Skeleton-bearing carbon shell-coated silicon-carbon negative electrode material and preparation method thereof

Abstract

The application relates to the technical field of lithium ion battery negative electrode materials, in particular to a skeleton bearing carbon shell coated silicon-carbon negative electrode material and a preparation method thereof. The negative electrode material comprises a rigid porous carbon skeleton, a silicon phase and a continuous carbon shell; the rigid porous carbon skeleton has bearing holes for fixing the silicon phase and buffer holes for reserving volume change space, the silicon phase is distributed at the bearing hole wall and the skeleton node, and the buffer holes remain connected and reserve a residual pore volume of not less than 20% of the total pore volume after loading; the continuous carbon shell is coated on the outer surface and comprises an inner side buffer carbon layer and an outer side limiting carbon layer. The application can effectively solve the problems of silicon phase agglomeration, structure cracking and serious interface side reaction under high silicon loading, and improve the cycle stability and coulomb efficiency of the material.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery anode material technology, and in particular to a skeleton-supported carbon shell-coated silicon-carbon anode material and its preparation method. Background Technology

[0002] Silicon, due to its high theoretical specific capacity, is considered a key anode material for improving the energy density of lithium-ion batteries and has broad application prospects in consumer electronics and power batteries. To address the volume effect of silicon during lithium insertion and extraction, existing technologies typically employ techniques such as nano-sizing, carbon coating, porous carbon composites, or vapor deposition to construct silicon-carbon composite materials. Common technical solutions include dispersing nano-silicon particles in a porous carbon matrix, utilizing the pores of the carbon material to accommodate the volume expansion of silicon, and coating the particles with a carbon layer to isolate them from the electrolyte, thereby maintaining the integrity of the electrode structure and the continuity of the conductive network.

[0003] However, in the existing technology, when the silicon loading is high, the silicon phase is prone to agglomeration in local areas, which leads to stress concentration during cycling, and in turn causes carbon layer cracking and particle pulverization. At the same time, if the pores are completely filled by the silicon phase, there is not enough remaining space to buffer volume changes, which will lead to material structure damage. In addition, the existing structural design is difficult to balance high capacity, low expansion and low impedance growth, resulting in unsatisfactory capacity retention of the battery during long cycles. Summary of the Invention

[0004] This application provides a skeleton-supported carbon shell coated silicon-carbon anode material and its preparation method to solve the above problems.

[0005] In a first aspect, this application provides a framework-supported carbon shell-coated silicon-carbon anode material, comprising a rigid porous carbon framework, a silicon phase, and a continuous carbon shell; The rigid porous carbon framework has an interconnected pore structure, which includes a support pore and a buffer pore. The support pore is used to fix the silicon phase, and the buffer pore is used to reserve space for volume changes during the charging and discharging process of the silicon phase. The silicon phase is distributed on the pore walls of the bearing pores and at the skeleton nodes of the rigid porous carbon skeleton, and the buffer pores remain connected after the silicon phase is loaded. The continuous carbon shell covers the outer surface of the rigid porous carbon skeleton loaded with the silicon phase. The continuous carbon shell includes a buffer carbon layer near the rigid porous carbon skeleton and a limiting carbon layer on the outer side. The rigid porous carbon framework has a total pore volume of 0.55-1.35 cm³ / g before loading the silicon phase, and retains a residual pore volume of not less than 20% of the total pore volume after loading the silicon phase. The mass content of the silicon phase in the silicon-carbon anode material is 25%-65%, and the average thickness of the continuous carbon shell is 8-60 nm.

[0006] Optionally, the supporting pores and the buffer pores are interconnected within the rigid porous carbon skeleton, with the peak pore diameter of the supporting pores being 5-25 nm and the peak pore diameter of the buffer pores being 35-120 nm. The carrier holes are used to disperse and fix the silicon phase along the hole walls, and the buffer holes are used to provide releasable space when the silicon phase expands during lithium intercalation, so as to reduce the cracking of the pore walls of the rigid porous carbon skeleton.

[0007] Optionally, the silicon phase includes one or more of nano-silicon, silicon-oxygen phase, and silicon-carbon transition phase; The portion of the silicon phase located inside the rigid porous carbon skeleton accounts for more than 80% of the total mass of the silicon phase, and the area of ​​the free silicon phase on the outer surface of the silicon-carbon anode material is no more than 8%.

[0008] Optionally, the rigid porous carbon framework has oxygen-containing functional groups and / or nitrogen-containing sites at the pore walls and framework nodes. The oxygen-containing functional groups and / or nitrogen-containing sites are used to improve the adsorption and fixation ability of the silicon precursor at the pore walls and framework nodes, so that the silicon phase is preferentially formed at the pore walls and framework nodes.

[0009] Optionally, the thickness of the buffer carbon layer in the continuous carbon shell is 3-25 nm, and the thickness of the limiting carbon layer is 5-35 nm; The buffer carbon layer is fitted to the rigid porous carbon skeleton supported by the silicon phase to buffer the internal volume change of the particles. The boundary carbon layer is located outside the buffer carbon layer to reduce the direct contact between the electrolyte and the silicon phase.

[0010] Secondly, this application provides a method for preparing a framework-supported carbon shell-coated silicon-carbon anode material, comprising: S1. Prepare a rigid porous carbon framework with interconnected pore structures, wherein the pore structure includes support pores for fixing the silicon phase and buffer pores for reserving space for volume changes. S2. Surface activation treatment is performed on the rigid porous carbon framework to form active sites for adsorbing silicon precursors at the pore walls of the supporting pores and the framework nodes of the rigid porous carbon framework. S3. Introduce the silicon precursor into the surface-activated rigid porous carbon framework, so that the silicon precursor is preferentially adsorbed and fixed at the pore walls and framework nodes of the supporting pores. S4. Heat treatment is performed on the rigid porous carbon skeleton loaded with the silicon precursor to transform the silicon precursor into a silicon phase distributed in the pore walls and skeleton nodes of the support pores, thereby obtaining a silicon phase support skeleton. S5. Perform a channel clearing treatment on the silicon phase support skeleton to remove free silicon phase or byproducts that are not fixed to the support hole wall and skeleton node, so that the buffer hole remains connected after silicon phase loading. S6. A secondary carbon source is deposited on the outer surface of the silicon phase support skeleton, and the secondary carbon source is subjected to segmented carbonization treatment to form a continuous carbon shell on the outer surface of the silicon phase support skeleton. The continuous carbon shell includes a buffer carbon layer close to the silicon phase support skeleton and a boundary carbon layer on the outside, thereby obtaining the skeleton support carbon shell coating silicon-carbon anode material. The rigid porous carbon framework has a total pore volume of 0.55-1.35 cm³ / g before loading the silicon phase, and retains a residual pore volume of not less than 20% of the total pore volume after loading the silicon phase. The mass content of the silicon phase in the obtained silicon-carbon anode material is 25%-65%, and the average thickness of the continuous carbon shell is 8-60 nm.

[0011] Optionally, when preparing the rigid porous carbon framework, the carbon source, pore-forming agent and conductive reinforcing material are mixed and shaped, then carbonized in an inert atmosphere, and then the pore-forming agent is removed. In this process, by adjusting the particle size and amount of the pore-forming agent, the resulting rigid porous carbon skeleton is made to form carrier pores with smaller pore sizes and buffer pores with larger pore sizes, and the carrier pores and the buffer pores are kept in communication.

[0012] Optionally, the surface activation treatment includes one or more of oxidation treatment, ammonia treatment, urea heat treatment, melamine heat treatment, and dopamine deposition treatment; After the surface activation treatment, oxygen-containing functional groups and / or nitrogen-containing sites are formed on the pore walls and framework nodes of the rigid porous carbon framework, so that the subsequently introduced silicon precursor is preferentially adsorbed on the pore walls and framework nodes of the supporting pores, rather than being randomly deposited in all the pores.

[0013] Optionally, when introducing the silicon precursor into the surface-activated rigid porous carbon framework, vacuum wetting, pressure wetting, or cyclic wetting methods are used to allow the silicon precursor to enter the carrier pores. By controlling the silicon precursor concentration, wetting time, or number of wetting cycles, the silicon phase loading formed after heat treatment is matched with the remaining pore volume of the buffer pore.

[0014] Optionally, when performing segmented carbonization on the secondary carbon source, the secondary carbon source deposited on the outer surface of the silicon phase support skeleton is first pre-carbonized at 450-650℃ to form a buffer carbon layer that fits the silicon phase support skeleton; then, high-temperature carbonization is performed at 750-950℃ to form a boundary carbon layer on the outside of the buffer carbon layer, thereby obtaining a continuous carbon shell with inner buffering and outer boundary functions.

[0015] The beneficial effects achieved by this application through the above technical solution are as follows: This application employs a rigid porous carbon framework with interconnected channels. Within the framework, relatively small-diameter carrier pores and relatively large-diameter buffer pores are incorporated. The carrier pores are primarily used for dispersing and fixing the silicon phase, while the buffer pores are mainly used to reserve space for volume changes during silicon phase charging and discharging. This avoids the silicon phase disorderly filling all pores, reducing the risk of pore wall cracking and particle pulverization during silicon phase expansion.

