Silicon-carbon composite negative electrode material and preparation method and application thereof

Abstract

The invention relates to a silicon-carbon composite negative electrode material and a preparation method and application thereof. The silicon-carbon composite negative electrode material is of a core-shell-shell structure and sequentially comprises the following components from inside to outside: an inner core complex which comprises a porous carbon matrix and nano silicon particles dispersed in pores of the porous carbon matrix; the porous carbon matrix is used as a conductive supporting framework, and the nano silicon particles are uniformly dispersed on the conductive supporting framework; a three-dimensional interconnected pore structure with a sub-nanometer scale is arranged in the nanometer silicon particle; the passivation coating layer is coated on the outer surface of the inner core complex; and the outer surface of the passivation coating layer is coated with the carbon layer. According to the silicon-carbon composite negative electrode material provided by the invention, systematic regulation and control on the volume expansion behavior of silicon are realized through collaborative design of a cross-scale structure.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion battery materials, in particular to a silicon-carbon composite negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of electric vehicles and large-scale energy storage systems, the market has put forward higher requirements for the energy density of lithium ion batteries. Silicon (Si) material is considered as one of the most potential negative electrode materials for the next generation of high energy density lithium ion batteries due to its extremely high theoretical specific capacity (about 4200 mAh / g, more than 10 times that of traditional graphite negative electrode) and suitable lithium intercalation potential.

[0003] However, silicon material will experience a huge volume expansion (>300%) during charging and discharging, which leads to a series of serious problems: active material particle breakage and pulverization; continuous growth, breakage and reconstruction of the solid electrolyte interface film; and destructive changes in the electrode structure. These factors together cause rapid capacity decay and dramatic shortening of cycle life of the battery, which seriously restricts the commercial application of silicon-based negative electrode materials.

[0004] To alleviate the above-mentioned volume effect, the existing technology mainly adopts the design concept of "reserved space". The core of this concept is to artificially construct buffer pores or cavities inside or around the silicon particles to provide space for the expansion of silicon during charging and discharging and to absorb stress. Based on this concept, the industry has developed a variety of technical solutions, such as constructing three-dimensional reserved pore structures, designing core-shell multi-stage pores, using vapor grown carbon fiber (VGCF) to connect the buffer, and preparing large-size hollow structures. These technical solutions have improved the cycle performance of silicon negative electrodes to a certain extent.

[0005] However, in-depth research has found that the existing "reserved space" scheme is mostly based on "core-shell" or "coating" structure, and its essence still belongs to a kind of "external pressure" stress management method. This method mainly relies on the mechanical constraint of the external carbon shell to physically limit the expansion of the silicon core. This "hard constraint" mode has inherent limitations:

[0006] 1. High stress concentration

[0007] The expansion stress is mainly concentrated on the interface between silicon and carbon shell, rather than being effectively guided inside the material, which easily leads to interface fatigue failure in long-term cycling.

[0008] 2. Coating layer is easy to break

[0009] Repeated stress concentration makes the external carbon coating layer prone to cracking and even breaking, which destroys the integrity of the structure.

[0010] 3. Process requirements are harsh

[0011] In order to realize effective "external pressure type" constraint, high requirements are put forward to the uniformity, mechanical strength and binding force of the cladding layer and the core, which increases the process difficulty and manufacturing cost, and it is difficult to realize large-scale stable production.

[0012] Therefore, it is urgent to develop a new technical route, which can actively guide and release the expansion stress from the inside of the material, avoid interface stress concentration, and at the same time consider the feasibility of the process and the long-term stability of the structure, so as to fundamentally solve the cycle life problem of the silicon-carbon composite negative electrode material. SUMMARY

[0013] The purpose of the present application is to overcome the defects of the prior art, provide a silicon-carbon composite negative electrode material and a preparation method and application thereof, and construct a silicon-carbon composite negative electrode material with a core-shell-shell structure, introduce silicon particles with a sub-nanometer three-dimensional interconnected pore structure into a porous carbon support framework, and set a passivation cladding layer and a dense carbon layer outside, to realize internal guidance and external stable coordination and regulation of silicon volume expansion. The structure can effectively disperse the volume stress in the charging and discharging process, and significantly improve the cycle stability, structural integrity and interface safety of the silicon-carbon negative electrode material.

[0014] To achieve the above purpose, in a first aspect, the present application provides a silicon-carbon composite negative electrode material, which is a core-shell-shell structure, and comprises, from inside to outside:

[0015] The inner core complex comprises a porous carbon matrix and nano-silicon particles dispersed in the pores of the porous carbon matrix; the porous carbon matrix serves as a conductive support framework, and the nano-silicon particles are uniformly dispersed on the conductive support framework; the nano-silicon particles have a three-dimensional interconnected pore structure with a sub-nanometer scale inside;

[0016] The passivation cladding layer is coated on the outer surface of the inner core complex;

[0017] The carbon layer is coated on the outer surface of the passivation cladding layer.

[0018] Preferably, the pore structure inside the nano-silicon particles has a pore size of 0.2nm-2nm; a porosity of 38%-69%; the surface of the pore structure has in-situ doped F elements; the F elements are bonded to the silicon on the surface of the pore structure to form Si-F, which is used as a flexible buffer layer to absorb the volume expansion of the silicon negative electrode during charging and discharging.

[0019] Preferably, the porous carbon matrix comprises one or more of porous carbon, covalent organic framework material COF, carbon fiber, graphene, graphene oxide, graphene-like material with pore structure or expanded graphite;

[0020] The specific surface area of the porous carbon matrix is 1500m2 / g-2500m 2 / g, the width of the pores is 0.3nm-10nm, and the pore volume is 0.6cm 3 / g-1.7cm 3 / g, and the ash content is less than 0.1%.

[0021] Preferably, the thickness of the carbon layer is 3nm-10nm.

[0022] The passivation coating layer is a carbon material layer or a fluoride material layer; when the passivation coating layer is a carbon material layer, the thickness of the passivation coating layer is 1nm-50nm; when the passivation coating layer is a fluoride material layer, the thickness of the passivation coating layer is 2nm-30nm.

[0023] In a second aspect, the present application provides a preparation method of the silicon-carbon composite negative electrode material of the first aspect, and the preparation method comprises:

[0024] The porous carbon matrix is placed in a reactor, heated to a vapor deposition temperature under a protective atmosphere, and then an oxidation mixed gas composed of a silicon source gas and an oxidation gas is introduced, or the oxidation mixed gas and the silicon source gas are alternately introduced, to perform chemical vapor deposition, thereby co-depositing a composite of nanosilicon particles and silicon oxide in the pores of the porous carbon matrix, to obtain a primary silicon-carbon composite;

[0025] The primary silicon-carbon composite is placed in a reactor under a protective atmosphere at a passivation temperature, and a passivation gas is introduced into the reactor to perform surface passivation treatment on the primary silicon-carbon composite, thereby forming a passivation coating layer on the surface of the primary silicon-carbon composite, to obtain a surface-passivated primary silicon-carbon composite;

[0026] The surface-passivated primary silicon-carbon composite is mixed with a fluorine-containing etching solution, and a selective removal of the silicon oxide is performed through an etching reaction, thereby forming a sub-nanoscale three-dimensional interconnected pore structure inside the nanosilicon particles; at the same time, the fluorine element in the etching solution enters the pore structure through adsorption and / or interfacial reaction, bonds with the silicon on the surface of the pore structure, and forms Si-F in situ through doping, to obtain a core-shell structure intermediate;

[0027] The core-shell structure intermediate is subjected to carbon coating treatment, thereby forming a carbon layer on the outer surface of the core-shell structure intermediate and repairing the damage to the passivation coating layer caused by the etching reaction, to obtain a silicon-carbon composite negative electrode material with a core-shell-shell structure.

