Porous silicon carbon negative electrode material and preparation method thereof

Abstract

The invention provides a porous silicon-carbon negative electrode material and a preparation method thereof, the porous silicon-carbon negative electrode material comprises a porous carbon matrix material, nano silicon particles are formed in carbon pores of the porous carbon matrix material, a carbon coating layer is arranged on the outer surface of the porous carbon matrix material, and the nano silicon particles are formed by reducing silicon dioxide. The preparation method comprises the following steps: infiltrating estersil steam into carbon holes of a porous carbon matrix material, decomposing estersil in the carbon holes, depositing a generated silicon compound in the carbon holes, calcining the silicon compound to generate a thermal decomposition reaction to generate nano silicon dioxide particles which are uniformly and tightly attached into the carbon holes, and adopting active metal as a reducing agent to prepare the nano silicon dioxide composite material. And reducing the nano silicon dioxide particles into silicon, and carrying out carbon coating on the nano silicon porous carbon particles by adopting a chemical vapor deposition method.

Description

Technical Field

[0001] This invention relates to the field of batteries, and in particular to porous silicon-carbon anode materials and their preparation methods. Background Technology

[0002] Silicon is widely recognized as the next-generation high-capacity lithium-ion battery anode material due to its ultra-high theoretical specific capacity (~4200 mAh / g), moderate operating voltage, and abundant natural resources. However, silicon anodes undergo significant volume expansion (~300%) during charge and discharge, which can easily lead to material pulverization and peeling off from the current collector, resulting in decreased electrode cycle performance or even failure. Furthermore, silicon has a low intrinsic electronic conductivity (<10). -8 The low Ω / cm (Ω / cm) results in slow electrode reaction kinetics and poor electrode rate performance.

[0003] To address the aforementioned issues, current methods primarily focus on improving material structural stability and electrode cycling characteristics through material nano-sizing and composite materials.

[0004] For nanotechnology, smaller particle size can quickly dissipate the structural stress generated by particle expansion / contraction during lithium insertion / extraction, improving the structural stability of the material. On the other hand, the smaller particle size can effectively shorten the Li+ transport path, accelerate the Li+ transport rate in the Si anode, and thus improve the electrode reaction kinetics. For example, CN109755517A) describes an invention that uses mechanical sand milling to mix and grind raw materials such as silicon, carbon matrix, conductive agent, and binder, and obtains a nano-silicon-carbon composite material through subsequent drying and heat treatment steps. In this structure, nano-Si is well dispersed in the carbon matrix, preventing agglomeration during cycling and effectively utilizing the structural advantages of nano-Si, resulting in good cycling performance. However, the nano-Si particle size obtained by mechanical sand milling is generally around 100 nm, making it difficult to obtain smaller particle sizes, thus resulting in poor material cycling performance. In addition, since the two are bonded together by mechanical force, the bonding between nano-Si and the carbon matrix is ​​poor, causing it to peel off from the carbon matrix during cycling, leading to a decline in cycling performance.

[0005] Composite technology involves combining Si with a highly conductive carbon matrix to improve the electronic conductivity of the material. Currently, the main approach is to load nano-Si particles within a porous carbon framework to form porous silicon-carbon materials. On one hand, the carbon framework provides a three-dimensional electronic conductivity network, accelerating electron conduction between particles and improving electrode reaction kinetics. On the other hand, the carbon pores can "confine" the volume expansion of Si particles, preventing particles from peeling off from the carbon matrix due to expansion and improving the structural stability of Si materials.

[0006] US12155066B2 discloses an anode material and its preparation method, which involves permeating and pyrolyzing a silicon source gas, SiH4, into the pores of porous carbon to form small-sized nano-Si particles. The outer surface of these porous carbon particles is then coated with carbon to obtain a porous silicon-carbon anode material. Similarly, CN117317204A and CN114051663A disclose silicon-carbon anode materials with similar structures. However, in these methods, the nano-Si particles obtained from SiH4 pyrolysis lack functional groups that can bond with the porous carbon surface. Therefore, the bonding between the nano-Si and the porous carbon is weak, and Si easily separates from the carbon matrix during cycling, causing performance degradation. Furthermore, SiH4 is a highly flammable and explosive special gas, posing significant safety hazards during transportation and use. Summary of the Invention

[0007] To address the aforementioned problems, the present invention provides a porous silicon-carbon anode material comprising a porous carbon matrix material, wherein nano-silicon particles are formed within the carbon pores of the porous carbon matrix, and a carbon coating layer is present on the outer surface of the porous carbon matrix material, wherein the nano-silicon particles are formed by reducing silicon dioxide.

