Double-layer coated porous silicon material, preparation method and application thereof

Abstract

The invention provides a double-layer coated porous silicon material as well as a preparation method and application thereof, and relates to the technical field of lithium ion battery materials. The material is of a core-shell type composite structure and sequentially comprises a porous silicon core, an inner-layer-coated magnesium silicon nitride layer and an outer-layer-coated carbon shell. According to the structure, silicon magnesium nitride grows on the surface of a porous silicon framework in situ and is used for enhancing mechanical support and improving ion transmission efficiency; and the external uniform and compact carbon layer provides an electron conduction path and isolates the electrolyte, so that the interface stability is improved. The preparation method comprises the steps of alloying, ammonia nitrogen atmosphere step-by-step nitriding, carbon source coating, carbonization treatment, acid pickling and the like, the process is controllable, and the structural hierarchy is clear. The obtained material shows excellent electrochemical performance under high-rate and long-cycle conditions, and is suitable for preparation of a high-performance lithium ion battery negative electrode material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery materials, and particularly relates to a double-layer coated porous silicon material, a preparation method and application thereof. BACKGROUND

[0002] Silicon-based negative electrode materials have become a research hotspot for new generation of lithium ion battery negative electrode materials due to their high theoretical specific capacity of 4200 mAh / g, far exceeding traditional graphite materials, and good resource base. Although having obvious advantages, silicon materials will produce severe volume expansion (up to 300%) during charging and discharging, resulting in problems such as structure pulverization, poor cycle stability, and serious electrochemical polarization. In addition, the intrinsic conductivity is poor, which also limits the performance at high rate.

[0003] To overcome the above technical bottlenecks, researchers have proposed various structure optimization and surface modification strategies, such as artificial construction of SEI film, nanocrystallization, porous structure design, carbon coating, and double-layer coating. Among them, double-layer coating has become a key research direction in recent years due to its simultaneous enhancement of conductivity and structural stability.

[0004] There are many published patents and documents that propose different design ideas for double-layer coated silicon-based materials. For example, Chinese patent CN202410183168.4 discloses a double-layer coated spherical silicon negative electrode material, which has an inner layer of aluminum oxide particles, an outer layer of amorphous carbon, and a spherical silicon core in between. The material is prepared by two-step ball milling and then coated with a carbon source, and finally dried. This technology realizes the inorganic and organic double-layer composite structure, which improves the electrochemical performance to some extent. However, the preparation process is complicated, requiring multiple ball milling and drying, and the uniformity of the coating layer is difficult to control, which limits the batch stability and large-scale production capacity of the material.

[0005] Chinese patent CN202410596621.4 discloses a double-layer coated silicon-based composite negative electrode material. A carbon layer is formed on the surface of the silicon material by chemical deposition, and then polyacrylonitrile is introduced as a conductive polymer precursor. After mixing, coating, and high-temperature heat treatment, the coating structure is constructed. This process generates a secondary conductive network through polymer carbonization to improve the transmission efficiency of electrons and ions. However, this method relies on the polymer precursor and complex heat treatment process, and has problems such as difficulty in controlling the carbon layer structure, limited ion conductivity efficiency, and unstable conductive performance improvement effect.

[0006] Chinese patent CN201810553712.4 provides a kind of in situ double-layer coated silicon-carbon negative electrode material, adopt the organic solvent containing hydroxyl group and organic carbon source and submicron silicon are blended, after pre-reaction, spray drying, low-temperature calcination, high-temperature carbonization etc., finally get the composite material with the double-layer coated structure of particle size less than 1 μm, with good continuity.But its overall process path is long, involves in situ reaction and multiple heat treatment, process energy consumption is high, equipment requirement is complex, it is not conducive to large-scale low-cost production.In addition, the particle of the material prepared is too small, also easy to cause low tap density, electrode processing performance is poor.