[0028] Preferably, the fluorine-containing etching solution is hydrofluoric acid and / or ammonium fluoride.

[0029] The silicon source gas comprises one or more of monosilane, disilane, or dichlorosilane.

[0030] The oxidizing gas comprises one or more of carbon dioxide, gaseous nitrogen oxide, compressed air or water vapor;

[0031] When the passivation coating layer is a carbon material layer, the passivation gas used in the passivation treatment is a mixed gas of carbon source gas and protective gas;

[0032] When the passivation coating layer is a fluoride material layer, the passivation gas used in the passivation treatment is a mixed gas of trimethylaluminum and C2F6.

[0033] Preferably, the vapor deposition temperature is 400-650℃, and the deposition time is 200-400min;

[0034] The corrosion reaction temperature is 50-150℃, and the corrosion time is 0.5-3h;

[0035] When the passivation coating layer is a carbon material layer, the passivation temperature is 400-800℃, the passivation time is 0.5-2h, and the thickness of the formed carbon material layer is 1-50nm;

[0036] When the passivation coating layer is a fluoride material layer, the passivation temperature is 100-300℃, the passivation time is 0.5-2h, and the thickness of the formed fluoride material layer is 2-30nm.

[0037] Preferably, the carbon coating treatment comprises:

[0038] The core-shell structure intermediate is placed in a reactor, heated to 500-750℃ under a protective atmosphere, and then a mixed gas composed of carbon source gas and inert carrier gas is introduced to perform chemical vapor deposition treatment for 1-4h; or,

[0039] The core-shell structure intermediate is immersed in a carbon precursor solution for 3-8h, dried, and then subjected to carbonization treatment at 800-1100℃ under a protective atmosphere for 1-4h.

[0040] In a third aspect, the present application provides a negative electrode sheet, which comprises the silicon-carbon composite negative electrode material of the first aspect described above, or the silicon-carbon composite negative electrode material prepared by the preparation method of the second aspect described above.

[0041] In a fourth aspect, the present application provides an energy storage device, which comprises: a lithium ion battery or a lithium ion capacitor;

[0042] The energy storage device comprises the silicon-carbon composite negative electrode material of the first aspect described above, or comprises the silicon-carbon composite negative electrode material prepared by the preparation method of the second aspect described above, or comprises the negative electrode tab of the third aspect described above.

[0043] The silicon-carbon composite negative electrode material provided by the application realizes systematic regulation of the volume expansion behavior of silicon through cross-scale structure synergistic design. On the one hand, the porous carbon matrix serves as a conductive support framework, and its nanoscale pore structure provides uniform dispersion and stable support space for silicon particles, which can effectively buffer the overall volume change and maintain a continuous electronic transmission channel during lithium intercalation; on the other hand, the silicon particles embedded in the pores of the porous carbon matrix themselves have a sub-nanoscale three-dimensional interconnected pore structure, which provides a preferential release space for the volume expansion of silicon during lithium intercalation, so that the expansion is changed from “external extrusion” to “internal dredging”, thereby reducing the interface stress concentration from the source.

[0044] Further, the F element doped in situ on the surface of the internal pore structure of the silicon particles forms a Si-F bonding structure with silicon, and the flexible buffer layer formed thereby can reversibly deform during lithium intercalation / delithiation, thereby continuously absorbing and regulating the volume change of silicon; the flexible buffer effect and the stress dredging effect of the internal pores are mutually synergistic, so that the silicon particles remain structurally stable during charging and discharging cycles. At the same time, the external passivation coating layer and the dense carbon layer play an auxiliary role in interface stabilization and mechanical constraint for the overall structure, avoiding direct erosion of the electrolyte on the active silicon and inhibiting the occurrence of side reactions.

[0045] Based on the synergistic effect of the cross-scale pore structure and the flexible buffer mechanism described above, the silicon-carbon composite negative electrode material of the application can absorb the volume expansion stress of silicon through the internal pore space and the Si-F flexible buffer layer during lithium intercalation, thereby systematically relieving the volume expansion problem of the silicon negative electrode. When applied to the negative electrode of a lithium ion battery, it can achieve extremely low volume expansion (close to zero expansion) while maintaining high specific capacity, significantly improving the energy density, cycle life and overall safety performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 A preparation flowchart of the silicon-carbon composite negative electrode material provided for the embodiments of the application is shown in the figure.

[0047] Figure 2 A structure diagram of the silicon-carbon composite negative electrode material provided for the embodiments of the application is shown in the figure.

[0048] Figure 3 A scanning electron microscope (SEM) image of the silicon-carbon composite negative electrode material prepared in Example 3 of the application is shown in the figure. DETAILED DESCRIPTION

[0049] The technical solutions of the present application are described in further detail below with reference to the accompanying drawings and examples.

[0050] The reagents, materials and the like used in the following examples and comparative examples are all conventional reagent products obtained by purchase, and can also be prepared by conventional methods. If the specific experimental steps or conditions are not specified in the examples, the conventional experimental steps and conditions in the art are used. If the equipment is not specially specified, the conventional equipment in the art is used.

[0051] It should be noted that the pore size of the porous carbon matrix in the present application refers to the average diameter of the internal pores in the porous carbon matrix, and the pore size of the nano-silicon particles refers to the average pore size of the pore structure inside the nano-silicon particles, which is the meaning commonly known in the art. The pore size of the porous carbon matrix and the pore size of the pore structure inside the nano-silicon particles in the examples of the present application can be determined by instruments commonly known in the art and conventional methods. Specifically, the gas adsorption method (Brunauer-Emmet-Teller, BET) can be used, and the specific surface area and pore size analyzer with the model number Micromeritics ASAP2460 of the American Micromeritics (Shanghai) Instrument Co., Ltd. can be used.

[0052] The example of the present application provides a silicon-carbon composite negative electrode material, which has a core-shell-shell structure and comprises, from inside to outside:

[0053] The inner core complex comprises a porous carbon matrix and nano-silicon particles dispersed in the pores of the porous carbon matrix; the porous carbon matrix serves as a conductive support framework, and the nano-silicon particles are uniformly dispersed on the conductive support framework; the nano-silicon particles have a three-dimensional interconnected pore structure of sub-nanometer scale inside, and the surface of the pore structure has in-situ doped F elements; the F elements are bonded to the silicon on the surface of the pore structure to form Si-F, which serves as a flexible buffer layer to absorb the volume expansion of the silicon negative electrode during the charging and discharging process;

[0054] The passivation coating layer is coated on the outer surface of the inner core complex.

[0055] The carbon layer is coated on the outer surface of the passivation coating layer.

[0056] The thickness of the carbon layer is 3-10 nm. The passivation coating layer is a carbon material layer or a fluoride material layer; when the passivation coating layer is a carbon material layer, the thickness of the passivation coating layer is 1-50 nm; when the passivation coating layer is a fluoride material layer, the thickness of the passivation coating layer is 2-30 nm.

[0057] The porous carbon matrix comprises one or more of porous carbon, covalent organic framework material COF, carbon fiber, graphene, graphene oxide, graphene-like material with pore structure or expanded graphite.

