Silicon-based composite material and preparation method thereof

Abstract

The invention provides a silicon-based composite material and a preparation method thereof, and particularly relates to the technical field of secondary batteries. The silicon-based composite material comprises a core and a protective layer coated on the surface of the core, the core comprises porous carbon, nano silicon and metal silicate; the porous carbon comprises a first porous carbon matrix and a second porous carbon matrix, the surface of the first porous carbon matrix is wrapped with the second porous carbon matrix, pore channels of the first porous carbon matrix and the second porous carbon matrix are filled with nanometer silicon and metal silicate MSO in a dispersed mode, and the chemical formula of the metal silicate MSO is aMOx.bSiOy, x is less than or equal to 1, 0lt; y < = 2, 0lt; a / b is less than or equal to 2, and M is Li and / or Mg. Silicon expansion is effectively relieved on the porous structure, the first porous carbon matrix and the second porous carbon matrix, the structural stability of the negative electrode material in the circulation process is greatly improved, the irreversible lithium ion amount is further reduced through the metal silicate, and the first effect of the silicon-based composite material is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a silicon-based composite material and a preparation method thereof. Background Art

[0002] In recent years, with the rapid popularization and development of portable electronic devices, electric vehicles, etc., the traditional graphite anode can no longer meet the current market demand for high energy density and long cycle life, and it is urgent to find the next-generation anode material for lithium-ion batteries.

[0003] Silicon is an alloying lithium-inserting material, and its theoretical lithium-insertion capacity is as high as 4200 mAh / g. Moreover, it is rich in resources, safe and non-toxic, and is the most promising anode material for lithium-ion batteries. However, silicon will undergo a huge volume expansion during the process of lithium deinsertion and insertion, up to about 300%, which will cause the following adverse effects: the structure of the silicon-based anode material collapses, and the SEI film repeatedly experiences "formation - destruction - reformation", continuously consuming lithium ions, thereby leading to battery capacity attenuation and Coulomb efficiency reduction.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] One of the purposes of the present invention is to provide a silicon-based composite material to solve the problems of battery capacity attenuation and Coulomb efficiency reduction caused by large volume changes of silicon anode materials in the prior art.

[0006] Another purpose of the present invention is to provide a preparation method of the silicon-based composite material.

[0007] In order to achieve the above purposes of the present invention, the following technical solutions are specifically adopted:

[0008] The first aspect of the present invention provides a silicon-based composite material, which includes a core and a protective layer coated on the surface of the core; the core in the core-shell structure includes porous carbon, nano-silicon and metal silicate; the porous carbon includes a first porous carbon matrix and a second porous carbon matrix, the second porous carbon matrix wraps the surface of the first porous carbon matrix, and nano-silicon and metal silicate MSO are dispersedly filled in the pores of the first porous carbon matrix and the second porous carbon matrix, and the chemical formula of the metal silicate MSO is aMO x ·bSiO y , where 0 < x ≤ 1, 0 < y ≤ 2, 0 < a / b ≤ 2, and the M is Li and / or Mg.

[0009] Further, the median particle size d 50 of the first porous carbon matrix is 5 - 15 μm, and the thickness of the second porous carbon matrix is 0.5 - 2 μm.

[0010] Further, the particle size of the nano-silicon is 0.5 to 10 nm.

[0011] Further, based on the mass of the silicon-based composite material being 100%, the mass content of the metal silicate is 3 to 10%.

[0012] Further, the proportion of the mesopore volume of the first porous carbon matrix in the total pore volume of the first porous carbon is 25 to 60%, and the proportion of the mesopore volume of the second porous carbon matrix in the total pore volume of the second porous carbon is 5 to 15%.

[0013] Further, the proportion of the mesopore volume of the first porous carbon matrix in the total pore volume of the first porous carbon is 5 to 15%, and the proportion of the mesopore volume of the second porous carbon matrix in the total pore volume of the second porous carbon is 25 to 60%.

[0014] Further, the porous carbon has micropores, and the proportion of the micropore volume of the porous carbon in the total pore volume of the porous carbon is 30 to 90%.

[0015] Further, the specific surface area of the porous carbon is 200 to 3000 m 2 / g, and the total pore volume is 0.2 to 3.0 cm 3 / g.

[0016] Further, the core includes (C-MSO)-(Si-MSO-C), C-MSO is a porous carbon matrix with MSO on the surface, wherein the content of MSO in the porous carbon matrix is 0 to 5 wt.%; Si-MSO-C is located in the pores of the porous carbon and includes nano-silicon, metal silicate and optional carbon, and the nano-silicon, metal silicate and optional carbon are dispersedly distributed in the pores of the porous carbon, wherein the content of MSO in Si-MSO-C is 0 to 5 wt.%, and the content of optional carbon in Si-MSO-C is 0 to 5 wt.%.

[0017] Further, the nano-silicon is at least one of elemental silicon particles, SiO δ (0 < δ ≤ 2) particles or elemental silicon particles with at least partially oxidized surface.

[0018] Further, the protective layer includes at least one of silicon carbide, silicon nitride, carbon, and ion-conducting materials.

