Composite negative electrode material, preparation method thereof and lithium ion battery

Abstract

The invention discloses a composite negative electrode material, a preparation method thereof and a lithium ion battery, and belongs to the technical field of batteries, the composite negative electrode material comprises a core body and a shell wrapping the core body, the core body comprises Fe-B co-doped Si with Si-Fe alloy attached to the surface, and the shell comprises an amorphous carbon layer with SiC growing on the surface. According to the composite negative electrode material, Fe-B is co-doped with Si to form a core body, and B doping can increase the hole concentration so as to improve the carrier mobility, so that the electron and ion transfer rate of silicon is improved, and the intrinsic conductivity and multiplying power of silicon are improved; meanwhile, the Si-Fe alloy phase, the amorphous carbon layer and SiC growing on the surface of the amorphous carbon layer are matched with one another, so that volume expansion of silicon during charging and discharging can be effectively inhibited, and a transmission channel for lithium ions is provided, so that the cycle performance and the rate capability of a lithium ion battery prepared from the silicon-based composite negative electrode material are improved.

Description

A composite anode material and its preparation method, and a lithium-ion battery Technical Field

[0001] This application belongs to the field of lithium-ion battery technology, and in particular, this application relates to a composite negative electrode material and its preparation method, and a lithium-ion battery. Background Technology

[0002] The key to improving the energy density of lithium-ion batteries lies in the development of high-capacity electrode materials. For the anode, the theoretical capacity limit of traditional commercial graphite anodes is 372 mAh / g, making further energy density improvements difficult. Silicon (Si), with its ultra-high theoretical specific capacity (4200 mAh / g), low redox potential (<0.4V vs. Li / Li+), and abundant reserves in the Earth's crust, has become a recognized ideal next-generation anode material. However, the significant volume expansion and contraction effect of Si, along with its relatively low intrinsic conductivity, limits the performance of high-capacity silicon anode materials, severely hindering their widespread industrial application.

[0003] To address these two issues, researchers have proposed structural design strategies such as nanostructuring and porous structures to mitigate the volume expansion of silicon anodes and release some stress; silicon-carbon composite structures to buffer volume expansion and enhance the system's conductivity; and surface modification strategies to improve reaction kinetics and accelerate ion transport. These strategies have all improved the electrochemical performance of silicon anodes to some extent. However, several problems remain to be solved. The design process for nanostructuring and porous silicon structures is complex and often requires multiple steps, making it difficult to meet the needs of large-scale applications; silicon-carbon composite designs still suffer from slow kinetics; and the complex and diverse structures of surface modification layers make it difficult to achieve uniform, scalable, and multifunctional surface modification. Furthermore, the impact of surface modification layers on overall structural stability, interface structure, and ion / electron transport requires further investigation. The silicide structure formed during silicon alloying can serve as a dispersed buffer phase matrix, alleviating expansion and contraction stress, helping to stabilize the overall structure and improve cycle performance. In addition, doping can improve the band structure of silicon from the bulk phase, reduce the band gap of silicon, improve intrinsic conductivity, thereby accelerating the rapid transfer of charge and improving rate performance. Finally, continuous or discontinuous carbon layers can be constructed on the surface of silicon particles through carbon coating technology to achieve buffering of volume expansion, improvement of conductivity, and regulation of interface stability. For example, patent document CN117638029A describes a three-layer composite structure silicon-based alloy anode material consisting of a carbon matrix core, a silicon alloy layer, and a carbon layer. This material does not affect the battery's initial efficiency while significantly reducing the expansion rate of the anode material after lithium intercalation. However, it involves a fluidized bed spray device, making the required equipment relatively complex. Another example is patent document CN117810434A, which reports a method for preparing a porous silicon alloy anode material. First, titanium tailings, industrial silicon, and reducing metals are mixed and smelted in an oxygen-free atmosphere to obtain alloy powder. Then, the alloy powder is ball-milled and acid-etched to obtain the anode material. Although the electrochemical performance is improved, the preparation process uses solvents such as hydrochloric acid, which may have adverse environmental effects, and the preparation process is relatively cumbersome. Additionally, the document "Morphology Control of Ball-Milled Si-Ti Alloy Anode Materials for Li-Ion Batteries" reports the preparation of a series of Si-Ti alloy anode materials via ball milling, finding that Si85Ti15 exhibits superior cycle stability. However, titanium is expensive, which hinders its commercial development.

[0004] The above processes can all produce silicon-based composite anode materials, but they suffer from drawbacks such as high equipment requirements, unsustainable development, and expensive raw materials. Furthermore, large-scale production faces significant safety risks and cost pressures, and the performance of the resulting anode materials remains limited, hindering widespread adoption and thus restricting their practical application. Therefore, there is a need to find an innovative method for producing high-performance silicon alloy composite anode materials that is simple, low-cost, and suitable for large-scale production. Summary of the Invention

[0005] This application provides an example of a composite anode material and its preparation method, as well as a lithium-ion battery, which can improve the cycle stability and first coulombic efficiency of silicon-based anodes.