[0016] This application enhances the adsorption and fixation capacity of silicon precursors at the above-mentioned locations by surface activation of rigid porous carbon framework, forming oxygen-containing functional groups and / or nitrogen-containing sites at the pore walls and framework nodes. This results in silicon phase preferentially distributed at the pore walls and nodes inside the framework, reducing free silicon phase and local silicon phase agglomeration on the outer surface of particles, which is beneficial to improving the uniformity of silicon phase distribution and the stability of interfacial bonding.

[0017] This application incorporates a pore-clearing treatment after silicon phase formation to remove unfixed free silicon phase and reaction byproducts that clog the pores, ensuring that the buffer pores and connecting pore necks remain open after silicon phase loading. By controlling the remaining pore volume to more than 20% of the total pore volume before loading, necessary expansion and release space can be maintained under higher silicon loading, and lithium-ion transport channels within the particles can be preserved.

[0018] The rigid porous carbon framework of this application forms a continuous conductive network, which can maintain electrical contact between silicon phases and between the silicon phase and the external conductive system during repeated lithium insertion and extraction processes. Even if some silicon phases undergo volume changes, the framework can still support and constrain the silicon phases, thereby mitigating the problems of conductive contact failure and continuous increase in cycling impedance.

[0019] This application features a continuous carbon shell on the outer surface of a silicon-phase support framework, and forms an inner buffer carbon layer and an outer boundary carbon layer through segmented carbonization. The buffer carbon layer adheres to the silicon-phase support framework, absorbing and dispersing local expansion stress generated within the particles; the boundary carbon layer has a relatively dense structure, reducing direct contact between the electrolyte and the silicon phase, inhibiting repeated rupture and regeneration of the SEI film, and improving the stability of the particle's external interface.

[0020] This application achieves this by synergistically controlling the silicon phase mass content, the proportion of remaining pore volume, and the thickness of the continuous carbon shell. This avoids insufficient buffer space caused by simply increasing the silicon content, and also avoids increased lithium-ion diffusion resistance caused by simply thickening the carbon shell. Using a thinner carbon shell when the silicon content is low is beneficial for improving rate performance; while retaining a larger buffer pore volume and appropriately increasing the carbon shell thickness when the silicon content is high is beneficial for balancing material capacity and cycle stability.

[0021] A comparison of the examples and comparative examples shows that, under similar conditions of silicon content and carbon coating amount, materials lacking a rigid porous carbon framework are more prone to silicon phase agglomeration, particle cracking, and conductive contact failure; when the remaining pore volume after silicon phase loading is less than 20%, the material is more prone to pore wall rupture and rapid capacity decay; without surface activation, the amount of free silicon phase on the outer surface of the particles increases; and with ordinary single-layer carbon coating, it is difficult to simultaneously achieve internal stress buffering and external interface protection. This demonstrates that the various structural and process steps in this application have a synergistic effect.

[0022] The silicon-carbon anode material prepared in this application can maintain a relatively complete particle structure and a relatively stable internal pore structure at a high silicon phase mass content, reducing particle expansion, carbon shell cracking, surface side reactions and impedance growth during cycling. Therefore, it is beneficial to improve the first coulombic efficiency, cycle capacity retention and rate performance, and is suitable for lithium-ion battery anode materials that require high capacity and long cycle performance. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the overall structure of the skeleton supporting the carbon shell-coated silicon-carbon anode material of this application. Figure 2 This is a schematic diagram of the pore structure of the rigid porous carbon framework of this application; Figure 3 These are comparison diagrams showing the changes in pore size distribution and pore volume before and after silicon phase loading in Examples 1, 3, and Comparative Example 2 of this application; Figure 4 This is a comparison diagram of the silicon phase distribution in Example 1 and Comparative Example 3 of this application; Figure 5 This is a comparison diagram of the external carbon shell structure of the particles in Example 1 and Comparative Example 4 of this application; Figure 6 This is a comparison chart of the cycle performance of Examples 1-3 and Comparative Examples 2-4 of this application; Figure 7 This is a comparison of the AC impedance spectra before and after cycling in Example 1 and Comparative Examples 2-4 of this application; Figure 8 These are comparison images of particle morphology after cycling in Examples 1, 2, and 4 of this application. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0026] Furthermore, the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0027] The embodiments of this application will now be described in further detail with reference to the accompanying drawings.

[0028] Example 1

[0029] like Figure 1 As shown in the figure, this embodiment provides a skeleton-supported carbon shell coated silicon-carbon anode material, the preparation method of which includes the following steps.

[0030] (1) Preparation of rigid porous carbon framework Weigh out 100 parts of phenolic resin, 30 parts of first silica template, 60 parts of second silica template and 3 parts of carbon nanotubes by weight. The average particle size of the first silica template is 15 nm and the average particle size of the second silica template is 70 nm.

[0031] Phenolic resin was dissolved in a mixed solvent of ethanol and water. A first silica template, a second silica template, and carbon nanotubes were added. After stirring for 2 hours, the mixture was ultrasonically dispersed to ensure uniform dispersion of all components, resulting in a mixed slurry. The mixed slurry was then spray-dried to obtain a spherical framework precursor.

[0032] The spherical framework precursor was placed in a tube furnace and heated to 900°C at a rate of 3°C / min under nitrogen protection, and held at this temperature for 3 hours to carbonize the phenolic resin and form a continuous carbon structure. After the furnace temperature naturally cooled to room temperature, the carbonized product was placed in a 2 mol / L sodium hydroxide solution and treated at 80°C for 6 hours to remove the first and second silica templates. After treatment, the product was repeatedly washed with deionized water until the washing solution was neutral, and then vacuum dried at 100°C for 12 hours to obtain a rigid porous carbon framework.

[0033] like Figure 2 and Figure 3 As shown, nitrogen adsorption-desorption testing revealed that the total pore volume of the obtained rigid porous carbon framework was 0.92 cm³ / g. The pore size distribution exhibited two peak values: a smaller peak at 15 nm, corresponding to the carrier pores, and a larger peak at 72 nm, corresponding to the buffer pores. The carrier and buffer pores were interconnected via pore necks. Carbon nanotubes were distributed within the carbon framework, forming a continuous conductive pathway together with the carbonized phenolic resin.

[0034] (2) Surface activation of rigid porous carbon framework The obtained rigid porous carbon framework was added to a 20% urea aqueous solution at a mass ratio of 1:10, and stirred and impregnated at room temperature for 4 hours. After impregnation, it was dried at 80°C to constant weight.

[0035] The dried material was placed in a nitrogen atmosphere and heated to 750°C at a heating rate of 5°C / min, and held for 2 hours. This allowed the nitrogen-containing components generated by the pyrolysis of urea to combine with the surface of the carbon skeleton, forming nitrogen-containing sites on the pore walls and skeleton nodes. At the same time, some oxygen-containing functional groups on the surface of the carbon skeleton were retained, resulting in an activated rigid porous carbon skeleton.

[0036] X-ray photoelectron spectroscopy analysis revealed that the nitrogen content on the activated rigid porous carbon framework surface was 3.2 at and the oxygen content was 5.1 at. The nitrogen-containing sites and oxygen-containing functional groups formed can enhance the adsorption and fixation capacity of subsequent silicon precursors on the pore walls and framework nodes.

[0037] (3) Impregnation and fixation of silicon precursor Tetraethyl orthosilicate, anhydrous ethanol, deionized water and ammonia were mixed in a volume ratio of 1:6:2:0.08 and stirred for 30 min to obtain a silicon precursor impregnation solution.

[0038] The activated rigid porous carbon framework was placed in the silicon precursor wetting solution, ensuring complete immersion. The wetting system was then placed in a vacuum container and maintained at a gauge pressure of -0.08 MPa for 15 minutes to expel gas from the pores. Subsequently, the pressure was restored to atmospheric pressure and wetting continued for 30 minutes, allowing the silicon precursor to enter the carrier pores and adjacent pore necks. This vacuum-atmospheric pressure wetting process was repeated three times.

[0039] After impregnation, the material is removed from the silicon precursor impregnation solution and dried at 60°C for 6 hours, and then dried at 120°C for 4 hours to allow tetraethyl orthosilicate to hydrolyze, condense and fix near nitrogen-containing sites and oxygen-containing functional groups, thus obtaining a rigid porous carbon framework supported on silicon precursor.

[0040] (4) Formation of silicon phase and pore clearing A rigid porous carbon framework supporting a silicon precursor was uniformly mixed with magnesium powder, and the amount of magnesium powder added was controlled according to a molar ratio of magnesium to silicon dioxide formed by the conversion of the silicon precursor to 2.2:1. The mixture was placed in an argon-protected tube furnace and heated to 650°C at a heating rate of 3°C / min, and held at that temperature for 3 hours, so that the silicon precursor fixed on the pore walls and framework nodes was converted into the silicon phase by magnesothermic reduction.