[0019] The second aspect of the present invention provides a method for preparing the silicon-based composite material, including:

[0020] Step S1, providing porous carbon, the porous carbon includes a first porous carbon matrix and a second porous carbon matrix, the second porous carbon matrix wraps the surface of the first porous carbon matrix, and the first porous carbon matrix and the second porous carbon matrix have mesopores;

[0021] Step S2: Using a silicon precursor and an oxygen precursor as deposition gases, deposit nano-silicon in the pores of the first porous carbon matrix and the second porous carbon matrix through a chemical vapor deposition process;

[0022] Step S3: Mix the material obtained in Step S2 with an M source and perform high-temperature sintering to obtain a pre-lithiated or pre-magnesiated silicon-based composite material; the M source contains Li and / or Mg elements;

[0023] Step S4: Coat a protective layer on the surface of the pre-lithiated or pre-magnesiated silicon-based composite material obtained in Step S3 to obtain the silicon-based composite material.

[0024] Compared with the prior art, the present invention has at least the following beneficial effects:

[0025] For the silicon-based composite material provided by the present invention, the pore structure distributions of the first porous carbon matrix and the second porous carbon matrix are different. After silicon deposition, the particle sizes of the formed nano-silicon materials are non-uniform. The silicon loaded in the porous carbon matrix with a relatively large proportion of mesopores has a larger size. The introduction of an oxygen spacer can inhibit the excessive growth of silicon particles. The silicon loaded in the porous carbon matrix with a relatively large proportion of micropores has a smaller size. Therefore, the sizes of the nano-silicon materials filled in the pores of the composite material particles form a gradient distribution, resulting in different degrees of volume expansion in the first porous carbon matrix and the second porous carbon matrix during the lithium insertion process. This unbalanced expansion stress is partially offset, effectively alleviating the silicon expansion on the porous structure, the first porous carbon matrix, and the second porous carbon matrix, and greatly improving the structural stability of the negative electrode material during the cycling process. At the same time, the metal silicate distributed in the pores further reduces the irreversible amount of lithium ions and improves the initial efficiency of the silicon-based composite material.

[0026] The preparation method of the silicon-based composite material provided by the present invention has a simple process and a large batch processing capacity, and is suitable for industrial production.

[0027] The secondary battery provided by the present invention uses a silicon-based composite material with better performance, improves the capacity stability and Coulomb efficiency of the secondary battery, and promotes the development of downstream industries. Detailed Embodiments

[0028] The following will describe the implementation schemes of the present invention in detail in combination with embodiments and examples. However, those skilled in the art will understand that the following embodiments and examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0029] In the first aspect of the present invention, a silicon-based composite material is provided. The silicon-based composite material includes a core and a protective layer coated on the surface of the core. The core in the core-shell structure includes porous carbon, nano-silicon, and metal silicate. The porous carbon includes a first porous carbon matrix and a second porous carbon matrix. The second porous carbon matrix wraps around the surface of the first porous carbon matrix. Nano-silicon and metal silicate MSO are dispersedly filled in the pores of the first porous carbon matrix and the second porous carbon matrix. The chemical formula of the metal silicate MSO is aMO x ·bSiO y , where 0 < x ≤ 1, 0 < y ≤ 2, 0 < a / b ≤ 2, and M is Li and / or Mg.

[0030] In the silicon-based composite material provided by the present invention, the pore structure distributions of the first porous carbon matrix and the second porous carbon matrix are different. After silicon deposition, the particle sizes of the formed nano-silicon materials are non-uniform. The silicon particles loaded in the porous carbon matrix with a relatively large proportion of mesopores are larger in size. The introduction of an oxygen interval can inhibit the excessive growth of silicon particles. The silicon particles loaded in the porous carbon matrix with a relatively large proportion of micropores are smaller in size. Therefore, the sizes of the nano-silicon materials filled in the pores of the porous carbon in the composite material particles form a gradient distribution. During the lithium insertion process, different degrees of volume expansion occur in the first porous carbon matrix and the second porous carbon matrix. This unbalanced expansion stress is partially offset, effectively alleviating the silicon expansion on the porous structure, the first porous carbon matrix, and the second porous carbon matrix, and greatly improving the structural stability of the negative electrode material during the cycling process. At the same time, the metal silicate distributed in the pores further reduces the irreversible amount of lithium ions and improves the initial efficiency of the silicon-based composite material.

[0031] In some embodiments, the median particle size d of the first porous carbon matrix 50 is 5 - 15 μm, and the thickness of the second porous carbon matrix is 0.5 - 2 μm. Typically but not restrictively, the median particle size d of the first porous carbon matrix 50 can be, for example, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, or 15 μm; the thickness of the second porous carbon matrix can be, for example, 0.5 μm, 1 μm, 1.5 μm, or 2 μm.

[0032] In some embodiments, the particle size of the nano-silicon is 0.5 - 10 nm. Typically but not restrictively, the particle size of the nano-silicon can be 0.5 nm, 1 nm, 3 nm, 5 nm, 7 nm, 9 nm, or 10 nm.