[0006] The solution presented in this application is implemented through the following steps.

[0007] In a first aspect, this application discloses a composite anode material comprising a core and a shell covering the core. The core comprises Fe-B co-doped Si with Si-Fe alloy attached to its surface, and the shell comprises an amorphous carbon layer with SiC grown on its surface.

[0008] Optionally, the mass ratio of Si, Fe and B in the nucleus is 90%~94%:5%~8%:1%~2%, and / or;

[0009] The mass ratio of SiC to amorphous carbon in the shell is 10%~40%:60%~90%.

[0010] Optionally, the diameter of the core is 200 nm to 800 nm, the thickness of the shell is 10 nm to 35 nm, and / or the particle size of the composite negative electrode material is 0.2 μm to 2 μm.

[0011] In a second aspect, this application also discloses a method for preparing the composite anode material as described above, which includes: first, ball milling and mixing silicon powder, ferroborone alloy powder and carbon source powder, drying to obtain a precursor material, and then heat-treating the precursor material under inert gas protection.

[0012] Optionally, the heat treatment temperature is 600℃~1500℃, and the heat treatment holding time is 1h~7h.

[0013] Optionally, the mass ratio of silicon powder, ferroborone alloy powder and carbon source powder is 48%~53%:6%~9%:41%~43%.

[0014] Optionally, the ferroborone alloy includes one or more of Fe-10B, Fe-20B, Fe-30B and Fe-40B, and / or; the carbon source includes one or more of phenolic resin, pitch, glucose, sucrose and citric acid.

[0015] Optionally, the DV50 particle size of silicon powder is 5μm to 30μm, and / or; the DV50 particle size of boron-iron alloy powder is 10μm to 40μm, and / or; and the DV50 particle size of carbon source powder is 1μm to 15μm.

[0016] Optionally, when ball milling and mixing silicon powder, ferroborone alloy powder, and carbon source powder, the silicon powder and ferroborone alloy powder are first ball milled to obtain a first mixture, and then the first mixture and carbon source powder are ball milled a second time to obtain a second mixture; optionally, the first ball milling and / or the second ball milling is wet ball milling; optionally, the process control agent for wet ball milling includes one or more of deionized water, ethanol, n-hexanol, ethylene glycol, and propylene glycol; optionally, the mass ratio of the mixed powder to the process control agent in wet ball milling is 1:3~12; optionally, the ball milling time for the first ball milling and / or the second ball milling is 0.5h~12h, and the ball milling speed is 250rpm~1000rpm; optionally, the second mixture is dried at 50℃~100℃ for 5h~15h to obtain a precursor material.

[0017] In a third aspect, this application also discloses a lithium-ion battery comprising the aforementioned composite negative electrode material or a composite negative electrode active material prepared according to the aforementioned method for preparing the composite negative electrode material.

[0018] This application has at least the following beneficial effects: The composite anode material of this application forms a core by Fe-B co-doping Si. B doping can increase the hole concentration, thereby improving the carrier mobility, which in turn improves the electron and ion transfer rate of silicon, thereby improving the intrinsic conductivity and rate capability of silicon. At the same time, the iron element of the boron-iron alloy and the Si-Fe alloy generated by silicon alloying can adhere to the surface of Fe-B co-doped Si. The Si-Fe alloy phase, the amorphous carbon layer and the SiC grown on the surface of the amorphous carbon layer work together to effectively suppress the volume expansion of silicon during charging and discharging and provide lithium ion transport channels, thereby improving the cycle performance and rate capability of lithium-ion batteries made with silicon-based composite anode materials.

[0019] The method for preparing the composite anode material of this application adopts a composite strategy of silicon bulk phase and surface modification, and the preparation process is gradual. It has the advantages of low cost, simple and feasible synthesis process and green environmental protection. Attached Figure Description

[0020] Figure 1 shows a flowchart of the preparation process of the composite anode material in the example of this application.

[0021] Figure 2 shows a schematic diagram of the synthesis of the composite anode materials obtained in Examples 1-3 of this application.

[0022] Figure 3 shows the XRD analysis diagrams of the composite anode materials prepared in Examples 1-3 of this application.

[0023] Figure 4 shows SEM images of the composite anode materials prepared in Examples 1-3 of this application.

[0024] Figure 5 shows the Raman diagram of the composite negative electrode material prepared in Example 2 of this application.

[0025] Figure 6 shows a TEM image of the composite anode material prepared in Example 1 of this application.

[0026] Figure 7 shows a TEM image of the composite anode material prepared in Example 2 of this application.

[0027] Figure 8 shows a TEM image of the composite anode material prepared in Example 3 of this application.

[0028] Figure 9 shows a comparison of the cycle performance of lithium-ion batteries made of the composite negative electrode materials of Examples 1-3 of this application at different temperatures at 0.1C (1C=1000mA / g).

[0029] Figure 10 shows a comparison of the rate performance of lithium-ion batteries made of composite negative electrode materials in Examples 1-3 of this application at different rates and temperatures.