[0041] After the furnace temperature dropped to room temperature, the obtained product was added to a 1 mol / L hydrochloric acid solution and stirred at 60°C for 2 hours to remove magnesium oxide, unreacted magnesium, and other soluble byproducts. The product was then washed with deionized water until the washing solution was neutral, followed by a second wash with ethanol, and finally vacuum dried at 80°C for 12 hours.

[0042] Through the above acid pickling and washing process, loose particles not fixed to the walls of the support holes and the nodes of the skeleton, as well as byproducts that block the necks of the holes, are removed, allowing some buffer holes and necks to reopen, thus obtaining a silicon phase support skeleton.

[0043] Inductively coupled plasma (ICP-PAP) testing and thermogravimetric analysis revealed that the silicon content in the silicon-supported framework, after subsequent carbon shell formation, was approximately 45% of the final product mass. Pore structure testing showed that the remaining pore volume after silicon phase loading was approximately 32% of the initial total pore volume of the rigid porous carbon framework, and the buffer pores remained interconnected.

[0044] like Figure 4 As shown, cross-sectional scanning electron microscopy and silicon elemental distribution analysis revealed that the silicon phase is mainly distributed in the pore walls and skeleton nodes of the supporting pores. The silicon phase located inside the rigid porous carbon skeleton accounts for about 87% of the total mass of the silicon phase, while the free silicon phase on the outer surface of the particles accounts for about 4.3% of the area.

[0045] (5) Formation of continuous carbon shell Ten parts of a silicon-phase support framework were added to 500 parts of a 10 mmol / L, pH 8.5 tris(hydroxymethyl)aminomethane buffer solution, along with two parts of dopamine hydrochloride. The mixture was stirred at room temperature for 6 hours to allow dopamine to self-aggregate and deposit on the outer surface of the silicon-phase support framework particles. After deposition, the mixture was filtered, washed with deionized water, and vacuum dried at 80°C for 12 hours.

[0046] The dried material was placed in a tube furnace under nitrogen protection. It was first heated to 600°C at a heating rate of 2°C / min and held for 1 hour to pre-carbonize the dopamine deposition layer near the silicon phase support skeleton, forming a buffer carbon layer with more defects and a certain stress buffering capacity. Then, the temperature was further increased to 850°C and held for 2 hours to further carbonize and densify the carbon layer on the outer side, forming a boundary carbon layer.

[0047] After cooling, the obtained material was slightly depolymerized and sieved to obtain a framework-supported carbon shell-coated silicon-carbon anode material with a particle size D50 of 8.6 μm.

[0048] like Figure 5 As shown, transmission electron microscopy revealed that the obtained continuous carbon shell uniformly coated the outer surface of the silicon-phase support framework particles. The average total thickness of the continuous carbon shell was approximately 25 nm. Specifically, the buffer carbon layer near the silicon-phase support framework had an average thickness of approximately 10 nm, while the boundary carbon layer on the outer side had an average thickness of approximately 15 nm. No obvious through-cracks were observed in the continuous carbon shell, and the free silicon phase on the particle surface was covered by the continuous carbon shell.

[0049] (6) Material structure and electrochemical performance testing The obtained skeleton-supported carbon shell-coated silicon-carbon anode material, conductive carbon black, and sodium carboxymethyl cellulose / styrene-butadiene rubber composite binder were mixed at a mass ratio of 80:10:10, and a negative electrode slurry was prepared using deionized water as the dispersion medium. The negative electrode slurry was uniformly coated on the surface of copper foil, and after drying, rolling, and stamping, a negative electrode sheet was formed.

[0050] A coin cell was assembled in an argon-filled glove box using a lithium metal sheet as the counter electrode, and charge-discharge tests were conducted within a voltage range of 0.01–1.5V.

[0051] like Figure 6 As shown, the test results indicate that the initial coulombic efficiency of the silicon-carbon anode material obtained in this embodiment is 88.1%; after 100 cycles at 0.5C, the capacity retention rate is 89.2%. The thickness growth rate and particle cracking degree of the electrode sheet after cycling are lower than those of materials without residual buffer pore volume or without forming a continuous carbon shell.

[0052] Combination Figure 7 The AC impedance spectrum shown is Figure 8The morphology of the particles after cycling is shown. The above results show that in this embodiment, the silicon phase is fixed to the hole wall and skeleton node through the support hole, the buffer hole that keeps the connection open provides release space for the lithium intercalation expansion of the silicon phase, and the continuous carbon shell composed of the buffer carbon layer and the boundary carbon layer reduces the direct contact between the electrolyte and the silicon phase, thereby taking into account the silicon phase loading, particle structure stability and cycling performance.

[0053] Example 2

[0054] This embodiment provides a low-silicon-load, thin-shell carbon-supported carbon-coated silicon-carbon anode material. The preparation process of this embodiment is basically the same as that of Example 1, with the main differences being the number of wetting cycles of the silicon precursor and the amount of secondary carbon source added.

[0055] (1) Preparation of rigid porous carbon framework Following the method described in Example 1, phenolic resin, a first silica template, a second silica template, and carbon nanotubes were mixed, and then subjected to spray drying, carbonization in a nitrogen atmosphere, and alkaline solution treatment to remove the template, thereby obtaining a rigid porous carbon framework with interconnected pore structures.

[0056] Nitrogen adsorption-desorption tests revealed a total pore volume of 0.90 cm³ / g for the obtained rigid porous carbon framework, with a peak pore size of 14 nm for the carrier pores and 70 nm for the buffer pores. The carrier pores and buffer pores are interconnected through pore necks.

[0057] (2) Surface activation of rigid porous carbon framework The obtained rigid porous carbon framework was added to a 20% urea aqueous solution at a mass ratio of 1:10 and stirred and impregnated at room temperature for 4 hours. After impregnation, it was dried to constant weight at 80°C and then heat-treated at 750°C for 2 hours in a nitrogen atmosphere to obtain an activated rigid porous carbon framework.

[0058] Testing revealed that the activated rigid porous carbon framework surface contained 3.1 at% nitrogen and 5.0 at% oxygen. The nitrogen-containing sites and oxygen-containing functional groups formed are distributed on the pore walls and framework nodes, serving to adsorb and immobilize the silicon precursor.

[0059] (3) Impregnation and fixation of silicon precursor A silicon precursor impregnation solution was prepared by mixing tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia in a volume ratio of 1:6:2:0.08.

[0060] The activated rigid porous carbon framework was completely immersed in a silicon precursor wetting solution and vacuum-treated for 15 min at a gauge pressure of -0.08 MPa. The pressure was then restored to ambient pressure and wetting continued for another 30 min. Unlike Example 1, this example repeats the vacuum-atmospheric pressure wetting process twice to reduce the amount of silicon precursor entering the rigid porous carbon framework.

[0061] After impregnation, the material is dried at 60°C for 6 hours and then at 120°C for 4 hours to hydrolyze, condense and fix the silicon precursor at the pore walls and framework nodes, thus obtaining a rigid porous carbon framework loaded with silicon precursor.

[0062] (4) Formation of silicon phase and pore clearing A rigid porous carbon framework supporting a silicon precursor was mixed with magnesium powder to achieve a molar ratio of magnesium to silicon dioxide formed from the silicon precursor at 2.2:1. The mixture was placed in an argon-protected tube furnace and heated to 650°C at a heating rate of 3°C / min, and held at that temperature for 3 hours to convert the silicon precursor into a silicon phase distributed on the pore walls and framework nodes.

[0063] After cooling, the obtained product was added to a 1 mol / L hydrochloric acid solution and stirred at 60 °C for 2 h to remove magnesium oxide, unreacted magnesium, and byproducts clogging the pores. The product was then washed with deionized water until the washing solution was neutral and vacuum dried at 80 °C for 12 h to obtain a silicon-phase support framework.

[0064] Tests showed that the silicon phase was mainly distributed on the walls of the supporting pores and at the nodes of the skeleton. The silicon phase loaded inside the rigid porous carbon skeleton accounted for about 86% of the total mass of the silicon phase, and the free silicon phase on the outer surface of the particles accounted for about 3.8% of the area.

[0065] After silicon phase loading and pore channel clearing, the remaining pore volume of the material is approximately 40% of the initial total pore volume of the rigid porous carbon skeleton, and the buffer pores and pore necks remain connected.

[0066] (5) Formation of continuous carbon shell Ten parts of the silicon phase support framework were added to 500 parts of a 10 mmol / L, pH 8.5 tris(hydroxymethyl)aminomethane buffer solution, and 1.2 parts of dopamine hydrochloride were added. The mixture was stirred at room temperature for 6 hours to allow dopamine to form a uniform deposition layer on the outer surface of the silicon phase support framework particles.