[0008] This invention also provides a method for preparing porous silicon-carbon anode material, comprising the following steps:

[0009] S1: Silicon ester vapor is generated by heating silicon ester material as a silicon source to form silicon ester vapor;

[0010] S2: The porous carbon matrix material is placed in a silicon ester vapor environment and a certain temperature and pressure are applied to carry out a silicon ester evaporation-condensation reaction, so that the silicon ester vapor permeates into the carbon pores of the porous carbon matrix material, the silicon ester in the carbon pores decomposes, and the generated silicon compounds are deposited in the carbon pores to form porous carbon containing silicon compounds; the porous carbon matrix material is porous carbon particles.

[0011] S3: The porous carbon containing silicon compounds in step S2 is calcined in an inert gas atmosphere, so that the silicon compounds therein undergo a thermal decomposition reaction to generate uniform and tightly attached nano-silica particles in the carbon pores, thus obtaining porous carbon containing nano-silica.

[0012] S4: Using an active metal as a reducing agent, the nano-silica particles in porous carbon containing nano-silica are reduced to silicon;

[0013] S5: The product obtained in step S4 is placed in a dilute hydrochloric acid solution and stirred at room temperature for several hours, then filtered to obtain a solid product; the solid product is then washed with deionized water and finally dried to obtain nano-silicon porous carbon particles.

[0014] S6: Coating: The nano-silicon porous carbon particles obtained in step S5 are coated with carbon using chemical vapor deposition to form a carbon coating layer on their surface, thereby forming a porous silicon-carbon anode material.

[0015] Further, in step S4, the active metal powder, alkali metal salt and porous carbon containing nano-silica are mixed in a certain mass ratio and calcined in an inert gas atmosphere.

[0016] Furthermore, in step S4, the heating rate for calcination in an inert gas atmosphere is 2–10 °C / min, the holding temperature is 650–800 °C, and the holding time is 1–20 h.

[0017] Furthermore, in step S3, the porous carbon containing silicon compounds is calcined in an inert gas atmosphere at a heating rate of 2–10 °C / min, a holding temperature of 700–900 °C, and a holding time of 1–20 h.

[0018] Further, in step S6, the nano-silicon porous carbon particles are heated in a mixed gas atmosphere of carbon source gas and inert gas at a temperature of 600-800℃ for 0.5-5h, so that the elemental carbon decomposed from the carbon source gas is deposited on the nano-silicon porous carbon particles to obtain the final silicon-carbon material product.

[0019] Furthermore, the carbon source gas is one or more of methane, ethane, acetylene, and toluene, and the volume fraction of the carbon source gas in the mixed gas is 5% to 30%.

[0020] Furthermore, the silicone ester material is one or more of methyl orthosilicate and ethyl orthosilicate.

[0021] Furthermore, the mass ratio of the active metal powder, alkali metal salt, and porous carbon containing nano-silica is 0.4–2:5–20:1.

[0022] This invention utilizes liquid silicon ester with low-temperature volatility as the silicon source to replace methane silicon gas in the preparation of porous silicon-carbon anode materials, resulting in higher safety and significantly reduced storage, transportation, and usage costs. Benefiting from the dehydration condensation and bonding reactions between the hydroxyl and alkoxy groups in the silicon compounds from the silicon ester pyrolysis products and the oxygen- and hydroxyl-containing functional groups on the carbon matrix surface, the nano-Si particles in the silicon-carbon material exhibit strong chemical bonding with the porous carbon matrix. This enhances the adhesion of Si particles to the carbon matrix, preventing Si particles from detaching from the carbon matrix surface due to repeated expansion / contraction during lithium insertion / extraction, thus improving the structural stability and cycle performance of the material. The uniform deposition of nano-Si within the pores of the porous carbon structure effectively buffers the volume expansion of nano-Si during charge-discharge reactions and "confines" the nano-Si, preventing breakage and detachment from the carbon matrix, thereby improving the structural stability of the composite material and obtaining a silicon-carbon anode material with long-term cycle stability. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the porous silicon-carbon anode material structure;