[0058] The specific surface area of the porous carbon matrix is 1500 m 2 / g-2500 m 2 The pore size of the pore structure of the porous carbon matrix is 0.3 nm-10 nm, and the pore volume is 0.6 cm 3 / g-1.7 cm 3 / g, and the ash content is less than 0.1%. The pore size of the pore structure inside the nanosilicon particles is 0.2 nm-2 nm, and the porosity of the nanosilicon particles is 38%-69%.

[0059] The silicon-carbon composite negative electrode material provided by the application contains 40wt%-60wt% of carbon, 35wt%-55wt% of silicon, 0.1wt%-3wt% of F element, 0.5wt%-4wt% of oxygen, and 0.1wt%-2wt% of aluminum element and / or unavoidable impurities.

[0060] The silicon-carbon composite negative electrode material provided by the application realizes systematic regulation of the volume expansion behavior of silicon through cross-scale structure synergistic design. On the one hand, the porous carbon matrix serves as a conductive support framework, and the nanoscale pore structure thereof provides uniform dispersion and stable support space for the silicon particles, can effectively buffer the overall volume change during lithium intercalation, and maintains a continuous electron transmission channel; on the other hand, the silicon particles embedded in the pores of the porous carbon matrix have a sub-nanoscale three-dimensional interconnected pore structure, and the internal pore network provides a preferential release space for the volume expansion of silicon during lithium intercalation, so that the expansion is changed from “external extrusion” to “internal dredging”, thereby reducing the interface stress concentration from the source.

[0061] Further, the F element doped in situ on the surface of the internal pore structure of the silicon particles forms a Si-F bonding structure with silicon, and the flexible buffer layer formed thereby can reversibly deform during lithium intercalation / delithiation, thereby continuously absorbing and regulating the volume change of silicon; the flexible buffer effect and the stress dredging effect of the internal pores are mutually synergistic, so that the silicon particles remain structurally stable during charging and discharging cycles. At the same time, the external passivation coating layer and the dense carbon layer play an auxiliary role in interface stabilization and mechanical constraint for the overall structure, avoiding direct erosion of the electrolyte to the active silicon and inhibiting the occurrence of side reactions.

[0062] Based on the synergistic effect of the above cross-scale pore structure and flexible buffer mechanism, the silicon-carbon composite negative electrode material of the application can absorb the volume expansion stress of silicon through the internal pore space and the Si-F flexible buffer layer during lithium intercalation, thereby systematically relieving the volume expansion problem of the silicon negative electrode. When applied to the negative electrode of a lithium ion battery, it can achieve extremely low volume expansion (close to zero expansion) while maintaining high specific capacity, thereby significantly improving the energy density, cycle life and overall safety performance of the battery.

[0063] In addition, the embodiment of the present application also provides a preparation method of the above-mentioned silicon-carbon composite negative electrode material, and the main steps are as follows Figure 1 as shown in the figure, comprising the following steps:

[0064] In step 110, the porous carbon matrix is placed in a reactor, and under a protective atmosphere, the temperature is raised to the gas phase deposition temperature, and then the oxidation mixed gas composed of the silicon source gas and the oxidation gas is introduced, or the oxidation mixed gas and the silicon source gas are alternately introduced, and chemical vapor deposition is carried out to co-deposit the composite of nanosilicon particles and silicon oxide in the pores of the porous carbon matrix, thereby obtaining a primary silicon-carbon composite.

[0065] The introduction of the oxidation gas is to promote the deposit to be a composite phase of nanosilicon particles and silicon oxide, and to create a template for subsequent selective etching. Therefore, the purpose of this step is to deposit nanosilicon particles and silicon oxide (SiO x , 0 < x < 2) in the pores of the porous carbon matrix by chemical vapor deposition technology, to construct a primary silicon-carbon composite with a core-shell structure, and to provide a precursor for subsequent pore etching and fluorine doping.

[0066] Specifically, the porous carbon matrix can be placed in a chemical vapor deposition device (sulfidation bed or rotary furnace) under a protective atmosphere, and the temperature is raised to the gas phase deposition temperature of 400-650°C at a temperature raising rate of 3-10°C / min. Then the reaction gas is introduced for chemical vapor deposition, and the deposition time is 200-400 min.

[0067] The porous carbon matrix includes one or more of porous carbon, covalent organic framework material COF, carbon fiber, graphene, graphene oxide, graphene-like material with pore structure, or expanded graphite. The protective atmosphere includes one or more of argon, nitrogen, or helium; wherein the oxygen content in the nitrogen is less than 5 ppm. The silicon source gas includes one or more of monosilane, disilane, or dichlorosilane, and the flow rate is controlled at 0.5-5 min / L. The oxidation gas includes one or more of carbon dioxide, gas phase nitrogen oxide (such as NO, NO2, etc.), compressed air, or water vapor. The oxidation gas can be diluted with inert gas as needed before use, and the inert gas includes one or more of argon or helium.

[0068] The gas introduction mode has the following two modes:

[0069] The oxidation mixed gas composed of the silicon source gas and the oxidation gas is introduced. The volume fraction of the oxidation gas in the oxidation mixed gas is 0.1%-30%. Alternatively,

[0070] The oxidation mixed gas and the single silicon source gas are alternately introduced into the above. For example, in each cycle, the oxidation mixed gas is introduced for 30-60s, and then the pure silicon source gas is introduced for 20-40s, and the cycle number is 50-200. The above-mentioned gas introduction mode of each cycle is only an example and is not a limitation of the gas introduction mode of the present application. The gas introduction mode of each cycle can be determined according to the actual situation.

[0071] The reaction mechanism of this step is:

[0072] At high temperature, the silicon source gas (such as SiH4) undergoes thermal decomposition reaction: SiH4→ Si + 2H2↑, and the generated silicon atoms deposit in the pores of the porous carbon matrix to form nanosilicon particles. At the same time, the oxidation gas (such as CO2) decomposes to generate active oxygen species at high temperature, which oxidize part of the silicon atoms or silane: Si + O2→ SiO x or SiH4+ O2→ SiO x + 2H2↑, to generate silicon oxide (SiO x , 0 < x < 2). By controlling the proportion and gas introduction mode of the oxidation gas, the proportion and distribution state of silicon and silicon oxide can be adjusted.

[0073] Step 120, under a protective atmosphere, the primary silicon-carbon composite is placed at a passivation temperature, a passivation gas is introduced into the reactor, and the primary silicon-carbon composite is subjected to surface passivation treatment to form a passivation coating layer on the surface of the primary silicon-carbon composite, thereby obtaining a surface-passivated primary silicon-carbon composite.

[0074] To ensure that the prepared primary silicon-carbon composite is not oxidized during mixing with the etching solution, a passivation coating layer needs to be formed on the surface of the primary silicon-carbon composite before mixing.

[0075] The primary silicon-carbon composite has high-reactivity silicon and silicon oxide, and exposure to air will quickly form amorphous SiO2 and release heat, which will pose a serious safety risk. To prevent the prepared primary silicon-carbon composite from being oxidized, a passivation coating layer needs to be prepared on the outer surface of the silicon-carbon composite.

[0076] Specifically, when the passivation coating layer is a carbon material layer, the passivation gas used for passivation treatment is a mixed gas of a carbon source gas and a protective gas; the passivation temperature is 400-800°C, the passivation time is 0.5-2h, and the thickness of the formed carbon material layer is 1-50nm.

