Silicon-based composite materials, their preparation methods and applications

Abstract

This application relates to a silicon-based composite material, its preparation method, and its application. The composite material includes a core and an inner and outer coating layer sequentially covering the surface of the core from the inside out. The core comprises a silicon-based core, the inner coating layer is an MXene nanosheet layer, and the outer coating layer is a carbon network layer. The silicon-based core has a mass fraction of 40%–55% in the silicon-based composite material; the MXene nanosheet layer has a mass fraction of 5%–10% in the silicon-based composite material; and the carbon network layer has a mass fraction of 35%–50% in the silicon-based composite material. The silicon-based core is composed of a silicon framework with a gradient porosity. The MXene nanosheets are modified with amino and alkyl groups. The carbonization precursor of the carbon network includes a repair agent. The silicon-based composite material provided by this application exhibits a low cycle volume expansion rate, maintains the integrity and interfacial stability of the negative electrode, and thus enables the battery to exhibit excellent initial coulombic efficiency and cycle performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to silicon-based composite materials, their preparation methods, and applications. Background Technology

[0002] Lithium-ion batteries are widely used in 3C digital products, power tools, aerospace, energy storage, and electric vehicles due to their advantages such as high specific energy, no memory effect, and long cycle life. The rapid development of electronic information technology and consumer products has placed higher demands on the electrochemical performance of lithium-ion batteries.

[0003] To overcome the energy density bottleneck of lithium-ion batteries, silicon-carbon anode materials have become a core development direction due to their theoretical specific capacity of up to 4200 mAh / g (far exceeding graphite's 372 mAh / g). However, their massive volume expansion of over 300% during charge-discharge cycles not only easily leads to the pulverization of active materials, electrode structure cracking, and continuous interface deterioration, resulting in a sharp increase in interfacial impedance and a decrease in cycle life, but also causes a large amount of irreversible lithium consumption due to the repeated rupture and regeneration of the solid electrolyte interphase (SEI) film, severely limiting the battery's initial coulombic efficiency.

[0004] Therefore, there is an urgent need to develop a new type of anode active material to synergistically optimize the specific capacity, initial coulombic efficiency, and cycle stability of batteries. Summary of the Invention

[0005] To address or partially address the problems existing in related technologies, this application provides a silicon-based composite material, its preparation method, and its application. This silicon-based composite material exhibits a low cycle volume expansion rate, which can maintain the integrity of the negative electrode and the interface stability, thereby enabling the battery to exhibit excellent first coulombic efficiency and cycle performance.

[0006] The first aspect of this application provides a silicon-based composite material, wherein the silicon-based composite material includes a core and an inner coating layer and an outer coating layer that are sequentially coated on the surface of the core from the inside to the outside, the core includes a silicon-based core, the inner coating layer is an MXene nanosheet layer, and the outer coating layer is a carbon network layer; The silicon-based core comprises 40% to 55% by mass in the silicon-based composite material; the MXene nanosheets comprise 5% to 10% by mass in the silicon-based composite material; and the carbon network layer comprises 35% to 50% by mass in the silicon-based composite material. The silicon-based core is composed of a silicon framework with a gradient pore structure; the surface of the MXene nanosheets is modified with amino and oxyalkyl groups; the carbonization precursor of the carbon network includes a repair agent, and the thermally triggered repair temperature of the repair agent is 90℃~100℃.

[0007] As described in the first aspect, the silicon-based composite material has surface pores, middle pores and central pores distributed sequentially from the outside to the inside of the silicon skeleton, and the central pore is filled with elastic carbon spheres. Preferably, the pore size of the surface pores in the silicon-based core is 0.1 μm to 0.3 μm; preferably, the pore size of the middle pores in the silicon-based core is 0.5 μm to 1 μm; preferably, the pore size of the central pore in the silicon-based core is 2 μm to 3 μm; preferably, the porosity of the silicon-based core is 45% to 55%.

[0008] Preferably, the carbonization precursor of the elastic carbon spheres comprises carbon nanospheres coated with an elastic polymer; the elastic polymer comprises at least one of polyimide, polyamide-imide, and polyurethane-polyimide copolymer. Preferably, the elastic carbon spheres constitute 60% to 80% of the mass fraction of the silicon-based core.

[0009] The silicon-based composite material as described in the first aspect, wherein the amino grafting rate of the MXene nanosheets is 20% to 30%; And / or, the grafting rate of oxyalkyl groups in the MXene nanosheets is 15%~25%; And / or, the interlayer spacing of the MXene nanosheets is 1.2~1.5 nm; And / or, the thickness of the MXene nanosheets is 1 nm to 5 nm; And / or, the MXene nanosheets are doped with titanium nitride nanoparticles; preferably, the particle size of the titanium nitride nanoparticles is 5 nm to 20 nm; preferably, the mass fraction of the titanium nitride nanoparticles in the MXene nanosheets is 5% to 8%.

[0010] The silicon-based composite material as described in the first aspect, wherein the repair agent comprises maleimide-modified phenolic resin; preferably, the carbonization precursor of the carbon network further comprises carbon nanotubes and graphene oxide; more preferably, the mass ratio of the carbon nanotubes, the graphene oxide, and the maleimide-modified phenolic resin is (3-5):(1-3):1. And / or, the mass fraction of silicon in the silicon-based composite material is 35% to 55%.

[0011] A second aspect of this application provides a method for preparing a silicon-based composite material, comprising the following steps: S1. According to the corresponding mass fraction, the silicon-based core is added to the first dispersion containing the MXene nanosheets, and the dispersion is carried out. The mixture is then dried to obtain a silicon-based core coated with an MXene nanosheet layer. The MXene nanosheets form the inner coating layer on the surface of the silicon-based core. The silicon-based core is composed of a silicon framework with a gradient pore structure. The surface of the MXene nanosheets is modified with amino and oxyalkyl groups. S2. The silicon-based core coated with the MXene nanosheets is added to a second dispersion of a carbonized precursor including a carbon network, and carbonized after spray granulation to obtain the silicon-based composite material; wherein the carbon network forms an outer coating layer on the surface of the MXene nanosheets.

[0012] The method for preparing silicon-based composite materials as described in the second aspect, wherein the dispersion treatment includes ultrasonic dispersion treatment, with an ultrasonic power of 100W~300W and an ultrasonic time of 20min~40min; And / or, the particle size of the particles obtained by the spray granulation treatment is 5μm~10μm; And / or, the carbonization process includes: pre-carbonization at 300℃~350℃ for 2h~3h, followed by high-temperature carbonization at 800℃~900℃ for 2h~5h; And / or, the first dispersion includes the titanium nitride nanoparticles, such that the titanium nitride nanoparticles are doped into the MXene nanosheets.

[0013] The method for preparing silicon-based composite materials as described in the second aspect, wherein the method for preparing the silicon-based core includes the following steps: sequentially subjecting nano-silicon powder to a first etching treatment and a second etching treatment to obtain a silicon framework; subsequently filling the silicon framework with carbon nanospheres coated with the elastic polymer using an impregnation method, and performing a curing treatment to obtain the silicon-based core; Preferably, the first etching process includes: etching the nano-silicon powder with a sodium hydroxide solution of 0.3M to 0.7M molar concentration for 0.5h to 1.5h; preferably, the second etching process includes: etching the nano-silicon powder after the first etching process with a sodium hydroxide solution of 1.5M to 2.5M molar concentration for 1.5h to 2.5h; preferably, the curing temperature of the curing process is 100℃ to 140℃, and the curing time is 2h to 4h. And / or, the method for preparing the MXene nanosheets includes the following steps: mixing pure MXene dispersion with 3-aminopropyltriethoxysilane and refluxing to obtain the MXene nanosheets with amino and oxyalkyl groups on their surface; Preferably, the reflux reaction temperature is 50℃~70℃, and the reaction time is 3h~5h.

[0014] A third aspect of this application provides a negative electrode sheet, wherein the negative electrode sheet includes a negative current collector and a negative electrode coating loaded on at least one side of the negative current collector, the negative electrode coating comprising a silicon-based composite material as described in the first aspect or a silicon-based composite material prepared by a method for preparing a silicon-based composite material as described in the second aspect.

[0015] A fourth aspect of this application provides a battery, wherein the battery includes a negative electrode as described in the third aspect.

[0016] The battery as described in the fourth aspect, wherein the battery includes an electrolyte; the electrolyte includes fluoroethylene carbonate; Preferably, the mass fraction of the fluoroethylene carbonate in the electrolyte is 3% to 8%.

