic layer deposition (ALD) in a cyclic reaction, thus obtaining a Si@TiO2 intermediate.

[0020] S3: Preparation of ZIF-8 derived carbon shell

[0021] S301: The Si@TiO2 intermediate was dispersed in methanol, ultrasonically treated, dissolved in Zn(NO3)2, and then 2-methylimidazolium ligand was added and stirred.

[0022] S302: The solution treated in step S301 is centrifuged and washed with methanol to remove unreacted Zn. 2+ The Si@TiO2@ZIF-8 precursor was prepared by vacuum drying with 2-methylimidazolium ligand.

[0023] S303: The Si@TiO2@ZIF-8 target material is obtained by heating, holding and cooling the Si@TiO2@ZIF-8 precursor under Ar gas protection.

[0024] Preferably, in step S101, the concentration of the NaOH solution is 1.5-2.5 mol / L, the stirring temperature is 70-90℃, the stirring time is 3.5-4.5 h, the vacuum drying temperature is 55-65℃, and the vacuum drying time is 7-9 h.

[0025] Preferably, in step S201, the alternating pulse duration of tetraisopropoxy titanium and deionized water is 0.1s for tetraisopropoxy titanium and 0.1s for deionized water, the temperature of the cyclic reaction is 130-160℃, and the number of cycles is 50-80.

[0026] Preferably, in step S301, the mass ratio of the Si@TiO2 intermediate to Zn(NO3)2 is (3-4):1; the Zn 2 + :2-Methylimidazole = 1:2.

[0027] Preferably, in step S302, the vacuum drying temperature is 55-65℃ and the vacuum drying time is 11-13h.

[0028] Preferably, in step S303, the heating rate is 2-4℃ / min, the holding temperature is 700-900℃, the holding time is 2.5-3.5h, and the temperature is allowed to cool naturally.

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

[0030] (1) The core-shell silicon-carbon anode material of the present invention controls the volume expansion rate to within 60% through the synergistic effect of the three-layer structure, solving the problem of expansion >100% in the prior art. The conductivity of ZIF-8 derived carbon is ≥200S / cm, no excessive conductive agent is required, the volume energy density is increased by 20%, the TiO2 layer isolates the side reaction, the nitrogen doped carbon induces the stabilization of the SEI film, the initial coulombic efficiency is increased to more than 85%, and there is no significant decay after 1000 cycles. The ALD and MOFs carbonization process can be integrated into existing production lines in steps, and there are no highly toxic reagents (such as HF), which is suitable for large-scale production.

[0031] (2) The preparation method of the core-shell silicon-carbon anode material of the present invention adopts a three-step method of "alkali etching to create pores - ALD coating - MOFs carbonization". Alkali etching controls the uniformity of nanopores and avoids capacity loss caused by excessive etching; ALD self-limiting growth ensures that the thickness deviation of TiO2 layer is ≤2nm, covering the silicon core surface and the inner wall of the pores; ZIF-8 in-situ growth and carbonization synergistically regulate the porosity of carbon layer and ZnO dispersion.

[0032] (3) This invention uses micron-sized silicon as the silicon core. Compared with nano-sized silicon (larger specific surface area, more lithium consumption in SEI film), micron-sized silicon reduces specific surface area and electrolyte side reactions, while providing high theoretical capacity. The nanopores etched by alkaline etching have good uniformity. The pores reserve elastic space for volume expansion during silicon lithium intercalation, disperse stress concentration, shorten the lithium ion diffusion path, alleviate high-rate polarization, and the rough porous surface significantly improves the chemical anchoring force of the subsequent TiO2 layer (such as the formation of Si-O-Ti bonds). The pores accelerate ion diffusion, and the capacity retention rate is significantly improved under high current density. The alkaline etching parameters (temperature, time, NaOH concentration) are strictly controlled to avoid excessive dissolution of silicon particles leading to capacity loss. No harmful etchant is used. The process is compatible with existing lithium battery production lines, and the production cost is controllable. Low-temperature vacuum drying prevents surface oxidation and maintains the cleanliness of the pores.

