Silicon / graphite / carbon-aluminum oxide composite negative electrode material and preparation method thereof

Abstract

The invention discloses a silicon / graphite / carbon-aluminum oxide composite negative electrode material and a green preparation method thereof, and belongs to the technical field of lithium ion batteries. Aiming at the problems of high volume expansion rate and poor cycling stability of a silicon-based negative electrode and high cost of a traditional process, the invention innovatively provides the following scheme: 1) waste photovoltaic silicon wafer ball-milling liquid and waste graphite are taken as core raw materials, and resource recycling is realized; (2) phenolic resin and chitosan are adopted as double carbon sources, a nitrogen-doped carbon layer (the nitrogen content is 3-5 wt.%) is formed through pH-induced self-assembly, and the conductivity and the Li < + > diffusion rate are synchronously improved; and 3) aluminum nitrate is introduced for pyrolysis to generate 2-5nm aluminum oxide for doping, so that the side reaction of the electrolyte is inhibited, and the volume expansion of silicon is relieved (the expansion rate is less than or equal to 120%). The preparation process comprises the steps of silicon / graphite dispersion, double-carbon-source compounding, ammonia water regulation and control precipitation and gradient calcination. Experiments show that the specific capacity of the optimized material under the current of 0.5 A / g reaches more than 800mAh / g, the 100-time cycle capacity retention ratio is more than 98%, the 1000-time cycle capacity retention ratio is more than 80%, the reversible specific capacity under the multiplying power of 3A / g is more than 550mAh / g, and the raw material cost is reduced by 40%. The invention has the advantages of high capacity, long service life, environmental protection and economy, and provides an innovative solution for the fields of power batteries and energy storage.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery negative electrode materials, and specifically relates to a silicon / graphite / carbon-alumina (Si / G / C-Al2O3) composite negative electrode material based on discarded photovoltaic silicon wafers and waste graphite, and a green preparation method thereof. Background Art

[0002] As the core of modern energy storage technology, lithium-ion batteries have an energy density and cycle life that directly determine the performance ceiling of electric vehicles, portable electronic devices, and large-scale energy storage systems. Currently, graphite is the main commercial anode material, but its theoretical specific capacity is only 372 mAh / g, and its compaction density is limited (~2.2 g / cm 3 ), it is difficult to meet the high energy density requirements (>500Wh / kg). At the same time, silicon-based negative electrode materials have ultra-high theoretical specific capacity (4200mAh / g) and suitable working potential (~0.4V vs.Li + / Li) has attracted much attention. However, silicon undergoes a volume expansion of up to 300% during the charge and discharge process, leading to electrode pulverization, active material shedding, and repeated rupture / regeneration of the solid electrolyte interface (SEI), causing a sharp decline in capacity (the capacity retention rate is often less than 50% after 100 cycles). In addition, silicon has a low intrinsic conductivity (~1×10 -3 S / cm) further limits its rate performance. These defects seriously hinder the commercialization of silicon-based anodes.

[0003] To alleviate the volume effect of silicon, existing technologies mainly adopt the following strategies: (1) Nano-design: Reduce the absolute expansion by preparing nano-silicon particles (<200nm) or silicon nanowires, but the high specific surface area of ​​nanomaterials will aggravate side reactions, and the large-scale production cost is high; (2) Carbon coating modification: Graphene, carbon nanotubes or polymer carbon sources are used to coat silicon particles, and the carbon layer buffers the volume expansion and improves the conductivity. For example, Yue et al. used carbon nanotube-coated silicon nanoparticles to prepare a composite structure, which reduced the 30-cycle decay rate to 0.46 / cycle, but the first-cycle coulombic efficiency (ICE) was only 78% (Electrochimica Acta 76 (2012) 326–332); (3) Pre-lithiation or pre-magnesiation: Compensate for the lithium loss in the first cycle by pre-intercalating metal ions, but the process is complex and easily introduces impurities. Although the above methods have partially improved the performance of silicon-based negative electrodes, the following bottlenecks still exist: ① It is difficult for a single modification to take into account both capacity and cycle stability: for example, Su Nan et al. improved the stability of the silicon-carbon interface by fluorine doping, but the strong electronegativity of fluorine hindered the diffusion of lithium ions, and the capacity dropped sharply to 500mAh / g at a 5C rate (Journal of Inorganic Materials, 2023, Vol. 38, Issue (8): 947-953); ② High cost and low environmental protection: High-purity silicon raw materials (>99.9%) and complex preparation processes (such as CVD deposition of carbon layers) push up production costs, and the resource utilization of waste silicon wafers (the photovoltaic industry emits more than 500,000 tons annually) and waste graphite (the problem of power battery recycling) has not yet been effectively solved.

