A composite anode material and its preparation method and its application in batteries

Abstract

This invention discloses a composite anode material comprising silicon nanoparticles, carbon nanotubes, and silicon oxide material. The carbon nanotubes are curved or spiral-shaped, and transition metal nanoparticles are present at their ends. The carbon nanotubes have a hollow structure. This composite anode material, when used in batteries, offers advantages such as high silicon loading and high specific capacity.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, specifically to a composite anode material and its preparation method and its application in batteries, and more specifically to an in-situ synthesis of carbon nanotube composite silicon anode material and its preparation method and its application in batteries. Background Technology

[0002] Carbon nanotubes possess excellent mechanical and electrical properties, thus their composite composition can effectively buffer stress generated during material cycling. Simultaneously, the three-dimensional conductive network structure formed by carbon nanotubes effectively binds the material particles, maintaining the overall continuity and conductivity of the electrode. Therefore, carbon nanotube composite active materials offer significant advantages as electrode materials.

[0003] Silicon anodes offer a significant theoretical specific capacity advantage. Currently, graphite is the most widely used anode material, but commercially available graphite anodes have already reached near their theoretical specific capacity (372 mAh / g), limiting their further applications. Therefore, there is an urgent need to develop anode materials with higher specific capacity. Silicon anodes, on the other hand, possess a very high theoretical specific capacity (4200 mAh / g) and a low electrochemical lithium intercalation potential, along with excellent fast-charging performance. This is precisely what is urgently needed in a range of emerging technology fields such as portable electronics, drones, new energy vehicles, and energy storage battery systems. However, silicon anodes experience significant volume expansion during charge and discharge. This substantial volume expansion during cycling leads to poor cycle life and irreversible capacity, severely hindering their commercial application. Summary of the Invention

[0004] This invention addresses the problems in existing technologies by disclosing a composite anode material. Silicon nanoparticles are loaded onto carbon nanotubes to form a three-dimensional network structure, serving as the inner layer of the composite anode material. The excellent conductivity and mechanical properties of carbon nanotubes improve the conductivity of silicon nanoparticles and constrain their expansion. Furthermore, the suitable helical structure of the carbon nanotubes further prevents silicon particle breakage during long-cycle batteries, acting as a link between silicon particles and suppressing cycle failure. Silicon oxide serves as the outer layer of the composite anode material. On one hand, silicon oxide is a cross-linked network structure formed by calcining a resin template, resulting in a relatively stable structure that helps maintain anode cycle stability. On the other hand, the silicon-containing outer layer does not reduce the anode capacity. Therefore, the outer silicon oxide layer encapsulates the silicon nanoparticles loaded on the carbon nanotubes in the inner layer, exhibiting a robust structure that effectively encapsulates the silicon particles. The dual effects of the inner and outer layers suppress the volume expansion of the silicon material. Simultaneously, the overall structure prevents contact between silicon nanoparticles and the electrolyte, reducing battery side reactions, improving initial efficiency and capacity, reducing internal resistance, and enhancing cycle stability.

[0005] This invention is achieved through the following technical solution:

[0006] The present invention provides a composite anode material comprising silicon nanoparticles, carbon nanotubes and silicon oxide materials, wherein the carbon nanotubes are curved or spiral, and transition metal nanoparticles are present at the ends of the carbon nanotubes, and the carbon nanotubes have a hollow structure.