[0033] (4) The TiO2 film completely covers the silicon core surface and the inner wall of the nanopores, densely isolating electrolyte penetration. TiO2 is chemically bonded to the silicon core through Si-O-Ti chemical bonds, providing rigid mechanical support. The high Young's modulus (~200 GPa) of TiO2 inhibits silicon lithiation expansion and avoids electrode structure collapse. The micron-sized silicon core + nanopores + TiO2 transition buffer layer reduces the overall expansion rate to a controllable range. The stable TiO2 surface induces a uniform thin SEI film (thickness fluctuation ±5 nm), reducing lithium consumption. Furthermore, TiO2 also has ionic conductivity (Li... + The TiO2 transition buffer layer provides mechanical support and interface stability, balancing charge transport and expansion buffer requirements. This ensures the integrity of the electrode structure for more than 1000 cycles, resulting in a capacity retention rate significantly higher than that of traditional silicon anodes. The rigid TiO2 layer disperses and transfers the expansion stress of the silicon core to the ZIF-8 carbon shell. With the synergy of the composite shell, the overall expansion rate is ≤60%, reducing direct contact between silicon and electrolyte. This reduces side reactions, resulting in a thinner and more uniform SEI film. The initial coulombic efficiency and cycle stability are both improved. The self-limiting growth of ALD ensures uniform layer thickness, making it particularly suitable for complex porous silicon core surfaces. The low-temperature process (130-160℃) avoids excessive oxidation of the silicon core or collapse of the pore structure.

[0034] (5) The ZIF-8 derived carbon shell retains the inherent micropores (2-5 nm) of ZIF-8 and the mesopores generated by the decomposition of MOFs after carbonization, providing a buffer space for silicon expansion. Nitrogen doping improves electronic conductivity, and ZnO catalyzes carbon graphitization for further optimization. The carbon layer is chemically bonded to the TiO2 layer through Zn-O-Ti bonds, enhancing the overall structural stability of the core and shell. The elastic deformation of porous carbon accommodates the expansion stress transmitted by the TiO2 layer. The rigid constraint of TiO2 and the flexible deformation of the carbon layer make the electrode volume expansion rate ≤60%. Porous carbon shortens the ion diffusion path, and the capacity retention rate is ≥65% at 4A / g. The continuous carbon network realizes an efficient conductive pathway from the silicon core to the current collector. The chemical groups on the carbon surface (pyridine nitrogen, etc.) induce a stable SEI film and reduce lithium consumption. The high conductivity network maintains the electrical contact of the active material. The multi-level channels alleviate the accumulation of expansion stress, so that conductivity and structural stability are both taken into account. The ZIF-8 synthesis ratio (Zn 2+ The ratio of 2-methylimidazole to 1:2 ensures uniform crystal growth. The carbonization temperature (700-900℃) balances the degree of carbon graphitization and controls silicon volatilization to avoid silicon loss. The ZIF-8 precursor is inexpensive and readily available, and the process is green (no HF / strong acid is used), making it suitable for large-scale production. Attached Figure Description

[0035] Figure 1 The XRD pattern of the sample prepared in Example 1 of this invention. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to specific examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Example 1.

[0038] The core-shell silicon-carbon anode material of this embodiment consists of a silicon core, a TiO2 transition buffer layer, and a ZIF-8 derived carbon shell layer, from the inside to the outside.

[0039] The silicon core consists of micron-sized silicon particles with porous surfaces. The particle size of the silicon core is 1-3 μm, and the surface nanopores have a pore size of 50-80 nm and a porosity of 25%.

[0040] The TiO2 transition buffer layer was formed by atomic layer deposition (ALD), with a thickness of 25 nm and a density of 96%.

[0041] The ZIF-8 derived carbon shell is a nitrogen-doped porous carbon shell formed by carbonizing ZIF-8. The ZIF-8 derived carbon shell contains 6 wt% ZnO nanoparticles, has a thickness of 40 nm, a pore size of 2-5 nm, and a nitrogen doping amount of 6 at.