[0033] In some embodiments, the metal silicate MSO is selected from at least one of lithium silicate, magnesium silicate, and lithium magnesium silicate. Exemplarily, lithium silicate can include Li 2 Si 2 O 5 、Li 2SiO 3 or Li 4 SiO 4 or one or more of the following; Exemplarily, magnesium silicate may include MgSiO 3 , Mg 2 SiO 4 or Mg 4 SiO 6 or one or more of the following.

[0034] In some embodiments, based on the mass of the silicon-based composite being 100%, the mass content of the metal silicate is 3-10%, such as 3%, 5%, 7%, 9% or 10%.

[0035] The first porous carbon matrix and the second porous carbon matrix have different mesopore distributions. In some embodiments, the proportion of the mesopore volume of the first porous carbon matrix in the total pore volume of the first porous carbon is 25-60%, and the proportion of the mesopore volume of the second porous carbon matrix in the total pore volume of the second porous carbon is 5-15%. In some embodiments, the proportion of the mesopore volume of the first porous carbon matrix in the total pore volume of the first porous carbon is 5-15%, and the proportion of the mesopore volume of the second porous carbon matrix in the total pore volume of the second porous carbon is 25-60%.

[0036] The pore structure of the porous carbon serves as the region for nano-silicon deposition and determines the size and quality of the loaded nano-silicon. The porous carbon matrix with a relatively large proportion of mesopore volume has larger-sized filled nano-silicon, a high silicon loading, and a larger volume expansion; on the contrary, the porous carbon matrix with a relatively small proportion of mesopore volume has a small volume expansion, bears part of the external or internal expansion of the high-silicon-content carbon matrix, effectively alleviates the silicon expansion, and improves the cycle stability of the silicon-based composite. In some embodiments, for the porous carbon matrix with more mesopores, the Si and heteroatoms can be deposited simultaneously or alternately. In the resulting composite material, the heteroatom layer can form a good separation and confinement for the silicon nanoparticles, thus solving the problems of increased silicon particles, large electrode expansion, and poor cycle stability in the composite material caused by the mesoporous carbon matrix. However, if too much oxygen is introduced, the initial efficiency of the material will decrease, and means such as pre-lithiation and pre-magnesium are needed to improve the initial efficiency of the composite material. Therefore, the mesopore ratio and oxygen content in each component of the composite material need to be within a suitable range.

[0037] In some embodiments, the porous carbon has micropores, and the proportion of the micropore volume of the porous carbon in the total pore volume of the porous carbon is 30-90%. The micropores provide a buffer space, and an appropriate amount of micropores is beneficial for the porous carbon to alleviate the volume expansion and contraction of silicon.

[0038] In some embodiments, the specific surface area of the porous carbon is 200-3000 m 2 / g, and the total pore volume is 0.2-3.0 cm 3 / g. Typically but not restrictively, the specific surface area of the porous carbon can be, for example, 200 m 2 / g, 500 m 2 / g, 1000 m 2 / g, 2000 m 2 / g or 3000 m 2 / g; the total pore volume can be, for example, 0.2 cm 3 / g, 0.5 cm 3 / g, 0.8 cm 3 / g, 1 cm 3 / g, 2 cm 3 / g or 3 cm 3 / g.

[0039] In some embodiments, the core includes (C-MSO)-(Si-MSO-C), where C-MSO is a porous carbon matrix with MSO on its surface, and the content of MSO in the porous carbon matrix is 0 to 5 wt.%; Si-MSO-C is located in the pores of the porous carbon and includes nanosilicon, metal silicate, and optionally carbon, and the nanosilicon, metal silicate, and optionally carbon are dispersedly distributed in the pores of the porous carbon, where the content of MSO in Si-MSO-C is 0 to 5 wt.%, and the content of optionally carbon in Si-MSO-C is 0 to 5 wt.%.

[0040] In the above core, the nanosilicon is uniformly dispersed in the pores of the porous carbon and is separated and confined by the metal silicate and optionally carbon. A protective layer is coated on the surface of the core, and the obtained silicon-based composite material has a high specific capacity, an excellent initial Coulomb efficiency, and good cycle stability.

[0041] The above-mentioned C-MSO is a metal silicate on the surface of porous carbon and porous carbon, mainly composed of carbon material C. The metal silicate MSO is distributed between the porous carbon and the nanosilicon and can be formed in various ways. MSO is directly introduced in the carbon precursor stage. During the carbonization and activation processes, the sites containing MSO have higher activity and are more likely to form pores. In the obtained carbon material, MSO is located on the pore surface of the porous carbon. By performing silicon deposition and pre-lithiation / pre-magnesiation on the porous carbon material with a surface oxide layer, MSO is formed by the transformation of the surface oxide layer. After depositing silicon and oxygen in the pores of the porous carbon matrix, by controlling the depth of pre-lithiation or pre-magnesiation, the metal silicate MSO can also be formed on the surfaces of the porous carbon and the nanosilicon. The above-mentioned Si-MSO-C is nanosilicon uniformly dispersed in the pores of the porous carbon, metal silicate and optional carbon. The nanosilicon, the metal silicate and the optional carbon are uniformly interlaced. There are metal silicate and optional carbon between the nanosilicons. A small amount of metal silicate and optional carbon are used to disperse and bind the nanosilicon. The amount of the metal silicate and the optional carbon should not be too much, as too much will cause a decrease in the overall Si content, and both the specific capacity per gram and the first efficiency of the composite electrode will decrease. Therefore, the content of MSO in Si-MSO-C is controlled to be 0 to 5 wt.%, and the content of the optional carbon in Si-MSO-C is controlled to be 0 to 5 wt.%.