[0007] In addition, the document "Improved cycling performance of SiO x / MgO / Mg2SiO4 / C composite anode materials for lithium-ion battery" reports a construction method of SiO x / MgO / Mg2SiO4 / C composite material.The document uses porous SiO2 as a template, adds magnesium powder to react to form magnesium oxide and silicate intermediates, and performs polymerization and carbonization in a CTAB, ammonia, resorcinol and formaldehyde system, to finally form a composite material containing a multi-phase coating layer.Although this system has made some progress in improving cycle stability, its synthesis process is complex and involves potentially harmful chemicals such as formaldehyde, limiting its process greenness and promotion.

[0008] In summary, although the existing double-layer coated silicon-based negative electrode material has made some progress in improving the electrochemical performance of silicon material, it generally has problems such as unstable coating structure, complicated preparation process, insufficient coordination of conductive and buffer structure, difficulty in balancing high rate capability and long cycle life, etc.Therefore, it is urgent to develop a double-layer coated porous silicon material with more reasonable structure, more simple process and more excellent performance, and a preparation method thereof, to realize the unification of high energy density, high rate capability and long cycle stability. SUMMARY

[0009] The present application provides a double-layer coated porous silicon material, a preparation method and its application, aiming to solve the technical problems of short cycle life and poor rate performance of silicon-based negative electrode material due to poor conductivity and severe volume expansion.The composite coating structure with synergistic effect is constructed to improve the conductivity and buffer stress of the material, optimize the interface stability and lithium ion transmission efficiency, so as to realize the stable operation of porous silicon negative electrode material under the conditions of high specific capacity, high rate and long cycle, and promote its practical application process in lithium ion battery.

[0010] To achieve the above-mentioned purposes, the technical solutions adopted by the present application are as follows: The first aspect of the present application provides a double-layer coated porous silicon material, which is a core-shell composite structure and sequentially comprises a porous silicon core, an inner layer of magnesium silicon nitride (MgSiN2) and an outer layer of carbon shell; wherein the porous silicon is the core part and has a three-dimensional interconnected pore structure like an ant nest; the inner layer of magnesium silicon nitride is formed continuously along the inner wall of the pores and the surface of the framework of the porous silicon; and the carbon coating layer is located at the outermost layer and has a continuous and uniform carbon shell structure, which entirely coats the porous silicon and the inner magnesium silicon nitride layer.

[0011] Further, the D50 particle size of the porous silicon is 3-8 μm, the inner layer of magnesium silicon nitride accounts for 5-10% of the total mass, and the outer layer of carbon shell accounts for 5-10% of the total mass.

[0012] The double-layer coated porous silicon material designed in the present application does not coat the framework of the porous silicon with a carbon layer, but coats the whole porous silicon particles in the carbon layer to form a core-shell structure. The porous structure can relieve the volume expansion of silicon in the cycle process, the carbon layer coating can improve the electrical conductivity of the material and the coulomb efficiency, and also slows down the contact between the porous silicon and the electrolyte and reduces the side reactions caused by the contact with the electrolyte. In addition, the inner layer of magnesium silicon nitride coating along the framework of the porous silicon forms a fast ion conductor in the process of lithium extraction and insertion, improves the electrical conductivity of the material, and forms a rigid material to relieve the volume expansion of silicon in the cycle process. The two complement each other, retain the advantages of the porous silicon structure, and better play the electrochemical performance of the porous silicon.

[0013] The second aspect of the present application provides a preparation method of the double-layer coated porous silicon material, comprising the following steps: (1) mixing silicon powder and magnesium powder at a molar ratio of 1:1.5-2.0, and then heating and reacting under argon protection to obtain a silicon-magnesium alloy; (2) heating and reacting the silicon-magnesium alloy in an atmosphere containing ammonia, then switching the ammonia to nitrogen to continue the heating and reaction, and obtaining a porous silicon intermediate containing magnesium silicon nitride; (3) placing the intermediate obtained in step (2) in a solution containing phenolic resin, and performing evaporation drying treatment under negative pressure to obtain a precursor coated with a carbon source on the surface; (4) performing two-stage heating reaction of the precursor coated with the carbon source in an argon atmosphere to obtain a carbon-coated material; (5) performing acid washing and drying of the carbon-coated material to obtain the double-layer coated porous silicon material.