[0007] The inventors discovered that the morphology of hollow carbon nanotubes can be adjusted, enabling the further construction of electron transport networks with relatively small amounts of carbon nanotubes. By adjusting the helicity of the carbon nanotubes, different active sites and specific surface areas can be achieved, which is beneficial to improving the conductivity and electrolyte absorption rate of the carbon nanotube material. The uniform diameter distribution of the carbon nanotubes avoids material agglomeration and promotes dispersion. Ultimately, only a small amount needs to be added to improve the battery's resistance, rate performance, and cycle performance. The transition metal nanoparticles at the ends of the carbon nanotubes are the nucleation sites for the formation of the carbon nanotube material of this invention. During the in-situ synthesis of carbon nanotubes, the morphology of the carbon nanotubes can be controlled to be linear, curved, or helical to varying degrees by controlling the synthesis conditions. In the helical form, the pitch is relatively uniform, and the helix angle varies with the number of helical turns, thereby maintaining the stability of the carbon nanotubes. When carbon nanotubes are used to improve the conductivity of silicon materials, since silicon materials have poorer conductivity and higher expansion compared to carbon materials, carbon nanotubes with a high degree of helicity are more beneficial for improving the conductivity of silicon materials and suppressing expansion. When carbon nanotubes are used in cathode active materials containing transition metals, since the cathode expansion is relatively small and the conductivity requirement is less than that of silicon materials, carbon nanotubes in the form of straight, curved, or low-degree helical shapes can meet the performance requirements. When carbon nanotubes are used in sulfur-containing cathodes, similar to the mechanism of silicon anodes, the morphology of high-degree helical shapes has greater advantages.

[0008] The inventors further combined carbon nanotubes with other materials to prepare a silicon nanoparticle / carbon nanotube@silicon oxide composite structure. The " / " indicates that the silicon nanoparticles and carbon nanotubes are in a mixed state. Due to the longer diameter of the carbon nanotubes, this mixed state represents silicon nanoparticles loaded onto the carbon nanotubes. The silicon nanoparticles loaded onto the carbon nanotubes form a three-dimensional network structure, serving as the inner layer of the composite anode material. This improves the volume expansion of silicon and enhances its conductivity, fully utilizing the high specific capacity of silicon. The "@" indicates that silicon oxide is on the outer layer, with the outer layer of silicon oxide coating the inner layer of "silicon nanoparticles / carbon nanotubes." This exhibits a robust structure that encapsulates the silicon particles. The dual effect of the inner and outer layers suppresses the volume expansion of the silicon material. Simultaneously, the overall structure prevents contact between the silicon nanoparticles and the electrolyte, reducing battery side reactions, improving initial efficiency and capacity, reducing internal resistance, and enhancing cycle stability.

[0009] As a further option, the diameter of the carbon nanotube material is 5nm-100nm. This not only improves the electrical properties of the carbon nanotubes but also facilitates the bending of the carbon nanotube material.

[0010] As a further improvement, the diameter of the carbon nanotube material is 20nm-40nm.

[0011] As a further option, the pitch of the carbon nanotubes is 90nm-100nm. A suitable pitch not only makes it easier to achieve stability after the carbon nanotubes are rolled up, but also avoids the problem of conductive materials agglomerating and becoming difficult to disperse when the pitch is too small, thus affecting the battery's internal resistance. When the pitch is too large, it cannot further reduce the spacing between the active material particles after mixing with the active material, resulting in an increase in the amount of binder used and an increase in the battery's internal resistance.

[0012] In this invention, the pitch of carbon nanotubes represents the straight-line distance between two points corresponding to the center of the plane of the helical loop between two adjacent helices.

[0013] As a further improvement, the helix angle of the carbon nanotube material is 30°-120°.

[0014] In this invention, the helix angle of carbon nanotubes represents the bending angle formed when the carbon nanotube, which is growing from a near-straight direction with the tube end as the starting point, begins to bend and grows. The helical bending angles that appear in sequence can be represented as θ1, θ2, θ3, ...

[0015] As a further option, the aspect ratio of the carbon nanotubes is 3-1000. If the aspect ratio is too small, the performance is similar to that of spherical particulate conductive agents, and the performance of the conductive battery is reduced; if the aspect ratio is too large, the nanotube structure is prone to instability.

[0016] As a further refinement, the particle size ratio of the transition metal nanoparticles to the diameter of the carbon nanotubes is (1-1.5):(1-1.5). The similar diameters of the two particles ensure stable attachment of the transition metal particles to the carbon nanotube ports, promoting the formation of longer helical morphologies through stable chemical bonding.

[0017] As a further embodiment, the transition metal nanoparticles include one or more transition metals.

[0018] As a further embodiment, the transition metal includes one or more of iron, cobalt, nickel, copper, and manganese.