[0017] The technical solution provided in this application can include the following beneficial effects: by synergistically working the silicon-based core, MXene nanosheet layer, and carbon network layer in the silicon-based composite material, and simultaneously controlling the mass fraction of these three components, integrated functional integration and performance optimization are achieved. On the one hand, the silicon-based core adopts a unique gradient pore structure design, which not only provides sufficient buffer space to accommodate the huge volume expansion generated during lithium ion insertion / extraction, effectively releasing internal stress and preventing the crushing of silicon-based composite material particles, but also shortens the diffusion path of lithium ions. On the other hand, the MXene nanosheets introduce dual functional groups through surface chemical grafting, fundamentally eliminating interface peeling during cycling, and constructing a flexible stress buffer layer that can adapt to volume changes. Furthermore, the external carbon network layer not only constructs an efficient three-dimensional electron transport channel, but also performs in-situ repair of micro-damage to the silicon-based core and inner coating layer during the preparation process. Through precise mass fraction control, the three components form a tightly coupled system of "core buffering expansion stress - inner coating layer with rigid and flexible anchoring - outer coating layer conductive repair", which together suppresses the occurrence of side reactions and maintains the long-term integrity of the electrode structure, thereby significantly improving the battery's initial coulombic efficiency and long-term cycle stability.

[0018] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Detailed Implementation

[0019] To facilitate understanding of this application, it will be described in detail below. However, before describing this application in detail, it should be understood that this application is not limited to the specific embodiments described. It should also be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to be restrictive.

[0020] Where a numerical range is provided, it should be understood that every intermediate value between the upper and lower limits of the range and any other specified or intermediate value within the specified range is covered within this application. The upper and lower limits of these smaller ranges may be independently included in the smaller range and are also covered within this application, subject to any explicitly excluded limits within the specified range. Where the specified range includes one or two limits, the range excluding any or both of those included limits is also included within this application.

[0021] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. While the methods and materials described herein, or any equivalent methods and materials, may also be used in the implementation or testing of this application, preferred methods and materials are now described.

[0022] To overcome the energy density bottleneck of lithium-ion batteries, silicon-carbon anode materials have become a core development direction due to their theoretical specific capacity of up to 4200 mAh / g (far exceeding graphite's 372 mAh / g). However, their massive volume expansion of over 300% during charge-discharge cycles not only easily leads to the pulverization of active materials, electrode structure cracking, and continuous interface deterioration, resulting in a sharp increase in interfacial impedance and a decrease in cycle life, but also causes a large amount of irreversible lithium consumption due to the repeated rupture and regeneration of the solid electrolyte interphase (SEI) film, severely limiting the battery's initial coulombic efficiency.

[0023] Numerous explorations have been conducted on silicon-based anode modification technologies, but significant limitations remain: some technologies employ a multi-layer coating structure of "porous carbon / oxide / silicon / oxide / carbon," which can suppress volume expansion to some extent, but due to the excessive rigidity of the oxide interface layer, it is prone to brittle cracking under cyclic stress, thus accelerating the destruction of the electrode structure and causing a sharp deterioration in cycle performance; other technologies attempt to introduce aluminosilicate nano-additives to buffer stress through physical filling, but this mechanism lacks chemical bonding support, resulting in limited suppression of volume expansion, and the material still exhibits a high expansion rate; there is also a technology using double carbon coating, but this technology lacks effective chemical bonding at the interface, making silicon and carbon prone to separation during charging and discharging, leading to a sharp increase in interface impedance; while the method of introducing a SiO2 protective film can form an initial protective layer, it lacks the ability to dynamically repair cyclic microcracks, leading to the breakage of the conductive network, ultimately severely restricting the long-term cycle stability of the battery.

[0024] To address the aforementioned issues, this application provides a silicon-based composite material comprising a core and an inner and outer coating layer sequentially covering the surface of the core from the inside out. The core comprises a silicon-based core, the inner coating layer is an MXene nanosheet layer, and the outer coating layer is a carbon network layer. The silicon-based core comprises 40% to 55% of the silicon-based composite material by mass; the MXene nanosheet layer comprises 5% to 10% of the silicon-based composite material by mass; and the carbon network layer comprises 35% to 50% of the silicon-based composite material by mass. The silicon-based core is composed of a silicon framework with a gradient pore structure. The surface of the MXene nanosheets is modified with amino and alkyl groups. The carbonization precursor of the carbon network includes a repair agent, and the thermal triggering repair temperature of the repair agent is 90°C to 100°C.

[0025] The silicon-based composite material of this application has a three-layer core-shell structure, consisting of a core, an inner coating layer covering the surface of the core, and an outer coating layer covering the surface of the inner coating layer, from the inside out.

[0026] The core of this application is a silicon-based core, which is mainly composed of silicon (Si). This silicon-based core is not a solid block, but rather a three-dimensional silicon framework with gradient porosity. This allows the pore size within the silicon framework to exhibit a gradient distribution from the inside out. This maintains overall mechanical support strength while effectively buffering internal stresses caused by volume expansion, optimizing mass transport paths, and maximizing specific surface area through the non-uniform porosity distribution. Ultimately, this significantly improves the structural stability and lithium-ion transport efficiency of the silicon-based composite material.

[0027] The inner coating layer of this application is an MXene nanosheet layer, that is, a coating layer formed from MXene nanosheets. The MXene nanosheets in this application are formed by successfully introducing amino functional groups (-NH2) and alkyl oxo groups onto the surface or interlayer of pure MXene nanosheets through chemical grafting. Pure MXene nanosheets are a transition metal carbide / nitride, namely Ti3C2T. x T xThe surface functional groups are represented by -O, -OH, and -F. This modification not only significantly improves the dispersion stability of MXene in polar solvents by utilizing the strong polarity of amino groups, preventing their stacking and aggregation, but more importantly, the amino groups, as active sites, can form dynamic and reversible hydrogen bonds with the outer carbon network, achieving dynamic interfacial response and stress buffering. This results in a strong interfacial bond between the MXene nanosheets and the carbon network, effectively alleviating the interfacial delamination problem during charge and discharge and synergistically improving the overall electrochemical performance. In addition, the alkyl groups on the surface of the MXene nanosheets can hydrolyze to generate silanol groups (-Si-OH), which then undergo a dehydration condensation reaction with the silanol groups on the surface of the silicon-based core to form a stable Si-O-Si covalent bond. This chemically anchors the MXene nanosheets to the surface of the silicon-based core, constructing a rigid basic framework that resists volume expansion and prevents the MXene nanosheets from physically detaching from the silicon-based core during cycling, ultimately ensuring the long-term cycle stability of the battery.

[0028] The outer coating layer of this application is a carbon network layer. The carbon network in this application refers to a conductive carbon skeleton with three-dimensional continuous interconnection characteristics, obtained by carbonization of a carbonization precursor including a repair agent. The thermal trigger repair temperature of this repair agent is 90℃~100℃. This carbon network layer not only acts as a physical barrier to effectively isolate the direct erosion of the internal active material by the electrolyte and inhibit side reactions, but its high conductivity and porous network structure also construct transport channels throughout the entire particle, reducing interfacial contact resistance and ensuring rapid electron transport between the gradient porous silicon skeleton and the MXene nanosheets. Furthermore, its excellent mechanical toughness further constrains the volume expansion of the internal material, preventing electrode pulverization, thereby further improving the cycle stability and rate performance of the silicon-based composite material. In addition, during the preparation of the silicon-based composite material, when the temperature reaches the thermal trigger repair temperature of the repair agent, the chemical bonds in the repair agent undergo addition polymerization, achieving the repair of potential structural damage to the silicon core and MXene nanosheets, thus significantly improving the overall structural stability and cycle life of the silicon-based composite material.

[0029] The mass fraction of the silicon-based core in the silicon-based composite material of this application is 40% to 55%, for example, the mass fraction of the silicon-based core can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, etc.

[0030] The mass fraction of MXene nanosheets in the silicon-based composite material of this application is 5% to 10%, for example, the mass fraction of MXene nanosheets can be 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0031] The mass fraction of the carbon network layer in the silicon-based composite material of this application is 35% to 50%, for example, the mass fraction of the carbon network layer can be 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, etc.