[0014] Further, the particle size D50 of the silicon powder in step (1) is controlled at 1-10 μm, and the heating reaction temperature is 500-600 ℃, and the reaction time is 4-6 h.

[0015] Further, in step (2), the silicon-magnesium alloy is heated to 700-800 DEG C at a heating rate of 2-10 DEG C / min under an atmosphere containing ammonia and is kept for 3-6 h.

[0016] Further, in step (2), the silicon-magnesium alloy is heated to 700-800 DEG C at a heating rate of 2-10 DEG C / min under an atmosphere containing ammonia and is kept for 3-6 h.

[0017] Further, in step (3), the mass ratio of the intermediate to the phenolic resin is 30-40:1.

[0018] Further, in step (4), the first stage of the two-stage heating reaction is heated to 250-350 DEG C at a heating rate of 2-4 DEG C / min and is kept for 2-3 h.

[0019] Further, in step (4), the second stage of the two-stage heating reaction is heated to 800-1000 DEG C at a heating rate of 4-6 DEG C / min and is kept for 3-4 h.

[0020] In the carbonization step, the two-stage heating strategy is adopted, the first stage is kept at a lower temperature of 250-350 DEG C, which can realize the sufficient pre-carbonization of the phenolic resin, remove small molecular products in the decomposition process, promote the full adhesion of the carbon precursor to the surface of the porous silicon, avoid the peeling or discontinuous structure of the carbon layer due to the intense pyrolysis, and help to form a uniform and dense initial coating layer; and then the main carbonization reaction is completed at a high temperature stage of 800-1000 DEG C, which further improves the graphitization degree of the carbon layer, enhances the electronic conductivity and structural stability. The two-stage heating process not only helps to obtain a continuous carbon shell with moderate thickness, strong adhesion and uniform distribution, but also effectively inhibits the agglomeration or local carbonization of the carbon source at high temperature, improves the conductivity and interface integrity of the final material, and is a key process measure to realize the synergistic optimization of the stability and electrochemical performance of the double-layer coated structure.

[0021] The third aspect of the present application provides the application of the above-mentioned double-layer coated porous silicon material in the preparation of negative electrode materials for electrochemical energy storage devices, which is especially suitable for the field of lithium ion battery negative electrode materials.

[0022] Compared with the prior art, the present application has the following beneficial effects: The double-layer coated porous silicon material has the comprehensive advantages of high structural stability and excellent ion and electron conductivity. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 A scanning electron microscope (SEM) image of the double-layer coated porous silicon material prepared in Example 1.

[0024] Figure 2 A thermogravimetric analyzer (TGA) image of the double-layer coated porous silicon material prepared in Example 1.

[0025] Figure 3 An X-ray photoelectron spectrometer (XPS) image of the double-layer coated porous silicon material prepared in Example 1.

[0026] Figure 4 A transmission electron microscope (TEM) image of the double-layer coated porous silicon material prepared in Example 1.

[0027] Figure 4 b Figure 4 f An energy dispersive spectroscopy (EDS) image of the double-layer coated porous silicon material prepared in Example 1.

[0028] Figure 5 a The cycle performance test curve of the double-layer coated porous silicon material prepared in Example 1 at a current density of 2 A / g.

[0029] Figure 5 b The rate performance test results of the double-layer coated porous silicon material prepared in Example 1.

[0030] Figure 6 The electrochemical impedance spectroscopy (EIS) variation of the double-layer coated porous silicon material prepared in Example 1 at different cycle numbers.

[0031] Figure 7A scanning electron microscope (SEM) image of the double-layer coated porous silicon material prepared for Comparative Example 2. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application. In addition, it is worth mentioning that the raw materials involved in the present application are all ordinary commercially available products unless otherwise specified. EMBODIMENT

[0033] The present embodiment provides a double-layer coated porous silicon material, and a preparation method thereof includes the following steps: (1) Commercial silicon powder is ball milled to control the particle size at 3 μm. The silicon powder and magnesium powder are uniformly mixed at a molar ratio of 1:1.8, and then placed in an alloy kettle container. The silicon-magnesium alloy is obtained by heating to 550 ℃ at a heating rate of 5 ℃ / min in a tube furnace under an argon protective atmosphere, and then holding for 6 h.