[0030] Figure 11 shows a comparison of the electrochemical impedance spectroscopy (EIS) diagrams of lithium-ion batteries made from the composite anode materials of Examples 1-3 of this application and the original Si at different temperatures. Detailed Implementation

[0031] This invention discloses a composite anode material and its preparation method, as well as a lithium-ion battery. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0032] This application provides a composite anode material, which includes a core and a shell covering the core. The core includes Fe-B co-doped Si with Si-Fe alloy attached to its surface, and the shell includes an amorphous carbon layer with SiC grown on its surface.

[0033] It should be noted that the core body is Fe-B co-doped Si, and the surface of the core body is attached with Si-Fe alloy, which includes FeSi2 alloy and / or Fe5Si3 alloy.

[0034] Optionally, the Si-Fe alloy includes the FeSi2 alloy.

[0035] Optionally, the mass ratio of Si, Fe and B elements in the nucleus is 90%~94%:5%~8%:1%~2%.

[0036] As an example, the mass ratio of Si, Fe and B elements in the nucleus can be 90%:8%:2%, 91%:8%:1%, 92%:7%:1%, 93%:6%:1% or 94%:5%:1%.

[0037] Optionally, the mass ratio of SiC to amorphous carbon in the shell is 10%~40%:60%~90%.

[0038] As an example, the mass ratio of SiC to amorphous carbon in the shell can be 10%:90%, 20%:80%, 30%:70%, or 40%:60%.

[0039] Optionally, the diameter of the core is 200nm~800nm, and the thickness of the shell is 10nm~35nm.

[0040] As an example, the diameter of the core can be 200nm, 300nm, 400nm, 500nm, 600nm, 700nm or 800nm, and the thickness of the shell can be 10nm, 15nm, 20nm, 25nm, 30nm or 35nm.

[0041] Optionally, the particle size of the composite negative electrode material is 0.2 μm to 2 μm.

[0042] As an example, the particle size of the composite anode material can be 0.2μm, 0.3μm, 0.5μm, 1μm, 1.5μm or 2μm.

[0043] The composite anode material of this application forms a core by Fe-B co-doping Si. B doping can increase the hole concentration, thereby improving the carrier mobility, which in turn enhances the electron and ion transfer rate of silicon, thus improving the intrinsic conductivity and rate capability of silicon. At the same time, the iron element of the boron-iron alloy and the Si-Fe alloy formed by silicon alloying can adhere to the surface of Fe-B co-doped Si. The Si-Fe alloy phase, the amorphous carbon layer, and the SiC grown on the surface of the amorphous carbon layer work together to effectively suppress the volume expansion of silicon during charging and discharging and provide lithium ion transport channels, thereby improving the cycle performance and rate capability of lithium-ion batteries made with silicon-based composite anode materials.

[0044] Please refer to Figure 1. This application also provides a method for preparing a composite negative electrode material, which includes the following steps: S1. Preparing a first mixture: First, silicon powder and ferroborone alloy powder are subjected to a first ball milling process to obtain a first mixture.

[0045] Optionally, the silicon powder has a purity of 99.99%.

[0046] Optionally, the DV50 particle size of the silicon powder is 5μm to 30μm.

[0047] As an example, the DV50 particle size of silicon powder can be 5μm, 10μm, 15μm, 20μm, 25μm or 30μm.

[0048] Optionally, the ferroborone alloy includes one or more of Fe-10B, Fe-20B, Fe-30B, and Fe-40B.

[0049] Optionally, the DV50 particle size of the ferroborone alloy powder is 10 μm to 40 μm.

[0050] As an example, the DV50 particle size of the ferroborone alloy powder can be 10 μm, 20 μm, 30 μm or 40 μm.

[0051] Optionally, the first ball milling process is wet ball milling.

[0052] Optionally, the process control agent for wet ball milling includes one or more of deionized water, ethanol, n-hexanol, ethylene glycol, and propylene glycol.

[0053] Optionally, the mass ratio of the mixed powder to the process control agent in the wet ball mill is 1:3 to 12.

[0054] As an example, the mass ratio of the mixed powder, including silicon powder and ferroborone alloy powder, to the process control agent in wet ball milling can be 1:3, 1:5, 1:8, 1:10, or 1:12.

[0055] Optionally, the ball milling time for the first ball milling treatment is 0.5h to 12h, and the ball milling speed is 250rpm to 1000rpm.

[0056] As an example, the ball milling time for the first ball milling process can be 0.5h, 1h, 2h, 5h, 8h, 10h or 12h, and the ball milling speed can be 250rpm, 300rpm, 500rpm, 800rpm or 1000rpm.

[0057] S2. Preparation of the second mixture: The first mixture and the carbon source powder are subjected to a second ball milling process to obtain the second mixture.

[0058] Optionally, the carbon source includes one or more of phenolic resins, asphalt, glucose, sucrose, and citric acid.

[0059] Optionally, the DV50 particle size of the carbon source powder is 1μm~15μm.

[0060] As an example, the DV50 particle size of the carbon source powder can be 1 μm, 2 μm, 5 μm, 8 μm, 10 μm or 15 μm.