[0077] When the passivation coating layer is a fluoride material layer, the passivation gas used in the passivation treatment is a mixed gas of trimethylaluminum and C2F6 in a volume ratio of (0-3]: (0-3]; the passivation temperature is 100-300°C, the passivation time is 0.5-2h, and the thickness of the formed fluoride material layer is 2-30nm.

[0078] In step 130, the surface-passivated primary silicon-carbon composite is mixed with a fluorine-containing etching solution to selectively remove silicon oxide through an etching reaction, thereby forming a sub-nanoscale three-dimensionally interconnected pore structure inside the nanosilicon particles; meanwhile, fluorine elements in the etching solution enter the pore structure through adsorption and / or interfacial reaction and bond with silicon on the surface of the pore structure to form Si-F in situ, thereby obtaining a core-shell intermediate.

[0079] The purpose of this step is to selectively remove the silicon oxide phase and build a three-dimensionally interconnected sub-nanoscale pore structure inside the nanosilicon particles, while achieving in-situ doping of fluorine elements.

[0080] Specifically, the surface-passivated primary silicon-carbon composite is mixed with a fluorine-containing etching solution, and an etching reaction is performed at 50-150°C for 0.5-3h. The fluorine-containing etching solution is an aqueous solution of hydrofluoric acid and / or ammonium fluoride, and the concentration is 0.1-30wt%.

[0081] Taking hydrofluoric acid as an example, the reaction between the nanosilicon particles and HF is very slow and the nanosilicon particles are hardly etched at room temperature. The passivation coating layer also has a very slow reaction rate with HF. However, HF has high selective etching ability for silicon oxide. The passivation coating layer physically blocks the fluorine-containing etching solution from directly and rapidly washing the internal silicon particles, but the passivation coating layer is not completely sealed and may have microscopic defects or itself have a slow reaction with HF. These locations become selective channels for the penetration of HF. Since the reaction rate of silicon oxide with HF is much higher than that of silicon and the passivation coating layer, HF will preferentially diffuse to the location of silicon oxide through these channels and react. This guides the etching reaction to proceed along the distribution path of the silicon oxide, thereby forming an interconnected pore network structure, rather than random and destructive etching (as shown in FIG. 1C). Therefore, the passivation coating layer not only prevents the primary silicon-carbon composite from being oxidized, but also effectively protects the silicon particles from being excessively etched in the subsequent etching process, while providing a selective channel for the etching reaction. Figure 2

[0082] ​During the corrosion reaction, fluorine element in the corrosion solution enters the newly formed pore structure through adsorption and / or interfacial reaction, bonds with silicon atoms on the surface of the pore structure, and is in-situ doped to form Si-F. After the reaction, solid-liquid separation is performed by vacuum filtration, and repeated washing with deionized water is performed until the filtrate is neutral. Finally, the solid product is dried in a vacuum drying oven to obtain a core-shell structure intermediate with three-dimensionally interconnected pore structure and fluorine element doping. For example, drying in a vacuum drying oven at 80°C for 4h, the specific drying temperature and drying time can be determined according to the actual situation.

[0083] The F element does not enter the silicon lattice to form substitutional doping, but preferentially bonds to the silicon surface of the sub-nanometer pore structure, mainly to low-coordination Si atoms (defect sites, boundary sites). The Si-F bond has the characteristics of high bond energy, adjustable bond angle, and high coordination flexibility. At the nanometer / sub-nanometer scale, it does not form a continuous rigid network, but exists as a dispersed and deformable surface bond.

[0084] Therefore, the Si-F bond formed is not a rigid structure support in the traditional sense, but a surface bonding structure with high polarity and low coordination constraints.

[0085] During the lithium intercalation process, silicon will undergo a phase transition from Si→Li x Si, accompanied by significant volume expansion, and the stress sources mainly include: Si-Si bond stretching and breaking, and local stress concentration caused by non-uniform lithium concentration. Si-F can achieve the following functions:

[0086] (1) Relieve surface stress concentration: delay crack initiation.

[0087] Because the Si-F bond preferentially distributes on the inner surface of the pore; when the silicon skeleton expands, the Si-F bond absorbs part of the strain through bond angle change and local rearrangement, reducing the direct transmission of stress to the Si-Si main skeleton. Thus, it is equivalent to introducing a "stress buffer interface" on the outer layer of the silicon skeleton, reducing the probability of crack initiation.

[0088] (2) Cooperate with the sub-nanometer pore structure to achieve "inward deformation" rather than "outward extrusion".

[0089] The sub-nanometer pore structure provides a preferential release space for silicon expansion, and the Si-F bonding layer is located on the inner wall of the pore. During lithium intercalation, it can deform cooperatively with the pore wall, and during lithium extraction, it can partially recover to the original configuration. This deformation is not macroscopic elastic recovery, but configuration reversible adjustment at the atomic scale, exhibiting "pseudo-reversible" deformation behavior.

[0090] (3) Inhibit irreversible structure collapse and pulverization.

[0091] Unmodified porous silicon is prone to pore wall collapse and particle pulverization during repeated expansion / contraction. The Si-F bond stabilizes the chemical environment of surface Si atoms, reduces the irreversible rupture of Si-Si bonds, and improves the integrity of the pore wall structure during cycling.

[0092] That is, unlike the conventional method of relying on an external rigid carbon shell or dense coating layer to limit the volume expansion of silicon, the Si-F bond structure formed in situ in the present application is located on the surface of the sub-nanometer pore structure inside the silicon particles. During the lithium intercalation / deintercalation process, it can buffer the volume change of silicon by changing the bond angle and rearranging the local configuration, and cooperate with the three-dimensional interconnected pore structure inside the silicon particles to continuously absorb and regulate the volume expansion stress.

[0093] Therefore, the Si-F bonding interface is not a traditional rigid support phase, but through its high bond energy to ensure chemical stability, and its adjustable bond angle and coordination flexibility, it absorbs strain by local configuration rearrangement during the lithium intercalation / deintercalation process, thereby achieving dynamic stress buffering at the sub-nanometer scale. This interface cooperates with the three-dimensionally interconnected pore structure to guide the volume change to be released in an "inward deformation" manner rather than causing the particles to "extrude outward" and break, thereby significantly inhibiting material pulverization and improving cycle stability.

[0094] Step 140, carbon coating treatment is performed on the core-shell structure intermediate to form a carbon layer on the outer surface of the core-shell structure intermediate and repair the damage to the passivation coating layer caused by the corrosion reaction, thereby obtaining a silicon-carbon composite negative electrode material with a core-shell-shell structure.

[0095] The purpose of this step is to form a dense carbon layer on the outer surface of the core-shell structure intermediate, repair the damage to the passivation coating layer caused by the corrosion process, improve the electrical conductivity and structural stability of the material, and ultimately obtain a silicon-carbon composite negative electrode material with a core-shell-shell structure.

[0096] The introduction of the carbon layer repairs the damage to the passivation coating layer caused by the corrosion process and forms an intact core-shell-shell structure. The carbon layer improves the electrical conductivity of the material, promoting the rapid transmission of electrons. At the same time, the carbon layer acts as a mechanical buffer layer, effectively inhibiting the volume expansion and pulverization of silicon during charging and discharging, significantly improving the cycle stability and rate performance of the material.