[0034] (2) The silicon-magnesium alloy is placed in a porcelain boat and heated to 750 ℃ at a heating rate of 5 ℃ / min in a tube furnace containing an ammonia atmosphere, and then holding for 6 h to obtain silicon and magnesium nitride obtained after nitriding of the silicon-magnesium alloy. Subsequently, the ammonia gas is switched to nitrogen gas, and the silicon-containing magnesium nitride porous silicon intermediate is obtained by heating to 880 ℃ at a heating rate of 10 ℃ / min and holding for 30 min.

[0035] (3) 0.028 g of phenolic resin is added to 100 ml of alcohol and ultrasonically dispersed. Then, 1 g of the silicon-containing magnesium nitride porous silicon intermediate obtained in step (2) is added to the alcohol solution containing the phenolic resin, and ultrasonic dispersion is continued for 10 min. The surface-coated carbon source precursor is obtained by drying under negative pressure at a temperature of 80 ℃.

[0036] (4) The surface-coated carbon source precursor is placed in a porcelain boat and heated to 300 ℃ at a heating rate of 3 ℃ / min under an argon atmosphere, and then holding for 3 h. Subsequently, the carbon-coated material is obtained by heating to 900 ℃ at a heating rate of 5 ℃ / min and holding for 4 h.

[0037] (5) The obtained carbon-coated material is placed in 1 mol / L hydrochloric acid in a 60 ℃ water bath for acid pickling for 4 h. After filtration and freeze-drying, the double-layer coated porous silicon material PSi@MgSiN2@C is obtained.

[0038] The surface morphology of the prepared material is observed by scanning electron microscope (SEM), as shown in Figure 1As shown, the material has regular particle morphology and uniform particle size distribution, with an average size of about 3 to 5 μm. The surface is covered by a continuous thin carbon shell, exhibiting typical core-shell structure characteristics.

[0039] The carbon content in the sample was tested using a thermogravimetric analyzer (TGA), such as... Figure 2 As shown, the carbon content is approximately 5.067%.

[0040] X-ray photoelectron spectroscopy (XPS) test results are as follows Figure 3 As shown, multiple chemical bond signals such as CN, CC, Si-Si, Si-O, Mg-N and Si-N were detected in the sample, indicating that magnesium silicon nitride (MgSiN2) phase was successfully formed in the sample and the overall phase structure was stable.

[0041] Transmission electron microscopy (TEM) images such as Figure 4 As shown in Figure a, the sample exhibits a distinct three-layer structure: the innermost layer is a porous silicon framework with a channel structure, the middle layer is a magnesium silicon nitride layer uniformly coated along the channels with an average coating thickness of about 3.9 nm, and the outermost layer is a uniform and dense carbon layer with a thickness of about 5.16 nm.

[0042] Combination Figure 4 b~ Figure 4 The energy dispersive spectroscopy (EDS) results shown in f further confirm the spatial distribution of elements in the material. Silicon is mainly concentrated inside the particles, while carbon is uniformly distributed on the outer layer without obvious agglomeration, confirming that the carbon layer is a complete coating structure. Magnesium and nitrogen are co-distributed with silicon on the framework surface, indicating that the magnesium silicon nitride layer is uniformly distributed without local deposition or phase separation.

[0043] Figure 5 a represents the cycling performance test curve of the material in this embodiment under a current density of 2 A / g. The first three cycles were activated using a current of 0.1 A / g, followed by long-term cycling at a high current density. The results showed that the material maintained a reversible capacity of 1029 mAh / g after 700 cycles, and reached a capacity of 1484 mAh / g at the 350th cycle, demonstrating excellent cycling stability.