[0067] After deposition, the material was filtered, washed, and vacuum dried at 80°C for 12 hours. The dried material was then placed in a nitrogen-protected tube furnace and heated to 600°C at a rate of 2°C / min and held for 1 hour to pre-carbonize the dopamine deposition layer near the silicon phase support framework to form a buffer carbon layer. The temperature was then further increased to 850°C and held for 2 hours to further densify the outer carbon layer to form a boundary carbon layer.

[0068] After cooling, the obtained material is depolymerized and sieved to obtain a silicon-carbon anode material with a carbon shell supporting the skeleton.

[0069] Testing revealed that the particle size D50 of the obtained material was 8.3 μm, and the silicon phase comprised approximately 38% of the silicon-carbon anode material by mass. The average total thickness of the continuous carbon shell was approximately 15 nm, with the buffer carbon layer having an average thickness of approximately 6 nm and the boundary carbon layer having an average thickness of approximately 9 nm. The continuous carbon shell uniformly covered the outer surface of the silicon phase support framework, and no obvious localized detachment or through-cracks were observed.

[0070] (6) Material property testing Following the method described in Example 1, the silicon-carbon anode material with a carbon shell supporting the skeleton obtained in this example was used to make a negative electrode sheet, and a coin cell was assembled for electrochemical performance testing.

[0071] like Figure 6 As shown, the test results indicate that the initial coulombic efficiency of the material obtained in this embodiment is 89.0%. After 100 cycles at 0.5C, the capacity retention rate is 91.1%. The discharge capacity retention rate at 2C is higher than that in Example 1, and the capacity of the material can be basically recovered after returning to 0.2C.

[0072] Compared with Example 1, this example reduces the number of wetting cycles of the silicon precursor, thereby reducing the silicon phase content and retaining a larger residual buffer pore volume. At the same time, by reducing the amount of secondary carbon source to form a thinner continuous carbon shell, the transport resistance of lithium ions passing through the carbon shell is reduced.

[0073] The above results show that, under the condition of relatively low silicon phase mass content, controlling the thickness of the continuous carbon shell within a small range and retaining a high proportion of interconnected buffer pore volume can improve the rate performance and cycle stability of the material while maintaining the stability of the particle structure.

[0074] Example 3

[0075] This embodiment provides a high silicon load, thick carbon shell skeleton-supported carbon shell silicon-carbon anode material, the preparation method of which includes the following steps.

[0076] (1) Preparation of rigid porous carbon framework Weigh out 100 parts by weight of phenolic resin, 35 parts by weight of the first silica template, 75 parts by weight of the second silica template, and 3 parts by weight of carbon nanotubes. The average particle size of the first silica template is 18 nm, and the average particle size of the second silica template is 90 nm.

[0077] Phenolic resin was dissolved in a mixed solvent of ethanol and water. A first silica template, a second silica template, and carbon nanotubes were added, and the mixture was stirred for 2 hours followed by ultrasonic dispersion to obtain a homogeneous slurry. The slurry was then spray-dried to obtain a spherical framework precursor.

[0078] The spherical framework precursor was placed in a tube furnace and heated to 900°C at a rate of 3°C / min under nitrogen protection, and held at that temperature for 3 hours. After the furnace temperature cooled to room temperature, the carbonized product was placed in a 2 mol / L sodium hydroxide solution and treated at 80°C for 6 hours to remove the first and second silica templates. After treatment, the product was repeatedly washed with deionized water until the washing solution was neutral, and then vacuum dried at 100°C for 12 hours to obtain a rigid porous carbon framework.

[0079] like Figure 3 As shown, nitrogen adsorption-desorption tests revealed that the total pore volume of the obtained rigid porous carbon framework was 1.10 cm³ / g, the peak pore size of the carrier pores was 18 nm, and the peak pore size of the buffer pores was 91 nm. The carrier pores, buffer pores, and their necks are interconnected, forming a three-dimensional porous carbon structure with continuous conductive pathways.

[0080] (2) Surface activation of rigid porous carbon framework The obtained rigid porous carbon framework was added to a 20% urea aqueous solution at a mass ratio of 1:10, and stirred and impregnated at room temperature for 4 hours. After impregnation, it was dried at 80°C to constant weight.

[0081] The dried material was placed in a nitrogen atmosphere and heated to 750°C at a heating rate of 5°C / min, and held for 2 hours to form nitrogen-containing sites on the pore walls and framework nodes, while retaining some oxygen-containing functional groups, thus obtaining an activated rigid porous carbon framework.

[0082] X-ray photoelectron spectroscopy analysis revealed that the nitrogen content on the activated rigid porous carbon framework surface was 3.3 at and the oxygen content was 5.2 at. These nitrogen-containing sites and oxygen-containing functional groups enhance the adsorption and fixation capacity of the silicon precursor at the pore walls and framework nodes.

[0083] (3) Impregnation and fixation of silicon precursor Tetraethyl orthosilicate, anhydrous ethanol, deionized water and ammonia were mixed in a volume ratio of 1:6:2:0.08 and stirred for 30 min to obtain a silicon precursor impregnation solution.

[0084] The activated rigid porous carbon framework was completely immersed in the silicon precursor wetting solution and vacuum-treated for 15 min at a gauge pressure of -0.08 MPa. Then, it was restored to atmospheric pressure and wetting continued for 30 min. The above vacuum-atmospheric pressure wetting process was repeated 4 times to increase the amount of silicon precursor entering the carrier pores and fixed on the pore walls and framework nodes.

[0085] After impregnation, the material was dried at 60°C for 6 hours and then at 120°C for 4 hours to allow tetraethyl orthosilicate to hydrolyze, condense and fix near the active sites, thus obtaining a rigid porous carbon framework loaded with silicon precursor.

[0086] (4) Formation of silicon phase and pore clearing The rigid porous carbon framework supporting the silicon precursor was uniformly mixed with magnesium powder, and the amount of magnesium powder added was controlled according to the molar ratio of magnesium to silicon dioxide formed by the conversion of the silicon precursor was 2.2:1.

[0087] The mixture was placed in an argon-protected tube furnace and heated to 650°C at a heating rate of 3°C / min, and held for 3 hours to allow the silicon precursor fixed to the walls of the support holes and the skeleton nodes to be converted into the silicon phase by magnesothermic reduction.

[0088] After cooling, the obtained product was added to a 1 mol / L hydrochloric acid solution and stirred at 60 °C for 2 h to remove magnesium oxide, unreacted magnesium, and other byproducts. It was then washed with deionized water until the washing solution was neutral, followed by a second wash with ethanol, and finally vacuum dried at 80 °C for 12 h.

[0089] By acid washing and washing, free silicon phases not fixed to the walls of the support holes and the nodes of the skeleton, as well as byproducts blocking the necks, are removed, allowing the buffer holes and necks to reopen, thus obtaining a silicon phase support skeleton.

[0090] Tests showed that the silicon phase was mainly distributed on the walls of the supporting pores and at the nodes of the skeleton. The silicon phase inside the rigid porous carbon skeleton accounted for about 88% of the total mass of the silicon phase, while the free silicon phase on the outer surface of the particles accounted for about 4.8% of the area.

[0091] After silicon phase loading and pore clearing, the remaining pore volume of the material is approximately 30% of the initial total pore volume of the rigid porous carbon framework, and the buffer pores and pore necks remain connected. The remaining buffer pore volume can provide release space for the lithium intercalation expansion of the silicon phase under high silicon loading conditions.

[0092] (5) Formation of continuous carbon shell Ten parts of silicon-phase support framework were added to 500 parts of tris(hydroxymethyl)aminomethane buffer solution with a concentration of 10 mmol / L and a pH of 8.5. 2.8 parts of dopamine hydrochloride were added and stirred at room temperature for 6 hours to allow dopamine to self-aggregate and deposit on the outer surface of the silicon-phase support framework particles.

[0093] After deposition, the material was filtered and washed with deionized water, and then vacuum dried at 80°C for 12 hours. The dried material was then placed in a nitrogen-protected tube furnace and heated to 600°C at a rate of 2°C / min, and held for 1 hour to pre-carbonize the dopamine deposition layer near the silicon phase support framework to form a buffer carbon layer. Subsequently, the temperature was further increased to 850°C and held for 2 hours to further carbonize and densify the outer carbon layer, forming a boundary carbon layer.

[0094] After cooling, the obtained material is depolymerized and sieved to obtain a silicon-carbon anode material with a carbon shell supporting the skeleton.

[0095] The obtained material was found to have a particle size D50 of 8.9 μm, and the silicon phase content in the silicon-carbon anode material was approximately 52% by mass. The average total thickness of the continuous carbon shell was approximately 32 nm, of which the average thickness of the buffer carbon layer was approximately 14 nm and the average thickness of the boundary carbon layer was approximately 18 nm.

[0096] Transmission electron microscopy revealed that a continuous carbon shell uniformly coated the outer surface of the silicon-phase support framework particles, with no obvious through cracks. The buffer carbon layer was bonded to the silicon-phase support framework, and the boundary carbon layer was continuously distributed on the outer side of the buffer carbon layer.