[0097] Specifically, the carbon coating treatment includes:

[0098] The core-shell structure intermediate is placed in a reactor, heated to 500-750°C under a protective atmosphere, and then a mixture of carbon source gas and inert carrier gas is introduced for chemical vapor deposition treatment for 1-4 hours; or,

[0099] The core-shell structure intermediate is immersed in the carbon precursor solution for 3-8 hours, dried, and then carbonized at 800-1100℃ for 1-4 hours under a protective atmosphere.

[0100] The protective atmosphere includes one or more of argon, nitrogen, or helium, and the oxygen content of the nitrogen is less than 5ppm. The carbon source gas includes one or more of methane, acetylene, or propylene. The temperature increase rate is 3-10℃ / min.

[0101] The carbon precursor includes one or more of sucrose, glucose, or phenolic resin. The concentration of the carbon precursor solution is adjusted according to actual needs, such as 5-30wt%.

[0102] The preparation method provided by the application is based on the synergistic design of "in-situ oxidation-selective corrosion-secondary coating". First, an oxidizing atmosphere is introduced in the deposition stage to create a corrodible phase in-situ in the silicon particles, as a subsequent corrodible template. Then, the high selectivity of the corrosion solution containing fluorine is used to accurately construct internal buffer channels (three-dimensionally interconnected pore structures), and simultaneously realize in-situ doping of fluorine elements to form Si-F with flexible buffer performance. Finally, the densification and structural reinforcement of the outer surface are realized through secondary coating. The method is controllable, and the prepared material has a unique structure of "internal tightness and external stability", which can essentially alleviate the volume expansion problem of silicon, thereby significantly improving the electrochemical cycle stability and safety performance of the silicon-carbon composite negative electrode.

[0103] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings. 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 a person of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0104] Example 1

[0105] In this example 1, the volume fraction of the oxidizing gas in the oxidizing mixed gas is 3%.

[0106] 1. 800g of commercially available coconut shell-based porous carbon was weighed and packed into a fluidized bed reactor. Under the protection of nitrogen (oxygen content <5ppm, flow rate 10L / min), the temperature was raised to 500℃ at a rate of 5℃ / min. Then, the chemical vapor deposition was carried out by introducing an oxidizing mixed gas composed of a silicon source gas and an oxidizing gas: the silane flow rate was 2.5L / min; the pure carbon dioxide flow rate was about 0.15L / min, which was diluted with nitrogen to a total flow rate of 2.5L / min before being introduced, and the deposition reaction lasted for 320min. A primary silicon-carbon composite was prepared.

[0107] 2. Still under the protection of nitrogen atmosphere, the temperature was raised to 550℃ at a rate of 4℃ / min, and acetylene gas was introduced for passivation treatment for 80 min. A carbon material layer was formed on the surface of the primary silicon-carbon composite, and a passivated primary silicon-carbon composite was obtained.

[0108] 3. The primary silicon-carbon composite was transferred to a polytetrafluoroethylene reactor, 4000 mL of deionized water was added, and stirring was performed at a speed of 200 rpm for 30 min to fully wet the primary silicon-carbon composite. Subsequently, a 30wt% hydrofluoric acid aqueous solution was slowly added, and a selective etching reaction was performed at a constant temperature of 80℃ for 2 h. After the reaction was completed, solid-liquid separation was performed by vacuum filtration, and the solid product was repeatedly washed with deionized water until the pH of the filtrate was about 7. Finally, the solid product was placed in a vacuum drying oven at 80℃ for 4 h, and a silicon-carbon intermediate with an internal porous structure and fluorine element doping was obtained.

[0109] 4. The silicon-carbon intermediate was placed in a rotary kiln and heated to 600℃ at a rate of 10℃ / min under the protection of nitrogen. Subsequently, acetylene gas was introduced (the flow rate ratio of nitrogen to acetylene was 1:1, both were 3 L / min) for carbon coating for 2 h. After the coating was completed, the sample was naturally cooled to room temperature, and a final silicon-carbon composite negative electrode material (denoted as sample E1) was obtained.

[0110] The sample material prepared was characterized and tested:

[0111] Composition and structure analysis:

[0112] Carbon content, measured by a carbon-sulfur analyzer (CS-2800G);

[0113] Silicon content, calculated by combining X-ray fluorescence spectroscopy (XRF) analysis;

[0114] Oxygen content, detected by a carbon-nitrogen-hydrogen analyzer.

[0115] After testing, the total carbon content of the material was 51.00wt%, the mass content of silicon element was 46.21wt%, and the content of oxygen element was 2.20wt%.

[0116] Example 2

[0117] The difference between this example 2 and example 1 is:

[0118] In the oxidation mixed gas, the volume fraction of the oxidizing gas was 6%.

[0119] The obtained material was denoted as sample E2, and the characterization and testing method of the sample material was the same as that of example 1.

[0120] The total carbon content of the material is 52.58wt%, the silicon content of the material is 43.52wt%, and the oxygen content is 2.65wt%.

[0121] Example 3

[0122] The difference between this example 3 and example 1 is that:

[0123] The volume fraction of the oxidizing gas in the oxidizing mixed gas is 9%.

[0124] The obtained material is recorded as sample E3, and the sample material characterization and test method are the same as example 1.

[0125] The total carbon content of the material is 53.58wt%, the silicon content of the material is 40.47wt%, and the oxygen content is 2.80wt%.

[0126] Figure 3 The scanning electron microscope (SEM) image of sample E3 is shown. As can be clearly seen from the figure, the material presents uniform size and regular morphology of spherical secondary particles, and the particles are well dispersed without obvious agglomeration. The particle surface is smooth, dense and complete, and no cracks, damage or obvious pore exposure is observed. This morphology directly proves the success of the "in-situ oxidation-selective corrosion-secondary coating" preparation process:

[0127] 1. The integrity of the shell layer: the smooth and dense particle surface confirms that the outermost carbon layer has been formed completely and uniformly, which can effectively isolate the direct contact of the electrolyte and the active material, and provide mechanical constraint for the particles.

[0128] 2. The embodiment of process controllability: uniform particle size and morphology reflect that the process has good repeatability and controllability.

[0129] Example 4

[0130] The difference between this example 4 and example 1 is that:

[0131] The volume fraction of the oxidizing gas in the oxidizing mixed gas is 12%.

[0132] The obtained material is recorded as sample E4, and the sample material characterization and test method are the same as example 1.

[0133] The total carbon content of the material is 54.56wt%, the silicon content of the material is 37.94wt%, and the oxygen content is 3.10wt%.

[0134] Example 5

[0135] The difference between this example 5 and example 1 is that:

[0136] The passivation treatment was carried out using a passivation gas mixture of trimethylaluminum and C2F6in a volume ratio of 1 :2 at a temperature of 200°C for 1 h.

[0137] The resulting material was designated as sample E5, and the sample material was characterized and tested according to the methods of Example 1.

[0138] The material was tested and found to have a total carbon content of 51.28 wt%, a silicon content of 44.59 wt%, and an oxygen content of 2.15 wt%.

[0139] Example 6

[0140] This Example 6 differs from Example 3 in that the passivation time was 0.5 h.

[0141] The resulting material was designated as sample E6, and the sample material was characterized and tested according to the methods of Example 1.

[0142] The material was tested and found to have a total carbon content of 53.10 wt%, a silicon content of 41.05 wt%, and an oxygen content of 2.70 wt%.

[0143] Example 7

[0144] This Example 7 differs from Example 3 in that the passivation time was 2 h.