[0032] This application achieves integrated functionality and performance optimization by leveraging the synergistic effect of the silicon-based core, MXene nanosheets, and carbon network layer in a silicon-based composite material, while simultaneously controlling the mass fraction of these three components. On one hand, the silicon-based core employs a unique gradient pore structure design, which not only provides ample buffer space to accommodate the massive volume expansion generated during lithium-ion insertion / extraction, effectively releasing internal stress and preventing the crushing of silicon-based composite material particles, but also shortens the lithium-ion diffusion path. On the other hand, the MXene nanosheets introduce dual-functional groups through surface chemical grafting, fundamentally eliminating interfacial delamination during cycling, while simultaneously constructing a flexible stress buffer layer capable of adapting to volume changes. Furthermore, the external carbon network layer not only constructs an efficient three-dimensional electron transport channel but also performs in-situ repair of micro-damage to the silicon-based core and inner coating layer during the preparation process. Through precise mass fraction control, the three components form a tightly coupled system of "core buffering expansion stress - inner coating layer with rigid and flexible anchoring - outer coating layer conductive repair", which together suppresses the occurrence of side reactions and maintains the long-term integrity of the electrode structure, thereby significantly improving the battery's initial coulombic efficiency and long-term cycle stability.

[0033] Specifically, the mass fractions of silicon-based core, MXene nanosheets, and carbon network layers in silicon-based composite materials can be obtained by thermogravimetric analysis (TG / TGA) of the silicon-based composite materials.

[0034] In one specific embodiment, the silicon framework has surface pores, middle pores, and a central pore distributed sequentially from the outside to the inside, with the central pore filled with elastic carbon spheres. The silicon-based core of this application is a composite structure of silicon framework and elastic spheres; wherein, different continuous channels are formed inside the silicon framework, with small pores on the surface, medium pores in the middle layer, and large pores in the center, and the channels are interconnected without clear shell material boundaries; while the elastic carbon spheres fill the interior of the central large pore, rather than being dispersed in the silicon framework.

[0035] When the silicon-based core has the above structure, it forms a composite structure of "rigid silicon framework + flexible filling," exhibiting a synergistic buffering mechanism of "gradient porosity - elastic core" to match the staged volume expansion and stress evolution during lithium-ion intercalation: In the early stage of lithiation, the reaction starts from the surface layer, and the micropore structure directly accommodates the surface expansion through "pore compression." At the same time, the pore diameter is smaller than the critical nucleation size of lithium dendrites, which can suppress surface cracking caused by lithium dendrite growth. In addition, the high specific surface area of ​​the micropores can increase lithium-ion adsorption sites, improve ion transport uniformity, and avoid stress concentration caused by local over-lithiation. As lithiation penetrates into the middle layer region, the mesopore structure in the middle layer region provides sufficient buffer space, while the pore diameter is precisely matched with the length of the lithium-ion diffusion path, avoiding ion transport obstruction due to excessively large pores or insufficient buffering due to excessively small pores. When entering the core region of the final lithiation stage, the macroporous structure provides a "cavity-level" buffer space, while the inner wall of the macropores provides attachment sites for elastic carbon spheres, forming a composite structure of "rigid silicon skeleton + flexible filling". The elastic deformation of the carbon spheres further dissipates residual stress. Furthermore, the continuous and interconnected nature of the macropores ensures that lithium ions can quickly reach the core region, avoiding capacity loss caused by insufficient lithiation of the core silicon. This achieves efficient electrochemical activity utilization across the entire particle range, thereby improving the cycle stability of the battery.

[0036] In a preferred embodiment, the pore size of the surface pores in the silicon-based core is 0.1 μm to 0.3 μm, for example, the pore size of the surface pores in the silicon-based core can be 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, etc.

[0037] In a preferred embodiment, the pore size of the intermediate layer pores in the silicon-based core is 0.5μm to 1μm, for example, the pore size of the intermediate layer pores can be 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, etc.

[0038] In a preferred embodiment, the diameter of the central hole in the silicon-based core is 2μm to 3μm. For example, the diameter of the central hole can be 2μm, 2.1μm, 2.2μm, 2.3μm, 2.4μm, 2.5μm, 2.6μm, 2.7μm, 2.8μm, 2.9μm, 3μm, etc.

[0039] When the pore sizes of the surface pores, middle pores, and central pores in the silicon-based core are within the aforementioned ranges, the silicon-based core can buffer expansion stress to a greater extent, while improving the lithium-ion transport efficiency, thereby enabling the battery to exhibit better first coulombic efficiency and cycle performance.

[0040] Specifically, the pore sizes of the surface pores, middle pores, and central pores in the silicon-based core can be obtained by performing mercury intrusion porosimetry (MIP) testing on the silicon-based core.

[0041] In a preferred embodiment, the porosity of the silicon-based core is 45% to 55%, for example, the porosity of the silicon-based core can be 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, etc. When the porosity of the silicon-based core is within the above range, the silicon-based core ensures sufficient free volume to efficiently accommodate the lithium intercalation expansion stress while avoiding structural collapse problems caused by excessive porosity, thereby improving the stability of the silicon-based composite material and ultimately ensuring the high cycle performance of the battery.

[0042] Specifically, the porosity of the silicon-based core can be obtained by performing a BET test (Brunauer-Emmett-Teller Method) on the silicon-based core.

[0043] In one specific embodiment, the carbonization precursor of the elastic carbon spheres includes carbon nanospheres coated with an elastic polymer. The elastic carbon spheres of this application are obtained by carbonizing carbon nanospheres coated with an elastic polymer. After carbonization, the elastic polymer fuses with the carbon spheres to form an elastic carbon core. This not only retains the original buffering performance but also enhances the bonding strength between the elastic carbon spheres and the inner wall of the silicon core, thereby improving the buffering performance of the silicon-based core while also enhancing its stability.

[0044] In one specific embodiment, the elastic polymer includes at least one of polyimide (PI), polyamide-imide (PAI), and polyurethane-polyimide copolymer (PU-PI). The elastic polymer possesses both a high glass transition temperature and excellent thermal decomposition stability, and exhibits a high carbonization residue rate, ensuring that it does not undergo phase loss during high-temperature heat treatment, but instead transforms in situ into a dense elastic carbon layer. Furthermore, during the carbonization process, the aromatic ring structure in the elastic polymer undergoes a deep cross-linking reaction, forming nitrogen-containing amorphous carbon. This nitrogen-containing amorphous carbon layer fuses with carbon spheres to form an elastic carbon core, thereby successfully preparing elastic carbon spheres.

[0045] In one specific embodiment, the mass fraction of the elastic carbon spheres in the silicon-based core is 60% to 80%, for example, the mass fraction of the elastic carbon spheres in the silicon-based core can be 60%, 65%, 70%, 75%, 80%, etc. When the mass fraction of the elastic carbon spheres is within the above range, the elastic carbon spheres can dissipate residual stress to a greater extent and can avoid excessively occupying the space of the central macropore, providing sufficient buffer space for volume expansion, thereby ensuring the cycle stability of the silicon-based core to a greater extent and improving the electrochemical performance of the battery.

[0046] Specifically, the mass fraction of elastic carbon spheres in the silicon-based core can be obtained by performing thermogravimetric analysis (TG / TGA) on the silicon-based core.

[0047] In one specific embodiment, the amino grafting rate in the MXene nanosheets is 20% to 30%, for example, the amino grafting rate in the MXene nanosheets can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, etc. When the amino grafting rate in the MXene nanosheets is within the above range, a stronger interfacial bond is formed between the MXene nanosheets and the carbon network, which can better achieve dynamic interfacial response and stress buffering, further alleviate the interfacial delamination problem during charge and discharge, and ultimately improve the electrochemical performance of the battery to a greater extent.

[0048] Specifically, the method for testing the amino grafting rate in MXene nanosheets includes the following steps: (1) Sample pretreatment: Weigh 0.1g of MXene nanosheets (if modified with 3-aminopropyltriethoxysilane (APTES)) (denoted as m1, accurate to 0.0001g), add 50mL of anhydrous ethanol, and ultrasonically disperse for 30min (to ensure that the surface amino groups are fully exposed and to avoid interlayer aggregation and encapsulation); centrifuge (8000r / min, 10min), collect the precipitate, wash it 3 times with anhydrous ethanol, and vacuum dry it at 60℃ for 2h for later use. (2) Titration process: Place the pretreated sample into an Erlenmeyer flask, add 30 mL of anhydrous ethanol and 10 mL of deionized water, and sonicate for 10 min to form a uniform suspension; add 3 drops of bromophenol blue indicator, at which point the solution is blue (amino alkaline); titrate with 0.01 mol / L hydrochloric acid standard solution, stirring while titrating, until the solution changes from blue to yellow and does not fade within 30 s (titation endpoint), and record the volume of hydrochloric acid consumed, V1 (unit: L). (3) Blank experiment: Take 0.1 g of pure MXene nanosheets, repeat the above steps, and record the volume of hydrochloric acid consumed, V0. The amino grafting rate in MXene nanosheets was calculated using the following formulas: Amino molar mass (n_NH2) = (V1-V0) × c_HCl, where c_HCl is the concentration of the hydrochloric acid standard solution (mol / L); APTES grafting mass (m_APTES) = n_NH2 × M_APTES, where M_APTES is the molecular weight of APTES; Amino grafting rate (G) = (m_APTES / m1) × 100%.

