Silicon-based material with core-shell structure, preparation method of silicon-based material, negative plate and lithium ion battery

Abstract

The invention provides a silicon-based material with a core-shell structure, a preparation method of the silicon-based material, a negative plate and a lithium ion battery. The core-shell structure silicon-based material comprises a silicon core and a SiOC shell coating the surface of the silicon core. The free carbon component in the SiOC shell layer has good electron conduction capability, and is beneficial to promoting charge transmission in the electrode material, thereby being beneficial to improving the conductivity of the core-shell structure silicon-based material. Due to the existence of the SiOC shell layer, the direct contact between the silicon core and the electrolyte is reduced, the overgrowth and repeated damage of the SEI layer are inhibited, and the continuous consumption of the electrolyte and the loss of active Li < + > are reduced. Therefore, the silicon-based material with the core-shell structure has relatively low volume change in the charge-discharge process, and the silicon-based material with the core-shell structure has excellent ion transmission and electron transmission performance, so that the specific capacity and the cycling stability of the battery can be improved when the silicon-based material with the core-shell structure is applied to the battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a core-shell structure silicon-based material, a preparation method thereof, a negative electrode sheet and a lithium ion battery. BACKGROUND

[0002] With the increasing demand for mobile electronic products, electric vehicles and large-scale energy storage systems, high energy density has become the focus of lithium ion battery development. However, the capacity of traditional graphite negative electrode is limited (372 mAh·g -1 ), so it is necessary to develop negative active materials with higher theoretical capacity. Silicon-based negative electrodes have attracted great interest from researchers. Silicon is not only abundant (accounting for 28% of the mass of the earth's crust), low-cost, non-toxic, but also has a theoretical capacity of ≈3579 mAh·g -1 and an ideal lithiation / delithiation potential. However, silicon usually undergoes large volume expansion and contraction during cycling, leading to electrode pulverization and collapse, repeated destruction and reconstruction of the solid electrolyte interface layer (SEI), and continuous consumption of electrolyte and loss of active Li + , which severely limits the cycle life of the electrode. Therefore, it is urgent to develop a high-performance silicon-based negative electrode material with stable structure and limited volume expansion.

[0003] To solve these problems of silicon-based negative electrodes and induce stress release and improve cycle performance, some researchers have developed nano-silicon structures. However, these nanostructures result in low volume energy density of the electrode and low initial coulombic efficiency (ICE), and in addition, the cost of preparing silicon materials of nano-size is usually high. Another common approach is to reserve some buffer space inside the electrode material to accommodate the volume change of silicon during cycling. However, the introduction of voids inside the electrode reduces the loading of active silicon, thereby reducing the energy density of the electrode, so this method is not very practical. A more attractive alternative is to coat a layer of carbon on the surface of silicon. However, the specific capacity of the coated carbon material is usually low and easy to break, and it is difficult to prepare a thin and uniform carbon coating. SUMMARY

[0004] The main purpose of the present application is to provide a core-shell structure silicon-based material, a preparation method thereof, a negative electrode sheet and a lithium ion battery, to solve the problem of poor cycle stability of the silicon-based negative electrode sheet in the prior art.

[0005] In order to achieve the above-mentioned purpose, according to one aspect of the present application, a core-shell structure silicon-based material is provided, which comprises a silicon core and a SiOC shell coated on the surface of the silicon core.

[0006] The free carbon component in the SiOC shell has good electron conduction capacity, which helps to promote the charge transmission inside the electrode material, thus helping to improve the conductivity of the core-shell structure silicon-based material. Meanwhile, the Li + in the SiOC shell can be alloyed with the Si atoms and combined with the O atoms, thus helping to improve the Li + storage capacity of the core-shell structure silicon-based material, reducing the diffusion resistance of lithium ions in the electrode material, thus helping to improve the rate performance of the lithium ion battery, and further helping to improve the discharge capacity and energy density of the battery. In addition, the presence of the SiOC shell reduces the direct contact between the silicon core and the electrolyte, inhibits the excessive growth and repeated damage of the SEI layer, and reduces the continuous consumption of the electrolyte and the loss of active Li + .