[0044] Figure 5 b represents the rate performance test results. As the current density gradually increases from 0.1 A / g to 5 A / g, the capacity retention rate is approximately 52%, reflecting that the material has good rate retention capability.

[0045] Figure 6The electrochemical impedance spectroscopy (EIS) changes of the material under different cycle numbers are shown. The interface resistance after the first cycle is about 87 Ω, and the impedance gradually decreases to about 17 Ω at the 50th cycle, indicating that the electrode interface gradually stabilizes and the ion / electron transmission resistance decreases.

[0046] Comprehensive analysis shows that the double-layer coated porous silicon material has good structure regulation ability. Among them, the porous structure provides an effective volume expansion buffer space, the silicon magnesium nitride coating layer can generate lithium magnesium alloy during the lithium intercalation process, forming a rigid interface to further limit the structure collapse; the outer carbon shell constructs an electron conduction channel and stabilizes the electrolyte interface, and the synergistic effect of the two significantly improves the electron / ion transmission efficiency and rate performance of the material.

[0047] Comparative Example 1 The preparation method of the present comparative example comprises the following steps: (1) The commercial silicon powder is ball milled to control the particle size to 3 μm. The silicon powder and magnesium powder are uniformly mixed at a molar ratio of 1:1.8, then put into an alloy kettle container, and then heated to 550 ℃ at a heating rate of 5 ℃ / min in a tube furnace in an argon protective atmosphere, and kept for 6 h to obtain a silicon-magnesium alloy.

[0048] (2) The silicon-magnesium alloy is placed in a porcelain boat in a tube furnace containing ammonia gas and heated to 750 ℃ at a heating rate of 5 ℃ / min and kept for 6 h to obtain silicon and magnesium nitride after nitriding of the silicon-magnesium alloy. Then, the ammonia gas is switched to nitrogen at a heating rate of 10 ℃ / min and heated to 800 ℃ for 30 min. Due to the too low secondary nitriding temperature, the melting reaction of magnesium nitride and silicon cannot be carried out, and the inner silicon magnesium nitride coating layer cannot be generated.

[0049] Comparative Example 2 The preparation method of the present comparative example comprises the following steps: (1) The commercial silicon powder is ball milled to control the particle size to 3 μm. The silicon powder and magnesium powder are uniformly mixed at a molar ratio of 1:1.8, then put into an alloy kettle container, and then heated to 550 ℃ at a heating rate of 5 ℃ / min in a tube furnace in an argon protective atmosphere, and kept for 6 h to obtain a silicon-magnesium alloy.

[0050] (2) The silicon-magnesium alloy is placed in a porcelain boat in a tube furnace containing ammonia gas and heated to 750 ℃ at a heating rate of 5 ℃ / min and kept for 6 h to obtain silicon and magnesium nitride after nitriding of the silicon-magnesium alloy. Then, the ammonia gas is switched to nitrogen at a heating rate of 10 ℃ / min and heated to 880 ℃ for 30 min to obtain a porous silicon intermediate containing silicon magnesium nitride.

[0051] (3) 0.055 g phenolic resin was added into 100 ml alcohol and ultrasonic dispersed, then 1 g of the porous silicon intermediate obtained above was added into the alcohol solution containing phenolic resin and ultrasonic dispersed for 10 min, and then the surface coated precursor of carbon source was obtained by negative pressure drying at 80 ℃.

[0052] (4) The surface coated precursor of carbon source was placed in a porcelain boat and heated to 300 ℃ at a heating rate of 3 ℃ / min under an argon atmosphere and kept for 3 h, and then heated to 900 ℃ at a heating rate of 5 ℃ / min and kept for 4 h to obtain a carbon coated material.

[0053] (5) Then the carbon coated material was put into 1 mol / L hydrochloric acid in a 60 ℃ water bath for 4 h, and then filtered to obtain a double-layer coated porous silicon material.

[0054] Due to too much phenolic resin being added, the thickness of the carbon layer was too thick, cracks appeared in the carbon layer, and the long cycle performance was affected, Figure 7 The scanning electron microscope diagram of the carbon layer of the present comparative example.