[0049] In one specific embodiment, the grafting rate of alkyl groups in the MXene nanosheets is 15% to 25%, for example, the grafting rate of alkyl groups in the MXene nanosheets can be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, etc. When the grafting rate of alkyl groups in the MXene nanosheets is within the above range, the Si-O-Si covalent bonds formed by the hydrolysis of alkyl groups and the silanol groups on the surface of the silicon-based core can better chemically anchor the MXene nanolayers to the surface of the silicon-based core, further buffering volume expansion, and preventing the MXene nanolayers from physically detaching from the silicon-based core during cycling, ultimately resulting in higher cycle stability of the battery.

[0050] The method for testing the oxyalkyl grafting rate in the MXene nanosheets of this application is similar to the method for testing the amino grafting rate described above.

[0051] In one specific embodiment, the interlayer spacing of MXene nanosheets is 1.2 nm to 1.5 nm, for example, the interlayer spacing of MXene nanosheets can be 1.2 nm, 1.25 nm, 1.3 nm, 1.35 nm, 1.4 nm, 1.45 nm, 1.5 nm, etc. When the interlayer spacing of MXene nanosheets is within the above range, the diffusion barrier of lithium ions is significantly reduced, the ion transport kinetics are accelerated, and amino and alkyl groups can be more easily grafted onto the MXene nanosheets. This allows the MXene nanosheets to fully utilize their performance in buffering expansion stress and dynamic anchoring, ultimately improving the initial coulombic efficiency and cycle performance of the battery.

[0052] Specifically, the interlayer spacing of MXene nanosheets can be obtained by X-ray diffraction (XRD) testing. The testing and analysis steps are as follows: 1) The dried MXene nanosheet powder is evenly spread on the XRD sample stage and flattened until the surface is smooth; 2) Using a Cu-Kα radiation source, continuous scanning is performed within the range of 2θ = 3° to 60° to obtain the XRD pattern; 3) The characteristic diffraction peaks corresponding to the MXene (002) crystal plane are located in the pattern, and their 2θ diffraction angles are read; 4) According to the Bragg equation: 2dsinθ = nλ, where d is the interplanar spacing (i.e., the MXene interlayer spacing), θ is half of the diffraction angle, λ is the X-ray wavelength (Cu-Kα radiation, λ = 1.5406 Å), and n = 1 is the diffraction order; the measured 2θ value is substituted into the formula to calculate the interlayer spacing d of the MXene nanosheets.

[0053] In one specific embodiment, the thickness of the MXene nanosheets is 1 nm to 5 nm, for example, the thickness of the MXene nanosheets can be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, etc. When the thickness of the MXene nanosheets is within the above range, the aggregation or stacking of the MXene nanosheets can be avoided, thereby shortening the diffusion path of lithium ions. At the same time, the MXene nanosheets are endowed with excellent flexibility to adapt to the volume deformation of silicon-based composite materials, thereby improving the electrochemical performance of the battery.

[0054] Specifically, the thickness of the MXene nanosheets can be obtained by X-ray diffraction (XRD) testing of the MXene nanosheets.

[0055] In one specific embodiment, titanium nitride nanoparticles are doped into MXene nanosheets. Doping MXene nanosheets with titanium nitride nanoparticles serves two main purposes. First, TiN acts as a high-modulus, high-strength, nano-rigid reinforcing phase, uniformly embedded within the flexible MXene layers. This significantly improves the overall stiffness and interlayer bonding of the MXene nanosheets, effectively suppressing the propagation of interfacial microcracks and the delamination of MXene nanosheet layers caused by the volume expansion of silicon-based composite materials during intense charge-discharge cycles. Second, thanks to the excellent metallic conductivity of titanium nitride, it can construct stable three-dimensional high-speed electron transport channels between the MXene nanosheet layers. This compensates for the potential decrease in conductivity at the dynamic interface during hydrogen bond breakage and layer slippage, stabilizing the interfacial impedance and ensuring that the electrode maintains excellent electron transport efficiency and structural integrity throughout long-term operation.

[0056] In one specific embodiment, the titanium nitride nanoparticles have a particle size of 5nm to 20nm, for example, the particle size of titanium nitride nanoparticles can be 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, 16nm, 17nm, 18nm, 19nm, 20nm, etc. Within this size range, titanium nitride nanoparticles can avoid interlayer slippage in MXene nanosheets, hindering microcrack propagation, and because their size is much smaller than the lateral scale of MXene sheets, they avoid stress concentration points that could lead to matrix brittle fracture. At the same time, the ultra-small particle size endows titanium nitride nanoparticles with extremely high specific surface area, allowing them to be more uniformly dispersed between MXene layers, constructing a dense and continuous three-dimensional conductive permeation network, reducing battery impedance, and thus effectively improving battery cycle performance.

[0057] Specifically, the particle size of titanium nitride nanoparticles can be obtained by transmission electron microscopy (TEM) testing of titanium nitride nanoparticles.

[0058] In one specific embodiment, the mass fraction of titanium nitride nanoparticles in the MXene nanosheets is 5% to 8%, for example, the mass fraction of titanium nitride nanoparticles can be 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, etc. Within this range, the titanium nitride nanoparticles are sufficient to construct a continuous and efficient three-dimensional electron permeation network between the MXene nanosheet layers, and provide sufficient rigid support to suppress volume expansion. At the same time, it avoids the sharp drop in specific capacity caused by excessive crowding of electrochemical active sites or blockage of ion transport channels due to excessive addition, thereby enabling the battery to exhibit better initial coulombic efficiency and cycle performance.

[0059] Specifically, the mass fraction of titanium nitride nanoparticles in MXene nanosheets can be obtained by inductively coupled plasma optical emission spectroscopy (ICP-OES). The specific steps are as follows: 1) Take a certain mass of MXene powder sample and use a mixed acid system for microwave digestion to completely dissolve the metal elements in the sample and convert them into ionic states; 2) Adjust the volume of the digestion solution to a fixed volume to prepare the test solution; 3) Use an inductively coupled plasma optical emission spectrometer to test the test solution and determine the intensity of the characteristic spectral lines of Ti element in the solution; 4) Calculate the total concentration of Ti element in the test solution according to the standard curve, and calculate the total mass fraction of Ti element in the sample by combining the sample mass and the fixed volume; Based on the total mass fraction of Ti element and the Ti content contained in MXene itself, calculate the Ti content in titanium nitride nanoparticles by difference, and then convert it to the mass fraction of titanium nitride nanoparticles in MXene nanosheets.

[0060] In one specific embodiment, the repair agent includes maleimide-modified phenolic resin, and the carbonization precursor of the carbon network also includes carbon nanotubes and graphene oxide. One-dimensional carbon nanotubes, acting as a conductive framework, are interspersed between two-dimensional graphene oxide sheets, effectively preventing the stacking and aggregation of graphene oxide during the reduction process, thus constructing a long-range, continuous, three-dimensional high-speed electron transport channel. Secondly, the maleimide groups in the maleimide-modified phenolic resin can form strong covalent bonds with the oxygen-containing functional groups on the surface of graphene oxide, significantly enhancing the interfacial bonding force between graphene oxide and the maleimide-modified phenolic resin. Finally, during the high-temperature carbonization process, the maleimide-modified phenolic resin transforms into an amorphous carbon matrix, firmly welding the carbon nanotubes and graphene together to form a dense carbon network with high conductivity, high mechanical strength, and abundant pore structure, thereby providing stable physical support and a rapid ion / electron transport path for the silicon-based composite material.