[0042] Preparation method of core-shell silicon-carbon anode material:

[0043] S1: Preparation of silicon cores

[0044] S101: 20g of microsilica powder was dispersed in 200ml of 2mol / L NaOH solution and stirred on a magnetic stirrer at 80℃ for 4h at a stirring rate of 500r / min. After the reaction, the mixture was repeatedly washed with deionized water until the pH of the filtrate was 7 to remove residual NaOH and silicate byproducts. The product was then placed in a vacuum drying oven and dried under vacuum at 60℃ for 8h to obtain porous silicon cores. Scanning electron microscopy showed that the surface nanopores had a pore size of 50-100nm and a porosity of 25%.

[0045] S2: Preparation of TiO2 transition buffer layer

[0046] S201: A silicon nucleus was placed in an atomic layer deposition (ALD) reaction chamber, evacuated to 1 Pa, and heated to 150 °C. Using tetraisopropoxy titanium as a precursor and deionized water as an oxidant, tetraisopropoxy titanium and deionized water were alternately pulsed onto the silicon nucleus surface by atomic layer deposition (ALD) for 0.1 s. After each pulse, Ar gas was used to purge for 30 s to remove unreacted precursors. This process was repeated 50 times to form a 25 nm thick TiO2 layer on the silicon nucleus surface, thus obtaining a Si@TiO2 intermediate. X-ray fluorescence spectroscopy analysis showed that the TiO2 purity was ≥99% and the density was 96%.

[0047] S3: Preparation of ZIF-8 derived carbon shell

[0048] S301: Disperse 10g of Si@TiO2 intermediate in 200ml of methanol and sonicate for 30min (300W) to form a uniform suspension; add 3g of Zn(NO3)2 and 2.4g of 2-methylimidazole ligand and stir at room temperature for 12h to allow ZIF-8 to grow in situ on the surface of TiO2 layer.

[0049] S302: The solution treated in step S301 was centrifuged, washed three times with methanol to remove unreacted zinc ions and ligands, and then vacuum dried at 60℃ for 12 h to obtain the Si@TiO2@ZIF-8 precursor.

[0050] S303: The Si@TiO2@ZIF-8 precursor was heated to 800℃ at a rate of 3℃ / min under Ar protection and held for 3h for carbonization. After natural cooling, a core-shell silicon-carbon anode material was obtained. The ZIF-8 derived carbon shell was 40nm thick with a pore size of 2-5nm containing 6wt% ZnO nanoparticles and 6at nitrogen doping.

[0051] Example 2.

[0052] The core-shell silicon-carbon anode material of this embodiment consists of a silicon core, a TiO2 transition buffer layer, and a ZIF-8 derived carbon shell layer, from the inside to the outside.

[0053] The silicon core consists of micron-sized silicon particles with porous surfaces. The particle size of the silicon core is 1-3 μm, and the surface nanopores have a pore size of 70-100 nm and a porosity of 30%.

[0054] The TiO2 transition buffer layer was formed by atomic layer deposition (ALD), with a thickness of 30 μm and a density of 98%.

[0055] The ZIF-8 derived carbon shell is a nitrogen-doped porous carbon shell formed by carbonizing ZIF-8. The ZIF-8 derived carbon shell contains 8wt% ZnO nanoparticles, has a thickness of 50nm, a pore size of 2-5nm, and a nitrogen doping amount of 4at.

[0056] Preparation method of core-shell silicon-carbon anode material:

[0057] S1: Preparation of silicon cores

[0058] S101: 20g of microsilica powder was dispersed in 200ml of 1.5mol / L NaOH solution and stirred on a magnetic stirrer at 90℃ for 4.5h at a stirring speed of 500r / min. After the reaction, the mixture was repeatedly washed with deionized water until the pH of the filtrate was 7 to remove residual NaOH and silicate byproducts. The product was then placed in a vacuum drying oven and dried under vacuum at 55℃ for 9h to obtain porous silicon cores. Scanning electron microscopy showed that the surface nanopores had a pore size of 70-100nm and a porosity of 30%.