[0097] (6) Material property testing Following the method described in Example 1, the skeleton-supported carbon shell-coated silicon-carbon anode material, conductive carbon black, and sodium carboxymethyl cellulose / styrene-butadiene rubber composite binder obtained in this example were mixed at a mass ratio of 80:10:10 to form an anode sheet, and then assembled into a coin cell for electrochemical performance testing.

[0098] like Figure 6 As shown, the test results indicate that the material obtained in this embodiment has a higher initial discharge specific capacity than that in Examples 1 and 2, with an initial coulombic efficiency of 86.8%. After 100 cycles at 0.5C, the capacity retention rate is 87.6%.

[0099] Post-cycle observations of the electrode showed that the material particles remained largely intact, without large-area pulverization or detachment. Although the silicon phase content in this embodiment was relatively high, approximately 30% of the residual pore volume remained after silicon phase loading, allowing silicon phase expansion to be absorbed by the interconnected buffer pores. Simultaneously, the thicker buffer carbon layer and boundary carbon layer limited internal particle expansion and external interface side reactions.

[0100] The above results show that, while increasing the silicon phase content, by increasing the initial pore volume of the rigid porous carbon skeleton, retaining no less than 20% of the remaining buffer pore volume, and appropriately increasing the thickness of the continuous carbon shell, the material capacity can be improved while mitigating pore wall cracking, particle pulverization, and cycle capacity decay caused by high silicon loading.

[0101] Comparative Example 1

[0102] This comparative example provides a carbon-coated silicon-carbon anode material without a rigid porous carbon framework. Except for not preparing a rigid porous carbon framework, not selectively loading the silicon precursor into the pores, and not performing pore clearing treatment, its carbon coating and electrochemical testing conditions are set as in Example 1.

[0103] (1) Preparation of silicon-carbon mixed particles Weigh out 45 parts by weight of nano-silicon with an average particle size of 80 nm, 30 parts of phenolic resin, and 3 parts of conductive carbon black. Dissolve the phenolic resin in a mixed solvent of ethanol and water, add the nano-silicon and conductive carbon black, stir for 2 hours and ultrasonically disperse for 30 minutes to obtain a silicon-carbon mixed slurry.

[0104] The silicon-carbon mixed slurry was spray-dried to obtain a silicon-carbon mixed precursor. The silicon-carbon mixed precursor was placed in a tube furnace under nitrogen protection and heated to 750°C at a heating rate of 3°C / min, and held at that temperature for 2 hours to carbonize the phenolic resin and form mixed particles of nano-silicon and carbon materials.

[0105] After cooling, the obtained material is lightly depolymerized and sieved to obtain silicon-carbon mixed particles.

[0106] and Figure 1 and Figure 2 Compared to the skeleton support structure shown, since no pore-forming agent was used to construct a rigid porous carbon skeleton in this comparative example, the resulting material does not have interconnected support pores and buffer pores. The nano-silicon is mainly distributed in the carbon material in a mixed and locally embedded manner.

[0107] (2) Formation of the external carbon coating layer To make the carbon coating amount in this comparative example similar to that in Example 1, 10 parts of silicon-carbon mixed particles were added to 500 parts of a 10 mmol / L, pH 8.5 tris(hydroxymethyl)aminomethane buffer solution, and 2 parts of dopamine hydrochloride were added. The mixture was stirred at room temperature for 6 hours to allow dopamine to self-aggregate and deposit on the outer surface of the silicon-carbon mixed particles.

[0108] After deposition, the material was filtered and washed with deionized water, and then vacuum dried at 80°C for 12 hours. Subsequently, the dried material was placed in a nitrogen-protected tube furnace, heated to 600°C at a heating rate of 2°C / min, held for 1 hour, and then heated to 850°C and held for 2 hours to carbonize the dopamine deposition layer and form an external carbon coating layer.

[0109] After cooling, the obtained material is depolymerized and sieved to obtain a carbon-coated silicon-carbon anode material without a rigid porous carbon framework.

[0110] The particle size D50 of the obtained material was found to be 8.5 μm, the mass content of silicon phase in the obtained material was about 45%, and the average thickness of the outer carbon coating layer was about 24 nm, which is basically close to the silicon content and continuous carbon shell thickness in Example 1.

[0111] (3) Material structure testing Scanning electron microscopy and silicon elemental distribution analysis revealed that the nano-silicon in the obtained material did not form a support structure distributed along the pore walls and framework nodes. Some nano-silicon formed localized agglomeration regions inside the particles and on the surface. No support pores or buffer pores with clearly defined pore sizes were detected inside the particles, nor were there any residual buffer pores that remained connected after silicon phase loading.

[0112] Transmission electron microscopy revealed that the outer carbon coating covered the outer surface of the silicon-carbon hybrid particles, but the contact between the internal nano-silicon and carbon materials was mainly random. The spacing between some adjacent nano-silicon particles was small, failing to provide a stable and continuous release space for the volume expansion during the lithium intercalation process of the nano-silicon.

[0113] (4) Material performance testing Following the method described in Example 1, the carbon-coated silicon-carbon anode material, conductive carbon black, and sodium carboxymethyl cellulose / styrene-butadiene rubber composite binder obtained in this comparative example were mixed at a mass ratio of 80:10:10 to form an anode sheet, and then assembled into a coin cell for electrochemical performance testing.

[0114] Test results show that the initial coulombic efficiency of the material obtained in this comparative example is 84.3%; after 100 cycles at 0.5C, the capacity retention rate is 72.8%, which is lower than the capacity retention rate of Example 1.

[0115] The observation results of the electrode cross-section and particle morphology after cycling showed that the silicon-carbon mixed particles in this comparative example exhibited obvious local cracking and pulverization, and gaps were formed between some nano-silicon agglomerates and the surrounding carbon material, disrupting the conductive contact between particles. The charge transfer impedance of the material after cycling also increased more significantly than that in Example 1.

[0116] The above results indicate that while mixing nano-silicon with carbon materials and forming a carbon coating layer on the outer surface of the particles can reduce the direct contact between the electrolyte and the silicon phase to some extent, the lack of a rigid porous carbon framework with supporting and buffering pores within the material prevents the dispersion and fixation of the silicon phase and fails to provide a continuous internal buffer space for volume changes in the silicon phase. Therefore, with essentially the same silicon content and external carbon coating thickness, the particle integrity, impedance stability, and cycle capacity retention of this comparative example are all lower than those of Example 1.

[0117] Comparative Example 2

[0118] This comparative example provides a framework-supported carbon shell-coated silicon-carbon anode material with insufficient residual buffer pore volume. The preparation of the rigid porous carbon framework, surface activation, silicon phase transformation, external carbon shell formation, and electrochemical testing conditions are basically the same as those in Example 1. The difference lies in increasing the number of wetting cycles of the silicon precursor, so that the silicon phase excessively fills the pores inside the rigid porous carbon framework.

[0119] (1) Preparation and surface activation of rigid porous carbon framework Following the method described in Example 1, 100 parts of phenolic resin, 30 parts of a first silica template with an average particle size of 15 nm, 60 parts of a second silica template with an average particle size of 70 nm, and 3 parts of carbon nanotubes were mixed, and then spray-dried, carbonized in a nitrogen atmosphere, and treated with an alkaline solution to remove the template, to obtain a rigid porous carbon framework.

[0120] Nitrogen adsorption-desorption tests showed that the total pore volume of the obtained rigid porous carbon framework was 0.91 cm³ / g, the peak pore size of the carrier pores was 15 nm, and the peak pore size of the buffer pores was 71 nm.

[0121] The obtained rigid porous carbon framework was immersed in a 20% urea aqueous solution for 4 hours, dried, and then heat-treated at 750°C for 2 hours in a nitrogen atmosphere to form nitrogen-containing sites on the pore walls and framework nodes, while retaining some oxygen-containing functional groups, thus obtaining an activated rigid porous carbon framework.

[0122] (2) Excessive wetting of silicon precursor A silicon precursor impregnation solution was prepared by mixing tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia in a volume ratio of 1:6:2:0.08.

[0123] The activated rigid porous carbon framework was completely immersed in the silicon precursor wetting solution and vacuum-treated for 15 min at a gauge pressure of -0.08 MPa. Then, it was restored to atmospheric pressure and continued to be immersed for 30 min.

[0124] Unlike Example 1, this comparative example repeats the above vacuum-atmospheric pressure impregnation process 6 times, and performs a 60°C pre-drying after each two impregnations, so that more silicon precursors can continuously enter the carrier hole, buffer hole and hole neck.

[0125] After impregnation, the material was dried at 60°C for 6 hours and then at 120°C for 4 hours to obtain a rigid porous carbon framework with an overloaded silicon precursor.

[0126] (3) Silicon phase formation and pore treatment The rigid porous carbon framework supporting the silicon precursor was uniformly mixed with magnesium powder, and the amount of magnesium powder added was controlled according to the molar ratio of magnesium to silicon dioxide formed by the conversion of the silicon precursor was 2.2:1.