[0061] In one specific embodiment, the mass ratio of carbon nanotubes, graphene oxide, and maleimide-modified phenolic resin is (3-5):(1-3):1. For example, the mass ratio of carbon nanotubes, graphene oxide, and maleimide-modified phenolic resin can be 3:1:1, 4:1:1, 5:1:1, 3:2:1, 4:2:1, 5:2:1, 3:3:1, 4:3:1, 5:3:1, etc. When the mass ratio of carbon nanotubes, graphene oxide, and maleimide-modified phenolic resin is within the above range, the carbon network formed by the carbonization of carbon nanotubes, graphene oxide, and maleimide-modified phenolic resin has higher density, conductivity, and mechanical strength, further improving the cycle performance of the battery.

[0062] In one specific embodiment, the mass fraction of silicon in the silicon-based composite material is 35% to 55%, for example, the mass fraction of silicon in the silicon-based composite material can be 35%, 40%, 45%, 50%, 55%, etc. When the mass fraction of silicon is within the above range, it can provide high energy density for the battery, while the remaining MXene nanosheets or carbon network are sufficient to construct a continuous and tough conductive network and mechanical support framework, effectively accommodating the high volume expansion of silicon, preventing particle pulverization, shedding, and repeated rupture and regeneration of the solid electrolyte interface film, thereby enabling the battery to achieve both high energy density, high initial coulombic efficiency, and long cycle stability.

[0063] This application provides a method for preparing a silicon-based composite material, comprising the following steps: S1. According to the corresponding mass fraction, a silicon-based core is added to a first dispersion containing MXene nanosheets, and the dispersion is carried out. After drying, a silicon-based core coated with an MXene nanosheet layer is obtained. The MXene nanosheet layer forms an inner coating layer on the surface of the silicon-based core. The silicon-based core is composed of a silicon framework with a gradient pore structure. The surface of the MXene nanosheets is modified with amino and oxyalkyl groups. S2. Add a silicon-based core coated with MXene nanosheets to a second dispersion of a carbonized precursor including a carbon network, and perform carbonization treatment after spray granulation to obtain a silicon-based composite material; wherein, the carbon network forms an outer coating layer on the surface of the MXene nanosheets.

[0064] Specifically, in step S1, MXene nanosheets with amino and oxyalkyl groups modified on their surface are dispersed in ethanol to form a first dispersion. Subsequently, a silicon-based core with a gradient pore structure is added to the first dispersion. After dispersion, the mixture is filtered and dried, allowing the MXene nanosheets to form an inner coating layer on the surface of the silicon-based core, resulting in a silicon-based core coated with an MXene nanosheet layer. The amounts of silicon-based core and MXene nanosheets added are adjusted so that the mass ratio of the silicon-based core to the MXene nanosheet layer in the prepared silicon-based core coated with the MXene nanosheet layer is (40~55):(5~10).

[0065] This application does not limit the specific parameters of distributed processing; they can be selected according to actual needs.

[0066] This application does not limit the specific parameters of the filtering process; they can be selected according to actual needs.

[0067] This application does not limit the specific parameters of the drying process, which can be selected according to actual needs.

[0068] In step S2, the carbonized precursor of the carbon network is dispersed in a solvent to form a second dispersion. Then, the silicon-based core coated with MXene nanosheets obtained in step S1 is added to the second dispersion. After mixing evenly, the mixture is sprayed and granulated. Subsequently, the sprayed particles are carbonized to transform the carbonized precursor of the carbon network into a carbon network, thus obtaining a silicon-based composite material. The amount of silicon-based core coated with MXene nanosheets and the amount of carbonized precursor added to the carbon network are adjusted so that the mass ratio of silicon-based core, MXene nanosheets, and carbon network layer in the silicon-based composite material is (40~55):(5~10):(35~50).

[0069] This application does not limit the specific choice of solvent, which can be selected according to actual needs.

[0070] The silicon-based composite material prepared by the preparation method of this application includes a silicon-based core, MXene nanosheets, and a carbon network layer. The components of the silicon-based composite material are proportioned as follows: 40%~55% silicon-based core, 5%~10% MXene nanosheets, and 35%~50% carbon network layer.

[0071] The method for preparing the silicon-based composite material provided in this application involves sequentially coating the surface of a silicon-based core with MXene nanosheets and a carbonized precursor of a carbon network, followed by carbonization treatment of the carbonized precursor of the carbon network, to obtain a silicon-based composite material with excellent stability and lithium conductivity. This silicon-based composite material, with its low cycle volume expansion rate, effectively ensures the integrity of the negative electrode structure and interface stability, thereby endowing the battery with excellent first-cycle coulombic efficiency and long-cycle performance. Furthermore, its preparation process is simple, requires low-barrier equipment, and uses readily available raw materials, showing potential for large-scale mass production and commercial application.

[0072] In one specific embodiment, the dispersion treatment includes ultrasonic dispersion, with an ultrasonic power of 100W~300W and an ultrasonic time of 20min~40min. For example, the ultrasonic power can be 100W, 150W, 200W, 250W, 300W, etc., and the ultrasonic time can be 20min, 25min, 30min, 35min, 40min, etc. Through the above ultrasonic dispersion treatment, MXene nanosheets can be successfully coated on the surface of the silicon-based core to form an MXene nanosheet layer, thereby laying the foundation for the successful preparation of subsequent silicon-based composite materials.

[0073] In one specific embodiment, the particle size of the particles obtained by spray granulation is 5μm to 10μm, for example, the particle size can be 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, etc. When the particle size of the particles obtained by spray granulation is within the above range, the carbonization treatment can fully transform the carbonization precursor on its surface into a carbon network, ensuring the successful preparation of silicon-based composite materials. At the same time, it effectively avoids the huge specific surface area and the accompanying surge of side reactions caused by excessively small particle size, and also prevents the lithium-ion transport obstruction caused by excessively large particle size, thereby enabling the battery to achieve both high coulombic efficiency and high cycle performance.

[0074] In one specific embodiment, the carbonization process includes: pre-carbonizing at 300℃~350℃ for 2h~3h under an inert atmosphere, followed by high-temperature carbonization at 800℃~900℃ for 2h~5h. For example, the pre-carbonization temperature can be 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, etc., and the pre-carbonization time can be 2h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h, 3h, etc. The high-temperature carbonization temperature can be 800℃, 820℃, 840℃, 860℃, 880℃, 900℃, etc., and the high-temperature carbonization time can be 2h, 3h, 4h, 5h, etc. The low-temperature pre-carbonization stage removes volatiles from the carbonization precursor of the carbon network and initiates preliminary cross-linking, allowing the carbon source to form a viscous, semi-charred framework on the surface of the MXene nanosheets. The subsequent high-temperature carbonization stage provides sufficient thermal energy to drive the transformation of amorphous carbon into graphite-like microcrystals, significantly improving the conductivity and mechanical strength of the carbon network. Simultaneously, it avoids the porous and porous structure or structural collapse of the carbon layer caused by the instantaneous explosive release of volatiles during a single high-temperature treatment. This gradient heating method ensures the successful preparation of silicon-carbon composite materials with excellent conductivity and structural stability.

[0075] In one specific embodiment, the first dispersion includes titanium nitride nanoparticles, such that the titanium nitride nanoparticles are doped into MXene nanosheets. Specifically, in step S1, MXene nanosheets with amino and oxyalkyl groups on their surfaces and titanium nitride nanoparticles are dispersed in ethanol according to the corresponding mass fractions to form a first dispersion; subsequently, a silicon-based core is added to the first dispersion, and after dispersion treatment, it is filtered and dried, so that the MXene nanosheets form an inner coating layer on the surface of the silicon-based core, while the titanium nitride nanoparticles are doped into the MXene nanosheets, resulting in a silicon-based core coated with an MXene nanosheet layer.

[0076] In one specific embodiment, the method for preparing a silicon-based core includes the following steps: performing a first etching treatment and a second etching treatment on nano-silicon powder in sequence to obtain a silicon framework; then filling the silicon framework with carbon nanospheres coated with an elastic polymer by an impregnation method and performing a curing treatment to obtain a silicon-based core.

[0077] Specifically, the nano-silicon powder is subjected to a first etching process and a second etching process in sequence, so that the nano-silicon powder has surface micropores, middle layer mesopores and central macropores distributed from the outside to the inside, which is the silicon skeleton; then, carbon nanospheres coated with elastic polymer are filled into the central macropore of the silicon skeleton by supercritical CO2 assisted impregnation method, and solidification process is performed to obtain silicon-based core.

[0078] During the subsequent carbonization process, the elastic polymer in the carbon nanospheres coated with elastic polymer is transformed into amorphous carbon and undergoes interfacial fusion with the internal carbon nanospheres to form an elastic carbon core.