[0019] As a further option, the transition metal is one or both of iron and copper. Iron and copper are conducive to the formation of multi-coordinate bonds.

[0020] As a further refinement, the transition metal is a mixture of iron and copper. The combination of iron and copper is more conducive to promoting the helical formation of carbon nanotubes.

[0021] A second aspect of the present invention is to provide a method for preparing a composite negative electrode material, comprising the following steps:

[0022] S1: Grind a mixture of carbon source, nitrogen source and transition metal source, and calcine it once under an inert gas to obtain material A1;

[0023] S2: The A1 material obtained in S1 is mixed with an electrophilic reagent and a protic acid solution, and then dried to obtain A2 carbon nanotube material;

[0024] S3: Add silicon nanoparticles and A2 carbon nanotubes to the dispersant and ultrasonically stir to disperse them evenly to obtain mixture A3;

[0025] S4: Under inert atmosphere conditions, A3 is calcined a second time to obtain the final composite anode material product.

[0026] In the method of this invention, the high-temperature calcination of a mixture of carbon source, nitrogen source, and transition metal salt tends to yield non-helical and relatively short straight carbon nanotubes (CNTs). Then, under the combined action of an electrophilic reagent and a protic acid, the CNTs are treated to obtain curved, elongated, or even helical morphologies. During high-temperature calcination, the carbon source reduces the transition metal salt to generate transition metal nanoparticles. The transition metal and nitrogen form coordination bonds, adsorbing more carbon and nitrogen fragments that migrate to the transition metal nanoparticles and gradually grow. During this process, other irregularly structured fragments or particles are also formed. In step S2, the electrophilic reagent and protic acid treatment further promotes the formation of nanotubes with a certain degree of helicity through the breaking and recombination of chemical bonds, resulting in curved nanotubes with varying degrees of stability, or even helical structures. Furthermore, in step S2, defect sites in the nanotubes are filled with fragments, and the metal complexation effect promotes the bonding of previously separate nanotubes, resulting in elongated, denser, and more uniform morphologies. The combined and coordinated action of multiple bonds allows the carbon nanotubes to crosslink and form stable structures. In step S3, the dispersant is a silicon carbide compound precursor. In step S4, during the second calcination, silicon carbide coats silicon nanoparticles and carbon nanotubes to form a silicon nanoparticle / carbon nanotube@silicon carbide composite structure. This structure exhibits a robust coating of silicon particles. The dual effects of the inner and outer layers can suppress the volume expansion of silicon materials. At the same time, the overall structure can prevent silicon nanoparticles from contacting the electrolyte, reduce battery side reactions, improve battery initial efficiency and capacity, reduce internal resistance, and improve cycle stability.

[0027] As a further option, the nitrogen source in S1 includes one or more nitrogen-containing heterocycles and their derivatives, guanidine salt derivatives.

[0028] As a further embodiment, the carbon source in S1 includes one or more of biomass carbon sources, carbon powder, graphene, and activated carbon.

[0029] As a further embodiment, the transition metal source in S1 includes at least one of transition metal chloride, transition metal nitrate, transition metal acetate, and transition metal fluoride.

[0030] As a further embodiment, the electrophilic reagent solution in S2 includes at least one of aluminum chloride, aluminum sulfate, boron trifluoride, sulfur trioxide, ferric bromide, titanium bromide, tin chloride, zinc chloride, ferric chloride, and molybdenum pentachloride.

[0031] As a further embodiment, the protic acid solution in step S2 includes at least one of formic acid, trifluoroacetic acid, oxalic acid, hydrochloric acid, sulfuric acid, citric acid, camphor sulfonic acid, and benzoic acid. Without the combined action of electrophilic reagents and protic acids, carbon nanotubes are relatively short and linear. The addition of step S2 increases the helicity, and this increased helicity creates new topological active sites at the tube's bends, thereby improving conductivity.