[0007] Further, the D50 particle size of the above-mentioned core-shell structure silicon-based material is 1-5 μm; and / or, the thickness of the SiOC shell is 3-15 nm.

[0008] Controlling the D50 particle size of the core-shell structure silicon-based material within the above-mentioned range helps to maintain the structural stability of the electrode material, reduces the breakage and collapse during the cycle process, thus helping to prolong the cycle life of the battery. Controlling the thickness of the SiOC shell within the above-mentioned range helps to make the core-shell structure silicon-based material have a higher energy density while having a lower volume change.

[0009] Further, the above-mentioned core-shell structure silicon-based material is prepared by in-situ reaction of supercritical carbon dioxide on the surface of silicon particles and calcination; and / or, the conductivity of the core-shell structure silicon-based material is 5.04×10 -4 -9.04×10 -4 S / cm.

[0010] The special properties of supercritical carbon dioxide help to uniformly form the SiOC shell on the surface of the silicon core, avoiding the uneven or excessive thickness phenomenon that may occur in traditional coating methods, which is beneficial to maintaining a high proportion of active silicon, and also helps to improve the relief of the volume expansion effect of the SiOC shell. The core-shell structure silicon-based material with the above-mentioned conductivity is more suitable for use as a negative active material of a battery.

[0011] According to another aspect of the present application, a preparation method of the aforementioned core-shell structure silicon-based material is provided, which comprises: step S1, passing carbon dioxide into a reactor containing a suspension of silicon particles, adjusting the carbon dioxide to a supercritical state, and then allowing it to react in-situ with the surface of the silicon particles to obtain a silicon-coated precursor; and step S2, calcining the silicon-coated precursor in a protective gas to obtain the core-shell structure silicon-based material.

[0012] The application utilizes the supercritical state carbon dioxide fluid which has similar diffusion of gas, nearly zero surface tension and excellent mass transfer characteristics, which can be well dispersed with silicon particles, fully contact and react to generate uniform coated precursor interface layer, and then calcined to prepare SiOC coated silicon-based anode material.

[0013] Further, the temperature of the in-situ reaction is 40-60℃; and / or, the time of the in-situ reaction is 1-4h; and / or, the pressure of the in-situ reaction is 8-10MPa.

[0014] Controlling the temperature and pressure of the reaction within the above range helps to make the carbon dioxide in a supercritical state. Controlling the time of the reaction within the above range helps to control the thickness of the SiOC shell, and a suitable SiOC shell thickness helps to make the core-shell structure silicon-based material have a higher energy density while having a lower volume change.

[0015] Further, the in-situ reaction is carried out in a stirring state, and the stirring speed of the in-situ reaction is 200-1000rpm; and / or, the D50 particle size of the silicon particles is 1-5μm; and / or, the purity of the silicon particles is greater than 99.9%.

[0016] Controlling the stirring speed within the above range helps to promote uniform contact of the supercritical carbon dioxide fluid with the silicon particles, promoting the uniformity of the reaction, thereby helping to improve the uniform formation of the SiOC shell layer on the surface of the silicon core. Controlling the D50 particle size of the silicon particles within the above range helps to optimize ion transport efficiency and improve the rate performance and cycle stability of the material.

[0017] Further, the mass of the silicon particles to the volume of the carbon dioxide is (1-10g):(0.25-5Nm 3 ).

[0018] Controlling the mass of the silicon particles to the volume of the carbon dioxide within the above range helps to promote the formation of the SiOC shell layer and improve the utilization of resources.

[0019] Further, the temperature of the calcination is 800-1000℃; and / or, the time of the calcination is 3-12h.