[0079] In one specific embodiment, the first etching process includes: etching the nano-silicon powder with a sodium hydroxide solution of 0.3M to 0.7M molar concentration for 0.5h to 1.5h. For example, the molar concentration of the sodium hydroxide solution can be 0.3M, 0.4M, 0.5M, 0.6M, 0.7M, etc., and the etching time can be 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, etc.

[0080] In one specific embodiment, the second etching process includes: etching the nano-silicon powder that has undergone the first etching process with a sodium hydroxide solution with a molar concentration of 1.5M to 2.5M for an etching time of 2h to 4h. For example, the molar concentration of the sodium hydroxide solution can be 1.5M, 1.6M, 1.7M, 1.8M, 1.9M, 2M, 2.1M, 2.2M, 2.3M, 2.4M, 2.5M, etc., and the etching time can be 2h, 2.5h, 3h, 3.5h, 4h, etc.

[0081] A hierarchical porous structure with "surface micropores, middle layer mesopores, and central macropores" can be constructed through a two-step NaOH gradient etching process: the first step uses a low-concentration NaOH solution for gentle etching, mainly acting on the surface of the nano-silicon powder to form dense micropores; the second step uses a high-concentration NaOH solution for deep etching, utilizing its stronger corrosiveness to penetrate deep into the interior of the nano-silicon powder, forming interconnected mesopore channels in the middle layer, and etching out macro-pore structures in the central region.

[0082] In one specific embodiment, the curing temperature is 100℃~140℃, and the curing time is 2h~4h. For example, the curing temperature can be 100℃, 110℃, 120℃, 130℃, 140℃, etc., and the curing time can be 2h, 2.5h, 3h, 3.5h, 4h, etc. The curing process can promote irreversible cross-linking of the elastic polymer chain segments, which not only forms a tight interlocking network between the carbon spheres, but also generates strong chemical bonding or mechanical interlocking force with the silicon-based pore walls, thereby eliminating the risk of displacement, detachment, or even loss of the carbon spheres due to vibration or volume expansion stress. In addition, the stable structure of the elastic carbon spheres embedded in the central macropore of the silicon-based core ensures that the elastic buffer unit in the central macropore is always effective in situ. It can continuously absorb the huge volume expansion stress of the silicon core, prevent the pore structure from collapsing, and maintain the integrity of the internal conductive network, significantly improving the overall structural stability and long cycle life of the silicon-based composite material.

[0083] In one specific embodiment, the preparation method of MXene nanosheets includes the following steps: mixing pure MXene dispersion with 3-aminopropyltriethoxysilane and refluxing to obtain MXene nanosheets with amino and alkyl groups modified on the surface. Specifically, pure MXene nanosheets are dispersed in an ethanol-water mixed solvent, ultrasonically dispersed, and the pH of the system is adjusted to 4-5 to obtain a pure MXene dispersion; then, 3-aminopropyltriethoxysilane (APTES) is added to the pure MXene dispersion, mixed evenly, and refluxing is performed. After the reaction, the nanosheets are centrifuged, washed, and dried to obtain MXene nanosheets with amino and alkyl groups modified on the surface. Through the above preparation method, amino and alkyl functional groups can be successfully grafted onto the surface of MXene nanosheets, laying the foundation for the subsequent preparation of silicon-based composite materials.

[0084] In one specific embodiment, the reflux reaction temperature is 50℃~70℃, and the reaction time is 3h~5h. For example, the reaction temperature can be 50℃, 55℃, 60℃, 65℃, 70℃, etc., and the reaction time can be 3h, 3.5h, 4h, 4.5h, 5h, etc. When the reflux reaction temperature and reaction time are within the above range, sufficient activation energy can be provided to accelerate the reaction rate between amino and alkyl functional groups and MXene nanosheets, ensuring that the reaction proceeds efficiently under mild conditions, while avoiding side reactions. This achieves uniform, dense, and stable chemical anchoring of amino and alkyl functional groups on the surface of MXene nanosheets.

[0085] A third aspect of this application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode coating loaded on at least one side of the negative electrode current collector. The negative electrode coating comprises the aforementioned silicon-based composite material or a binder prepared by the aforementioned method of preparing the silicon-based composite material. This negative electrode sheet has a stable structure and can improve the initial coulombic efficiency and cycle performance of the battery.

[0086] In one specific embodiment, the negative electrode coating further includes a conductive agent and a binder. The conductive agent and binder in the negative electrode coating of this application can be selected from conventional materials in the art.

[0087] A fourth aspect of this application provides a battery comprising the aforementioned negative electrode. This battery exhibits excellent initial coulombic efficiency and cycle performance.

[0088] In one specific embodiment, the battery of this application further includes an electrolyte comprising fluoroethylene carbonate (FEC), wherein the mass fraction of FEC in the electrolyte is 3% to 8%, for example, the mass fraction of FEC in the electrolyte can be 3%, 4%, 5%, 6%, 7%, 8%, etc. The FEC in the electrolyte can form a dense and mechanically strong SEI film on the surface of the negative electrode, effectively suppressing the continuous consumption of electrolyte by side reactions, reducing the irreversible consumption of active lithium, and alleviating the volume expansion of the silicon-based negative electrode material. More importantly, it can form a "structure-interface" synergy with the silicon-based negative electrode material: the structure of the silicon-based negative electrode material can accommodate volume deformation and maintain the conductive path, while the SEI film can provide chemical protection and mechanical support. The two complement each other, further improving the stability of the negative electrode sheet, thereby further improving the battery's initial coulombic efficiency and cycle performance.

[0089] In one specific embodiment, the electrolyte further includes a lithium salt and an organic solvent. The lithium salt and organic solvent are lithium salts and organic solvents known in the art that can be used in electrolytes to improve the electrochemical performance of the battery. The specific composition can be determined as needed.

[0090] In one specific embodiment, the battery of this application further includes a positive electrode sheet, which comprises a positive current collector and a positive electrode coating coated on the positive current collector. The positive electrode coating comprises a positive electrode active material, which includes at least one of lithium cobalt oxide, ternary material NCA, ternary material NCM, lithium iron phosphate, and lithium manganese iron phosphate. When the above-mentioned compounds are selected as the positive electrode active material, the positive electrode active material can fully exert its performance and improve the electrochemical performance of the lithium-ion battery.

[0091] In this application embodiment, there is no particular limitation on the type of positive electrode current collector; it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper. Preferably, the positive electrode current collector is a metallic material.

[0092] In one specific embodiment, the positive electrode coating further includes a conductive agent and a binder. The conductive agent includes at least one of carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, graphene, and vapor-grown carbon fiber (VGCF). The binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, polyvinylidene fluoride, and polytetrafluoroethylene.

[0093] In one specific embodiment, the lithium-ion battery further includes a separator. The embodiments of this application do not have any particular restrictions on the material and shape of the separator, as long as it does not significantly impair the effect of this application. It may include porous sheet-like or non-woven fabric-like materials with excellent liquid retention. The materials of the resin or glass fiber separator include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc., and can be set according to needs.

[0094] In one embodiment, the battery may include an outer packaging that can be used to encapsulate the electrode assembly and electrolyte.

[0095] In one specific embodiment, the outer packaging of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0096] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0097] This application does not impose any particular restrictions on the application fields of lithium-ion batteries, and they can be used in fields such as consumer batteries, power batteries for new energy vehicles, and energy storage batteries.

[0098] The present application will be further described in detail below through specific embodiments.

[0099] Example 1 1. Preparation of silicon-based composite materials 1) Fabrication of silicon-based cores The nano-silicon powder was etched with a 0.5 M sodium hydroxide solution for 1 h; then, the nano-silicon powder after the first etching treatment was etched with a 2 M sodium hydroxide solution for 2 h to obtain a silicon framework; then, polyimide-coated carbon nanospheres were filled into the central macropores of the silicon framework by supercritical CO2-assisted impregnation and cured at 120 °C for 3 h to obtain a silicon-based core.

[0100] 2) Preparation of MXene nanosheets with amino and oxyalkyl groups on their surface Pure MXene nanosheets were dispersed in an ethanol-water mixed solvent, ultrasonically dispersed, and the pH of the system was adjusted to 4-5 to obtain a pure MXene dispersion. Subsequently, 3-aminopropyltriethoxysilane (APTES) was added to the pure MXene dispersion, mixed evenly, and refluxed at 60°C for 4 h. After the reaction was completed, the nanosheets were centrifuged, washed, and dried to obtain MXene nanosheets with amino and alkyl groups on their surface.