[0145] The resulting material was designated as sample E7, and the sample material was characterized and tested according to the methods of Example 1.

[0146] The material was tested and found to have a total carbon content of 54.25 wt%, a silicon content of 39.80 wt%, and an oxygen content of 2.85 wt%.

[0147] Example 8

[0148] This Example 8 differs from Example 3 in that the etching time was 0.5 h.

[0149] The resulting material was designated as sample E8, and the sample material was characterized and tested according to the methods of Example 1.

[0150] The material was tested and found to have a total carbon content of 52.85 wt%, a silicon content of 41.60 wt%, and an oxygen content of 2.60 wt%.

[0151] Example 9

[0152] This Example 9 differs from Example 3 in that the etching time was 3 h.

[0153] The resulting material was designated as sample E9, and the sample material was characterized and tested according to the methods of Example 1.

[0154] Tests showed that the material contained 54.40 wt% carbon, 39.20 wt% silicon, and 2.10 wt% oxygen.

[0155] Comparative Example 1

[0156] The difference between Comparative Example 1 and Example 3 is that corrosion was omitted.

[0157] The obtained material is designated as sample CE1. The characterization and testing methods for the sample material are the same as in Example 1.

[0158] Tests showed that the material contained 50.14 wt% carbon, 45.86 wt% silicon, and 2.60 wt% oxygen.

[0159] Comparative Example 2

[0160] The difference between Comparative Example 2 and Example 3 is that in the initial chemical vapor deposition stage, no oxidizing gas is introduced, and only silane is introduced for deposition.

[0161] The resulting material is designated as sample CE2. The characterization and testing methods for the sample material are the same as in Example 1.

[0162] Tests showed that the material contained 51.29 wt% carbon, 47.91 wt% silicon, and 1.80 wt% oxygen.

[0163] Various performance tests were conducted on the silicon-carbon anode materials of the above embodiments and comparative examples, and the test results are recorded in Table 1.

[0164] X-ray fluorescence spectroscopy analysis was used to calculate the fluorine and Al content, respectively.

[0165] The specific surface area was measured using a specific surface area analyzer (model: BSD-660S) via nitrogen adsorption-desorption method.

[0166] Electrochemical performance testing:

[0167] The material was made into a negative electrode sheet, and then assembled into a coin cell using metallic lithium as the counter electrode for testing.

[0168] Silicon-carbon composite anode material, conductive additive carbon black, and binder (sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 1:1) were weighed at a mass ratio of 95:2:3 and placed in a pulping machine at room temperature to prepare a slurry. The prepared slurry was evenly coated onto copper foil and dried in a forced-air drying oven at 50°C for 2 hours. Then, it was cut into 8×8mm electrode sheets and placed in a vacuum drying oven at 100°C for 10 hours. The dried electrode sheets were then transferred to a glove box for later use in battery assembly.

[0169] The assembled button cell is specifically: the assembly of the simulation cell is carried out in a glove box containing a high-purity Ar atmosphere, using metal lithium as a counter electrode, using a 1 mol / L LiPF6-containing ethylene carbonate (EC) / dimethyl carbonate (DMC) (volume ratio v:v = 1:1) solution as an electrolyte, and using polyethylene as a separator, to assemble a battery.

[0170] The constant current charge and discharge mode test is carried out using a charge and discharge instrument, the discharge cutoff voltage is 0.005 V, the charge cutoff voltage is 2 V, and the charge and discharge routine test is carried out at a C / 10 current density. The discharge and charge specific capacities are recorded, and the first week coulombic efficiency is calculated and recorded in Table 1. Among them, the first week coulombic efficiency: the ratio of the first week charge specific capacity to the discharge specific capacity is the first week coulombic efficiency.

[0171] The first expansion rate of the negative electrode in the button cell half-battery is tested, and the test method is: at room temperature, the prepared negative electrode sheet is subjected to ion beam cutting, and the cross section is photographed using a scanning electron microscope. The thickness of the negative electrode sheet before assembling the button cell half-battery is measured and recorded as T1; the thickness of the copper foil current collector substrate is measured and recorded as T2; under the condition of 0.1C current density, the charge cutoff voltage is 2V, the button cell half-battery is fully charged, then the battery is disassembled in the glove box, the negative electrode sheet is taken out, after ion beam cutting, the cross section is photographed using a scanning electron microscope and the thickness of the negative electrode sheet at this time is measured and recorded as T3. Then the button cell expansion rate is calculated according to the formula: button cell expansion rate = (T3-T1) / (T1-T2) x 100%, and the unit lithium intercalation expansion rate = button cell expansion rate / discharge specific capacity*100%.

[0172]

[0173]

[0174] Table 1

[0175] According to Table 1, it can be known from the comparison of the test data of Examples 1-4 and Comparative Examples 1-2 that the volume expansion rate of the silicon-carbon negative electrode material prepared by the "in-situ oxidation-selective corrosion" method provided by the application is significantly inhibited. Specifically, as the proportion of oxidizing gas in the deposition atmosphere increases from 3% to 12%, the button cell expansion rate of the obtained material (E1-E4) decreases from 141.05% to 41.02%, and the unit lithium intercalation expansion rate also decreases synchronously. This shows that the higher the proportion of oxidizing gas, the more silicon oxide (SiO x, The more "sacrificial templates" there are in the range of 0 < x < 2), the more three-dimensional interconnected sub-nanometer pores are etched inside the nanosilicon particles in the subsequent etching step, and the higher the porosity (porosity of Example 4 is 61.47% > porosity of Example 1 is 41.4%). The core function of these internal pores is to provide a reserved accommodation space for the huge volume expansion of silicon during lithium intercalation. The higher the porosity, the stronger the stress buffering ability, which is macroscopically directly manifested as a significant reduction in the expansion rate.

[0176] Comparative Example 2 (CE2, without introducing oxidation gas) failed to form a silicon oxide phase that can serve as a sacrificial template due to the lack of oxidation gas introduction in the chemical vapor deposition stage, resulting in the inability to effectively construct a three-dimensional interconnected pore structure inside the nanosilicon particles in the subsequent etching step. Its fluorine element content is extremely low (821 ppm), and the charge-discharge expansion rate is as high as 165.07%, indicating that the lack of "internal hydrophobic" buffering leads to the inability to relieve volume stress and poor cycle stability. Comparative Example 1 (CE1, without undergoing selective etching) formed a SiO x phase but omitted the etching step and failed to convert the precursor into actual pores. Its charge-discharge expansion rate (126.52%) is lower than that of CE2 but still much higher than that of the Example materials, proving that only having the oxidation premise but lacking the etching realization still cannot solve the expansion problem.

[0177] In terms of capacity and efficiency, the initial discharge specific capacity of Examples 1 - 4 remains at a relatively high level of 1618.95 - 1961.79 mAh / g, the initial charge specific capacity is 1311.35 - 1639.24 mAh / g, and the first-week Coulomb efficiency is stable at 81.00% - 82.30%. The decrease in capacity (from 1639.24 mAh / g of E1 to 1311.35 mAh / g of E4) is an inevitable result of the structural change and is a reasonable and controllable sacrifice. Part of the silicon is oxidized to SiO x and is etched and removed subsequently, resulting in a slight reduction in the total amount of pure silicon phase available for lithium storage in the material per unit mass. This shows that while effectively suppressing volume expansion, the present invention still maintains the high-capacity advantage of the silicon material and a stable and excellent first-week Coulomb efficiency.