[0061] Optionally, the second ball milling process is wet ball milling.

[0062] Optionally, the ball milling time for the second ball milling treatment is 0.5h to 12h, and the ball milling speed is 250rpm to 1000rpm.

[0063] As an example, the ball milling time for the second ball milling process can be 0.5h, 1h, 2h, 5h, 8h, 10h or 12h, and the ball milling speed can be 250rpm, 300rpm, 500rpm, 800rpm or 1000rpm.

[0064] Optionally, the mass ratio of silicon powder, ferroborone alloy powder and carbon source powder is 48%~53%:6%~9%:41%~46%.

[0065] As an example, the mass ratio of silicon powder, ferroborone alloy powder and carbon source powder can be 48%:6%:46%, 49%:7%:44%, 50%:9%:41%, 51%:8%:41%, 52%:6%:42% or 53%:6%:41%.

[0066] S3. Preparation of precursor material: The second mixture is dried in a forced-air drying oven to obtain the precursor material.

[0067] Optionally, the drying temperature is 50℃~100℃, and the drying time is 5h~15h.

[0068] As an example, the drying temperature can be 50℃, 60℃, 70℃, 80℃, 90℃ or 100℃, and the drying time can be 5h, 8h, 10h, 12h or 15h.

[0069] S4. Preparation of composite anode material: The precursor material is placed in a tube furnace and heat-treated under inert gas protection. After the heat treatment is completed, it is cooled to room temperature to obtain the composite anode material.

[0070] Optionally, the heat treatment temperature is 600℃~1500℃, and the heat treatment holding time is 1h~7h.

[0071] Optionally, the heat treatment temperature is 800℃~1050℃.

[0072] As an example, the heat treatment temperature can be 600℃, 800℃, 1000℃, 1200℃ or 1500℃, and the heat treatment holding time can be 1h, 2h, 3h, 5h or 7h.

[0073] Optionally, the inert gas includes at least one of nitrogen, helium, neon, and argon.

[0074] The method for preparing the composite anode material of this application adopts a composite strategy of silicon bulk phase and surface modification, and the preparation process is gradual. It has the advantages of low cost, simple and feasible synthesis process and green environmental protection.

[0075] This application also provides a lithium-ion battery comprising the above-described composite negative electrode material or a composite negative electrode active material prepared according to the above-described method for preparing composite negative electrode material.

[0076] It should be noted that the lithium-ion battery of this application uses the above-mentioned composite negative electrode material as the negative electrode active material, but does not limit the type of lithium-ion battery. The lithium-ion battery can be a liquid battery, a semi-solid battery or a solid battery.

[0077] The present invention will be further described below with reference to the embodiments: Embodiment 1 This application embodiment provides a composite negative electrode material and its preparation method, which includes the following steps: S1, Preparation of the first mixture First, weigh 1.5g of silicon powder and 0.3g of ferroboron alloy powder. The purity of the silicon powder is 99.99% and the DV50 particle size of the silicon powder is 15μm. The ferroboron alloy is Fe-20B and the DV50 particle size of the ferroboron alloy powder is 25μm. Add the weighed silicon powder, ferroboron alloy powder and 20mL of ethanol into a ball mill jar for the first ball milling treatment. The grinding balls in the ball mill jar are zirconia balls. The ball-to-material mass ratio is 20:1. The first ball milling treatment time is 6h and the rotation speed is 600rpm to obtain the first mixture.

[0078] S2. To prepare the second mixture, first weigh 1g of carbon source powder, which is pitch, and the DV50 particle size of the carbon source powder is 5μm. Add the weighed carbon source powder and the first mixture prepared above into a ball mill jar for a second ball milling treatment. The grinding balls in the ball mill jar are zirconia balls, and the ball-to-material mass ratio is 20:1. The second ball milling treatment time is 6h, and the rotation speed is 600rpm to obtain the second mixture.

[0079] S3. Preparation of precursor material: The second mixture obtained above is dried in a forced-air drying oven at a temperature of 60°C for 12 hours to obtain the precursor material.

[0080] S4. Preparation of composite anode material: The precursor material obtained above is placed in a tube furnace and heat-treated under helium protection. The heat treatment temperature is 800℃ and the heat treatment holding time is 3h. After the heat treatment is completed, it is cooled to room temperature to obtain the composite anode material.

[0081] Example 2 This application provides a composite negative electrode material and its preparation method, which is based on Example 1, except that the heat treatment temperature is changed to 950°C, and everything else remains the same.

[0082] Example 3 This application provides a composite negative electrode material and its preparation method, which is based on Example 1, except that the heat treatment temperature is changed to 1050℃, and everything else remains the same.

[0083] Example 4 This application provides a composite negative electrode material and its preparation method, which is based on Example 1, except that the boron-iron alloy is changed to Fe-30B, the heat treatment temperature is 950℃, and everything else remains the same.

[0084] Example 5 This application provides a composite negative electrode material and its preparation method, which is based on Example 1, except that the carbon source is changed to phenolic resin and the heat treatment temperature is 950°C, while other aspects remain unchanged.