[0042] In some embodiments, the above-mentioned nanosilicon is at least one of elemental silicon particles, SiO δ (0 < δ ≤ 2) particles or at least partially surface-oxidized elemental silicon particles.

[0043] In some embodiments, the protective layer includes at least one of silicon carbide, silicon nitride, carbon, and ion-conducting materials. Exemplarily, the carbon in the protective layer includes one or more of hard carbon, soft carbon, graphite, carbon nanotubes, graphene, carbon black, and carbon fiber. Exemplarily, the ion-conducting materials include one or more of lithium fluoride, lithium hydroxide, lithium carbonate, lithium phosphate, lithium metaphosphate, lithium aluminum phosphate, aluminum hydroxide, aluminum phosphate, aluminum metaphosphate, aluminum oxide, silicon oxide, magnesium oxide, zinc oxide, titanium oxide, zirconium oxide, magnesium hydroxide, zinc hydroxide, LiPON (lithium phosphorus oxynitride), LLTO (lithium lanthanum titanium oxide), LLZO (lithium lanthanum zirconium oxide), and LATP (lithium aluminum titanium phosphate). Preferably, the thickness of the protective layer is 0.5 to 20 nm.

[0044] The second aspect of the present invention provides a method for preparing the silicon-based composite material, comprising: Step S1, providing porous carbon, the porous carbon comprising a first porous carbon matrix and a second porous carbon matrix, the second porous carbon matrix wrapping the surface of the first porous carbon matrix, and the first porous carbon matrix and the second porous carbon matrix having mesopores; Step S2, using a silicon-containing precursor and an oxygen-containing precursor as deposition gases, and depositing nano-silicon in the pores of the first porous carbon matrix and the second porous carbon matrix through a chemical vapor deposition process; Step S3, mixing the material obtained in Step S2 with an M source and performing high-temperature sintering to obtain a pre-lithiated or pre-magnesiated silicon-based composite material; Step S4, coating a protective layer on the surface of the pre-lithiated or pre-magnesiated silicon-based composite material obtained in Step S3 to obtain the silicon-based composite material.

[0045] The method for preparing the silicon-based composite material provided by the present invention has a simple process, a large batch processing capacity, and is suitable for industrial production.

[0046] In some embodiments, the porous carbon in the above Step S1 can be prepared by the following method: mixing a first carbon precursor and a second carbon precursor and performing carbonization, the carbonization being carried out in an inert gas, the inert gas being one or more of nitrogen, argon, and helium, preferably the carbonization temperature is 600-1800 °C and the time is 0.5-10 h. Optionally, the carbonized material is activated to obtain porous carbon with an increased pore volume, and the activator is selected from H 2 O, CO 2 、O 2 or O 3 or one or more of them, preferably the activation temperature is 300-1000 °C and the time is 0.5-20 h. Optionally, in some embodiments, the carbon precursor is pre-oxidized before carbonization, the pre-oxidation temperature is 100-300 °C, and the time is 0.1-48 h. In some embodiments, the first carbon precursor is selected from biomass precursors, and the second carbon precursor is selected from one or more of resin precursors, pitch precursors, or coal-based precursors.

[0047] During the carbonization and activation processes, by utilizing the different structures of the thermal decomposition products of the two carbon materials and the different reaction rates during physical activation, different mesopore volumes can be generated in the first porous carbon matrix and the second porous carbon matrix.

[0048] In some embodiments, the surface of the porous carbon contains an oxide layer. During the carbonization process, by adjusting the carbonization conditions, some oxygen-containing functional groups can be retained as the oxide layer. Or the porous carbon is oxidized to obtain a porous carbon material with a surface oxide layer, and the oxidation means are well-known to those skilled in the art and will not be elaborated here.

[0049] In step S2, the ways in which the silicon-containing precursor and the oxygen-containing precursor come into contact with the porous carbon and undergo chemical vapor deposition can be diverse, mainly including three types: (1) the silicon-containing precursor and the oxygen-containing precursor alternately come into contact with the porous carbon; (2) the silicon-containing precursor and the oxygen-containing precursor simultaneously come into contact with the porous carbon; (3) the silicon-containing precursor and a mixed gas containing the silicon-containing precursor and the oxygen-containing precursor alternately come into contact with the porous carbon; preferably, the silicon-containing precursor continuously comes into contact with the porous carbon, and the oxygen-containing precursor is intermittently introduced during this process.

[0050] In some embodiments, the temperature at which the silicon-containing precursor and the oxygen-containing precursor come into contact with the porous carbon is 150 - 1000 °C, and the time is 0.1 - 100 h.