[0178] In comparison, Comparative Example 2 (CE2) has the highest initial discharge specific capacity (2055.25 mAh / g), but its coulombic efficiency in the first cycle is 83.00%, even slightly higher than some of the inventive examples. However, this does not mean that its performance is better. On the contrary, its ultra-high discharge specific capacity is mainly due to the highest silicon content (47.91 wt%) and the absence of any buffer structure inside, resulting in the highest volume expansion rate (165.07%) among all samples. This combination of "high capacity and high expansion" means that the active material will quickly pulverize and the SEI film will continue to break and grow during cycling, and its actual cycle life will be extremely short. The so-called high initial efficiency will quickly decay in subsequent cycles, and the actual application value is extremely low. The coulombic efficiency in the first cycle of Comparative Example 1 (CE1) is 82.15%, but its volume expansion rate is still as high as 126.52%, and its long-term cycle stability is also far inferior to the inventive example materials.

[0179] The above data comparison strongly proves that simply pursuing high silicon content and high coulombic efficiency in the first cycle while ignoring the fundamental inhibition of volume expansion is futile. The core advantage of the present application is to achieve pore construction (stress dissipation) and fluorine doping (flexible buffer) in the silicon particles simultaneously through the "oxidation-corrosion" synergistic process, forming a unique "inward dissipation" and "flexible buffer" synergistic mechanism. This mechanism enables the material to maintain high capacity (>1300 mAh / g) and excellent initial efficiency (>81%) while achieving extremely low volume expansion rate (as low as 41.02%), thereby laying a solid foundation for high energy density and long cycle life. The results of Comparative Examples 1 and 2, on the other hand, demonstrate the innovativeness and effectiveness of the technical route of the present application in solving the contradiction between "high capacity" and "long life".

[0180] According to Table 1, from the performance change trend of Examples 1-4, the oxidation gas ratio is the core parameter to regulate the capacity and expansion balance of the material. When the oxidation gas ratio in the deposition atmosphere is increased by 12% in the range of 3%, the material can obtain the best expansion inhibition effect while maintaining high discharge specific capacity (>1311 mAh / g) and stable first-week coulomb efficiency (>81%). At the same time, the unit lithium intercalation expansion rate of all example materials is at a very low level (≤0.0719%), which is much lower than the comparative example. Notably, as the amount of fluorine element doping increases from 7131 ppm in Example 1 to 28622 ppm in Example 4, the material's discharge expansion rate and unit lithium intercalation expansion rate decrease significantly. This confirms that the fluorine element is successfully introduced into the pore structure inside the silicon particles during the selective corrosion process, forming a flexible buffer layer. The buffer layer can effectively dissipate the local stress generated by silicon during the lithium intercalation / delithiation process, and the "space dredging" effect of the internal three-dimensional interconnected pore structure produces a synergy, further inhibiting particle rupture and overall expansion at the micro level. This indicates that the "internal sparse" structure combined with element doping modification fundamentally reduces stress accumulation and structural strain during the cycle process, indicating that the material has a more stable electrode interface and better long-term cycle potential.

[0181] Examples 3 (passivation time 1 h), 6 (passivation time 0.5 h), and 7 (passivation time 2 h) remain consistent in oxidation gas ratio and corrosion time, only the passivation time is different. As the passivation time increases from 0.5 h (E6) to 1 h (E3) and then to 2 h (E7), in terms of performance, the discharge expansion rate decreases significantly from 58.50% (E6) to 46.82% (E3), and further to the lowest 40.11% (E7); at the same time, the fluorine element content increases from 19520 ppm (E6) to 23200 ppm (E7). This trend shows that appropriately extending the passivation time is conducive to forming a more complete and dense passivation layer, which can more effectively guide the selective penetration of corrosion agents (such as HF) in the corrosion solution during the subsequent corrosion step, thereby constructing a more efficient three-dimensional interconnected pore structure inside the silicon particles for stress buffering and promoting more fluorine element doping to form a Si-F flexible layer. However, the passivation time is not the longer the better, and a balance needs to be noted: although the first-week coulomb efficiency of Example 7 (2 h) (81.72%) is slightly higher than that of Example 6 (0.5 h, 80.92%), its charge specific capacity (1405.82 mAh / g) is slightly lower than that of Example 3 (1414.21 mAh / g), indicating that a too thick passivation layer may slightly increase the ion migration impedance. Therefore, there is an optimal range of passivation time (about 1 h-2 h) to achieve the best balance between expansion inhibition and capacity / efficiency.

[0182] Examples 3 (corrosion time 1 h), 8 (corrosion time 0.5 h) and 9 (corrosion time 3 h) are consistent in oxidation gas ratio and passivation time, only the corrosion time is different. With the corrosion time increasing from 0.5 h (E8) to 1 h (E3) and then to 3 h (E9), the performance, the charge expansion rate, decreases significantly from 65.78% (E8, insufficient corrosion) to 46.82% (E3) and further to the best 38.52% (E9, sufficient corrosion); the corresponding fluorine element content also increases significantly from 18540 ppm (E8) to 24800 ppm (E9). This proves that sufficient corrosion time is the key to build high-quality "inner sparse" buffer structure. Too short corrosion time (0.5 h), the silicon oxide sacrificial template is not completely removed, resulting in incomplete internal pore structure, insufficient buffer space, and high expansion rate; while sufficient corrosion time (1-3 h), the SiO X sacrificial template is completely removed, forming a developed three-dimensional interconnected pore channel and abundant Si-F bonding sites, thereby achieving excellent expansion inhibition. It is worth noting that with the extension of corrosion time, the capacity shows a downward trend (E9 charge specific capacity 1398.60 mAh / g), which is the inevitable price of the partial consumption of active silicon, but in exchange for the ultimate stability. Therefore, the corrosion time can be accurately regulated within 1 h-3 h according to the specific requirements of capacity and cycle life.

[0183] In addition, unlike embodiments 1-4 which use carbon material passivation coating layer, embodiment 5 uses aluminum fluoride (AlF3) as the passivation coating layer, which has a total fluorine content (16326 ppm) similar to that of embodiment 2 (15959 ppm), but a much higher volume expansion rate (135.28%) than that of embodiment 2 (92.06%). This phenomenon indicates that the passivation coating layer is not an inert barrier, and its chemical properties deeply affect the effectiveness of the subsequent "selective corrosion" step. The interaction of the aluminum fluoride layer with the corrosion agent (such as HF) in the corrosion solution results in a three-dimensional interconnected pore network constructed inside the silicon particles, which is slightly inferior to the pore structure formed under the protection of the carbon layer in terms of stress buffering efficiency. However, embodiment 5 achieves a higher specific charge capacity (1548.42 mAh / g vs 1517.89 mAh / g) while maintaining a similar initial coulombic efficiency (81.88% vs 81.90%) as embodiment 2, which reveals the effectiveness of the aluminum fluoride layer in interface protection. Therefore, the comparison of embodiment 5 with the embodiment series strongly proves that the "oxidation-passivation-corrosion" process of the present application is a highly synergistic system, and the selection of the passivation coating layer is a very critical variable. It not only provides initial protection, but also participates in and guides the corrosion process, ultimately affecting the quality of the "internal sparse" buffering structure. This provides clear experimental basis and flexibility for customizing the process for different performance focus requirements (such as higher capacity or lower expansion), further highlighting the precision and designability of the technical solution of the present application.