[0032] As a further embodiment, the dispersant in S2 is a silicone resin emulsion, preferably including a halogenated silicone resin emulsion, more preferably one or more of silicon tetrachloride, trihalogenated alkylsilane, dihalogenated dialkylsilane, and monohalogenated trialkylsilane resin emulsions. Silicone resin emulsion, as a dispersant, is a template that easily forms a uniform cross-linked network structure. As a precursor to silicon carbide, it can maintain the stability of the silicon carbide structure after calcination, which is beneficial for maintaining the negative electrode cycle. On the other hand, the silicon-containing outer layer does not reduce the negative electrode capacity. Furthermore, its excellent coating effect allows silicon nanoparticles and carbon nanotubes to coat each other during the second calcination in step S4, forming a silicon nanoparticle / carbon nanotube@silicon carbide composite structure. In particular, halogenated dispersants, because halogens and silicon easily form covalent bonds, are beneficial for promoting coating.

[0033] As a further embodiment, S1, in addition to being uniformly mixed with the carbon and nitrogen sources, also includes other non-metallic sources, namely one or more of phosphorus and fluorine sources. P and F readily form complex bonds with transition metals, further increasing the number of active sites and also contributing to improved conductivity of carbon nanotube materials and suppression of battery polarization reactions.

[0034] As a further embodiment, the phosphorus source is a phosphorus-containing organic or inorganic compound, and the fluorine source is a fluorine-containing organic or inorganic compound. Typically, but not limitingly, the phosphorus source is at least one of triphenylphosphine, tetraphenylphosphine bromide, sodium 1-butyl-3-methylimidazolium hexafluorophosphate, phosphine, sodium phosphate, sodium phosphite, elemental phosphorus, sodium hydrogen phosphate, sodium dihydrogen phosphate, sodium hydrogen phosphite, and phosphoric acid. The fluorine source is at least one of hydrogen fluoride, lithium fluoride, ammonium fluoride, sodium fluoride, ammonium fluoride, potassium fluoride, aluminum fluoride, magnesium fluoride, sodium fluoride, calcium fluoride, tetrabutylammonium fluoride, triethylmethoxymethylammonium fluoride, ammonium fluoroborate, tetrabutylammonium fluoroborate, and polyvinylidene fluoride.

[0035] As a further embodiment, the molar ratio of carbon, nitrogen, and metal elements in the carbon source, nitrogen source, and transition metal salt in S1 is (5-20):(50-150):(1-5).

[0036] As a further embodiment, when other non-metallic sources are added to S1, the molar ratio of carbon, nitrogen, other non-metallic sources, and transition metal sources containing carbon, nitrogen, other non-metallic sources, and transition metal elements is (5-20):(50-150):(5-50):(1-5).

[0037] As a further embodiment, the mass ratio of A1 material, electrophilic reagent solution and protic acid solution in S2 is 1:(5-10):(10-200).

[0038] As a further embodiment, the mass ratio of the dispersant, silicon nanoparticles, and A2 material in S3 is 100:(15-20):(1-15).

[0039] As a further embodiment, the calcination temperature in S1 is 500℃-800℃, and the calcination time is 2h-10h. If the calcination temperature is too high, the transition metal-nitrogen bonds will be destroyed, the carbon nanotube structure will be damaged, and the battery performance will decrease.

[0040] As a further refinement, in step S1, the calcination temperature is maintained at 550°C for 2 hours, followed by a further increase to 750°C and maintenance for 2 hours. Segmented calcination at different temperatures allows for more complete nucleation and tube growth reactions.

[0041] As a further option, the temperature of the process in S2 is 10℃-100℃, and the reaction time is 1h-20h.

[0042] As a further embodiment, the secondary calcination temperature in S4 is 600℃-800℃, and the calcination time is 2h-8h;

[0043] A third aspect of the present invention is to provide an electrode, battery, or electrochemical device having the aforementioned composite negative electrode material and the composite negative electrode material prepared by the aforementioned preparation method.

[0044] The features and beneficial effects of this invention are as follows:

[0045] (1) The hollow carbon nanotube material of the present invention can adjust its morphological characteristics, enabling the further construction of the electron transport network with a small amount of carbon nanotube material. By adjusting the helicity of the carbon nanotubes, different active sites and specific surface areas can be achieved, which is beneficial to improving the conductivity of the carbon nanotube material and the electrolyte absorption rate; the uniform distribution of carbon nanotube diameter can avoid material agglomeration and promote dispersion, and finally only a small amount of addition is needed to improve the battery resistance, rate performance and cycle performance.