[0059] S2: Preparation of TiO2 transition buffer layer

[0060] S201: A silicon nucleus was placed in an atomic layer deposition (ALD) reaction chamber, evacuated to 1 Pa, and heated to 160 °C. Using tetraisopropoxy titanium as a precursor and deionized water as an oxidant, tetraisopropoxy titanium and deionized water were alternately pulsed onto the silicon nucleus surface by atomic layer deposition (ALD) for 0.1 s. After each pulse, Ar gas was used to purge for 30 s to remove unreacted precursors. This process was repeated 80 times to form a 30 nm thick TiO2 layer on the silicon nucleus surface, thus obtaining a Si@TiO2 intermediate. X-ray fluorescence spectroscopy analysis showed that the TiO2 purity was ≥99% and the density was 98%.

[0061] S3: Preparation of ZIF-8 derived carbon shell

[0062] S301: Disperse 10g of Si@TiO2 intermediate in 200ml of methanol and sonicate for 30min (300W) to form a uniform suspension; add 3.3g of Zn(NO3)2 and 2.64g of 2-methylimidazole ligand and stir at room temperature for 12h to allow ZIF-8 to grow in situ on the surface of TiO2 layer.

[0063] S302: The solution treated in step S301 was centrifuged, washed three times with methanol to remove unreacted zinc ions and ligands, and then vacuum dried at 65℃ for 11 h to obtain the Si@TiO2@ZIF-8 precursor.

[0064] S303: The Si@TiO2@ZIF-8 precursor was heated to 900℃ at a rate of 2℃ / min under Ar protection and held for 3.5h for carbonization. After natural cooling, a core-shell silicon-carbon anode material was obtained. The ZIF-8 derived carbon shell was 50nm thick, containing 8wt% ZnO nanoparticles with a pore size of 2-5nm and a nitrogen doping amount of 4at.

[0065] Example 3.

[0066] The core-shell silicon-carbon anode material of this embodiment consists of a silicon core, a TiO2 transition buffer layer, and a ZIF-8 derived carbon shell layer, from the inside to the outside.

[0067] The silicon core consists of micron-sized silicon particles with porous surfaces. The particle size of the silicon core is 1-3 μm, and the surface nanopores have a pore size of 50-80 nm and a porosity of 20%.

[0068] The TiO2 transition buffer layer was formed by atomic layer deposition (ALD), with a thickness of 20 nm and a density of 96%.

[0069] The ZIF-8 derived carbon shell is a nitrogen-doped porous carbon shell formed by carbonizing ZIF-8. The ZIF-8 derived carbon shell contains 5 wt% ZnO nanoparticles, has a thickness of 30 nm, a pore size of 2-5 nm, and is nitrogen-doped by 5 at.

[0070] Preparation method of core-shell silicon-carbon anode material:

[0071] S1: Preparation of silicon cores

[0072] S101: 20g of microsilica powder was dispersed in 200ml of 1.5mol / L NaOH solution and stirred on a magnetic stirrer at 70℃ for 3.5h at a stirring rate of 500r / min. After the reaction, the mixture was repeatedly washed with deionized water until the pH of the filtrate was 7 to remove residual NaOH and silicate byproducts. The product was then placed in a vacuum drying oven and dried under vacuum at 65℃ for 7h to obtain porous silicon cores. Scanning electron microscopy showed that the surface nanopores had a pore size of 50-80nm and a porosity of 20%.

[0073] S2: Preparation of TiO2 transition buffer layer

[0074] S201: A silicon nucleus was placed in an atomic layer deposition (ALD) reaction chamber, evacuated to 1 Pa, and heated to 130 °C. Using tetraisopropoxy titanium as a precursor and deionized water as an oxidant, tetraisopropoxy titanium and deionized water were alternately pulsed onto the silicon nucleus surface by atomic layer deposition (ALD) for 0.1 s. After each pulse, Ar gas was used to purge for 30 s to remove unreacted precursors. This process was repeated 50 times to form a 20 nm thick TiO2 layer on the silicon nucleus surface, thus obtaining a Si@TiO2 intermediate. X-ray fluorescence spectroscopy analysis showed that the TiO2 purity was ≥99% and the density was 96%.