[0184] The preparation method provided by the present application is based on the synergistic design of "in-situ oxidation-selective corrosion-secondary coating". First, by introducing an oxidation atmosphere during the deposition stage, a corrodible phase is created in-situ inside the silicon particles, serving as a subsequent corrodible template. Then, using the high selectivity of the fluorine-containing corrosion solution, the internal buffering channels (three-dimensional interconnected pore structure) are precisely constructed, and the in-situ doping of fluorine element is simultaneously realized, forming Si-F with flexible buffering performance. Finally, the densification and structural reinforcement of the outer surface are realized through secondary coating. The method is controllable, and the prepared material has a unique structure of "internal sparse and external stable", which can fundamentally alleviate the volume expansion problem of silicon, thereby significantly improving the electrochemical cycle stability and safety performance of the silicon-carbon composite negative electrode.

[0185] In other words, the preparation method provided by the application solves the inherent volume expansion problem of the silicon negative electrode from the inside of the material through the innovative "inner loose-outer stable-doping toughening" structural design concept. The scheme not only relieves stress through internal channels and maintains integrity through external coating, but also improves the mechanical and electrochemical stability of the active material body by means of element doping. Therefore, the material not only maintains high specific capacity, but also has extremely low expansion rate and excellent first-cycle coulombic efficiency. The process flow involved is based on mature chemical vapor deposition and wet treatment technology, the raw materials are conventional and easy to obtain, and the process has good repeatability and large-scale production prospects, providing a high-performance and high-reliability negative electrode material solution for developing the next generation of lithium-ion batteries with high energy density and long cycle life.

[0186] The above specific embodiments further illustrate the purposes, technical solutions and beneficial effects of the application. It should be understood that the above description is only a specific embodiment of the application and is not intended to limit the protection scope of the application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the application shall be included in the protection scope of the application.

Claims

1. A silicon-carbon composite anode material, characterized in that, The silicon-carbon composite anode material has a core-shell-shell structure, comprising, from the inside out: The core composite comprises a porous carbon matrix and nano-silicon particles dispersed within the pores of the porous carbon matrix; the porous carbon matrix serves as a conductive support framework, and the nano-silicon particles are uniformly dispersed on the conductive support framework; the nano-silicon particles have a sub-nanometer scale three-dimensional interconnected pore structure inside. A passivation coating layer is applied to the outer surface of the core composite. A carbon layer is coated on the outer surface of the passivation coating layer.

2. The silicon-carbon composite anode material according to claim 1, characterized in that, The pore size of the internal pore structure of the nano-silicon particles is 0.2nm-2nm; the porosity is 38%-69%; the surface of the pore structure has in-situ doped F element; the F element is bonded to silicon on the surface of the pore structure to form Si-F, which is used as a flexible buffer layer to absorb the volume expansion of the silicon anode during charging and discharging.

3. The silicon-carbon composite anode material according to claim 1, characterized in that, The porous carbon matrix includes one or more of the following: porous carbon, covalent organic framework material COF, carbon fiber, graphene, graphene oxide, graphene-like material with a porous structure, or expanded graphite. The specific surface area of ​​the porous carbon matrix is ​​1500 m². 2 / g-2500m 2 / g, pore width 0.3nm-10nm, pore volume 0.6cm³ 3 / g-1.7cm 3 / g, ash content less than 0.1%.

4. The silicon-carbon composite anode material according to claim 1, characterized in that, The thickness of the carbon layer is 3nm-10nm; The passivation coating layer is a carbon material layer or a fluoride material layer; when the passivation coating layer is a carbon material layer, the thickness of the passivation coating layer is 1nm-50nm; when the passivation coating layer is a fluoride material layer, the thickness of the passivation coating layer is 2nm-30nm.

5. A method for preparing the silicon-carbon composite anode material according to any one of claims 1-4, characterized in that, The preparation method includes: A porous carbon matrix is ​​placed in a reactor and heated to the vapor deposition temperature under a protective atmosphere. Then, an oxidizing gas mixture consisting of silicon source gas and oxidizing gas is introduced, or the oxidizing gas mixture and silicon source gas are introduced alternately to perform chemical vapor deposition. A composite of nano-silicon particles and silicon oxides is co-deposited in the pores of the porous carbon matrix to obtain a primary silicon-carbon composite. Under a protective atmosphere, the primary silicon-carbon composite is placed at a passivation temperature and a passivation gas is introduced to perform surface passivation treatment on the primary silicon-carbon composite, thereby forming a passivation coating layer on the surface of the primary silicon-carbon composite to obtain a surface-passivated primary silicon-carbon composite. The passivated primary silicon-carbon composite is mixed with a fluorine-containing etching solution to selectively remove the silicon oxide through etching, forming a sub-nanometer-scale three-dimensional interconnected pore structure inside the nano-silicon particles. Simultaneously, fluorine in the etching solution enters the pore structure through adsorption and / or interfacial reaction, bonding with the silicon on the surface of the pore structure to form Si-F in situ, thus obtaining a core-shell structure intermediate. The core-shell structure intermediate is subjected to carbon coating treatment to form a carbon layer on the outer surface of the core-shell structure intermediate, which is used to repair the damage to the passivation coating layer caused by the corrosion reaction, thereby obtaining a silicon-carbon composite anode material with a core-shell structure.

6. The preparation method according to claim 5, characterized in that, The fluorine-containing corrosive liquid is hydrofluoric acid and / or ammonium fluoride; The silicon source gas includes one or more of silane, silane, or dichlorosilane; The oxidizing gas includes one or more of carbon dioxide, gaseous nitrogen oxides, compressed air, or water vapor; When the passivation coating layer is a carbon material layer, the passivation gas used in the passivation treatment is a mixture of carbon source gas and protective gas; When the passivation coating layer is a fluoride material layer, the passivation gas used in the passivation treatment is a mixture of trimethylaluminum and C2F6.

7. The preparation method according to claim 5, characterized in that, The vapor deposition temperature is 400℃-650℃, and the deposition time is 200min-400min; The corrosion reaction temperature is 50℃-150℃, and the corrosion time is 0.5h-3h; When the passivation coating layer is a carbon material layer, the passivation temperature is 400℃-800℃, the passivation time is 0.5h-2h, and the thickness of the formed carbon material layer is 1nm-50nm. When the passivation coating layer is a fluoride material layer, the passivation temperature is 100℃-300℃, the passivation time is 0.5h-2h, and the thickness of the formed fluoride material layer is 2nm-30nm.

8. The preparation method according to claim 5, characterized in that, The carbon coating process includes: The core-shell intermediate is placed in a reactor and heated to 500°C to 750°C under a protective atmosphere. A mixed gas consisting of a carbon source gas and an inert carrier gas is then introduced for chemical vapor deposition (CVD) treatment for 1-4 hours; or... The core-shell structure intermediate was immersed in a carbon precursor solution for 3-8 hours, dried, and then carbonized at 800-1100℃ for 1-4 hours under a protective atmosphere.

9. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the silicon-carbon composite negative electrode material according to any one of claims 1-4, or the silicon-carbon composite negative electrode material prepared by any one of claims 5-8.

10. An energy storage device, characterized in that, The energy storage device includes: for lithium-ion batteries or lithium-ion capacitors; The energy storage device includes the silicon-carbon composite negative electrode material according to any one of claims 1-4, or the silicon-carbon composite negative electrode material prepared by any one of claims 5-8, or the negative electrode sheet according to claim 9.

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