[0020] Controlling the temperature of the calcination within the above range helps to make the pyrolysis reaction of the silicon-coated precursor more sufficient, and helps to form a SiOC shell layer with good structure and performance. Controlling the time of the calcination within the above range helps to make the silicon-coated precursor fully pyrolyze to generate a stable and uniform SiOC shell layer.

[0021] According to another aspect of the application, a negative electrode sheet is provided, which contains the aforementioned core-shell structure silicon-based material.

[0022] Due to the fact that the negative electrode sheet contains the core-shell structured silicon-based material of the present application, the negative electrode sheet has high specific capacity and cycle stability.

[0023] According to yet another aspect of the present application, there is provided a lithium ion battery, a positive electrode sheet, an electrolyte and a negative electrode sheet, the negative electrode sheet being the aforementioned negative electrode sheet.

[0024] Due to the fact that the negative electrode sheet in the lithium ion battery contains the core-shell structured silicon-based material of the present application, the lithium ion battery has good rate capability and cycle stability.

[0025] By using the technical solution of the present application, silicon will experience significant volume expansion and shrinkage during the charging and discharging process of the lithium ion battery, leading to the destruction of the electrode structure. The SiOC shell layer can effectively buffer the volume change of the silicon core during the lithiation / delithiation process due to its flexibility, which helps to reduce the crushing and collapse of the electrode, thereby helping to improve the mechanical stability of the electrode material. The free carbon component in the SiOC shell layer has good electronic conductivity, which helps to promote the charge transport within the electrode material, thereby helping to improve the electrical conductivity of the core-shell structured silicon-based material. At the same time, Li + can be alloyed with Si atoms in SiOC and combined with O atoms, thereby helping to improve the Li + storage capacity of the core-shell structured silicon-based material, reducing the diffusion resistance of lithium ions in the electrode material, thereby helping to improve the rate capability of the lithium ion battery, and further helping to improve the discharge capacity and energy density of the battery. In addition, the presence of the SiOC shell layer reduces the direct contact between the silicon core and the electrolyte, inhibits the excessive growth and repeated destruction of the SEI layer, and reduces the continuous consumption of the electrolyte and the loss of active Li + Therefore, the core-shell structured silicon-based material of the present application has low volume change during the charging and discharging process, and the core-shell structured silicon-based material has excellent ion transport and electron transport performance, which helps to improve the specific capacity and cycle stability of the battery when it is applied to the battery. BRIEF DESCRIPTION OF DRAWINGS

[0026] The accompanying drawings, which form a part of the present application, are used to provide further understanding of the present application, and the illustrative embodiments of the present application and their description serve the purpose of explaining the present application. In the drawings:

[0027] Figure 1 The XRD pattern of the core-shell structured silicon-based material in Example 1 of the present application is shown. DETAILED DESCRIPTION

[0028] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments.

[0029] The term explanation: silicon oxycarbide (SiOC) is an amorphous structure mixed with Si, C and O atoms, and its chemical composition can be generally described as SiO 2(1-x) C x + elemental carbon, wherein 0 < x < 1.

[0030] As analyzed in the background of the present application, the prior art silicon-based negative electrode sheet has the problem of poor cycle stability. In order to solve the above problems, the present application provides a core-shell structure silicon-based material, a preparation method thereof, a negative electrode sheet and a lithium ion battery.

[0031] In a typical embodiment of the present application, a core-shell structure silicon-based material is provided, which comprises a silicon core and a SiOC shell coated on the surface of the silicon core.