[0055] Comparative Example 3 The preparation method of the present comparative example comprises the following steps: (1) Commercial silicon powder was ball milled to control the particle size to 3 μm, and then the silicon powder and magnesium powder were uniformly mixed at a molar ratio of 1:1.8, and then placed in an alloy kettle container, and then heated to 550 ℃ at a heating rate of 5 ℃ / min in a tube furnace under an argon protective atmosphere and kept for 6 h to obtain a silicon-magnesium alloy.

[0056] (2) The silicon-magnesium alloy was placed in a porcelain boat and heated to 750 ℃ at a heating rate of 5 ℃ / min in a tube furnace under an ammonia atmosphere and kept for 6 h to obtain silicon and nitrided magnesium obtained after nitriding of the silicon-magnesium alloy.

[0057] (3) 0.055 g phenolic resin was added into 100 ml alcohol and ultrasonic dispersed, then 1 g of the porous silicon intermediate obtained above was added into the alcohol solution containing phenolic resin and ultrasonic dispersed for 10 min, and then the surface coated precursor of carbon source was obtained by negative pressure drying at 80 ℃.

[0058] (4) The surface coated precursor of carbon source was placed in a porcelain boat and heated to 300 ℃ at a heating rate of 3 ℃ / min under an argon atmosphere and kept for 3 h, and then heated to 900 ℃ at a heating rate of 5 ℃ / min and kept for 4 h to obtain a carbon coated material.

[0059] (5) Then the carbon coated material was put into 1 mol / L hydrochloric acid in a 60 ℃ water bath for 4 h, and then filtered to obtain a carbon coated porous silicon material.

[0060] The difference between the present comparative example and Example 1 is that the secondary heating process in step (2) is removed, and the obtained is a carbon-coated porous silicon negative electrode material PSi@C without silicon magnesium nitride, as shown in FIG. b. Figure 5 b, the carbon-coated porous silicon negative electrode material has a lower electronic conductivity and ion conductivity and a poorer rate performance because there is no lithium nitride SEI layer converted from silicon magnesium nitride.

[0061] Comparative Example 4 (1) The commercial silicon powder is ball milled to control the particle size at 3 μm. The silicon powder and magnesium powder are uniformly mixed at a molar ratio of 1:1.8, and then placed in an alloy kettle container. The container is heated to 550 °C at a heating rate of 5 °C / min in a tube furnace under an argon protective atmosphere, and then heat-treated for 6 h to obtain a silicon-magnesium alloy.

[0062] (2) The silicon-magnesium alloy is placed in a porcelain boat and heated to 750 °C at a heating rate of 5 °C / min in a tube furnace containing ammonia gas, and then heat-treated for 6 h to obtain silicon and magnesium nitride obtained after nitriding of the silicon-magnesium alloy. Subsequently, the ammonia gas is switched to nitrogen, and the temperature is raised to 880 °C at a heating rate of 10 °C / min and heat-treated for 30 min to obtain a porous silicon intermediate containing silicon magnesium nitride.

[0063] (3) The obtained intermediate is placed in 1 mol / L hydrochloric acid in a 60 °C water bath kettle and acid washed for 4 h. After filtration and freeze-drying, a porous silicon material coated with silicon magnesium nitride is obtained.

[0064] The difference between the present comparative example and Example 1 is that the carbon coating process in steps (3) and (4) is removed, and the obtained is a silicon magnesium nitride-coated porous silicon negative electrode material PSi@MgSiN2, as shown in FIG. b. Figure 5 b, without the carbon layer to promote ion diffusion, the ion conductivity of the material is significantly reduced, and the rate performance is poor.

[0065] Comparative Example 5 The difference between the present comparative example and Example 1 is that the phenolic resin in step (3) is replaced by dopamine or glucose. The cycle stability of the obtained carbon-coated porous silicon negative electrode material is not as good as that of the phenolic resin, which is mainly due to the fact that the carbon layer produced by the phenolic resin has high strength, and the swelling relief effect of the porous silicon by dopamine and glucose carbon sources is obvious.