[0024] Figure 2 This is a SEM image of a porous silicon-carbon anode material;

[0025] Figure 3 This is the first charge-discharge curve of the porous silicon-carbon material. Detailed Implementation

[0026] like Figure 1-2 As shown, the porous silicon-carbon anode material structure mainly includes a porous carbon matrix material 1 (usually porous carbon particles), nano-silicon particles 3 formed within the carbon pores 2 of the porous carbon matrix 1, and a carbon coating layer 4 on the outer surface of the porous carbon matrix material 1. To form this structure, especially to penetrate the nano-Si particles into the carbon pores, this invention mainly employs a method of silicon ester vapor permeation precipitation, silicon ester decomposition to form silicon dioxide, and reduction of silicon dioxide using a reducing agent to form nano-silicon particles. Compared to the traditional SiH4 pyrolysis, this method is safer and has stronger silicon adhesion.

[0027] Specifically, the preparation method of the porous silicon-carbon anode material of the present invention includes the following steps:

[0028] S1: Generating silicon ester vapor. A suitable silicon ester material is selected as the silicon source and heated to form silicon ester vapor. Silicon ester is an ester compound containing silicon, which undergoes a thermal decomposition reaction at a certain temperature to generate SiO2. Preferably, a silicon ester liquid (methyl orthosilicate, ethyl orthosilicate) with a low vaporization temperature (<180℃) is selected as the silicon source.

[0029] S2: A porous carbon matrix material is placed in a silicon ester vapor environment, and a certain temperature and pressure are applied to carry out a silicon ester evaporation-condensation reaction, forming porous carbon containing silicon compounds. On the one hand, the silicon ester vapor permeates into the carbon pores of the porous carbon matrix material; on the other hand, the silicon ester within the carbon pores decomposes, and the resulting silicon compounds are deposited within the carbon pores. During the preparation process, the surface of the porous carbon has some oxygen-containing and hydroxyl-containing functional groups (e.g., prepared from biological raw materials; chemically activated (KOH, H3PO4) reacts with carbon to generate carboxyl groups, carbonyl groups, etc.; or pyridine nitrogen, thiols, etc., functional groups formed when nitrogen and sulfur are co-doped). The hydroxyl and alkoxy groups of the silicon compounds can react with the oxygen-containing and hydroxyl-containing functional groups in the porous carbon matrix material to form chemical bonds, causing the silicon compounds to adhere tightly to the carbon pores.

[0030] For example, for methyl orthosilicate, the temperature can be 130–250°C and the pressure 1–8 MPa; for ethyl orthosilicate, the temperature can be 170–300°C and the pressure 1–10 MPa.

[0031] Porous carbon matrix materials typically employ porous carbon particles with an average pore size of 0.5–10 nm and a particle size D50 of 2–10 μm. Furthermore, to accommodate a sufficient number of nano-Si particles, the specific pore volume of the porous carbon matrix material is 0.5–1.5 ml / g. Preferably, the specific surface area of ​​the porous carbon matrix material is ≥1600 m². 2 / g, tap density ≥0.6g / cm³ 3 .

[0032] S3: The porous carbon containing silicon compounds in step S2 is calcined in an inert gas atmosphere, causing the silicon compounds to undergo a thermal decomposition reaction, generating uniform and tightly attached nano-silica particles in the carbon pores, thus obtaining porous carbon containing nano-silica.

[0033] The inert atmosphere includes nitrogen and argon, 5-10% H2 / Ar; the calcination temperature is 600-1000℃.

[0034] Preferably, during calcination, the temperature can be gradually increased, for example, at a heating rate of 2–10 °C / min. An appropriate heating rate ensures good dispersion of the SiO2 particles. Furthermore, after reaching the calcination temperature, a holding period of 700–900 °C is implemented for 1–20 hours. Appropriate holding temperature and time promote the complete decomposition of the silicon compound, generating silicon dioxide.

[0035] S4: A reduction reaction is carried out on porous carbon containing nano-silica, reducing the nano-silica particles to silicon.