[0032] Silicon will experience significant volume expansion and contraction during the charging and discharging process of lithium ion batteries, leading to the destruction of the electrode structure. The SiOC shell layer can effectively buffer the volume change of the silicon core during lithiation / delithiation due to its flexibility, which helps to reduce the crushing and collapse of the electrode, thereby helping to improve the mechanical stability of the electrode material. The free carbon component in the SiOC shell layer has good electronic conductivity, which helps to promote the charge transfer inside the electrode material, thereby helping to improve the electrical conductivity of the core-shell structure silicon-based material. At the same time, Li + can be alloyed with Si atoms in SiOC and combined with O atoms, thereby helping to improve the Li + storage capacity of the core-shell structure silicon-based material, reducing the diffusion resistance of lithium ions in the electrode material, thereby helping to improve the rate performance of the lithium ion battery, and further helping to improve the discharge capacity and energy density of the battery. In addition, the presence of the SiOC shell layer reduces the direct contact between the silicon core and the electrolyte, inhibits the excessive growth and repeated destruction of the SEI layer, and reduces the continuous consumption of the electrolyte and the loss of active Li + Therefore, the core-shell structure silicon-based material of the present application has a lower volume change during charging and discharging, and the core-shell structure silicon-based material has excellent ion transport and electron transport performance, which helps to improve the specific capacity and cycle stability of the battery when applied to the battery.

[0033] In an embodiment of the present application, the D50 particle size of the above-mentioned core-shell structure silicon-based material is 1-5 μm; and / or, the thickness of the SiOC shell is 3-15 nm.

[0034] The smaller particle size helps to shorten the diffusion path of lithium ions inside the silicon material, thereby improving the rate of electrochemical reaction and the rate performance of the battery. The smaller particle size means an increase in specific surface area, which is beneficial for the formation of a SEI layer, making it thinner and more stable, reducing the consumption of electrolyte. Controlling the D50 particle size of the core-shell structure silicon-based material within the above range helps to maintain the structural stability of the electrode material, reducing the fragmentation and collapse during the cycle process, thereby helping to prolong the cycle life of the battery. The thin and uniform SiOC shell layer can effectively buffer the volume change of silicon during lithiation / delithiation, protecting the silicon core from mechanical stress damage, thereby helping to improve the cycle stability of the battery. The carbon component in the SiOC shell provides a good electron conduction path, enhancing the electrical conductivity of the material; at the same time, the internal pores and oxygen atom structure promote the storage and transport of lithium ions, improving the efficiency of electrochemical reaction. Controlling the thickness of the SiOC shell within the above range helps to make the core-shell structure silicon-based material have a higher energy density while having a lower volume change.

[0035] In an embodiment of the present application, the above-mentioned core-shell structure silicon-based material is prepared by supercritical carbon dioxide in-situ reaction and calcination on the surface of silicon particles; and / or, the electrical conductivity of the core-shell structure silicon-based material is 5.04 x 10 -4 ~ 9.04 x 10 -4 S / cm.

[0036] Supercritical carbon dioxide, as a special fluid, has high diffusivity, low surface tension and good mass transfer characteristics, fully contacts the surface of silicon particles and chemically reacts, generating a SiOC shell layer containing carbon, silicon and oxygen in-situ. The special properties of supercritical carbon dioxide help to uniformly form the SiOC shell layer on the surface of the silicon core, avoiding the uneven or excessive thickness phenomenon that may occur in traditional coating methods, which is beneficial to maintaining a high proportion of active silicon, while helping to improve the relief of the volume expansion effect of the SiOC shell. And the in-situ formed SiOC shell is more stable, thereby further helping to improve the cycle stability of the battery. The core-shell structure silicon-based material with the above electrical conductivity is more suitable for use as a negative active material of a battery.

[0037] In another typical embodiment of the present application, a preparation method of the aforementioned core-shell structure silicon-based material is provided, which comprises: step S1, passing carbon dioxide into a reactor containing a suspension containing silicon particles, adjusting the carbon dioxide to a supercritical state, and then allowing it to react in-situ with the surface of the silicon particles to obtain a silicon-coated precursor; step S2, calcining the silicon-coated precursor in a protective gas to obtain a core-shell structure silicon-based material.

[0038] The application utilizes the supercritical state carbon dioxide fluid which has similar gas diffusion, nearly zero surface tension and excellent mass transfer characteristics, which can be well dispersed with silicon particles, fully contact and react to generate a uniform coated precursor interface layer, and then calcined to prepare SiOC coated silicon-based anode material. The preparation method has simple and effective process, controllable cost and potential for large-scale production.