[0051] In step S3, the M source includes a lithium source and a magnesium source. The material obtained in step S3 is mixed with the M source and subjected to high-temperature sintering under the protection of an inert gas. Preferably, the temperature is 500 - 1000 °C to obtain the above-mentioned silicon-based composite material. The lithium source is selected from one or more of metallic lithium, lithium oxide, lithium hydroxide, lithium carbonate, lithium hydride, lithium nitride, lithium fluoride, lithium chloride, or lithium bromide; the magnesium source is selected from magnesium powder.

[0052] In step S4, the protective layer can be coated by chemical vapor deposition, atomic layer deposition, sol-gel method, or hydrothermal / solvent method, which are all well-known to those skilled in the art and will not be elaborated here.

[0053] The present invention will be further described below through specific examples and comparative examples. However, it should be understood that these examples are only for more detailed illustration and should not be construed as limiting the present invention in any way. For the raw materials used in the examples and comparative examples of the present invention, those without specific conditions stated are carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments without the manufacturer stated, they are all conventional products that can be obtained through commercial purchase.

[0054] Example 1

[0055] This example provides a silicon-based composite material, and the preparation method is as follows:

[0056] Step S1, commercially available coconut shell charcoal (iodine value 800, 20 - 60 mesh particles) and sucrose are mixed at a mass ratio of 20:1, and heated from room temperature to 800 °C at 2 °C / min in an N 2 atmosphere and held at this temperature for 2 h; CO 2 is introduced, and activation is carried out at 900 °C for 2 h, then cooled, and after crushing and classification, porous carbon is obtained. The obtained porous carbon is a core-shell structured porous carbon, with coconut shell-based porous carbon as the first porous carbon inner core and coated with a second porous carbon sucrose-based porous carbon;

[0057] Step S2, the porous carbon obtained in step S1 is placed in a tubular furnace, in an N 2The temperature was raised from room temperature to 600 °C at a rate of 2 °C / min in the atmosphere; then it was changed to a 20% SiH 4 -0.01% O 2 -N 2 mixed gas, and it was maintained at 600 °C for 30 h in this mixed atmosphere; it was changed to N 2 atmosphere and then cooled naturally, and crushed and classified;

[0058] Step S3: Mix the material obtained in Step S2 with Li powder evenly at a mass ratio of 100 / 1, place it in a high-temperature furnace, and in N 2 atmosphere, raise the temperature from room temperature to 600 °C at a rate of 2 °C / min, and maintain it at this temperature for 2 h. The Li powder reacts with SiO x to insert active lithium into the material to complete prelithiation, obtaining a core, where SiO x nanometer Si particles and lithium silicate are filled inside the porous carbon particles;

[0059] Step S4: Disperse the core particles in 5% AlPO 4 aqueous solution, ultrasonicate, stir and filter the excess liquid, dry it in an oven at 80 °C, and then sinter it in N 2 atmosphere at 400 °C for 2 h to form an AlPO 4 coating layer on the surface of the core to obtain a silicon-based composite material.

[0060] Example 2

[0061] This example provides a silicon-based composite material, and the preparation method is as follows:

[0062] Step S1: The same as Step S1 in Example 1;

[0063] Step S2: Place the porous carbon obtained in Step S1 in a tube furnace, and in N 2 atmosphere, raise the temperature from room temperature to 600 °C at a rate of 2 °C / min; then change it to 20% SiH 4 -N 2 , and keep it at 600 °C for 2 h for silicon deposition; then only introduce N 2 and cool down to 200 °C, then change to 1% O 2 -N 2 , and keep it at 200 °C for 0.1 h for oxygen atom deposition. Perform silicon deposition and oxygen atom deposition on the porous carbon in turn for 15 cycles, and cool down naturally under N 2 protection, and obtain a silicon-carbon composite material after crushing and classifying;

[0064] Step S3: The same as Step S3 in Example 1;

[0065] Step S4: The same as Step S4 in Example 1.

[0066] Example 3

[0067] This example provides a silicon-based composite material, and the preparation method is as follows:

[0068] Step S1 is the same as step S1 in Example 1;

[0069] Step S2 is the same as step S2 in Example 1;

[0070] Step S3: Mix the material obtained in step S2 with Li powder in a mass ratio of 100 / 0.6, place it in a high-temperature furnace, and heat it from room temperature to 600 °C at a rate of 2 °C / min in an N 2 atmosphere, and hold it at this temperature for 2 h. The Li powder reacts with SiO x at high temperature, and the active lithium inserts into the material to complete prelithiation, obtaining a core, where SiO x , nano-Si particles and lithium silicate are filled inside the porous carbon particles;

[0071] Step S4 is the same as step S4 in Example 1.

[0072] Example 4

[0073] This example provides a silicon-based composite material, and the preparation method is as follows:

[0074] Step S1 is the same as step S1 in Example 1;

[0075] Step S2 is the same as step S2 in Example 1;

[0076] Step S3: Mix the material obtained in step S2 with Li powder in a mass ratio of 100 / 2, place it in a high-temperature furnace, and heat it from room temperature to 600 °C at a rate of 2 °C / min in an N 2 atmosphere, and hold it at this temperature for 2 h. The Li powder reacts with SiO x at high temperature, and the active lithium inserts into the material to complete prelithiation, obtaining a core, where SiO x , nano-Si particles and lithium silicate are filled inside the porous carbon particles;

[0077] Step S4 is the same as step S4 in Example 1.