[0085] Comparative Example 1: Elemental silicon as the negative electrode active material.

[0086] Comparative Example 2 This application provides a composite negative electrode material and its preparation method, which includes the following steps: S1, preparing a first mixture: First, weigh 1.5g of silicon powder, 0.06g of boron powder and 0.24g of iron powder. The purity of the silicon powder is 99.99%, the DV50 particle size of the silicon powder is 15μm, the DV50 particle size of the iron powder is 25μm, and the DV50 particle size of the boron powder is 25μm. Add the weighed silicon powder, boron powder and iron powder and 20mL of ethanol into a ball mill jar for the first ball milling treatment. The grinding balls in the ball mill jar are zirconia balls, the ball-to-material mass ratio is 20:1, the first ball milling treatment time is 6h, and the rotation speed is 600rpm to obtain the first mixture.

[0087] S2. To prepare the second mixture, first weigh 1g of carbon source powder, which is pitch, and the DV50 particle size of the carbon source powder is 5μm. Add the weighed carbon source powder and the first mixture prepared above into a ball mill jar for a second ball milling treatment. The grinding balls in the ball mill jar are zirconia balls, and the ball-to-material mass ratio is 20:1. The second ball milling treatment time is 6h, and the rotation speed is 600rpm to obtain the second mixture.

[0088] S3. Preparation of precursor material: The second mixture obtained above is dried in a forced-air drying oven at a temperature of 60°C for 12 hours to obtain the precursor material.

[0089] S4. Preparation of composite anode material: The precursor material obtained above is placed in a tube furnace and heat-treated under helium protection. The heat treatment temperature is 950℃ and the heat treatment holding time is 3h. After the heat treatment is completed, it is cooled to room temperature to obtain the composite anode material.

[0090] Comparative Example 3 This application provides a composite negative electrode material and its preparation method, which includes the following steps: S1, preparing a first mixture: First, weigh 1.5g of silicon powder and 0.3g of iron powder. The purity of the silicon powder is 99.99%, the DV50 particle size of the silicon powder is 15μm, and the DV50 particle size of the iron powder is 25μm. Add the weighed silicon powder, iron powder and 20mL of ethanol into a ball mill jar for the first ball milling treatment. The grinding balls in the ball mill jar are zirconia balls, the ball-to-material mass ratio is 20:1, the first ball milling treatment time is 6h, and the rotation speed is 600rpm to obtain the first mixture.

[0091] S2. To prepare the second mixture, first weigh 1g of carbon source powder, which is pitch, and the DV50 particle size of the carbon source powder is 5μm. Add the weighed carbon source powder and the first mixture prepared above into a ball mill jar for a second ball milling treatment. The grinding balls in the ball mill jar are zirconia balls, and the ball-to-material mass ratio is 20:1. The second ball milling treatment time is 6h, and the rotation speed is 600rpm to obtain the second mixture.

[0092] S3. Preparation of precursor material: The second mixture obtained above is dried in a forced-air drying oven at a temperature of 60°C for 12 hours to obtain the precursor material.

[0093] S4. Preparation of composite anode material: The precursor material obtained above is placed in a tube furnace and heat-treated under helium protection. The heat treatment temperature is 950℃ and the heat treatment holding time is 3h. After the heat treatment is completed, it is cooled to room temperature to obtain the composite anode material.

[0094] Figure 2 is a schematic diagram of the synthesis of the composite anode materials obtained in Examples 1-3. As shown in Figure 2, initially, when Si is crushed by the shear force of a zirconium ball mill, the exposed surface contains a large number of unsaturated dangling bonds, which is conducive to the reaction between Si and Fe-B alloy powder to form a Si / Fe / B composite material. Subsequently, after the introduction of pitch, continuous grinding and stirring are carried out under the action of ball mill shear force, forming a special coating structure (Si / Fe / B)@Glucose with pitch on the surface of the silicon composite structure. Finally, during heat treatment, the internal Si / Fe / B composite structure undergoes alloying. This reaction generates a large amount of heat, forming Si-C bonds with the outermost pitch C layer, achieving further coating and enhancing the conductivity of the matrix.

[0095] Figure 3 shows the XRD analysis diagrams of the composite anode materials prepared in Examples 1-3. As can be seen from Figure 3, the composite anode materials of Examples 1-2 contain two silicon alloys, FeSi2 and Fe5Si3. Compared with the composite anode material of Example 1, the composite anode material of Example 2 has a relatively higher FeSi2 content and an increased SiC content. The XRD of the composite anode material of Example 3 shows that the intensity of the Si peak is significantly weakened, indicating the formation of more SiC.

[0096] Figure 4 shows the SEM images of the composite anode materials prepared in Examples 1-3. As can be seen from Figure 4, as the temperature increases, the carbon source continuously decomposes and carbonizes, and the particles become more and more dense on the relative surface. Combined with the previous XRD, it can be seen that Si-C bonding is mainly formed, which helps to enhance the surface coating effect.