[0039] In an embodiment of the application, the step S1 comprises: adding silicon particles into a ball mill tank for ball milling, then performing screening, magnetic removal and other operations, then dispersing the obtained silicon powder in a solvent to obtain a solution containing silicon particles; transferring the solution containing silicon particles into a reactor, heating the solution containing silicon particles to a required reaction temperature, then filling carbon dioxide gas into the reactor, pressurizing to make the carbon dioxide in a supercritical state, then performing reaction, reducing the pressure to normal pressure state after the reaction is completed, finally taking out the product from the reactor, drying the obtained powder at 50-70℃ overnight after filtration to obtain a silicon-coated precursor.

[0040] Including but not limited to, the solvent is anhydrous ethanol, which is miscible with carbon dioxide fluid in a supercritical state, so it is used as a solvent to facilitate subsequent processing and collection of powder samples. The amount of solvent can be sufficient to disperse the silicon particles.

[0041] Including but not limited to, the protective gas is selected from any one or more of nitrogen, argon and helium.

[0042] In an embodiment of the application, the in-situ reaction temperature is 40-60℃; and / or, the in-situ reaction time is 1-4h; and / or, the in-situ reaction pressure is 8-10MPa.

[0043] Controlling the temperature and pressure of the reaction within the above range helps to make the carbon dioxide in a supercritical state. Controlling the reaction time within the above range helps to control the thickness of the SiOC shell. A suitable SiOC shell thickness helps to make the core-shell structure silicon-based material have a higher energy density while having a lower volume change.

[0044] In an embodiment of the application, the in-situ reaction is performed under stirring, and the stirring speed of the in-situ reaction is 200-1000rpm; and / or, the D50 particle size of the silicon particles is 1-5μm; and / or, the purity of the silicon particles is greater than 99.9%.

[0045] Controlling the stirring speed in the above range helps to promote uniform contact of the supercritical carbon dioxide fluid with the silicon particles, promoting the uniformity of the reaction, thereby helping to improve the uniform formation of the SiOC shell layer on the surface of the silicon core. Controlling the D50 particle size of the silicon particles in the above range helps to optimize ion transport efficiency, improve the rate performance and cycle stability of the material. High-purity silicon particles help to improve the intrinsic electrochemical activity of the material, reduce the interference of impurity elements in the reaction process, and help to improve the formation quality of the SiOC shell layer and the electrochemical performance of the material.

[0046] To promote the formation of the SiOC shell layer and improve the utilization of resources, in an embodiment of the present application, the mass of the above-mentioned silicon particles to the volume of carbon dioxide is preferably (1-10 g):(0.25-5 Nm 3 )。

[0047] In an embodiment of the present application, the calcination temperature is 800-1000℃; and / or, the calcination time is 3-12h.

[0048] Controlling the calcination temperature in the above range helps to make the pyrolysis reaction of the silicon-coated precursor more complete, and helps to form a SiOC shell layer with good structure and performance. Controlling the calcination time in the above range helps to make the silicon-coated precursor pyrolyze sufficiently to generate a stable and uniform SiOC shell layer.

[0049] In another typical embodiment of the present application, a negative electrode sheet containing the aforementioned core-shell structure silicon-based material is provided.

[0050] Since the above-mentioned negative electrode sheet contains the core-shell structure silicon-based material of the present application, the negative electrode sheet has high specific capacity and cycle stability.

[0051] In another typical embodiment of the present application, a lithium ion battery is provided, which includes a positive electrode sheet, an electrolyte, and a negative electrode sheet, which is the aforementioned negative electrode sheet.

[0052] Since the negative electrode sheet in the above-mentioned lithium ion battery contains the core-shell structure silicon-based material of the present application, the lithium ion battery has good rate performance and cycle stability.

[0053] The beneficial effects of the present application will be further illustrated in conjunction with the examples below.