[0075] S3: Preparation of ZIF-8 derived carbon shell

[0076] S301: Disperse 10g of Si@TiO2 intermediate in 200ml of methanol and sonicate for 30min (300W) to form a uniform suspension; add 2.7g of Zn(NO3)2 and 2.16g of 2-methylimidazole ligand and stir at room temperature for 13h to allow ZIF-8 to grow in situ on the surface of TiO2 layer.

[0077] S302: The solution treated in step S301 was centrifuged, washed three times with methanol to remove unreacted zinc ions and ligands, and then vacuum dried at 55℃ for 11 h to obtain the Si@TiO2@ZIF-8 precursor.

[0078] S303: The Si@TiO2@ZIF-8 precursor was heated to 700℃ at a rate of 4℃ / min under Ar protection and held for 3h for carbonization. After natural cooling, a core-shell silicon-carbon anode material was obtained. The ZIF-8 derived carbon shell was 30nm thick, containing 5wt% ZnO nanoparticles with a pore size of 2-5nm and a nitrogen doping amount of 5at.

[0079] Comparative Example 1.

[0080] Unlike Example 1, the silicon used has a particle size of 300nm-400nm.

[0081] S1: Preparation of silicon cores

[0082] S101: 20g of nano-silicon powder was dispersed in 200ml of 2mol / L NaOH solution and stirred on a magnetic stirrer at 80℃ for 4h at a stirring rate of 500r / min. After the reaction, the product was repeatedly washed with deionized water until the pH of the filtrate was 7 to remove residual NaOH and silicate byproducts. The product was then placed in a vacuum drying oven and dried under vacuum at 60℃ for 8h to obtain porous silicon cores. Scanning electron microscopy showed that the surface nanopores had a pore size of 10-40nm and a porosity of 22%.

[0083] Comparative Example 2.

[0084] Unlike Example 1, there is no TiO2 transition buffer layer.

[0085] Comparative Example 3.

[0086] Unlike Example 1, in step S201, the silicon nucleus is placed in the atomic layer deposition (ALD) reaction chamber, evacuated to 1 Pa, and heated to 180°C.

[0087] Comparative Example 4.

[0088] Unlike Example 1, in step S301, the amount of 2-methylimidazole ligand used is 5g.

[0089] Comparative Example 5.

[0090] Unlike Example 1, in step S303, the Si@TiO2@ZIF-8 precursor is heated to 1200°C at a rate of 3°C / min under Ar gas protection and held at that temperature for 3 hours for carbonization.

[0091] Comparative Example 6.

[0092] Unlike Example 1, there is no ZIF-8 derived carbon shell.

[0093] Comparative Example 7.

[0094] Unlike Example 1, in step S101, the stirring time is 1 hour, and scanning electron microscopy shows that the surface nanopores have a diameter of 20 nm and a porosity of 10%.

[0095] Comparative Example 8.

[0096] Unlike Example 1, Zn(NO3)2 was not added in step S301, and the carbon shell contained no ZnO nanoparticles.

[0097] Comparative Example 9.

[0098] Unlike Example 1, in step 201, the cycle was repeated 20 times, resulting in a TiO2 layer with a thickness of only 10 nm and a density of 85%.

[0099] Examples 1-3 and Comparative Examples 1-5 underwent routine performance testing, including conductivity, cycling, and capacity performance. The test methods are as follows:

[0100] XRD pattern

[0101] The structural analysis of the samples was performed using a Rigaku U1tima IV X-ray scientific analyzer (Japan), with a scanning speed of 2° / min and a scanning range of 10-80°. The samples prepared in Example 1 were scanned, and the scanning results are as follows: Figure 1

[0102] Examples 1-3 and Comparative Examples 1-9 underwent routine performance testing, including conductivity, cycling, and capacity tests. The test methods are as follows:

[0103] Conductivity testing

[0104] The four-probe method was used for testing. The sample was pressed into a disc with a diameter of 10 mm and a thickness of 1 mm (pressure 5 MPa). The conductivity was measured at room temperature using a four-probe conductivity meter with a test current of 10 mA. The average value of three parallel tests was taken.