[0097] Figure 5 is the Raman spectrum of the composite anode material prepared in Example 2. As shown in Figure 5, the results are similar to those obtained by XRD, at 480 cm⁻¹. -1 and 938cm -1 The peak value corresponds to Si-O bonding, 520 cm⁻¹ -1 The peak value corresponds to silicon. Clearly, the peak value continuously shifts to lower wavenumbers after wet milling heat treatment, indicating that the reaction mainly occurs on the surface. Simultaneously, Fe-B solid solution or doping also contributes to the shift to lower wavenumbers.

[0098] Figure 6 shows a TEM image of the composite anode material prepared in Example 1. As shown in Figure 6(a), the nanoparticles of the composite anode material prepared in Example 1 are embedded on micron-sized particles. Combined with Figure 6(b) and the corresponding mapping diagram (b1), it can be seen that the micron-sized particles are still mainly composed of Si. Most of the added FeB undergoes an alloying reaction with Si to form Fe2Si (see Figure 6e2 lattice fringes). The remaining unreacted FeB particles are wrapped by an outer C layer, as shown in Figure 6(f) for morphology, corresponding elemental distribution, and lattice fringes. It is worth noting that a SiC structure was also found on the Si particles, corresponding to the (1 0 10) crystal plane as shown in Figure 6(c), which is consistent with the results of XRD phase analysis. These nanoparticles are coated with C, and the carbon layer is about 10-20 nm, as shown in Figure 6(d). As for the O element, it is mainly related to the oxide of Si and surface adsorbed oxygen. In summary, the microstructure of the composite anode material prepared in Example 1 is characterized by micron-sized Si embedded in nano-sized Fe2Si, SiC and residual FeB, i.e., a micro-nano multiphase composite structure with an external C coating layer.

[0099] Figure 7 is a TEM image of the composite anode material prepared in Example 2. As shown in Figures 7(a) and (b), the composite anode material prepared in Example 2 exhibits nanoparticles dispersed within micron-sized particles. Selected area diffraction was performed on the regions shown in Figures 7(b) and (c), and the patterns are shown in Figures 7(e) and (f). These diffraction rings correspond to the Si, SiC, FeSi2, and Fe5Si3 phases, respectively, which is consistent with the results of XRD, etc. However, no residual FeB phase was found, indicating that the increase in temperature promotes the complete decomposition of FeB, diffusion into the internal Si, and alloying reaction to form different silicon-iron compounds. Further lattice fringe analysis was performed on b1 in Figure 7(b). Regions h1 and h2 correspond to the SiC structure, while regions (h3) and (h4) correspond to the (101) crystal plane of Fe5Si3 and the (102) crystal plane of FeSi2, respectively. In addition, an amorphous carbon layer with a thickness of about 20-35 nm forms a good coating layer structure on the surface, as shown in Figure 7(d). Overall, in addition to nano-FeSi2 compounds, there is also an iron-rich phase Fe5Si3 formed by the complete dissolution of FeB, which is discretely distributed in the micron-sized Si matrix, surrounded by (SiC+C) structures.

[0100] Furthermore, as shown in Figure 7, the composite anode material is composed of Si, SiC, FeSi2, and Fe5Si3 phases, consistent with the XRD results. An amorphous carbon layer with a thickness of approximately 20–35 nm was also observed, forming a well-dense coating structure on the surface. Overall, in addition to the nano-FeSi2 compound, the iron-rich Fe5Si3 phase, formed by the complete dissolution of boron-iron compounds, is discretely distributed within the micron-sized Si matrix, surrounded by a (SiC+C) structure.

[0101] Figure 8 shows TEM images of the composite anode material prepared in Example 3. Figure 8(a) shows that the micron-sized particles appear to be integrally formed with numerous fibrous structures on the surface. These fibrous structures and the overall particles are mainly composed of Si and C elements, as shown in Figures 8(a1) and (a2). According to the corresponding lattice fringe analysis, the protruding fibers on the particle surface and the interior of the micron-sized particle region are mostly SiC structures (see Figure 8c), while the outer layer of the SiC fibers is covered by locally ordered graphitized carbon (see Figure 8d). This may be due to the heat released from the reaction of Si and C at a high temperature of 1100°C, which raises the local temperature and promotes the local amorphous-to-order transformation of the surface. Besides the majority of the SiC@C structure, a small amount of Fe5Si3 phase is found in Figure 8(e), but no obvious Si lattice fringes are observed, consistent with the XRD results, indicating that most Si is converted to SiC. In summary, this high-temperature heat treatment results in excessive Si consumption, which is very detrimental to high capacity performance, and the formation of a thick inert SiC phase also hinders the transport of lithium ions and electrons. Therefore, optimizing the material synthesis temperature has a significant impact on the internal doping, alloying structure, and surface coating structure of Si, thereby determining the electrochemical performance.