[0054] Example 1

[0055] (1) Micron silicon pretreatment:

[0056] Take 25 g of micron-sized silicon powder into a stainless steel ball mill tank, the purity of the silicon powder is 99.97%, add appropriate amount of ball milling beads and then ball mill for 24 hours, then take out the powder for screening and magnetic removal operation to obtain silicon powder with a D50 particle size of 2 μm, disperse the obtained silicon powder in anhydrous ethanol solution.

[0057] (2) Silicon-coated precursor preparation:

[0058] The solution obtained in step (1) is transferred to a sealed reactor, and the silicon powder-ethanol dispersion is heated to 50°C, then carbon dioxide gas is filled into the reactor, the mass of the silicon powder to the volume of the carbon dioxide is 1 g: 0.25 Nm 3 , pressurized to 10 MPa, so that the carbon dioxide is in a supercritical state, and the stirring speed is 800 rpm, the reaction is carried out for 3 hours, then the pressure is reduced to normal pressure, and finally the product is taken out from the sealed reactor, filtered and dried at 60°C overnight to obtain a silicon-coated precursor.

[0059] (3) Preparation of core-shell structure silicon-based material:

[0060] The dried powder of the silicon-coated precursor obtained in step (2) is weighed and then transferred to a tube furnace,

[0061] argon gas is introduced and calcined at 850°C for 3 hours, then annealed and cooled to room temperature to obtain a core-shell structure silicon-based material, the D50 particle size of the core-shell structure silicon-based material is 2 μm, and the thickness of the SiOC shell is 8 nm measured by transmission electron microscopy.

[0062] Example 2

[0063] The difference from Example 1 is that the ball milling time in step (1) is changed to 12 hours, and the D50 particle size of the silicon powder obtained is 5 μm, and finally a core-shell structure silicon-based material is obtained, the D50 particle size of the core-shell structure silicon-based material is 5 μm.

[0064] Example 3

[0065] The difference from Example 1 is that the stirring time of the silicon powder-ethanol dispersion in step (2) is changed to 1 hour, and finally a core-shell structure silicon-based material is obtained, the thickness of the SiOC shell is 3 nm.

[0066] Example 4

[0067] The difference from Example 1 is that the stirring time of the silicon powder-ethanol dispersion in step (2) is changed to 5 hours, and finally a core-shell structure silicon-based material is obtained, the thickness of the SiOC shell is 20 nm.

[0068] Example 5

[0069] The difference from Example 1 is that the ball milling time in step (1) is changed to 2 hours, and the D50 particle size of the micron silicon is 10 μm. Finally, a core-shell structure silicon-based material is obtained, and the D50 particle size of the core-shell structure silicon-based material is 10 μm.

[0070] Example 6

[0071] The difference from Example 1 is that the stirring time of the silicon powder-ethanol dispersion liquid in step (2) is changed to 4 hours. Finally, a core-shell structure silicon-based material is obtained, and the thickness of the SiOC shell is 15 nm.

[0072] Example 7

[0073] The difference from Example 1 is that the stirring speed is 200 rpm. Finally, a core-shell structure silicon-based material is obtained.

[0074] Example 8

[0075] The difference from Example 1 is that the stirring speed is 1000 rpm. Finally, a core-shell structure silicon-based material is obtained.

[0076] Example 9

[0077] The difference from Example 1 is that the stirring speed is 100 rpm. Finally, a core-shell structure silicon-based material is obtained.

[0078] Example 10

[0079] The difference from Example 1 is that the mass of the silicon powder to the volume of the carbon dioxide is 10 g:0.25 Nm 3 . Finally, a core-shell structure silicon-based material is obtained.

[0080] Example 11

[0081] The difference from Example 1 is that the mass of the silicon powder to the volume of the carbon dioxide is 10 g:0.15 Nm 3 . Finally, a core-shell structure silicon-based material is obtained.