[0105] Tests of first reversible capacity and first coulomb efficiency

[0106] Half-cell assembly and charge-discharge testing were conducted: The prepared silicon-carbon anode material was mixed with binder PVDF and conductive agent SuperP at a mass ratio of 8:1:1. N-methylpyrrolidone was added to form a slurry, which was then coated on copper foil (thickness 80-100μm). After vacuum drying (60℃, 12h), it was stamped into a disc with a diameter of 12mm as the working electrode. A lithium metal sheet was used as the counter electrode, Celgard 2400 was used as the separator, and 1mol / L LiPF5 (solvent with a volume ratio of EC:DMC:EMC=1:1:1, containing 5% FEC additive) was used as the electrolyte. CR2032 button half-cells were assembled in an argon glove box (water and oxygen content <0.1ppm).

[0107] The LANDCT2001A battery testing system was used in the voltage range of 0.01-3.0V (vs. Li). + / Li), and the first charge and discharge were performed at a current density of 0.1 A / g. The first reversible capacity was the first discharge capacity (average of 3 parallel experiments). The first coulombic efficiency = (first discharge capacity / first charge capacity) × 100%.

[0108] Testing of capacity retention after 1000 cycles and coulombic efficiency after 1000 cycles

[0109] Using the above half-cell system, after 1000 cycles at a current density of 1 A / g and a voltage range of 0.01-3.0 V, the capacity retention rate after 1000 cycles is calculated as (capacity discharged at the 1000th cycle / capacity discharged at the first cycle) × 100%, and the coulombic efficiency after 1000 cycles is calculated as (capacity discharged at the 1000th cycle / capacity charged at the 1000th cycle) × 100%.

[0110] 4A / g capacity retention test

[0111] In the above half-cell system, it was first activated 3 times with a current density of 0.1 A / g, and then cycled 5 times each with a current density of 0.5 A / g, 1 A / g, 2 A / g, and 4 A / g. The capacity retention rate at 4 A / g = (average discharge capacity at 4 A / g / average discharge capacity at 0.1 A / g) × 100%.

[0112] Test of volume expansion rate

[0113] Using an in-situ battery testing platform equipped with an optical microscope, the cells were charged and discharged to 0.01V (fully lithiated) at a current density of 0.1A / g. The volume expansion rate was calculated as follows: (Volume expansion rate = (Cycling thickness - Initial thickness) / Initial thickness × 100%).

[0114] The test results are as follows:

[0115]

[0116] Figure 1 The 25.3° characteristic peak corresponds to the (101) crystal plane diffraction peak of anatase TiO2, indicating the successful formation of the TiO2 transition buffer layer. The peak intensity is moderate, about 10-15% of the silicon peak intensity, indicating that the thickness of the TiO2 transition buffer layer is moderate (20-30nm), neither too thick nor too thin. The moderate peak height indicates that the ALD process is well controlled, avoiding the problems of over-deposition or under-deposition. This peak is wider than the silicon peak but narrower than the ZnO peak, indicating that the TiO2 transition buffer layer has good crystallinity, but the grain size is small (about 10-20nm), which is consistent with the characteristics of the nanoscale thin film formed by the ALD process. The existence of this peak directly proves the successful formation of the anatase TiO2 transition buffer layer. Its intensity and width characteristics indicate that the layer has good crystallinity and suitable thickness, which can effectively play the role of rigid constraint and interface bonding.

[0117] The 28.44° characteristic peak corresponds to the (111) crystal plane diffraction peak of the silicon nucleus, providing direct evidence of its presence. This peak is the highest in the spectrum, indicating that the silicon nucleus is the main component of the material and has good crystallinity. The narrow half-width at half-maximum (HWHM) of this peak indicates that the silicon nucleus has a large grain size (micrometer-level) and high crystallinity, consistent with the design of using micrometer-level silicon particles as the nucleus in this invention. This peak is direct evidence of the presence of the silicon nucleus, and its high and narrow characteristics indicate that the silicon nucleus maintains a good crystalline structure, providing a basis for the high specific capacity of the material. At the same time, the peak position did not shift significantly, indicating that the silicon nucleus did not undergo significant oxidation or structural changes, maintaining the stability of the material. The 47.30° characteristic peak corresponds to the (220) crystal plane diffraction peak of the silicon nucleus.