[0046] (2) In this invention, silicon nanoparticles are loaded on carbon nanotubes to form a three-dimensional network structure, which serves as the inner layer of the composite negative electrode material. This improves the volume expansion of silicon and enhances its conductivity, giving full play to the high specific capacity of silicon materials. The outer layer of silicon dioxide covers the inner layer of "silicon nanoparticles / carbon nanotubes", exhibiting a firm structure that covers silicon particles. Both the inner and outer layers can suppress the volume expansion of silicon materials. At the same time, the overall structure can prevent silicon nanoparticles from contacting the electrolyte, reduce battery side reactions, improve battery first efficiency and capacity, reduce internal resistance, and improve cycle stability.

[0047] (3) The method for preparing composite anode materials in this invention is simple, efficient and conducive to large-scale production. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 The images shown are: a 500nm scanning electron microscope (SEM) image, a 100nm transmission electron microscope (TEM) image, and a high-magnification (2nm) transmission electron microscope (HTEM) image of the carbon nanotube material corresponding to Example 1.

[0050] Figure 2 a is a transmission electron microscope (TEM) image of the carbon nanotube material corresponding to Example 2; Figure 2 b is a transmission electron microscope (TEM) image of the carbon nanotube material corresponding to Example 3; Figure 2 c is a transmission electron microscope (TEM) image of the carbon nanotube material corresponding to Comparative Example 2;

[0051] Figure 3 The image shows a scanning electron microscope (SEM) image of the silicon nanoparticle / carbon nanotube@silicon oxide composite anode material prepared in Example 1.

[0052] Figure 4 This is a schematic diagram of the structure of the silicon nanoparticle / carbon nanotube@silicon oxide composite anode material of the present invention, wherein 1 is a carbon nanotube, 2 is a silicon nanoparticle, 3 is a transition metal nanoparticle, and 4 is a silicon oxide micron-sized particle. Detailed Implementation

[0053] To facilitate understanding of the composite anode material of the present invention, a more comprehensive description of the composite anode material of the present invention will be given below, and embodiments of the present invention will be provided, but this does not limit the scope of the present invention.

[0054] Example 1:

[0055] A certain amount of sucrose, dicyandiamine, and ferric nitrate nonahydrate were weighed out according to a carbon, nitrogen, and transition metal molar ratio of 10:100:1 and dissolved in anhydrous ethanol. The resulting solid powder was then calcined at 550°C for 2 hours under a nitrogen atmosphere, followed by a further heating to 750°C for 2 hours to obtain a carbon nanotube precursor. After cooling to room temperature, aluminum trichloride solution and trifluoroacetic acid solution (mass ratio 1:10:200) were added to the carbon nanotube precursor and mixed thoroughly. The mixture was then heated at 45°C for 20 hours, filtered, and dried to obtain the carbon nanotube material.

[0056] Silicon nanoparticles and carbon nanotubes were added to a dispersant methyltrichlorosilane silicone resin emulsion (dispersant:silicon nanoparticles:carbon nanotubes mass ratio of 100:15:15), and ultrasonically stirred and dispersed for 30 minutes to obtain a mixed slurry. Under inert atmosphere, the mixed slurry was calcined twice and heat-treated at 800℃ for 6 hours to obtain the final composite anode material.

[0057] Example 2:

[0058] The carbon nanotube precursor, aluminum trichloride, and trifluoroacetic acid from Example 1 were mixed in a mass ratio of 1:5:200, and all other steps were the same.

[0059] Example 3:

[0060] The heating at 45°C for 20 hours in Example 1 was replaced with heating at 45°C for 10 hours, while everything else remained the same.

[0061] Example 4:

[0062] Replace the trifluoroacetic acid solution in Example 1 with an equal amount of dilute hydrochloric acid solution, and keep everything else the same.