[0101] 3) MXene nanosheets and titanium nitride nanoparticles with amino and alkyl groups modified on their surfaces were dispersed in ethanol to form a first dispersion. A silicon-based core was then added to the first dispersion, followed by ultrasonic dispersion, filtration, and drying to obtain a silicon-based core coated with MXene nanosheets. Carbon nanotubes, graphene oxide, and maleimide-modified phenolic resin were dispersed in a solvent to form a second dispersion. The silicon-based core coated with MXene nanosheets was then added to the second dispersion, mixed thoroughly, and subjected to spray granulation. The sprayed particles were then carbonized to obtain a silicon-based composite material. The ultrasonic power was 200W, and the ultrasonic time was 30 min. The particle size of the particles obtained after spray granulation was 8 μm. The carbonization process included pre-carbonization at 300℃ for 2 h under an inert atmosphere, followed by high-temperature carbonization at 800℃ for 4 h.

[0102] In the silicon-based composite material, the mass fraction of the silicon-based core is 48%; the mass fraction of the MXene nanosheet layer is 7%; and the mass fraction of the carbon network layer is 45%.

[0103] The surface pores in the silicon-based core have a diameter of 0.2 μm; the mesopores in the middle layer have a diameter of 0.8 μm; and the central macropores have a diameter of 2.5 μm; the porosity of the silicon-based core is 50%. The elastic carbon spheres constitute 70% of the silicon-based core by mass.

[0104] The amino grafting rate of MXene nanosheets is 25%; the alkyl grafting rate of MXene nanosheets is 20%; the interlayer spacing of MXene nanosheets is 1.3 nm; and the thickness of MXene nanosheets is 3 nm.

[0105] The titanium nitride nanoparticles have a particle size of 12 nm; the mass fraction of titanium nitride nanoparticles in MXene nanosheets is 6.5%.

[0106] The mass ratio of carbon nanotubes, graphene oxide, and maleimide-modified phenolic resin is 4:2:1.

[0107] The mass fraction of silicon in the silicon-based composite material is 42%.

[0108] 2. Preparation of negative electrode sheet Silicon-based composite material, conductive agent (Super P), dispersant (CMC), and binder (SBR) were mixed in a mass ratio of 92:5:1:2 and a negative electrode slurry was obtained under vacuum stirring. The negative electrode slurry was uniformly coated on the surface of a modified current collector, dried at 90°C, and rolled to obtain a negative electrode sheet with a negative electrode coating thickness of 85 μm. The above steps were then repeated on the other surface of the negative electrode sheet to obtain a negative electrode sheet with a negative electrode coating on both sides. The negative electrode sheet was cut into 100 mm × 70 mm dimensions and electrode tabs were welded on for later use.

[0109] 3. Preparation of positive electrode sheet The positive electrode active material (lithium cobalt oxide), conductive carbon black (SP), and polyvinylidene fluoride (PVDF) binder were mixed uniformly at a mass ratio of 8:1:1 and thoroughly stirred in N-methylpyrrolidone solvent to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated onto one surface of a 12 μm thick aluminum foil, dried at 90°C, and cold-pressed to obtain a positive electrode sheet with a coating thickness of 110 μm. The above steps were then repeated on the other surface of the same positive electrode sheet to obtain a positive electrode sheet with a double-sided coating. The positive electrode sheet was cut to a size of 76 mm × 851 mm and tabs were welded on for later use.

[0110] 4. Preparation of electrolyte Under conditions with a water content of less than 10 ppm, diethyl carbonate (DEC) and ethylene carbonate (EC) were mixed at a mass ratio of 2:1. Based on the total mass of the electrolyte, lithium hexafluorophosphate (LiPF6) was added to the solvent, dissolved, and mixed evenly. Then, electrolyte additives were added to obtain the electrolyte. The mass fraction of LiPF6 in the electrolyte was 12.5%, and the electrolyte additives included fluoroethylene carbonate (FEC), with a mass fraction of 5% in the electrolyte.

[0111] 5. Manufacturing of lithium-ion batteries The positive and negative electrode sheets are stacked in sequence, and the stacked electrode sheets are wound with the separator to obtain the electrode assembly. The electrode assembly is placed in the pre-formed aluminum-plastic film and dehydrated at 80°C. The prepared electrolyte is then injected, and the battery is obtained through vacuum sealing, settling, formation, and shaping processes.

[0112] The main difference between Examples 2-43, Comparative Examples 1-5 and Example 1 is the different parameters of the silicon-based composite material, as shown in Tables 1-6.

[0113] Table 1

[0114] Table 2

[0115] Table 3

[0116] Table 4

[0117] Table 5

[0118] Table 6

[0119] Test case The following performance tests were performed on the batteries prepared in the examples and comparative examples: 1. First Coulomb efficiency test After the lithium-ion battery formation and capacity grading are completed, the formation charging capacity C0, the first charge capacity of the capacity grading cycle C1, and the first discharge capacity of the capacity grading cycle C2 are recorded. The coulombic efficiency of the first cycle = C2 / (C0+C1)×100%.

[0120] The average cycle life of each group of 5 batteries is recorded in Table 7.

[0121] 2. Cyclic performance test In a constant temperature chamber at (25±2)℃, the lithium-ion battery was charged to 4.3V at a constant current and constant voltage of 0.2C, then charged to 0.05C at a constant voltage. After resting for 5 minutes, it was discharged to 3.0V at 0.5C. The capacity obtained in this step was taken as the initial capacity. Cyclic tests were performed using 1C charge / 1C discharge, and the capacity retention rate of the battery after 300 cycles was calculated.

[0122] Cycle capacity retention (%) = Discharge capacity at 300th cycle (mAh) / Discharge capacity at first cycle (mAh) × 100%.

[0123] Record the initial thickness data before battery cycling, and record the thickness data of the battery after 300 cycles when fully charged. Thickness expansion rate (%) = battery thickness (mm) after 300 cycles when fully charged / initial battery thickness (mm).

[0124] The average cycle life of each group of 5 batteries is recorded in Table 7.

[0125] Table 7

[0126] As shown in Table 7, based on the comparison between Examples 1-43 and Comparative Examples 1-5, when the silicon-based composite material is composed of a silicon-based core, MXene nanosheets, and a carbon network layer, the silicon-based composite material forms a tightly coupled system of "core buffering expansion stress - inner coating layer with rigid and flexible anchoring - outer coating layer conductive repair", thereby improving the battery's initial coulombic efficiency and long-cycle stability.

[0127] According to the comparison of Examples 1-7 and Comparative Examples 1-5, when the mass percentages of silicon-based core, MXene nanosheets, and carbon network layers in the silicon-based composite material are 40%-55%, 5%-10%, and 35%-50%, respectively, the initial coulombic efficiency and cycle performance of the battery are better.

[0128] According to the comparison of Examples 1, 8-10, when the ultrasonic power of ultrasonic dispersion treatment is 100W-300W and the ultrasonic time is 20min-40min, MXene nanosheets can be successfully coated on the surface of the silicon-based core to form an MXene nanosheet layer, which further improves the first coulombic efficiency and cycle performance of the battery.

[0129] According to the comparison of Examples 1 and 11-13, when the particle size of the particles obtained by spray granulation is 5μm to 10μm, the carbonized precursor can be fully transformed into a carbon network, ensuring the successful preparation of silicon-based composite materials, thereby making the battery's first coulombic efficiency and cycle performance better.

[0130] According to the comparison of Examples 1, 14-16, the carbonization process, which involves pre-carbonization at 300℃~350℃ for 2h~3h followed by high-temperature carbonization at 800℃~900℃ for 2h~5h, ensures the successful preparation of silicon-carbon composite materials with excellent conductivity and stable structure, thereby enabling the battery to exhibit higher first coulombic efficiency and cycle performance.

[0131] Based on the comparison of Examples 1 and 17-19, it can be seen that when the molar concentration of sodium hydroxide solution in the first etching treatment is 0.3M to 0.7M and the etching time is 0.5h to 1.5h, and the molar concentration of sodium hydroxide solution in the second etching treatment is 1.5M to 2.5M and the etching time is 1.5h to 2.5h, the silicon-based core can be constructed with a hierarchical porous structure of "surface micropores, middle layer mesopores, and central macropores", which ultimately improves the first coulombic efficiency and cycle performance of the battery.