[0127] The mixture was placed in an argon-protected tube furnace and heated to 650°C at a heating rate of 3°C / min, and held at that temperature for 3 hours to allow the silicon precursor to be converted into the silicon phase by magnesothermic reduction.

[0128] After cooling, the obtained product was added to a 1 mol / L hydrochloric acid solution and stirred at 60 °C for 2 h to remove magnesium oxide, unreacted magnesium, and other soluble byproducts. It was then washed with deionized water until the washing solution was neutral and vacuum dried at 80 °C for 12 h to obtain a silicon-phase support framework.

[0129] Because the silicon precursor undergoes multiple wetting cycles, some silicon phase, besides being distributed on the walls of the support holes and at the framework nodes, also enters the buffer holes and the necks of the holes, forming a continuous filling in localized areas. Although acid washing removes the reaction byproducts, it cannot release the buffer space occupied by the silicon phase.

[0130] like Figure 3 As shown, pore structure testing revealed that the remaining pore volume after silicon phase loading was only about 12% of the initial total pore volume of the rigid porous carbon framework, which is less than 20%. Some buffer pores and pore necks were blocked by the silicon phase, and the degree of pore connectivity was significantly lower than in Example 1.

[0131] (4) Formation of continuous carbon shell Ten parts of silicon-phase support framework were added to 500 parts of tris(hydroxymethyl)aminomethane buffer solution with a concentration of 10 mmol / L and a pH of 8.5. Two parts of dopamine hydrochloride were added, and the mixture was stirred at room temperature for 6 hours to allow dopamine to self-aggregate and deposit on the outer surface of the particles.

[0132] After deposition, the material was filtered, washed, and vacuum dried at 80°C for 12 hours. The dried material was then placed in a nitrogen-protected tube furnace, heated to 600°C and held for 1 hour, then heated to 850°C and held for 2 hours to form a continuous carbon shell on the outer surface of the particles.

[0133] After cooling, the obtained material is depolymerized and sieved to obtain a skeleton-supported carbon shell-coated silicon-carbon anode material with insufficient remaining buffer pore volume.

[0134] The obtained material was found to have a particle size D50 of 9.0 μm, a silicon phase mass content of approximately 58%, and an average total thickness of approximately 26 nm for the continuous carbon shell, of which the average thickness of the buffer carbon layer is approximately 10 nm and the average thickness of the boundary carbon layer is approximately 16 nm.

[0135] (5) Material structure testing Scanning electron microscopy and silicon elemental distribution analysis results show that the silicon phase in this comparative example is mainly located inside the rigid porous carbon framework, but some buffer pores are occupied by a large area of ​​silicon phase, and the local silicon phases are interconnected to form a relatively dense filling area.

[0136] Compared to Example 1, although this comparative example has a higher silicon loading, the remaining pore volume available for absorbing the volume expansion of the silicon phase is significantly reduced. Some pore necks become blocked, restricting the ion transport channels and expansion release channels within the particles.

[0137] (6) Material property testing Following the method described in Example 1, the material obtained in this comparative example was made into a negative electrode sheet and assembled into a coin cell. Charge and discharge tests were conducted within a voltage range of 0.01 to 1.5V.

[0138] like Figure 6 As shown, the test results indicate that the initial discharge specific capacity of the material obtained in this comparative example is higher than that of Example 1, and the initial coulombic efficiency is 85.6%; however, after 100 cycles at 0.5C, the capacity retention rate is only 68.9%, which is significantly lower than that of Example 1.

[0139] like Figure 7 and Figure 8 As shown, the particle morphology observation results after cycling indicate that some particles in this comparative example exhibit pore wall cracking, localized damage to the outer carbon shell, and particle edge pulverization. The growth rate of electrode thickness and charge transfer impedance after cycling are both higher than those in Example 1.

[0140] The above results indicate that while increasing the silicon phase loading can increase the initial capacity of the material, when the remaining pore volume after silicon phase loading is less than 20% of the initial total pore volume, the buffer pores cannot provide sufficient release space for the lithium intercalation expansion of the silicon phase. Local expansion stress easily concentrates on the pore walls and the outer carbon shell, leading to pore wall cracking, pore blockage, and deterioration of conductive contacts. Therefore, maintaining a remaining pore volume of no less than 20% of the initial total pore volume after silicon phase loading is an important condition for balancing silicon loading and cycle stability.

[0141] Comparative Example 3

[0142] This comparative example provides a silicon-carbon anode material with a framework-supported carbon shell that does not undergo surface activation treatment of the rigid porous carbon framework. Except for omitting the surface activation step of the rigid porous carbon framework, the other raw materials, silicon precursor wetting, silicon phase transformation, pore unblocking, continuous carbon shell formation, and electrochemical testing conditions are basically the same as in Example 1.

[0143] (1) Preparation of rigid porous carbon framework Weigh out 100 parts of phenolic resin, 30 parts of first silica template with an average particle size of 15 nm, 60 parts of second silica template with an average particle size of 70 nm, and 3 parts of carbon nanotubes by weight.

[0144] The above raw materials were dispersed in a mixed solvent of ethanol and water, stirred for 2 hours, and then ultrasonically dispersed to obtain a homogeneous slurry. The slurry was then spray-dried to obtain a spherical framework precursor.

[0145] The spherical framework precursor was placed in a tube furnace under nitrogen protection and heated to 900°C at a heating rate of 3°C / min, and held at that temperature for 3 hours. After cooling, the carbonized product was placed in a 2 mol / L sodium hydroxide solution and treated at 80°C for 6 hours to remove the first and second silica templates. It was then washed with deionized water until the washing solution was neutral, and vacuum dried at 100°C for 12 hours to obtain a rigid porous carbon framework without surface activation treatment.

[0146] Nitrogen adsorption-desorption tests showed that the total pore volume of the obtained rigid porous carbon framework was 0.91 cm³ / g, the peak pore size of the carrier pore was 15 nm, the peak pore size of the buffer pore was 71 nm, and the carrier pore and the buffer pore were interconnected through the pore neck.

[0147] This comparative example does not employ urea impregnation or subsequent nitrogen-containing heat treatment, and therefore does not actively construct nitrogen-containing sites for adsorbing silicon precursors at the pore walls and framework nodes.

[0148] (2) Wetting of silicon precursor A silicon precursor impregnation solution was prepared by mixing tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia in a volume ratio of 1:6:2:0.08.

[0149] The rigid porous carbon framework without surface activation treatment was completely immersed in the silicon precursor wetting solution and vacuum-treated for 15 min at a gauge pressure of -0.08 MPa. Then, it was restored to atmospheric pressure and wetting continued for 30 min. The above vacuum-atmospheric pressure wetting process was repeated 3 times.

[0150] After impregnation, the material is dried at 60°C for 6 hours and then at 120°C for 4 hours to allow tetraethyl orthosilicate to hydrolyze and condense, resulting in a rigid porous carbon framework supported on silicon precursors.

[0151] Due to the lack of activated oxygen-containing functional groups and nitrogen-containing sites on the pore walls and framework nodes, the adsorption between the silicon precursor and the carbon framework is weak. Some of the silicon precursor migrates to the outer surface of the particles during drying and undergoes non-selective deposition at the buffer pores and pore necks.

[0152] (3) Silicon phase formation and pore clearing The rigid porous carbon framework supporting the silicon precursor was uniformly mixed with magnesium powder, and the amount of magnesium powder added was controlled according to the molar ratio of magnesium to silicon dioxide formed by the conversion of the silicon precursor was 2.2:1.

[0153] The mixture was placed in an argon-protected tube furnace and heated to 650°C at a heating rate of 3°C / min, and held at that temperature for 3 hours to allow the silicon precursor to be converted into the silicon phase by magnesothermic reduction.

[0154] After cooling, the obtained product was added to a 1 mol / L hydrochloric acid solution and stirred at 60 °C for 2 h to remove magnesium oxide, unreacted magnesium, and other soluble byproducts. The product was then washed with deionized water until the washing solution was neutral and vacuum dried at 80 °C for 12 h to obtain a silicon-supported framework.

[0155] After pore structure testing, the remaining pore volume of the material after silicon phase loading and pore channel clearing was approximately 28% of the initial total pore volume of the rigid porous carbon framework. Although the remaining pore volume was still higher than 20%, some silicon phase was disorderedly distributed in the buffer pores and pore necks, resulting in lower pore connectivity than in Example 1.

[0156] like Figure 4 As shown, cross-sectional scanning electron microscopy and silicon elemental distribution analysis revealed that the silicon phase located inside the rigid porous carbon framework accounted for approximately 74% of the total silicon phase mass, which is lower than in Example 1; the free silicon phase area on the outer surface of the particles accounted for approximately 12.1%, which is significantly higher than in Example 1.

[0157] Some silicon phases form agglomerated regions on the outer surface of the particles and at the entrance of the channels, while the uniformity of silicon phase distribution at the pore walls and skeleton nodes is poor.