[0063] Example 5:

[0064] Change 100:15:15 in Example 1 to 100:20:1, and keep the rest the same.

[0065] Comparative Example 1:

[0066] The 750℃ in Example 1 was changed to 900℃, and everything else remained the same.

[0067] Comparative Example 2:

[0068] The aluminum trichloride was removed from Example 1, and everything else remained the same.

[0069] Comparative Example 3:

[0070] The silicon nanoparticles and carbon nanotubes obtained in Example 1 were calcined twice and the resulting slurry was heat-treated at 800°C for 6 hours under an inert atmosphere. The dispersant methyltrichlorosilane silicone resin emulsion (mass ratio of dispersant emulsion:silicon nanoparticles:carbon nanotubes was 100:15:15) was then heat-treated at 800°C for 6 hours under an inert atmosphere. The calcined product powders were then mixed uniformly to obtain the final composite anode material. Comparative Example 4:

[0071] Commercially available carbon nanotube material (purchased from Shenyang Huijing Nanotechnology Co., Ltd.) was used instead of the carbon nanotube material in Example 1, and everything else remained the same.

[0072] We also used the obtained composite anode material in the battery and tested the battery:

[0073] Battery manufacturing:

[0074] A negative electrode slurry was prepared by mixing composite negative electrode materials in a mass ratio of 9.6%:1%:0.5%:2.5% (carbon black (SP):SBR (styrene-butadiene rubber):PAA (polyacrylic acid) and then coated onto copper foil and dried to form an electrode sheet.

[0075] Lithium foil is used as the counter electrode.

[0076] Battery assembly: The separator is a 16μm thick PP membrane. The electrolyte is 1M LiPF6 (lithium hexafluorophosphate):EC (ethylene carbonate):EMC (ethyl methyl carbonate):DMC (dimethyl carbonate) (mass ratio of 1:1:1). Coin cells are assembled and constant current charge-discharge tests are performed in the voltage range of 0.005V-0.8V.

[0077] Test process for electrode sheet full charge expansion rate: Take the electrode sheet thickness before battery assembly as L1; discharge the battery with constant current to 0.005V, disassemble the cell and take the electrode sheet to test the electrode sheet thickness L2 at the same full charge state. Electrode sheet full charge expansion rate % = (L2-L1) / L1×100%.

[0078] Table 1 Test results of embodiments and comparative examples of the present invention

[0079]

[0080] The composite anode material obtained by this invention was used in a battery, and a series of tests were conducted. The test results are shown in Table 1. The additives obtained in Examples 1-5 of this invention exhibit significantly better electrical performance in the batteries than those in Comparative Examples 1-4. The main reason is that, firstly, the carbon nanotubes of this invention have a bent or helical morphology, such as... Figures 1-2As shown. At each bend and corner, new active sites can be formed, enabling the construction of a better electron transport network for the silicon anode. When the helical degree increases (…), Figure 1 (SEM image at 100nm) The increased number of active sites strengthens the suppression of negative electrode expansion, reducing capacity reduction caused by material fragmentation during charging and discharging. Under various material combinations and suitable conditions, a superior battery performance is achieved. Therefore, Examples 1-5 of this invention exhibit better electrical performance. Furthermore, the morphological characteristics of the carbon nanotubes in this invention increase the active sites and specific surface area, which is beneficial for improving the electrolyte absorption rate of the carbon nanotube material, thereby improving the wettability of the negative electrode, enhancing battery cycle performance, suppressing side reactions between the negative electrode material and the electrolyte, reducing gas production, and improving battery safety. In this invention, transition metal nanoparticles serve as nucleation sites for the carbon nanotube material, located at the ends of the carbon nanotubes. We can... Figure 1 and Figure 2 Discover.