[0078] Example 5

[0079] This example provides a silicon-based composite material, and the preparation method is as follows:

[0080] Step S1 is the same as step S1 in Example 1;

[0081] Step S2 is the same as step S2 in Example 1;

[0082] Step S3: Mix the material obtained in Step S2 with Mg powder in a mass ratio of 100 / 1 evenly, place it in a high-temperature furnace, and heat it from room temperature to 600 °C at a rate of 2 °C / min in an N 2 atmosphere, and keep it at this temperature for 2 h. The Li powder reacts with SiO x at high temperature, and the active lithium is inserted into the material to complete prelithiation, obtaining a core, where SiO x , nano-Si particles and magnesium silicate are filled inside the porous carbon particles;

[0083] Step S4: The same as Step S4 in Example 1.

[0084] Example 6

[0085] This example provides a silicon-based composite material, and the preparation method is as follows:

[0086] Step S1: Heat sucrose from room temperature to 900 °C at a rate of 2 °C / min in an N 2 atmosphere, and keep it at this temperature for 2 h; after crushing and grading, obtain the inner core carbon material; mix the inner core carbon material with asphalt in a mass ratio of 20:1, and heat the obtained mixture from room temperature to 900 °C at a rate of 2 °C / min in an N 2 atmosphere, then change to introduce CO 2 , and keep it for another 2 h, then cool down, and obtain the composite porous material after crushing and grading;

[0087] Step S2: The same as Step S2 in Example 1;

[0088] Step S3: The same as Step S3 in Example 1;

[0089] Step S4: The same as Step S4 in Example 1.

[0090] Example 7

[0091] This example provides a silicon-based composite material, and the preparation method is as follows:

[0092] Step S1: Mix commercially available coconut shell charcoal (iodine value 800, 20 - 60 mesh particles) with asphalt in a mass ratio of 20:1, and heat the obtained mixture from room temperature to 800 °C at a rate of 2 °C / min in an N 2 atmosphere, and keep it at this temperature for 2 h; introduce CO 2 , activate it at 900 °C for 2 h, cool down, and obtain porous carbon after crushing and grading;

[0093] Step S2: The same as Step S2 in Example 1;

[0094] Step S3: The same as Step S3 in Example 1;

[0095] Step S4: The same as Step S4 in Example 1.

[0096] Example 8

[0097] This example provides a silicon-based composite material, and the preparation method is as follows:

[0098] Step S1: Sucrose is heated from room temperature to 500 °C at a rate of 2 °C / min in an N 2 atmosphere and maintained at this temperature for 2 h; after crushing and grading, the core carbon material is obtained; the core carbon material and epoxy resin are mixed at a mass ratio of 20:1, and the resulting mixture is heated from room temperature to 900 °C at a rate of 2 °C / min in an N 2 atmosphere, then changed to introduce CO 2 , and then maintained for 2 h, cooled, and after crushing and grading, a composite porous material is obtained;

[0099] Step S2: The same as Step S2 in Example 1;

[0100] Step S3: The same as Step S3 in Example 1;

[0101] Step S4: The same as Step S4 in Example 1.

[0102] Example 9

[0103] Step S1: Commercial coconut shell charcoal (iodine value 800, 20 - 60 mesh particles), sucrose, and Li 2 SiO 3 powder are mixed at a mass ratio of 20:1:0.4, and heated from room temperature to 800 °C at a rate of 2 °C / min in an N 2 atmosphere and maintained at this temperature for 2 h; CO 2 is introduced and activated at 900 °C for 2 h, cooled, and after crushing and grading, porous carbon is obtained. The obtained porous carbon is core-shell structured porous carbon, with coconut shell-based porous carbon as the first porous carbon core, coated with second porous carbon sucrose-based porous carbon; the obtained porous carbon contains Li 2 SiO 3 ;

[0104] Step S2: The porous carbon obtained in Step S1 is placed in a tubular furnace and heated from room temperature to 600 °C at a rate of 2 °C / min in an N 2 atmosphere; then changed to a 20% SiH 4 -0.01% O 2 -N 2 mixed gas, and silicon deposition is carried out by maintaining at 600 °C for 2 h in this mixed atmosphere; changed to only introduce N 2 for purging for 1 h; changed to a 5% C 2 H 2 -N 2 mixed gas, and maintained for 0.1 h for carbon deposition. Silicon deposition, N 2 purging, and carbon deposition are successively carried out on the porous carbon, and the cycle is repeated 15 times, in N2 Cool down naturally under protection, and then crush and classify.