[0118] The 36.25° characteristic peak corresponds to the (101) crystal plane diffraction peak of ZnO, indicating the presence of ZnO nanoparticles in the ZIF-8 derived carbon shell. This peak is relatively weak, approximately 5-10% of the silicon peak intensity, indicating that the ZnO nanoparticle content in the ZIF-8 derived carbon shell is moderate (5-8 wt%), neither too high nor too low. The peak's wide half-width at half-maximum (HWHM) indicates that the ZnO nanoparticle size is small (5-10 nm), consistent with the characteristics of nanoscale ZnO particles formed during the ZIF-8 carbonization process. The presence of this peak directly proves the formation of ZnO nanoparticles in the ZIF-8 derived carbon shell, and its weak and wide characteristic indicates that ZnO is uniformly dispersed in the carbon matrix at the nanoscale, effectively catalyzing graphitization and stabilizing the SEI film.

[0119] From the above test results, it can be concluded that in this invention, Examples 1-3, due to the synergistic effect of the complete three-layer structure (silicon core + TiO2 transition layer + ZIF-8 derived carbon shell), have a stable SEI film, and the coulombic efficiency remains at 99.3%-99.5% after 1000 cycles, close to 100%, demonstrating that side reactions are effectively suppressed. Comparative Example 1 (using nano-silicon), due to its large specific surface area, has an SEI film that consumes more lithium, resulting in a coulombic efficiency of 98.11% after cycling, and an initial coulombic efficiency of 78.21%. Nanoparticle expansion is more difficult to control, with the volume expansion rate increasing to 75.69%. Conductivity and cycle stability decrease due to intensified interfacial reactions. In Comparative Example 2 (without a TiO2 layer), silicon directly contacts the electrolyte, leading to severe side reactions and a lower initial coulombic efficiency. Only 72.69%; lacking rigid constraints, the volume expansion rate reached 120.56%, the SEI film was unstable, and the coulombic efficiency after 1000 cycles was only 95.02%. Comparative Example 3 (ALD temperature 180℃): The ALD temperature was too high, and excessive oxidation of the silicon core led to the collapse of the pore structure, a decrease in the density of the TiO2 layer, a decrease in cycle retention rate to 75.23%, and an increase in volume expansion rate to 70.39%. Comparative Example 4 (excess 2-methylimidazolium ligand): This led to uneven growth of ZIF-8, disordered porous structure of the carbon shell, a decrease in conductivity to 174.58 S / cm, and a capacity retention rate of only 50.49% of 4 A / g (blocked ion diffusion path). Comparative Example 5 (carbonization temperature too high): The carbonization temperature of 1200℃ was too high, silicon volatilized, and ZnO... Aggregation and carbon layer structure disruption resulted in a cycle retention rate of 60.35% and a volume expansion rate of 80.55% (due to decreased carbon shell buffering capacity). Comparative Example 6 (without a ZIF-8 derived carbon shell) lacked the flexible buffering and highly conductive network of the carbon shell, leading to a conductivity decrease to 120.45 S / cm (below 200 S / cm) and a volume expansion rate of 90.56% (above 60%). Without nitrogen-doped carbon to stabilize the SEI film, the initial coulombic efficiency dropped to 76.34%, and the capacity retention rate after 1000 cycles was only 45.32%. This demonstrates that the ZIF-8 derived carbon shell is crucial for synergistically controlling expansion and improving conductivity. Comparative Example 7 (alkali etching with a stirring time of 1 hour) resulted in insufficient pores to accommodate expansion, leading to volume expansion. The efficiency increased to 68.34%; the ion diffusion path was long, and the 4A / g capacity retention decreased to 58.21%, demonstrating that alkaline etching to control the uniformity of nanopores is the basis for alleviating expansion and improving high-rate performance. In Comparative Example 8 (ZIF-8 derived carbon shell without ZnO), due to the lack of ZnO catalytic graphitization, the carbon layer conductivity decreased to 160.43 S / cm; the structural stability decreased, and the capacity retention after 1000 cycles decreased to 62.41%, indicating that ZnO nanoparticles can optimize the conductivity and structural stability of the carbon layer. In Comparative Example 9 (ALD cycle count of 20 times), the TiO2 layer was too thin, which could not completely isolate the electrolyte, increasing side reactions and reducing the initial coulombic efficiency to 81.09%; insufficient mechanical support led to a volume expansion rate of 65%.The 89% result indicates that ALD self-limiting growth, ensuring the uniformity of TiO2 layer thickness, is key to isolating side reactions and suppressing expansion.