[0132] According to the comparison of Examples 1 and 20-22, when the curing temperature of the curing treatment is 100℃~140℃ and the curing time is 2h~4h, the elastic carbon spheres can be embedded in the central large hole of the silicon-based core, resulting in higher initial coulombic efficiency and cycle performance of the battery.

[0133] According to the comparison of Examples 1 and 17-22, the surface pores in the silicon-based core have a diameter of 0.1μm to 0.3μm, the middle pores have a diameter of 0.5μm to 1μm, the central pores have a diameter of 2μm to 3μm, the porosity is 45% to 55%, and the mass fraction of elastic carbon spheres is 60% to 80%, resulting in higher initial coulombic efficiency and cycle performance of the battery.

[0134] Based on the comparison of Examples 1 and 23-25, it can be seen that when the reflux reaction temperature is 50℃~70℃ and the reaction time is 3h~5h, the amino grafting rate in MXene nanosheets is 20%~30% and the alkyl grafting rate is 15%~25%. MXene nanosheets grafted with amino and alkyl functional groups can be successfully prepared, thereby improving the first coulombic efficiency and cycle performance of the battery.

[0135] As can be seen from the comparison of Examples 1 and 26-28, when the interlayer spacing of MXene nanosheets is 1.2nm-1.5nm, it can accelerate ion transport kinetics, thereby improving the first coulombic efficiency and cycle performance of the battery.

[0136] As can be seen from the comparison of Examples 1 and 29-31, when the thickness of MXene nanosheets is 1nm-5nm, the diffusion path of lithium ions can be shortened, further improving the first coulombic efficiency and cycle performance of the battery.

[0137] According to the comparison of Examples 1 and 32-34, when the particle size of titanium nitride nanoparticles is 5nm-20nm, titanium nitride can improve the conductivity of MXene nanosheets, which is beneficial to improving the first coulombic efficiency and cycle performance of the battery.

[0138] Based on the comparison of Examples 1, 35-37, it can be seen that when the mass fraction of titanium nitride nanoparticles is 5%-8%, the electrochemical performance of the battery is better.

[0139] According to the comparison of Examples 1, 38-40, the mass ratio of graphene oxide to maleimide-modified phenolic resin is (3-5):(1-3):1, which results in higher density, conductivity, and mechanical strength of the carbon network, thereby improving the electrochemical performance of the battery.

[0140] According to the comparison of Examples 1 and 41-43, when the mass fraction of fluoroethylene carbonate in the electrolyte is 3% to 8%, it can form a dense SEI film with excellent mechanical strength, thereby improving the first coulombic efficiency and cycle performance of the battery to a greater extent.

[0141] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A silicon-based composite material, characterized by, The silicon-based composite material includes a core and an inner coating layer and an outer coating layer that are sequentially coated on the surface of the core from the inside out. The core includes a silicon-based core, the inner coating layer is an MXene nanosheet layer, and the outer coating layer is a carbon network layer. The silicon-based core comprises 40% to 55% by mass in the silicon-based composite material; the MXene nanosheets comprise 5% to 10% by mass in the silicon-based composite material; and the carbon network layer comprises 35% to 50% by mass in the silicon-based composite material. The silicon-based core is composed of a silicon framework with a gradient pore structure; the surface of the MXene nanosheets is modified with amino and oxyalkyl groups; the carbonization precursor of the carbon network includes a repair agent, and the thermally triggered repair temperature of the repair agent is 90℃~100℃.

2. The silicon-based composite material of claim 1, wherein The silicon framework has surface holes, middle holes and central holes distributed from the outside to the inside, and the central hole is filled with elastic carbon spheres. Preferably, the pore size of the surface pores in the silicon-based core is 0.1 μm to 0.3 μm; preferably, the pore size of the middle pores in the silicon-based core is 0.5 μm to 1 μm; preferably, the pore size of the central pore in the silicon-based core is 2 μm to 3 μm; preferably, the porosity of the silicon-based core is 45% to 55%. Preferably, the carbonization precursor of the elastic carbon spheres comprises carbon nanospheres coated with an elastic polymer; more preferably, the elastic polymer comprises at least one of polyimide, polyamide-imide, and polyurethane-polyimide copolymer. Preferably, the elastic carbon spheres constitute 60% to 80% of the mass fraction of the silicon-based core.

3. The silicon-based composite material of claim 1, wherein The amino grafting rate in the MXene nanosheets is 20%~30%; And / or, the grafting rate of oxyalkyl groups in the MXene nanosheets is 15%~25%; And / or, the interlayer spacing of the MXene nanosheets is 1.2 nm to 1.5 nm; And / or, the thickness of the MXene nanosheets is 1 nm to 5 nm; And / or, the MXene nanosheets are doped with titanium nitride nanoparticles; preferably, the particle size of the titanium nitride nanoparticles is 5 nm to 20 nm; preferably, the mass fraction of the titanium nitride nanoparticles in the MXene nanosheets is 5% to 8%.

4. The silicon-based composite material of claim 1, wherein The repair agent includes maleimide-modified phenolic resin; preferably, the carbonized precursor of the carbon network further includes carbon nanotubes and graphene oxide; more preferably, the mass ratio of the carbon nanotubes, the graphene oxide, and the maleimide-modified phenolic resin is (3-5):(1-3):

1. And / or, the mass fraction of silicon in the silicon-based composite material is 35% to 55%.

5. A method of producing the silicon-based composite material as claimed in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. According to the corresponding mass fraction, the silicon-based core is added to the first dispersion containing the MXene nanosheets, and the dispersion is carried out. The mixture is then dried to obtain a silicon-based core coated with an MXene nanosheet layer. The MXene nanosheets form the inner coating layer on the surface of the silicon-based core. The silicon-based core is composed of a silicon framework with a gradient pore structure. The surface of the MXene nanosheets is modified with amino and oxyalkyl groups. S2. The silicon-based core coated with the MXene nanosheets is added to a second dispersion of a carbonized precursor including a carbon network, and carbonized after spray granulation to obtain the silicon-based composite material; wherein the carbon network forms an outer coating layer on the surface of the MXene nanosheets.

6. The method of claim 5, wherein the silicon-based composite material is prepared by a process comprising: The dispersion treatment includes ultrasonic dispersion treatment, with an ultrasonic power of 100W~300W and an ultrasonic time of 20min~40min; And / or, the particle size of the particles obtained by the spray granulation treatment is 5μm~10μm; And / or, the carbonization process includes: pre-carbonizing at 300℃~350℃ for 2h~3h under an inert atmosphere, followed by high-temperature carbonization at 800℃~900℃ for 2h~5h; And / or, the first dispersion includes the titanium nitride nanoparticles, such that the titanium nitride nanoparticles are doped into the MXene nanosheets.

7. The method of claim 5, wherein the silicon-based composite material is prepared by a process comprising: The method for preparing the silicon-based core includes the following steps: after performing a first etching treatment and a second etching treatment on the nano-silicon powder in sequence, a silicon framework is obtained; then, the elastic polymer-coated carbon nanospheres are filled into the silicon framework by an impregnation method, and a curing treatment is performed to obtain the silicon-based core; Preferably, the first etching process includes: etching the nano-silicon powder with a sodium hydroxide solution of 0.3M to 0.7M molar concentration for 0.5h to 1.5h; preferably, the second etching process includes: etching the nano-silicon powder after the first etching process with a sodium hydroxide solution of 1.5M to 2.5M molar concentration for 1.5h to 2.5h; preferably, the curing temperature of the curing process is 100℃ to 140℃, and the curing time is 2h to 4h. And / or, the method for preparing the MXene nanosheets includes the following steps: mixing pure MXene dispersion with 3-aminopropyltriethoxysilane and refluxing to obtain the MXene nanosheets with amino and oxyalkyl groups on their surface; Preferably, the reflux reaction temperature is 50℃~70℃, and the reaction time is 3h~5h.

8. A negative electrode sheet characterized by comprising: The negative electrode sheet includes a negative electrode current collector and a negative electrode coating loaded on at least one side of the negative electrode current collector. The negative electrode coating includes a silicon-based composite material as described in any one of claims 1 to 4 or a silicon-based composite material prepared by the method of preparing the silicon-based composite material as described in any one of claims 5 to 7.

9. A battery, characterized by The battery includes the negative electrode sheet as described in claim 8.

10. The battery of claim 9, wherein, The battery includes an electrolyte; the electrolyte includes fluoroethylene carbonate; Preferably, the mass fraction of the fluoroethylene carbonate in the electrolyte is 3% to 8%.

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