[0102] The composite negative electrode materials prepared in Examples 1-5 and Comparative Examples 2-3, along with elemental silicon from Comparative Example 1, were used as negative electrode active materials to fabricate lithium-ion batteries. The preparation method is as follows: All electrochemical tests were conducted using CR2025 coin cell half-cells. The preparation process mainly consists of two parts: the preparation of the negative electrode sheet and the assembly of the coin cell half-cell. Specific details are as follows: Electrode sheet preparation: First, the active material, conductive agent (Super-P), and binder (4% PAA-Li) were weighed in a 6:2:2 mass ratio and placed in an agate mortar. An appropriate amount of deionized water was added for grinding. After uniform mixing, the slurry was evenly coated onto a copper foil current collector using a 200μm thick scraper. The electrode sheet was then placed in an 80℃ oven and vacuum dried for 8 hours. Finally, the thoroughly dried electrode sheet was cut into several round pieces (12mm in diameter) using a coin cell impactor and weighed.

[0103] Assembly of the coin cell: In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), the coin cells were arranged in the following order: positive electrode shell, positive electrode plate, separator, negative electrode plate, and negative electrode shell. An appropriate amount of electrolyte was added, and the cells were sealed and compacted using a manual sealing machine. The matching positive and negative electrode shells were CR2025, the positive electrode (reference electrode) was a lithium metal sheet, the separator was a single-layer microporous membrane (PP, Celgard® 2400), and the electrolyte was a silicon-carbon electrolyte. Finally, the assembled coin cell was allowed to stand for at least 12 hours to complete the subsequent electrochemical performance testing.

[0104] The specific discharge capacity of the composite anode materials prepared in Examples 1-5 and Comparative Examples 2-3, as well as the silicon elemental material in Comparative Example 1, as anode active material, were measured in coin half-cells at a current density of 0.05C (1C=1000mA / g, the same below), the capacity retention rate after 120 cycles at 0.1C (relative to the 6th cycle), and the capacity recovery rate of the rate performance (i.e., the specific discharge capacity when recovered to 0.05C compared with the initial 0.05C discharge capacity) were measured. The results are shown in Table 1.

[0105] The testing method is as follows: 1. The button half-cell prepared for the first discharge specific capacity is clamped onto the battery test frame. After setting the mass of the active material, the first discharge specific capacity is obtained by testing with a current density of 0.05C (voltage range of 0.01-2V).

[0106] 2. Capacity retention rate: The coin cell half-cell prepared is clamped onto the battery test rack. After setting the mass of the active material, the battery is first activated by cycling at a current density of 0.05C (voltage range of 0.01-2V) for 5 cycles. Then, a cycle test is performed at a current density of 0.1C (voltage range of 0.01-1.5V). The capacity retention rate after 120 cycles is obtained by comparing the discharge specific capacity of the 120th cycle with the discharge specific capacity of the 6th cycle.

[0107] 3. Capacity recovery rate for rate performance: The prepared coin cell half-cell is clamped onto the battery test frame. After setting the mass of the active material, the battery is first activated by cycling at a current density of 0.05C (voltage range of 0.01-2V) for 5 cycles. Then, it is cycled at current densities of 0.1C, 0.2C, 0.25C, 0.5C, 1C, 0.5C, 0.25C, 0.2C, 0.1C, and 0.05C for 5 cycles each. The capacity recovery rate for rate performance is obtained by comparing the average discharge specific capacity of the subsequent 0.05C cycles with the average discharge specific capacity of the first 5 cycles.

[0108] Table 1. Performance of coin half-cells made using composite anode materials from Examples 1-5 and Comparative Examples 2-3, and elemental silicon from Comparative Example 1 as the anode active material.

[0109] As can be seen from Examples 1 to 5, the coin cell prepared using the composite negative electrode material of this application as the negative electrode active material has a first-cycle discharge specific capacity of 881 mAh / g to 2167 mAh / g, a 120-cycle capacity retention rate of 52.1% to 93.7%, and a rate performance capacity recovery rate of 52.9% to 96.5%. Among them, the coin cell prepared with the composite negative electrode material of Example 2 has a relatively high first-cycle discharge specific capacity, 120-cycle capacity retention rate, and rate performance capacity recovery rate.

[0110] As can be seen from the comparison between Comparative Example 1 and Example 2, although the coin cell made by using silicon as the negative electrode active material in Comparative Example 1 has a high first-cycle discharge specific capacity, which is based on the high specific capacity of silicon itself, its capacity retention rate after 120 cycles is only 23.7%, and its capacity recovery rate in rate performance is only 32.7%, both of which are lower than that in Example 2 and have no application value.

[0111] Comparing Comparative Example 2 and Example 2, it can be seen that Comparative Example 2 uses boron and iron as raw materials to replace the ferroboron alloy. Its 120-cycle capacity retention rate is only 51.7%, and its rate performance capacity recovery rate is only 54.8%, both lower than Example 2. This shows that only by using ferroboron alloy as raw material can a composite negative electrode material including a core and a shell be obtained. The core includes Fe-B co-doped Si with Si-Fe alloy attached to the surface, and the shell includes an amorphous carbon layer with SiC grown on the surface.