[0082] Example 12

[0083] The difference from Example 1 is that the calcination temperature is 900°C, and the calcination time is 3 h. Finally, a core-shell structure silicon-based material is obtained.

[0084] Example 13

[0085] The difference from Example 1 is that the calcination temperature is 750°C, and the calcination time is 6 h. Finally, a core-shell structure silicon-based material is obtained.

[0086] Example 14

[0087] The difference from Example 1 is that the temperature of the reaction is 40℃, and the pressure of the reaction is 8 MPa. Finally, a core-shell structure silicon-based material is obtained.

[0088] Example 15

[0089] (1) Micro-silicon pretreatment:

[0090] Take 25 g of micron-sized silicon powder into a stainless steel ball mill tank. The purity of the silicon powder is 99.97%. Then, add appropriate amount of ball milling beads and ball mill for 24 hours. Then, take out the powder for screening and magnetic removal operation to obtain silicon powder with a D50 particle size of 2.1 μm.

[0091] (2) Preparation of silicon-coated precursor:

[0092] Transfer the silicon powder obtained in step (1) into a reactor. Coat a layer of polyvinylsilsesquioxane on the surface of the silicon particles by sol-gel method. After filtration, dry the obtained powder at 60℃ overnight to obtain a silicon-coated precursor.

[0093] (3) Preparation of core-shell structure silicon-based material:

[0094] Weigh the dried powder of the silicon-coated precursor obtained in step (2), and then transfer it to a tube furnace. Introduce argon flow and calcine at 850℃ for 3 hours. Then, anneal and cool to room temperature to obtain a core-shell structure silicon-based material. The D50 particle size of the core-shell structure silicon-based material is 2.1 μm, and the thickness of the SiOC shell is 20 nm.

[0095] Comparative Example 1

[0096] Directly use the raw material micron silicon as the silicon-based material.

[0097] Comparative Example 2

[0098] (1) Micro-silicon pretreatment:

[0099] Take 25 g of micron-sized silicon powder into a stainless steel ball mill tank. Then, add appropriate amount of ball milling beads and ball mill for 24 hours. Then, take out the powder for screening and magnetic removal operation to obtain silicon powder with a D50 particle size of 2.1 μm.

[0100] (2) Calcination treatment:

[0101] Transfer the silicon powder obtained in step (1) into a tube furnace. Introduce argon flow and calcine at 850℃ for 3 hours. Then, anneal and cool to room temperature to obtain a silicon-based material.

[0102] Performance test

[0103] The silicon-based material prepared in the examples and comparative examples was used as a negative active material to assemble a half battery: the negative active material, a conductive agent, and a binder were mixed in deionized water and ground thoroughly, and the slurry was cast and coated on a copper foil using a doctor blade. The conductive agent was acetylene black, and the binder was polyacrylic acid. The mass ratio of the negative active material, the conductive agent, and the binder was 8:1:1. After coating, the sample was dried at 120°C in a vacuum environment for 12 h, and then cut into a sheet to obtain a composite silicon-based negative electrode sheet. A lithium foil and a glass fiber membrane were used as the counter electrode and the separator, respectively. The electrolyte used was composed of 1M LiPF6 salt, a mixture of ethylene carbonate / diethyl carbonate solvents (volume ratio of 1:1), and 10wt% fluorine ethylene carbonate, and 60μL of electrolyte was added to each coin cell. The assembled half battery was cycled at a current density of 0.2A·g -1 for 400 cycles at 25°C, and the capacity retention rate was calculated. The initial specific capacity of the test half battery was tested at 2A·g -1 , and the test results are shown in Table 1.

[0104] Table 1

[0105]

[0106] Figure 1 The XRD pattern of the core-shell structure silicon-based material in Example 1 of the present application is shown in FIG. 1. Figure 1 As can be seen from FIG. 1, the core-shell structure silicon-based material still exhibits a silicon crystal structure (characteristic peaks at 28.4°, 47.4°, and 56.1°), and the SiOC layer exists in an amorphous state.