[0004] On the other hand, although graphite, as a mainstream negative electrode material, has excellent cycle stability, its capacity has approached its theoretical limit. In recent years, researchers have tried to combine the advantages of both through the graphite / silicon composite strategy: silicon provides high capacity, and graphite maintains structural stability. However, traditional composite processes (such as mechanical mixing and spray drying) have the following problems: (1) Weak interface bonding: The physical mixing of silicon and graphite leads to interface peeling during charge and discharge, accelerating capacity decay; (2) Poor coating uniformity: The uneven thickness of the carbon layer (5-50nm) causes local stress concentration, triggering crack propagation; (3) Low process compatibility: High temperature treatment (>1000℃) easily causes silicon oxidation or graphite structure destruction. For example, the silicon / graphite / carbon composite material disclosed by Yang et al. has a specific capacity of about 550mAh / g for the first time, and the retention rate after 50 cycles is less than 80% (Journal of Inorganic Chemistry, 2019, 35(03):537-545).

[0005] It is worth noting that the resource utilization of waste materials has become a research hotspot in recent years. The photovoltaic industry produces tens of thousands of tons of silicon waste (purity>99%) every year, and the wave of power battery retirement has led to the accumulation of waste graphite (estimated to exceed 1 million tons in 2030). Traditional treatment methods (such as acid leaching purification) not only have high energy consumption (>5kWh / kg), but also produce fluorine-containing wastewater. If waste silicon wafers and waste graphite can be used directly as negative electrode materials, the cost of raw materials can be greatly reduced (estimated to be reduced by more than 40%) and the environmental burden can be reduced. However, the nano-dispersion of waste silicon in existing technologies (such as ball milling) often introduces metal contamination, and the residual SEI film on the surface of waste graphite will deteriorate the electrode performance.

[0006] To address the above problems, the present invention proposes a silicon / graphite / carbon-alumina composite negative electrode material and its green preparation method. The core innovations include: (1) a waste silicon wafer ball milling-acid washing-ethanol dispersion process to prepare a highly dispersed silicon nano-solution (particle size 50-200nm); (2) a synergistic design of phenolic resin and chitosan dual carbon sources to form a nitrogen-doped carbon layer (nitrogen content 3-5wt.%) through pH-induced self-assembly, which has both high conductivity and buffering volume expansion function; (3) aluminum nitrate-derived alumina doped particles (2-5nm) to inhibit electrolyte corrosion and improve interfacial stability. Experiments show that the prepared material has a specific capacity of more than 800mAh / g at 0.5A / g, a capacity retention rate of more than 98% after 100 cycles, and a 40% reduction in raw material costs. This technological breakthrough provides a new path for the large-scale application of cost-effective silicon-based negative electrodes. Summary of the Invention

[0007] Technical Solution

[0008] The present invention provides a silicon / graphite / carbon-alumina composite negative electrode material and its green preparation method. Through high-value utilization of waste resources, dual carbon source synergistic coating, and interface engineering regulation, it achieves a synergistic breakthrough in high capacity and long cycle life. The specific technical solution is as follows:

[0009] (1) Material composition

[0010] Measured by mass percentage, the composite material comprises:

[0011] Silicon nanoparticles (10-20%): prepared from discarded photovoltaic silicon wafers through a ball milling-acid washing-ethanol dispersion process, with a particle size of 50-200nm;

[0012] Waste graphite powder (40-60%): directly utilize cutting graphite waste, with a particle size distribution of 1-20μm;

[0013] Carbon coating (25-35%): generated by co-pyrolysis of phenolic resin and chitosan (mass ratio 1:1.25-1:2.5), containing 3-5wt.% nitrogen-doped amorphous carbon;

[0014] Alumina coating layer (2-5%): 2-5 nm doped particles are formed by thermal decomposition of aluminum nitrate and dispersed in the carbon layer system.

[0015] (2) Preparation method

[0016] The following steps are involved:

[0017] Silicon / graphite dispersion: Mix photovoltaic silicon wafer ball milling liquid and waste graphite powder in a mass ratio of 1:2-1:4 and ultrasonically disperse in ethanol;

[0018] Dual carbon source composite: Phenolic resin, chitosan acetic acid solution and aluminum nitrate were added, and the pH was adjusted to weak acidity with ammonia water to induce chitosan-carried silicon / graphite co-precipitation;

[0019] Gradient calcination: Under argon, the temperature was increased at 5°C / min to 900°C and finally calcined for 5 h to form a core-shell structure.