[0112] Comparing Comparative Example 3 and Example 2, it can be seen that Comparative Example 3 uses elemental iron as a raw material to replace the ferroborone alloy, and its capacity retention rate after 120 cycles is only 50.3%, and its capacity recovery rate in rate performance is only 49.2%, both of which are lower than that in Example 2. This shows that when the core of the composite negative electrode material is only doped with iron, it does not significantly improve the cycle performance and rate performance.

[0113] Figure 9 shows the cycle performance of lithium-ion batteries made from the composite anode materials of Examples 1-3 at 0.1C (1C = 1000 mA / g). It can be seen that the cycle stability significantly improves with increasing temperature, and the capacity retention also shows an increasing trend. This indicates that the stress buffering effect of silicon inside the composite material gradually becomes more pronounced with increasing temperature, leading to continuously improved cycle stability. The lithium-ion battery made from the composite anode material of Example 2 maintains good capacity retention while also exhibiting a high specific capacity.

[0114] Figure 10 shows the rate performance of lithium-ion batteries made from the composite anode materials of Examples 1-3 and the original Si as the anode at different rates. It can be seen that the capacity recovery rate at 0.05C increases with increasing temperature. This indicates that the formation of the Si-Fe-B structure and the coupling of phase transformation, along with the increase in SiC content, have a certain improvement on the electronic conductivity of the modified composite electrode, which helps to accelerate electron transport and has a good effect on improving rate performance.

[0115] Figure 11 shows the electrochemical impedance of the composite anode materials of Examples 1-3 and the lithium-ion batteries made with original Si as the anode. It can be seen that the electrochemical impedance of the composite anode materials obtained after heat treatment modification at three different temperatures before cycling is significantly different from that of Si. As the temperature increases, the impedance gradually decreases. The lithium-ion battery made with the composite anode material of Example 2 has a relatively smaller impedance, which indicates that the charge transfer is faster after modification. This is because the introduction of the boron-iron alloy is beneficial to improving the silicon crystal structure and increasing electronic conductivity. At the same time, the introduction of carbon materials helps to improve the surface properties of silicon, accelerate the transport and charge transfer of lithium ions, and reduce the charge transfer impedance.

[0116] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A composite negative electrode material, characterized in that, The composite anode material includes a core and a shell covering the core. The core includes Fe-B co-doped Si with Si-Fe alloy attached to its surface, and the shell includes an amorphous carbon layer with SiC grown on its surface.

2. The composite negative electrode material according to claim 1, characterized in that, The mass ratio of Si, Fe and B in the core is 90%~94%:5%~8%:1%~2%, and / or; the mass ratio of SiC and amorphous carbon in the shell is 10%~40%:60%~90%.

3. The composite negative electrode material according to claim 1 or 2, characterized in that, The diameter of the core is 200nm~800nm, the thickness of the shell is 10nm~35nm, and / or; the particle size of the composite negative electrode material is 0.2μm~2μm.

4. A method for preparing the composite negative electrode material according to any one of claims 1 to 3, characterized in that, The preparation method of the composite anode material includes: first, ball milling and mixing silicon powder, ferroborone alloy powder and carbon source powder, drying to obtain a precursor material, and then heat-treating the precursor material under inert gas protection.

5. The composite negative electrode material according to claim 4, characterized in that, The heat treatment temperature is 600℃~1500℃, and the heat treatment holding time is 1h~7h.

6. The composite negative electrode material according to claim 4, characterized in that, The mass ratio of the silicon powder, the ferroborone alloy powder, and the carbon source powder is 48%~53%:6%~9%:41%~43%.

7. The composite negative electrode material according to claim 4, characterized in that, The ferroborone alloy includes one or more of Fe-10B, Fe-20B, Fe-30B, and Fe-40B, and / or; the carbon source includes one or more of phenolic resin, asphalt, glucose, sucrose, and citric acid.

8. The composite negative electrode material according to claim 4, characterized in that, The silicon powder has a DV50 particle size of 5μm to 30μm, and / or; the boron-iron alloy powder has a DV50 particle size of 10μm to 40μm, and / or; the carbon source powder has a DV50 particle size of 1μm to 15μm.

9. The composite negative electrode material according to claim 4, characterized in that, When ball milling and mixing silicon powder, ferroborone alloy powder, and carbon source powder, the silicon powder and ferroborone alloy powder are first ball milled to obtain a first mixture, and then the first mixture and the carbon source powder are ball milled a second time to obtain a second mixture. Optionally, the first ball milling and / or the second ball milling is wet ball milling. Optionally, the process control agent for the wet ball milling includes one or more of deionized water, ethanol, n-hexanol, ethylene glycol, and propylene glycol. Optionally, the mass ratio of the mixed powder to the process control agent in the wet ball milling is 1:3 to 12. Optionally, the ball milling time for the first ball milling and / or the second ball milling is 0.5 h to 12 h, and the ball milling speed is 250 rpm to 1000 rpm. Optionally, the second mixture is dried at 50°C to 100°C for 5 h to 15 h to obtain the precursor material.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the composite negative electrode material according to any one of claims 1 to 3 or the composite negative electrode active material prepared by the preparation method of the composite negative electrode material according to any one of claims 4 to 9.

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