[0158] (4) Formation of continuous carbon shell Ten parts of the silicon phase-supported framework were added to 500 parts of a 10 mmol / L, pH 8.5 tris(hydroxymethyl)aminomethane buffer solution, and two parts of dopamine hydrochloride were added. The mixture was stirred at room temperature for 6 hours to allow dopamine to self-aggregate and deposit on the outer surface of the particles.

[0159] After deposition, the material was filtered, washed, and vacuum dried at 80°C for 12 hours. The dried material was then placed in a nitrogen-protected tube furnace and heated to 600°C at a rate of 2°C / min, held for 1 hour, and then heated to 850°C and held for 2 hours to form a continuous carbon shell on the outer surface of the particles.

[0160] After cooling, the obtained material is depolymerized and sieved to obtain a silicon-carbon anode material with a skeleton-supported carbon shell that has not undergone skeleton surface activation treatment.

[0161] The obtained material was found to have a particle size D50 of 8.7 μm, a silicon phase content of approximately 44% by mass, and an average total thickness of approximately 25 nm for the continuous carbon shell, of which the average thickness of the buffer carbon layer is approximately 10 nm and the average thickness of the boundary carbon layer is approximately 15 nm.

[0162] (5) Material structure and electrochemical performance testing Following the method described in Example 1, the material obtained in this comparative example, conductive carbon black, and sodium carboxymethyl cellulose / styrene-butadiene rubber composite binder were mixed at a mass ratio of 80:10:10 to prepare a negative electrode sheet, and then assembled into a coin cell for electrochemical performance testing.

[0163] like Figure 6 As shown, the test results indicate that the initial coulombic efficiency of the material obtained in this comparative example is 82.6%; after 100 cycles at 0.5C, the capacity retention rate is 76.9%, both lower than that of Example 1.

[0164] Post-cycle particle morphology observations revealed significant expansion and cracking in the silicon phase agglomeration regions on the outer surface of the particles, with some continuous carbon shells being lifted or developing localized cracks. For example... Figure 7 As shown, the charge transfer impedance of the material after cycling increases more significantly than in Example 1.

[0165] The above results indicate that without surface activation treatment of the rigid porous carbon framework, silicon precursors are difficult to preferentially adsorb and fix onto the pore walls and framework nodes, and are prone to disordered deposition on buffer pores, pore necks, and the outer surface of particles. This leads to a decrease in the proportion of silicon phase inside the framework, an increase in free silicon phase on the surface, and exacerbates local expansion and interfacial side reactions during cycling.

[0166] Therefore, constructing oxygen-containing functional groups and / or nitrogen-containing sites at the pore walls and framework nodes of a rigid porous carbon framework is beneficial to improving the selective adsorption and fixation capacity of silicon precursors, reducing the free silicon phase on the outer surface of particles, and improving the initial coulombic efficiency and cycle stability of silicon-carbon anode materials.

[0167] Comparative Example 4

[0168] This comparative example provides a silicon-carbon anode material with a common single-layer carbon coating structure. Except that the secondary carbon source is not segmented carbonized and cannot form an inner buffer carbon layer and an outer boundary carbon layer, its rigid porous carbon framework preparation, surface activation, silicon precursor wetting, silicon phase transformation, pore unblocking, and electrochemical testing conditions are basically the same as those in Example 1.

[0169] (1) Preparation of rigid porous carbon framework Weigh out 100 parts of phenolic resin, 30 parts of first silica template with an average particle size of 15 nm, 60 parts of second silica template with an average particle size of 70 nm, and 3 parts of carbon nanotubes by weight.

[0170] The above raw materials were dispersed in a mixed solvent of ethanol and water, stirred for 2 hours, and then ultrasonically dispersed to obtain a mixed slurry. The mixed slurry was then spray-dried to obtain a spherical framework precursor.

[0171] The spherical framework precursor was placed in a tube furnace under nitrogen protection and heated to 900℃ at a heating rate of 3℃ / min, and held at that temperature for 3 h. After cooling, the carbonized product was placed in a 2 mol / L sodium hydroxide solution and treated at 80℃ for 6 h to remove the first and second silica templates. It was then washed with deionized water until the washing solution was neutral, and vacuum dried at 100℃ for 12 h to obtain a rigid porous carbon framework.

[0172] Nitrogen adsorption-desorption tests showed that the total pore volume of the obtained rigid porous carbon framework was 0.92 cm³ / g, the peak pore size of the carrier pores was 15 nm, the peak pore size of the buffer pores was 72 nm, and the carrier pores and buffer pores were interconnected through the pore neck.

[0173] (2) Surface activation of rigid porous carbon framework The obtained rigid porous carbon framework was added to a 20% urea aqueous solution at a mass ratio of 1:10, and stirred and impregnated at room temperature for 4 hours. After impregnation, it was dried at 80°C to constant weight.

[0174] The dried material was placed in a nitrogen atmosphere and heated to 750°C at a heating rate of 5°C / min, and held for 2 hours to form nitrogen-containing sites on the pore walls and framework nodes, while retaining some oxygen-containing functional groups, thus obtaining an activated rigid porous carbon framework.

[0175] (3) Impregnation and fixation of silicon precursor A silicon precursor impregnation solution was prepared by mixing tetraethyl orthosilicate, anhydrous ethanol, deionized water, and ammonia in a volume ratio of 1:6:2:0.08.

[0176] The activated rigid porous carbon framework was completely immersed in the silicon precursor wetting solution and vacuum-treated for 15 min at a gauge pressure of -0.08 MPa. Then, it was restored to atmospheric pressure and wetting continued for 30 min. The above vacuum-atmospheric pressure wetting process was repeated 3 times.

[0177] After impregnation, the material is dried at 60°C for 6 hours and then at 120°C for 4 hours to hydrolyze, condense and fix the silicon precursor at the pore walls and framework nodes, thus obtaining a rigid porous carbon framework loaded with silicon precursor.

[0178] (4) Formation of silicon phase and pore clearing The rigid porous carbon framework supporting the silicon precursor was uniformly mixed with magnesium powder, and the amount of magnesium powder added was controlled according to the molar ratio of magnesium to silicon dioxide formed by the conversion of the silicon precursor was 2.2:1.

[0179] The mixture was placed in an argon-protected tube furnace and heated to 650°C at a heating rate of 3°C / min, and held at that temperature for 3 hours to allow the silicon precursor to be converted into the silicon phase by magnesothermic reduction.

[0180] After cooling, the obtained product was added to a 1 mol / L hydrochloric acid solution and stirred at 60 °C for 2 h to remove magnesium oxide, unreacted magnesium, and byproducts clogging the pores. It was then washed with deionized water until the washing solution was neutral, followed by a second wash with ethanol, and finally vacuum dried at 80 °C for 12 h to obtain the silicon phase support framework.

[0181] Testing revealed that the silicon phase within the rigid porous carbon framework accounts for approximately 86% of the total silicon phase mass, while the free silicon phase on the outer surface of the particles accounts for approximately 4.5% of the total area. After the silicon phase is loaded and the pores are cleared, the remaining pore volume of the material is approximately 31% of the initial total pore volume of the rigid porous carbon framework, and the buffer pores and pore necks remain connected.

[0182] (5) Formation of ordinary single-layer carbon coating Ten parts of silicon-phase support framework were added to 500 parts of tris(hydroxymethyl)aminomethane buffer solution with a concentration of 10 mmol / L and a pH of 8.5. Two parts of dopamine hydrochloride were added, and the mixture was stirred at room temperature for 6 hours to allow dopamine to self-aggregate and deposit on the outer surface of the silicon-phase support framework particles.

[0183] After deposition, the material was filtered and washed with deionized water, and then vacuum dried at 80°C for 12 hours.

[0184] Unlike Example 1, this comparative example does not perform pre-carbonization in the range of 450-650°C. Instead, the dried material is placed directly in a tube furnace under nitrogen protection and continuously heated to 850°C at a heating rate of 5°C / min. The temperature is then maintained at 850°C for 2 hours to carbonize the dopamine deposition layer in one step, forming a common single-layer carbon coating layer.

[0185] After cooling, the obtained material is depolymerized and sieved to obtain a silicon-carbon anode material with a common single-layer carbon coating structure.

[0186] The obtained material was found to have a particle size D50 of 8.7 μm, a silicon phase content of approximately 45% by mass, and an average thickness of approximately 25 nm for the single-layer carbon coating, which is basically the same as the average total thickness of the continuous carbon shell in Example 1.

[0187] like Figure 5 As shown, transmission electron microscopy and Raman spectroscopy results indicate that the carbon coating layer exhibits a relatively uniform degree of carbonization, without the formation of a clearly distinguishable inner buffer carbon layer and outer boundary carbon layer. The monolayer carbon coating layer has a relatively dense structure, and the stress transition region between it and the silicon-phase support framework is not obvious.