[0036] Specifically, active metal powders (such as Li, Mg, and Al powders), alkali metal salts (NaCl, KCl, NaBr, etc.) are mixed with porous carbon containing nano-silica in a certain mass ratio (0.4–2:5–20:1) and calcined under an inert gas atmosphere. During calcination, SiO2 undergoes a redox reaction with the active metal powders, reducing nano-SiO2 to nano-Si and generating active metal oxides (Li2O, MgO, Al2O3) as byproducts. Since the alkali metal salts have suitable melting points, they can absorb the large amount of heat generated by the reduction reaction between SiO2 and the active metal powders through their own melting, controlling the reaction temperature and providing a constant temperature environment for the reaction. This avoids excessive growth of nano-Si grain size, improves the structural stability and electrochemical cycling performance of the material during cycling.

[0037] During the calcination process, the heating rate is 2–10 °C / min, the holding temperature is 650–800 °C, and the holding time is 1–20 h. Appropriate holding temperatures are beneficial for achieving sufficient reduction of SiO2 and smaller nano-Si sizes. Temperatures that are too low are not conducive to the diffusion of active metal vapor into the pores and its reduction reaction with SiO2, while temperatures that are too high can easily lead to the agglomeration of nano-Si particles.

[0038] The melting point range of alkali metal salts is 700–850℃. The melting point of alkali metal salts needs to be higher than the reduction treatment temperature in order to effectively absorb the released heat, but generally not higher than 850℃, in order to avoid excessively large nano-Si size.

[0039] S5: Remove the active metal oxides and excess alkali metal salts from the reduction reaction products. Place the product obtained in step S4 into a dilute hydrochloric acid solution and stir at room temperature for several hours, then filter to obtain a solid product. Wash the solid product with deionized water and finally dry to obtain nano-silicon porous carbon particles. At this point, the nano-silicon particles are uniformly attached to the carbon pores of the porous carbon particles.

[0040] During the stirring process in dilute hydrochloric acid, the active metal oxides and alkali metal salts of the reduction reaction products dissolve in the dilute hydrochloric acid, achieving the purpose of product purification; the purpose of the deionized water washing process is to remove all water-soluble byproducts and further purify the product.

[0041] S6: Coating. The nano-silicon porous carbon particles obtained in step S5 are coated with carbon using chemical vapor deposition to form a carbon coating layer on their surface, thereby forming a porous silicon-carbon anode material.

[0042] Specifically, porous silicon carbon particles are heated in a mixed gas atmosphere of carbon source gas (methane, ethane, acetylene, toluene) and inert gas (nitrogen, argon), preferably at a temperature of 600–800°C for 0.5–5 hours, so that elemental carbon decomposed from the carbon source gas is deposited on the porous silicon carbon particles, obtaining the final silicon-carbon material product. Preferably, the volume fraction of the carbon source gas in the mixed gas is 5–30%.

[0043] Appropriate temperature is beneficial for obtaining carbon coating with higher conductivity and avoiding excessive growth of nano-Si size (lower deposition temperature is not conducive to the complete decomposition of carbon source gas, resulting in carbon with more organic functional groups and insufficient conductivity); appropriate heating time can obtain a suitable amount of carbon coating. Thanks to the precise control of nano-Si size and carbon coating amount, the prepared silicon-carbon anode material exhibits high initial reversible specific capacity and initial coulombic efficiency. Figure 3 ).

[0044] The physical properties of the resulting porous silicon-carbon anode material are as follows:

[0045] Specific surface area <![CDATA[<3m 2 / g]]> Particle size range D50=3~10μm, D90=8~20μm Tap density <![CDATA[0.7~1.2g / cm 3 ]]> Silicon loading 20–80 wt% silicon particle size 1-200nm Carbon coating amount 2-5 wt%

[0046] The method for preparing porous silicon-carbon anode materials according to the present invention has the following advantages over existing technologies:

[0047] (1) Lower preparation cost and higher safety: This invention uses liquid silicon ester with low-temperature volatility as silicon source. Unlike flammable, explosive and expensive silane gas, silicon ester material has a higher ignition point and is less likely to burn or explode, thus improving the safety of the material preparation process. In addition, the inexpensive liquid silicon source greatly reduces storage, transportation and use costs.