[0066] Comparative Example 6 The difference between the present comparative example and Example 1 is that step (4) is replaced by: placing the precursor coated with a carbon source in a porcelain boat and heating to 900 °C at a heating rate of 3 °C / min under an argon atmosphere and heat-treating for 7 h to obtain a carbon-coated material.

[0067] Due to the lack of pre-carbonization step, the resulting carbon layer is destroyed in surface coating uniformity due to the cleavage of phenolic resin at high temperature.

[0068] Finally, it should be noted that the above content is only used to illustrate the technical solutions of the present application, and is not a limitation on the scope of protection of the present application. Simple modifications or equivalent replacements of the technical solutions of the present application made by those skilled in the art do not deviate from the essence and scope of the technical solutions of the present application.

Claims

1. A double-layer coated porous silicon material, wherein the material has a core-shell composite structure, comprising, sequentially, a porous silicon core, an inner layer of magnesium silicon nitride, and an outer layer of carbon shell; wherein, The core component is porous silicon, which has a three-dimensional interconnected pore structure resembling an ant nest. The inner coating layer of magnesium silicon nitride is continuously formed along the inner wall of the porous silicon pores and the surface of the framework. The outermost layer of carbon coating is located in a continuous carbon shell structure, which completely covers the porous silicon and the inner magnesium silicon nitride layer.

2. The double-layer coated porous silicon material according to claim 1, characterized in that: The D50 particle size of the porous silicon is 3-8 μm, with the inner layer of magnesium silicon nitride accounting for 5-10% of the total mass and the outer layer of carbon shell accounting for 5-10% of the total mass.

3. The method for preparing the double-layer coated porous silicon material according to claim 1 or 2, comprising the following steps: (1) Mix silicon powder and magnesium powder at a molar ratio of 1:1.5-2.0, and then heat the mixture under argon protection to obtain a silicon-magnesium alloy; (2) The silicon-magnesium alloy was heated and reacted in an ammonia atmosphere, and then the ammonia was switched to nitrogen and the reaction was continued to be heated to obtain a porous silicon intermediate containing silicon magnesium nitride. (3) The intermediate obtained in step (2) is placed in a solution containing phenolic resin and evaporated under negative pressure to obtain a precursor coated with carbon source. (4) The carbon-coated precursor is subjected to a two-stage heating reaction under an argon atmosphere to obtain a carbon-coated material. (5) The carbon-coated material is acid-washed and dried to obtain a double-layer coated porous silicon material.

4. The preparation method according to claim 3, characterized in that: In step (1), the particle size D50 of the silicon powder is controlled at 1-10 μm, the heating reaction temperature is 500-600 ℃, and the reaction time is 4-6 h.

5. The preparation method according to claim 3, characterized in that: In step (2), the silicon-magnesium alloy is heated to 700-800 ℃ at a heating rate of 2-10 ℃ / min in an ammonia atmosphere and held at that temperature for 3-6 h.

6. The preparation method according to claim 3, characterized in that: In step (2), the silicon-magnesium alloy is heated to 850-900 ℃ at a heating rate of 8-15 ℃ / min in a nitrogen atmosphere and held for 30-60 min to obtain a porous silicon intermediate containing silicon magnesium nitride.

7. The preparation method according to claim 3, characterized in that: The mass ratio of the intermediate to the phenolic resin in step (3) is 30-40:

1.

8. The preparation method according to claim 3, characterized in that: In step (4), the first stage of the two-stage heating reaction is heated to 250-350 ℃ at a heating rate of 2-4 ℃ / min and held for 2-3 h.

9. The preparation method according to claim 3, characterized in that: In step (4), the second stage of the two-stage heating reaction is heated to 800-1000 ℃ at a heating rate of 4-6 ℃ / min and held for 3-4 h.

10. The application of the double-layer coated porous silicon material according to claim 1 or 2 in the preparation of anode materials for electrochemical energy storage devices.

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