[0105] Step S3: Mix the material obtained in Step S2 with Li powder at a mass ratio of 100 / 0.6 evenly, place it in a high-temperature furnace, and heat it from room temperature to 600 °C at a rate of 2 °C / min in an N 2 atmosphere, and keep it at this temperature for 2 h. The Li powder reacts with SiO x at high temperature, and the active lithium inserts into the material to complete prelithiation, obtaining the core, where SiO x , nano-Si particles and lithium silicate are filled inside the porous carbon particles;

[0106] Step S4: The same as Step S4 in Example 1.

[0107] Comparative Example 1

[0108] Step S1: Heat sucrose from room temperature to 800 °C at a rate of 2 °C / min in an N 2 atmosphere, and keep it at this temperature for 2 h; introduce CO 2 , activate it at 900 °C for 2 h, cool down, and obtain porous carbon after crushing and classifying.

[0109] Step S2: The same as Step S2 in Example 1;

[0110] Step S3: The same as Step S4 in Example 1.

[0111] Comparative Example 2

[0112] Step S1: Heat sucrose from room temperature to 800 °C at a rate of 2 °C / min in an N 2 atmosphere, and keep it at this temperature for 2 h; introduce CO 2 , activate it at 900 °C for 2 h, cool down, and obtain porous carbon after crushing and classifying.

[0113] Step S2: The same as Step S2 in Example 1;

[0114] Step S3: The same as Step S3 in Example 1;

[0115] Step S4: The same as Step S4 in Example 1.

[0116] Test Example 1

[0117] Calculate the specific surface area of the porous carbon in Examples 1-9 and Comparative Examples 1-2 according to the multi-point BET method, calculate the total pore volume according to the adsorption amount at the maximum partial pressure (p / p0>0.99), and use the t-method for N 2The micropore and mesopore ratios of the porous carbon were calculated from the adsorption-desorption isotherm. The pore volumes of the first and second porous carbon matrices in the porous carbon were obtained by calculation: the pore volume of each part in the porous carbon was determined by separately measuring the pore volumes of the first and second porous carbon matrices that make up its structure. The results are shown in Tables 1 and 2 below.

[0118] Table 1

[0119]

[0120] Table 2

[0121]

[0122]

[0123] Test Example 2

[0124] Using the silicon-based composite materials provided in the above examples and comparative examples as the negative electrode active materials, negative electrode sheets were prepared separately, and CR2032 type button cells were prepared by conventional methods, and the electrical properties of the cells were tested. The specific test methods are as follows:

[0125] (1) Half-cell assembly: Assemble a CR2032 type button cell in a glove box, using a lithium metal sheet as the counter electrode, a polypropylene microporous membrane as the separator, and the electrolyte is LiPF 6 dissolved in a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio EC:DEC = 1:1), where the concentration of LiPF 6 is 1 mol / L.

[0126] Use a LAND battery test system to test the charge and discharge of the battery.

[0127] (2) Cycling specific capacity and first efficiency test: After the CR2032 type button cell is left standing for 6 h, it is discharged at 0.05C to 0.005V, and then discharged at 0.01C to 0.005V; after standing for 5 min, it is charged at a constant current of 0.05C to 1.5V; the first de-lithiation specific capacity is the specific capacity (or mass specific capacity) of the electrode material, and the ratio of the first de-lithiation capacity to the first lithium insertion capacity is the first Coulomb efficiency of the battery.

[0128] (3) Capacity retention rate test: After standing for 5 min, repeat the above charge and discharge steps twice; then discharge at 0.25C to 0.005V; after standing for 5 min, charge at a constant current of 0.25C to 1.5V for 100 cycles. The charge specific capacity of the 100th cycle / the charge capacity of the first cycle × 100%, and the capacity retention rate is calculated.

[0129] (4) Electrode expansion rate: After the CR2032 button cell stands still for 6 h, it is discharged to 0.005 V at 0.05 C, and then discharged to 0.005 V at 0.01 C; then the button cell is disassembled in a glove box, the electrode is cleaned with DEC, and the thickness of the electrode is measured. The calculation method of the expansion rate is: (electrode thickness in the first fully charged state - fresh electrode thickness) / fresh electrode thickness × 100%.

[0130] Table 3 Electrochemical performance data table

[0131]

[0132]

[0133] It can be seen from Table 1, Table 2 and Table 3 that, compared with Comparative Examples 1-2, the porous carbon in the silicon-based composites obtained in Examples 1-9 has micropores and mesopores, and there are differences in the mesopore distribution between the first porous carbon matrix and the second porous carbon matrix, so that the sizes of the nanosilicon materials filled in the pores of the porous carbon form a gradient distribution, and different degrees of volume expansion occur in the first porous carbon matrix and the second porous carbon matrix during the lithium intercalation process. This unbalanced expansion stress is partially offset, so that the silicon expansion is effectively alleviated on the porous structure, the first porous carbon matrix and the second porous carbon matrix, and the structural stability of the negative electrode material during the cycling process is greatly improved. Thanks to this, the electrode expansion rate of the silicon-based composites obtained in Examples 1-9 is significantly reduced compared with Comparative Examples 1-2, and the 100-cycle capacity retention rate is also significantly improved. Compared with Comparative Example 1, the silicon-based composites obtained in Examples 1-9 and Comparative Example 2 contain metal silicates, and the metal silicates are distributed between the porous carbon and the nanosilicon, as well as between the nanosilicon and the nanosilicon. Due to the spacer effect of the metal silicates, the nanosilicon is uniformly dispersed in the pores of the porous carbon, further alleviating the volume effect of the silicon and reducing the irreversible lithium ion amount. The composite electrode shows a relatively excellent initial efficiency. Therefore, the 1.5 V initial Coulomb efficiency is significantly improved and increases with the increase of the silicate content. However, due to the increase of the silicate content, the active silicon content in the silicon-based composite is reduced, resulting in a slight decrease in the 1.5 V specific capacity, and it decreases with the increase of the silicate content.