[0048] (2) The material has high cycle stability: thanks to the dehydration condensation and bonding reaction between the hydroxyl and alkoxy groups in the silicon compound of the silicon ester pyrolysis product and the oxygen-containing and hydroxyl-containing functional groups on the carbon matrix surface, the nano-Si particles in the silicon-carbon material have a strong chemical bonding force with the porous carbon matrix, which is conducive to enhancing the adhesion of Si particles on the carbon matrix and preventing Si particles from falling off the carbon matrix surface due to repeated expansion / contraction during lithium insertion / extraction, thereby improving the structural stability and cycle performance of the material.

[0049] (3) Since nano-Si is uniformly deposited in the structure of porous carbon pores, the porous carbon matrix can effectively buffer the volume expansion of nano-Si during the charge and discharge reaction process and “confine” the nano-Si, preventing the nano-Si particles from breaking and peeling off from the carbon matrix, thereby improving the structural stability of the composite material and obtaining silicon-carbon anode material with long cycle stability.

[0050] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention 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 the present invention.

Claims

1. A porous silicon-carbon anode material, characterized in that, The material includes a porous carbon matrix (1), in which nano-silicon particles (3) are formed within the carbon pores (2) of the porous carbon matrix (1), and a carbon coating layer (4) is present on the outer surface of the porous carbon matrix (1), wherein the nano-silicon particles (3) are formed by reducing silicon dioxide.

2. A method for preparing the porous silicon-carbon anode material as described in claim 1, characterized in that, Includes the following steps: S1: Silicon ester vapor is generated by heating silicon ester material as a silicon source to form silicon ester vapor; S2: The porous carbon matrix material is placed in a silicon ester vapor environment and a certain temperature and pressure are applied to carry out a silicon ester evaporation-condensation reaction, so that the silicon ester vapor permeates into the carbon pores of the porous carbon matrix material, the silicon ester in the carbon pores decomposes, and the generated silicon compounds are deposited in the carbon pores to form porous carbon containing silicon compounds; the porous carbon matrix material is porous carbon particles. S3: The porous carbon containing silicon compounds in step S2 is calcined in an inert gas atmosphere, so that the silicon compounds therein undergo a thermal decomposition reaction to generate uniform and tightly attached nano-silica particles in the carbon pores, thus obtaining porous carbon containing nano-silica. S4: Using an active metal as a reducing agent, the nano-silica particles in porous carbon containing nano-silica are reduced to silicon; S5: The product obtained in step S4 is placed in a dilute hydrochloric acid solution and stirred at room temperature for several hours, then filtered to obtain a solid product; the solid product is then washed with deionized water and finally dried to obtain nano-silicon porous carbon particles. S6: Coating: The nano-silicon porous carbon particles obtained in step S5 are coated with carbon using chemical vapor deposition to form a carbon coating layer on their surface, thereby forming a porous silicon-carbon anode material.

3. The preparation method according to claim 2, characterized in that, In step S4, active metal powder, alkali metal salt and porous carbon containing nano-silica are mixed in a certain mass ratio and calcined in an inert gas atmosphere.

4. The preparation method according to claim 3, characterized in that, In step S4, the heating rate for calcination under an inert gas atmosphere is 2–10 °C / min, the holding temperature is 650–800 °C, and the holding time is 1–20 h.

5. The preparation method according to claim 2, characterized in that, In step S3, the porous carbon containing silicon compounds is calcined in an inert gas atmosphere at a heating rate of 2–10 °C / min, a holding temperature of 700–900 °C, and a holding time of 1–20 h.

6. The preparation method according to claim 2, characterized in that, In step S6, the nano-silicon porous carbon particles are heated in a mixed gas atmosphere of carbon source gas and inert gas at a temperature of 600-800℃ for 0.5-5h, so that the elemental carbon decomposed from the carbon source gas is deposited on the nano-silicon porous carbon particles to obtain the final silicon-carbon material product.

7. The preparation method according to claim 6, characterized in that, The carbon source gas is one or more of methane, ethane, acetylene, and toluene, and the volume fraction of the carbon source gas in the mixed gas is 5% to 30%.

8. The preparation method according to claim 2, characterized in that, The silicone material is one or more of methyl orthosilicate and ethyl orthosilicate.

9. The preparation method according to claim 3, characterized in that, The mass ratio of the active metal powder, alkali metal salt, and porous carbon containing nano-silica is 0.4–2:5–20:1.

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