[0188] (6) Material structure and electrochemical performance testing Following the method described in Example 1, the silicon-carbon anode material obtained in this comparative example, conductive carbon black, and sodium carboxymethyl cellulose / styrene-butadiene rubber composite binder were mixed at a mass ratio of 80:10:10 to form an anode sheet, and then assembled into a coin cell for electrochemical performance testing.

[0189] like Figure 6 As shown, the test results indicate that the initial coulombic efficiency of the material obtained in this comparative example is 86.9%; after 100 cycles at 0.5C, the capacity retention rate is 80.7%, which is lower than that of Example 1.

[0190] like Figure 7 As shown, under 2C rate conditions, the discharge specific capacity of this comparative example is lower than that of Examples 1 and 2. AC impedance testing results indicate that the interface impedance of this comparative example before cycling is slightly higher than that of Example 1, and the increase in charge transfer impedance after 100 cycles is also higher than that of Example 1.

[0191] like Figure 8 As shown, the particle morphology observation results after cycling indicate that some particles exhibit localized cracks in the carbon coating layer or separation from the internal silicon phase support framework. Because the monolayer carbon coating layer lacks a stress buffer region near the silicon phase support framework, the localized stress generated by silicon phase expansion is easily and directly transmitted to the carbon coating layer. Simultaneously, the relatively dense monolayer carbon coating layer exerts significant resistance to lithium-ion transport.

[0192] The above results indicate that, under the condition that the total thickness of the carbon coating layer and the mass content of the silicon phase are basically the same, it is difficult for a conventional single-layer carbon coating layer formed by a single high-temperature carbonization to simultaneously provide internal expansion buffering and external interface protection. By first performing low-temperature pre-carbonization to form a buffer carbon layer that fits the silicon phase support framework, and then performing high-temperature carbonization to form an outer boundary carbon layer, a division of labor in terms of structure and function can be formed within the continuous carbon shell.

[0193] The buffer carbon layer absorbs the local stress generated by the cyclic expansion of the silicon phase, while the boundary carbon layer reduces the direct contact between the electrolyte and the silicon phase and stabilizes the external interface of the particles. Therefore, the double-layer continuous carbon shell formed by segmented carbonization helps to reduce carbon shell cracking, control impedance growth, and improve the cycle stability of silicon-carbon anode materials.

[0194] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A skeleton-supported carbon shell-coated silicon-carbon anode material, characterized in that, It includes a rigid porous carbon framework, a silicon phase, and a continuous carbon shell; The rigid porous carbon framework has an interconnected pore structure, which includes a support pore and a buffer pore. The support pore is used to fix the silicon phase, and the buffer pore is used to reserve space for volume changes during the charging and discharging process of the silicon phase. The silicon phase is distributed on the pore walls of the bearing pores and at the skeleton nodes of the rigid porous carbon skeleton, and the buffer pores remain connected after the silicon phase is loaded. The continuous carbon shell covers the outer surface of the rigid porous carbon skeleton loaded with the silicon phase. The continuous carbon shell includes a buffer carbon layer near the rigid porous carbon skeleton and a limiting carbon layer on the outer side. The rigid porous carbon framework has a total pore volume of 0.55-1.35 cm³ / g before loading the silicon phase, and retains a residual pore volume of not less than 20% of the total pore volume after loading the silicon phase. The mass content of the silicon phase in the silicon-carbon anode material is 25%-65%, and the average thickness of the continuous carbon shell is 8-60 nm.

2. The skeleton-supported carbon shell-coated silicon-carbon anode material according to claim 1, characterized in that, The supporting pores and the buffer pores are interconnected within the rigid porous carbon skeleton. The peak pore diameter of the supporting pores is 5-25 nm, and the peak pore diameter of the buffer pores is 35-120 nm. The carrier holes are used to disperse and fix the silicon phase along the hole walls, and the buffer holes are used to provide releasable space when the silicon phase expands during lithium intercalation, so as to reduce the cracking of the pore walls of the rigid porous carbon skeleton.

3. The skeleton-supported carbon shell-coated silicon-carbon anode material according to claim 1, characterized in that, include: The silicon phase includes one or more of nano-silicon, silicon-oxygen phase, and silicon-carbon transition phase; The portion of the silicon phase located inside the rigid porous carbon skeleton accounts for more than 80% of the total mass of the silicon phase, and the area of ​​the free silicon phase on the outer surface of the silicon-carbon anode material is no more than 8%.

4. The skeleton-supported carbon shell-coated silicon-carbon anode material according to claim 1, characterized in that, include: The rigid porous carbon framework has oxygen-containing functional groups and / or nitrogen-containing sites at the pore walls and framework nodes. The oxygen-containing functional groups and / or nitrogen-containing sites are used to improve the adsorption and fixation ability of silicon precursor at the pore walls and framework nodes, so that the silicon phase is preferentially formed at the pore walls and framework nodes.

5. The skeleton-supported carbon shell-coated silicon-carbon anode material according to claim 1, characterized in that, include: The thickness of the buffer carbon layer in the continuous carbon shell is 3-25 nm, and the thickness of the limiting carbon layer is 5-35 nm. The buffer carbon layer is fitted to the rigid porous carbon skeleton supported by the silicon phase to buffer the internal volume change of the particles. The boundary carbon layer is located outside the buffer carbon layer to reduce the direct contact between the electrolyte and the silicon phase.

6. A method for preparing a silicon-carbon anode material with a carbon shell supporting a framework, characterized in that, The preparation of the framework-supported carbon shell-coated silicon-carbon anode material as described in any one of claims 1-5 includes: S1. Prepare a rigid porous carbon framework with interconnected pore structures, wherein the pore structure includes support pores for fixing the silicon phase and buffer pores for reserving space for volume changes. S2. Surface activation treatment is performed on the rigid porous carbon framework to form active sites for adsorbing silicon precursors at the pore walls of the supporting pores and the framework nodes of the rigid porous carbon framework. S3. Introduce the silicon precursor into the surface-activated rigid porous carbon framework, so that the silicon precursor is preferentially adsorbed and fixed at the pore walls and framework nodes of the supporting pores. S4. Heat treatment is performed on the rigid porous carbon skeleton loaded with the silicon precursor to transform the silicon precursor into a silicon phase distributed in the pore walls and skeleton nodes of the support pores, thereby obtaining a silicon phase support skeleton. S5. Perform a channel clearing treatment on the silicon phase support skeleton to remove free silicon phase or byproducts that are not fixed to the support hole wall and skeleton node, so that the buffer hole remains connected after silicon phase loading. S6. A secondary carbon source is deposited on the outer surface of the silicon phase support skeleton, and the secondary carbon source is subjected to segmented carbonization treatment to form a continuous carbon shell on the outer surface of the silicon phase support skeleton. The continuous carbon shell includes a buffer carbon layer close to the silicon phase support skeleton and a boundary carbon layer on the outside, thereby obtaining the skeleton support carbon shell coating silicon-carbon anode material. The rigid porous carbon framework has a total pore volume of 0.55-1.35 cm³ / g before loading the silicon phase, and retains a residual pore volume of not less than 20% of the total pore volume after loading the silicon phase. The mass content of the silicon phase in the obtained silicon-carbon anode material is 25%-65%, and the average thickness of the continuous carbon shell is 8-60 nm.

7. The preparation method according to claim 6, characterized in that, In preparing the rigid porous carbon framework, the carbon source, pore-forming agent and conductive reinforcing material are mixed and shaped, then carbonized in an inert atmosphere, and then the pore-forming agent is removed. In this process, by adjusting the particle size and amount of the pore-forming agent, the resulting rigid porous carbon skeleton is made to form carrier pores with smaller pore sizes and buffer pores with larger pore sizes, and the carrier pores and the buffer pores are kept in communication.

8. The preparation method according to claim 6, characterized in that, The surface activation treatment includes one or more of the following: oxidation treatment, ammonia treatment, urea heat treatment, melamine heat treatment, and dopamine deposition treatment. After the surface activation treatment, oxygen-containing functional groups and / or nitrogen-containing sites are formed on the pore walls and framework nodes of the rigid porous carbon framework, so that the subsequently introduced silicon precursor is preferentially adsorbed on the pore walls and framework nodes of the supporting pores, rather than being randomly deposited in all the pores.

9. The preparation method according to claim 6, characterized in that, When introducing a silicon precursor into a surface-activated rigid porous carbon framework, vacuum wetting, pressure wetting, or cyclic wetting methods are used to allow the silicon precursor to enter the carrier pores. By controlling the silicon precursor concentration, wetting time, or number of wetting cycles, the silicon phase loading formed after heat treatment is matched with the remaining pore volume of the buffer pore.

10. The preparation method according to claim 6, characterized in that, When performing segmented carbonization on the secondary carbon source, the secondary carbon source deposited on the outer surface of the silicon phase support skeleton is first pre-carbonized at 450-650℃ to form a buffer carbon layer that fits the silicon phase support skeleton; then, high-temperature carbonization is performed at 750-950℃ to form a boundary carbon layer on the outside of the buffer carbon layer, thereby obtaining a continuous carbon shell with inner buffering and outer boundary functions.

Images