[0120] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0121] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A core-shell silicon-carbon anode material, characterized in that, From the inside out, the layers are a silicon core, a TiO2 transition buffer layer, and a ZIF-8 derived carbon shell. The silicon core is a micron-sized silicon particle with a porous surface; The TiO2 transition buffer layer is formed by atomic layer deposition (ALD); The ZIF-8 derived carbon shell is a nitrogen-doped porous carbon shell formed by carbonizing ZIF-8. The silicon core has a particle size of 1-3 μm, and the surface nanopores have a pore size of 50-100 nm and a porosity of 20-30%. The thickness of the TiO2 transition buffer layer is 20-30 nm, and its density is ≥95%. The ZIF-8 derived carbon shell contains 5-8 wt% ZnO nanoparticles, with a thickness of 30-50 nm, a pore size of 2-5 nm, and nitrogen doping of 4-6 at.

2. The method for preparing the core-shell silicon-carbon anode material according to claim 1, characterized in that, Includes the following steps: S1: Preparation of silicon cores S101: After dispersing microsilicon powder in NaOH solution and stirring for alkaline etching, wash with deionized water until neutral, and then vacuum dry to obtain a porous silicon core on the surface; S2: Preparation of TiO2 transition buffer layer S201: Using tetraisopropoxy titanium as a precursor and deionized water as an oxidant, a TiO2 layer is formed on the silicon core surface by alternating pulses of tetraisopropoxy titanium and deionized water through atomic layer deposition (ALD) in a cyclic reaction, thus obtaining a Si@TiO2 intermediate. S3: Preparation of ZIF-8 derived carbon shell S301: The Si@TiO2 intermediate was dispersed in methanol, ultrasonically treated, dissolved in Zn(NO3)2, and then 2-methylimidazolium ligand was added and stirred. S302: The solution treated in step S301 is centrifuged and washed with methanol to remove unreacted Zn. 2+ The precursor Si@TiO2@ZIF-8 was prepared by vacuum drying with 2-methylimidazolium ligand. S303: The Si@TiO2@ZIF-8 target material is obtained by heating, holding and cooling the Si@TiO2@ZIF-8 precursor under Ar gas protection.

3. The method for preparing the core-shell silicon-carbon anode material according to claim 2, characterized in that, In step S101, the concentration of the NaOH solution is 1.5-2.5 mol / L, the stirring temperature is 70-90℃, the stirring time is 3.5-4.5 h, the vacuum drying temperature is 55-65℃, and the vacuum drying time is 7-9 h.

4. The method for preparing the core-shell silicon-carbon anode material according to claim 2, characterized in that, In step S201, the alternating pulse duration of tetraisopropoxy titanium and deionized water is 0.1s for tetraisopropoxy titanium and 0.1s for deionized water, the temperature of the cyclic reaction is 130-160℃, and the number of cycles is 50-80.

5. The method for preparing the core-shell silicon-carbon anode material according to claim 2, characterized in that, In step S301, the mass ratio of the Si@TiO2 intermediate to Zn(NO3)2 is (3-4):1; Zn 2+ The molar ratio of 2-methylimidazole to 2-methylimidazole is 1:1.

84.

6. The method for preparing the core-shell silicon-carbon anode material according to claim 5, characterized in that, In step S302, the vacuum drying temperature is 55-65℃ and the vacuum drying time is 11-13h.

7. The method for preparing the core-shell silicon-carbon anode material according to claim 6, characterized in that, In step S303, the heating rate is 2-4℃ / min, the holding temperature is 700-900℃, the holding time is 2.5-3.5h, and then the temperature is allowed to cool naturally.

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