[0081] In the method of this invention, we further added an electrophilic reagent solution and a protic acid solution. We found that the mixed treatment of the electrophilic reagent solution and the protic acid, on the one hand, allows irregular ring fragments such as incompletely tubed particles, chains, five-membered or seven-membered rings, etc., to further promote the formation of carbon nanotubes with a certain degree of helicity through the breaking and recombination of chemical bonds, even forming stable helical shapes to varying degrees; on the other hand, it can also fill the defect sites in the carbon nanotubes with fragments, and the metal complexation effect promotes the bonding of originally separate tubes together, resulting in tube elongation, a denser and more uniform morphology. Furthermore, the combined and coordinated effect of multiple bonds enables the carbon nanotubes to crosslink and form a stable structure, such as... Figure 2 As shown. Comparing Example 1 and Comparative Example 2, after the electrophilic reagent was removed, it did not interact with the protic acid, which is not conducive to the formation of long nanotubes and reduces battery performance. Figure 2 c). Further comparison of Examples 1 and 4 shows that the electrical performance of Example 1 is superior to that of Example 4. We believe this is because the strong polarity of fluorine in trifluoroacetic acid promotes the formation of complex bonds, which in turn facilitates the formation of multiple helices in carbon nanotubes. Examples 1-3 demonstrate that reducing the amount of electrophilic reagent or decreasing the reaction temperature reduces the electrical performance. Sufficient reagents and conditions are conducive to further helical growth of nanotubes, thus maximizing battery performance.

[0082] In the method of this invention, both the outer layer of silicon dioxide and the inner layer of silicon nanoparticles loaded on carbon nanotubes are indispensable. A comparison of Example 1 and Comparative Example 3 reveals that without the outer layer of silicon dioxide coating, the electrochemical performance of the materials significantly decreases. We believe that the three-dimensional network structure formed by silicon nanoparticles loaded on carbon nanotubes improves the conductive path and extends cycle life. However, the outer layer of silicon dioxide further suppresses the volume expansion of the silicon material, prevents contact between the silicon nanoparticles and the electrolyte, reduces side reactions, and increases initial capacity efficiency and cycle performance. A comparison of Example 1 and Comparative Example 4 shows that the highly helical carbon nanotubes obtained by this invention exhibit significantly improved performance compared to commercially available carbon nanotubes. A comparison of Example 1 and Example 5 reveals that although reducing the relative content of carbon nanotubes lowers the electrical performance, it still shows a significant advantage over the batteries in Comparative Examples 1-4, indicating that even a small amount of carbon nanotubes can promote improved negative electrode performance.

[0083] Finally, the formation of carbon nanotubes is inseparable from the sintering process. By comparing Examples 1-5 with Comparative Example 1, we found that the electrical performance of the batteries in Examples 1-5 was superior to that in Comparative Example 1. We believe this is because when the temperature exceeds 800℃, most metal ions do not maintain coordination with the original nanotube structure, the metal-nitrogen bonds are broken, more fragments are formed, and it is not easy to form a tubular structure, leading to a decrease in performance.

[0084] In summary, the present invention uses bent or highly helical carbon nanotubes with a certain degree of helicity to load silicon nanoparticles in composite anode materials, and uses silicon oxide as an outer coating layer, which has the advantages of high silicon loading and high specific capacity.

[0085] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a composite negative electrode material, characterized in that, Includes the following steps: S1: Grind a mixture of carbon source, nitrogen source and transition metal source, and calcine it once under an inert gas to obtain material A1; S2: The A1 material obtained in S1 is mixed with an electrophilic reagent and a protic acid solution, and then dried to obtain A2 carbon nanotube material; S3: Add silicon nanoparticles and A2 carbon nanotubes to the dispersant and ultrasonically stir to disperse them evenly to obtain mixture A3; S4: Under inert atmosphere conditions, A3 is calcined a second time to obtain the final composite anode material product; The nitrogen source in S1 includes one or more of nitrogen-containing heterocycles and their derivatives, and guanidine salt derivatives; The electrophilic reagent in S2 includes one or more of the following: aluminum chloride, aluminum sulfate, boron trifluoride, sulfur trioxide, ferric bromide, titanium bromide, tin chloride, zinc chloride, ferric chloride, and molybdenum pentachloride. The protic acid solution in S2 includes one or more of formic acid, trifluoroacetic acid, oxalic acid, hydrochloric acid, sulfuric acid, citric acid, camphor sulfonic acid, and benzoic acid. The dispersant in S3 is a silicone resin emulsion; The composite anode material includes silicon nanoparticles, carbon nanotubes, and silicon oxide materials. The carbon nanotubes are curved or spiral-shaped, and transition metal nanoparticles are present at the ends of the carbon nanotubes. The carbon nanotube materials have a hollow structure. The silicon nanoparticles are loaded on the carbon nanotubes as the inner layer of the composite anode material, and the silicon dioxide carbon is used as the outer layer of the composite anode material. The calcination temperature in S1 is 500℃-800℃; The secondary calcination temperature in S4 is 600℃-800℃.

2. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The diameter of the carbon nanotubes is 5nm-100nm.

3. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The diameter of the carbon nanotubes is 20nm-40nm.

4. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The pitch of the carbon nanotubes is 90nm-100nm.

5. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The helix angle of the carbon nanotube material is 30°-120°.

6. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The aspect ratio of the carbon nanotube material is not less than 3.

7. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The ratio of the particle size of the transition metal nanoparticles to the diameter of the carbon nanotubes is (1-1.5):(1-1.5).

8. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The transition metal nanoparticles include one or more transition metals.

9. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The transition metal includes one or more of iron, cobalt, nickel, copper, and manganese.

10. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The transition metal is one or both of iron and copper.

11. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The carbon source in S1 includes one or more of biomass carbon sources, carbon powder, graphene, and activated carbon; The transition metal source in S1 includes one or more of transition metal chlorides, transition metal nitrates, transition metal acetates, and transition metal fluorides. The dispersant in S3 includes a halogenated silicone resin emulsion.

12. The method for preparing the composite negative electrode material according to claim 11, characterized in that, The halogenated silicone resin emulsion is one or more of silicon tetrachloride, trihalogenated alkylsilane, dihalogenated dialkylsilane, and monohalogenated trialkylsilane resin emulsions.

13. The method for preparing the composite negative electrode material according to claim 1, characterized in that, In addition to being mixed with carbon and nitrogen sources, S1 also includes other non-metallic sources, which are one or more of phosphorus and fluorine sources. The phosphorus source is a phosphorus-containing organic or inorganic compound, and the fluorine source is a fluorine-containing organic or inorganic compound. The molar ratio of carbon, nitrogen, and transition metal elements in the carbon source, nitrogen source, and transition metal salt in S1 is (5-20):(50-150):(1-5); The mass ratio of A1 material, electrophilic reagent and protic acid solution in S2 is 1:(5-10):(10-200); The mass ratio of the dispersant, silicon nanoparticles, and A2 carbon nanotube material in S3 is 100:(15-20):(1-15).

14. The method for preparing the composite negative electrode material according to claim 13, characterized in that, The phosphorus source is at least one of triphenylphosphine, tetraphenylphosphine bromide, sodium 1-butyl-3-methylimidazolium hexafluorophosphate, phosphine, sodium phosphate, sodium phosphite, elemental phosphorus, sodium hydrogen phosphate, sodium dihydrogen phosphate, sodium hydrogen phosphite, and phosphoric acid. The fluorine source is at least one of hydrogen fluoride, lithium fluoride, ammonium fluoride, sodium fluoride, potassium fluoride, aluminum fluoride, magnesium fluoride, sodium fluoride, calcium fluoride, tetrabutylammonium fluoride, triethylmethoxymethylammonium fluoride, ammonium fluoroborate, tetrabutylammonium fluoroborate, and polyvinylidene fluoride.

15. The method for preparing the composite negative electrode material according to claim 1, characterized in that, The calcination time in S1 is 2h-10h; The secondary calcination time in S4 is 2h-8h; The temperature of the process in S2 is 10℃-100℃, and the reaction time is 1h-20h.

16. The method for preparing the composite negative electrode material according to claim 1, characterized in that, In S1, the calcination temperature is maintained at 550℃ for 2 hours, and then the temperature is further increased to 750℃ and maintained for 2 hours.

17. The application of a composite negative electrode material prepared by the preparation method according to any one of claims 1-16 in an electrode, battery or electrochemical device.

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