[0107] As can be seen from the above description, the above examples of the present application achieve the following technical effects:

[0108] Silicon undergoes significant volume expansion and contraction during the charging and discharging process of lithium ion batteries, leading to the destruction of the electrode structure. The SiOC shell layer can effectively buffer the volume change of the silicon core during lithiation / delithiation due to its flexibility, which helps to reduce the crushing and collapse of the electrode, thereby helping to improve the mechanical stability of the electrode material. The free carbon component in the SiOC shell layer has good electronic conductivity, which helps to promote the charge transport within the electrode material, thereby helping to improve the electrical conductivity of the core-shell structure silicon-based material. At the same time, Li + can alloy with Si atoms in SiOC and combine with O atoms, thereby helping to improve the Li + storage capacity of the core-shell structure silicon-based material, reducing the diffusion resistance of lithium ions in the electrode material, thereby helping to improve the rate performance of the lithium ion battery, and further helping to improve the discharge capacity and energy density of the battery. In addition, the presence of the SiOC shell layer reduces the direct contact of the silicon core with the electrolyte, inhibits the excessive growth and repeated destruction of the SEI layer, and reduces the continuous consumption of the electrolyte and active Li+ Therefore, the core-shell structure silicon-based material of the present application has a lower volume change in the charging and discharging process, and the core-shell structure silicon-based material has excellent ion transmission and electron transmission performance, which helps to improve the specific capacity and cycle stability of the battery when applied to the battery.

[0109] The above merely illustrates the embodiments of the present application and is not intended to limit the present application. The present application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A core-shell structured silicon-based material, characterized in that: The core-shell structured silicon-based material includes a silicon core and a SiOC shell covering the surface of the silicon core.

2. The core-shell structure silicon-based material according to claim 1, characterized in that: The D50 particle size of the core-shell structured silicon-based material is 1 to 5 μm; and / or the thickness of the SiOC shell is 3 to 15 nm.

3. The core-shell structure silicon-based material according to claim 1 or 2, characterized in that: The core-shell structured silicon-based material is prepared by in-situ supercritical carbon dioxide reaction and calcination on the surface of silicon particles; and / or the electrical conductivity of the core-shell structured silicon-based material is 5.04×10 -4 ~9.04×10 -4 S / cm.

4. A method for preparing a core-shell structured silicon-based material according to any one of claims 1 to 3, characterized in that: The preparation method comprises: Step S1, introducing carbon dioxide into a reactor containing a suspension of silicon particles, adjusting the carbon dioxide to a supercritical state, and reacting the carbon dioxide with the surface of the silicon particles in situ to obtain a silicon-coated precursor; Step S2: calcining the silicon-coated precursor in a protective gas to obtain the core-shell structured silicon-based material.

5. The preparation method according to claim 4, characterized in that The temperature of the in-situ reaction is 40-60° C.; and / or, the time of the in-situ reaction is 1-4 hours; and / or, the pressure of the in-situ reaction is 8-10 MPa.

6. The preparation method according to claim 4, characterized in that The in-situ reaction is carried out under stirring, and the stirring speed of the in-situ reaction is 200-1000 rpm; and / or the D50 particle size of the silicon particles is 1-5 μm; and / or the purity of the silicon particles is greater than 99.9%.

7. The preparation method according to any one of claims 4 to 6, characterized in that The ratio of the mass of the silicon particles to the volume of the carbon dioxide is (1-10 g): (0.25-5 Nm 3 ).

8. The preparation method according to any one of claims 4 to 6, characterized in that The calcination temperature is 800-1000° C.; and / or the calcination time is 3-12 hours.

9. A negative electrode sheet, characterized in that: The negative electrode sheet contains the core-shell structure silicon-based material according to any one of claims 1 to 3.

10. A lithium-ion battery comprising a positive electrode sheet, an electrolyte and a negative electrode sheet, characterized in that: The negative electrode sheet is the negative electrode sheet according to claim 9.

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