[0134] Examples 6-8 use different carbon precursors from Examples 1-5 to obtain silicon-based composites with different pore structures. The first carbon precursor and the second carbon precursor of Example 7 are respectively selected as coconut shell carbon and pitch, and appropriate activation conditions are adopted. The obtained composite porous carbon matrix has the highest micropore ratio compared with others. Therefore, Example 7 has the lowest electrode expansion rate and the highest 100-cycle capacity retention rate.

[0135] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A silicon-based composite material, characterized in that, The silicon-based composite material includes a core and a protective layer coated on the surface of the core; the core includes porous carbon, nano-silicon and metal silicate; the porous carbon includes a first porous carbon matrix and a second porous carbon matrix, the second porous carbon matrix is wrapped on the surface of the first porous carbon matrix, and nano-silicon and metal silicate MSO are dispersedly filled in the pores of the first porous carbon matrix and the second porous carbon matrix. The chemical formula of the metal silicate MSO is aMO x ·bSiO y , where 0 < x ≤ 1, 0 < y ≤ 2, 0 < a / b ≤ 2, and the M is Li and / or Mg.

2. The silicon-based composite material according to claim 1, characterized in that, The median particle size d of the first porous carbon matrix V50 is 5 to 15 μm, and the thickness of the second porous carbon matrix is 0.5 to 2 μm; Preferably, the particle size of the nano-silicon is 0.5 to 10 nm; Preferably, based on the mass of the silicon-based composite material being 100%, the mass content of the metal silicate is 3 to 10%.

3. The silicon-based composite material according to claim 1, characterized in that, The proportion of the mesopore volume of the first porous carbon matrix in the total pore volume of the first porous carbon is 25 to 60%, and the proportion of the mesopore volume of the second porous carbon matrix in the total pore volume of the second porous carbon is 5 to 15%.

4. The silicon-based composite material according to claim 1, characterized in that, The proportion of the mesopore volume of the first porous carbon matrix in the total pore volume of the first porous carbon is 5 to 15%, and the proportion of the mesopore volume of the second porous carbon matrix in the total pore volume of the second porous carbon is 25 to 60%.

5. The silicon-based composite material according to claim 1, characterized in that, The porous carbon has micropores, and the proportion of the micropore volume of the porous carbon in the total pore volume of the porous carbon is 30 to 90%.

6. The silicon-based composite material according to any one of claims 1 to 5, characterized in that, The specific surface area of the porous carbon is 200 to 3000 m 2 / g, and the total pore volume is 0.2 to 3.0 cm 3 / g.

7. The silicon-based composite material according to any one of claims 1 to 5, characterized in that, The core includes (C-MSO)-(Si-MSO-C), C-MSO is a porous carbon matrix with MSO on its surface, where the content of MSO in the porous carbon matrix is 0 to 5 wt.%; Si-MSO-C includes nano-silicon, metal silicate and optional carbon, and the nano-silicon, metal silicate and optional carbon are dispersedly distributed in the pores of the porous carbon, where the content of MSO in Si-MSO-C is 0 to 5 wt.%, and the content of optional carbon in Si-MSO-C is 0 to 5 wt.%.

8. The silicon-based composite material according to any one of claims 1 to 5, characterized in that, The nano-silicon is at least one of elemental silicon particles, SiO δ (0 < δ ≤ 2) particles or elemental silicon particles with at least partially oxidized surfaces.

9. The silicon-based composite material according to claim 1, characterized in that, The protective layer includes at least one of silicon carbide, silicon nitride, carbon, and ion-conducting materials.

10. A method for preparing the silicon-based composite material according to any one of claims 1 to 9, characterized in that, comprises: Step S1, providing porous carbon, the porous carbon includes a first porous carbon matrix and a second porous carbon matrix, the second porous carbon matrix wraps the surface of the first porous carbon matrix, and the first porous carbon matrix and the second porous carbon matrix have mesopores; Step S2, using a silicon-containing precursor and an oxygen-containing precursor as deposition gases, and depositing nano-silicon in the pores of the first porous carbon matrix and the second porous carbon matrix by chemical vapor deposition; Step S3, mixing the material obtained in Step S2 with an M source and performing high-temperature sintering to obtain a pre-lithiated or pre-magnesiated silicon-based composite material; the M source contains Li and / or Mg elements; Step S4, coating a protective layer on the surface of the pre-lithiated or pre-magnesiated silicon-based composite material obtained in Step S3 to obtain the silicon-based composite material.

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