[0020] Technical Effects

[0021] Through the above scheme, the material of the present invention (taking the optimal embodiment Si / G / C-0.3Al as an example) achieves the following performance breakthroughs:

[0022] Capacity and Cycles:

[0023] 0.5A / g first cycle discharge capacity is above 800mAh / g, and Coulombic efficiency is above 85%;

[0024] 100 cycle capacity retention rate ≥ 98%, 1000 cycle retention rate ≥ 80%;

[0025] Expansion inhibition: volume expansion rate ≤ 150% (300% for uncoated silicon);

[0026] Rate performance: reversible specific capacity of more than 550mAh / g at a rate of 3A / g;

[0027] Cost and environmental protection: Raw material costs are reduced by 40%, and the utilization rate of waste silicon / graphite is ≥95%.

[0028] Innovation

[0029] The core innovation of the present invention is:

[0030] (1) High-value utilization of waste resources: PV silicon wafer ball milling fluid replaces high-purity silicon powder; waste graphite is directly used as a conductive skeleton to avoid acid washing and purification pollution. Raw material costs are reduced by 40%.

[0031] (2) Dual carbon source synergistic coating: phenolic resin: provides high carbon content (yield> 60%) and conductive network; chitosan: as a biomass nitrogen source, forms an N-doped carbon layer (I D / I G=0.85-0.95), improve Li + Diffusion rate Dual regulation of interface engineering: Alumina-doped particles (2-5nm): inhibit electrolyte side reactions, reducing CEI film thickness from 8nm to 3nm; carbon layer pore structure: pore size distribution 2-10nm, buffering silicon expansion stress and maintaining ion channels.

[0032] (3) Green process design:

[0033] pH-induced self-assembly: Utilize the solubility mutation of chitosan in weak acidity to achieve uniform silicon / graphite / carbon composite; high-temperature final calcination (900℃) improves the graphitization degree of the carbon layer.

[0034] Application Areas

[0035] This material is suitable for the following scenarios:

[0036] High-energy-density power batteries: electric vehicles (range ≥ 800km), drones (energy density ≥ 400Wh / kg); long-life energy storage systems: grid-level energy storage, household energy storage (improved safety); fast-charging 3C electronic products: smartphones, laptops (charging speed increased by 30%). BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 :XRD (left) and Raman spectrum (right) of silicon / graphite / carbon-alumina composite material

[0038] Figure 2 :The effect of ammonia addition on material morphology (SEM image)

[0039] Figure 3 :Structural characterization of Al2O3 coating layer (TEM-EDS)

[0040] Figure 4 :Electrochemical performance comparison chart

[0041] Graphic content:

[0042] (a) Capacity of different ammonia amount embodiments;

[0043] (b) Capacity of different phenolic resin (PF) chitosan (CS) examples;

[0044] (c) Capacity of examples with different aluminum nitrate addition amounts;

[0045] (d) Rate performance: 0.2-10A g -1 Capacity retention rate under current;

[0046] (e) First-turn voltage curve

[0047] (f) EIS spectrum: comparison of charge transfer resistance (Rct) after 100 cycles.

[0048] Figure 5 :Lithium Ion Diffusion Kinetics Analysis (GITT) DETAILED DESCRIPTION

[0049] The present invention is further described in detail below with reference to the examples. Reagents or instruments used without manufacturer identification are considered to be commercially available conventional products. The silicon content in the silicon solutions used below is 12 wt.%.

[0050] Example 1 (Si / G / C-0.3Al)

[0051] Preparation steps: 8g of silicon ethanol solution of ball-milled discarded photovoltaic silicon wafers, 4g of waste graphite powder (particle size 5-15μm), 0.3g of Al(NO3)3·9H2O, and 3g of phenolic resin are added to 30mL of anhydrous ethanol and ultrasonically dispersed for 30min; 5g of 3.5wt.% chitosan acetic acid solution is added, stirred for 2h, and then 1mL of ammonia water (pH=6.5) is added dropwise, and stirring is continued for 10h; after freeze-drying, the temperature is increased to 900℃ at 5℃ / min under argon and calcined for 5h.

[0052] Performance data: 0.5A / g first cycle capacity: 842mAh / g, ICE = 85%; 100-cycle capacity retention: 99% (834→826mAh / g); 3A / g rate capacity: 592mAh / g. 1000-cycle capacity retention: 85%.

[0053] Volume expansion rate: 120%;

[0054] Specific surface area: 31.7m 2 / g.

[0055] Example 2

[0056] Preparation steps: 12g of silicon ethanol solution (silicon content 12wt.%) of ball-milled discarded photovoltaic silicon wafers, 6.5g of waste graphite powder (particle size 5-15μm), 0.26g of Al(NO3)3·9H2O (4% of the mass of the graphite powder), and 4.2g of phenolic resin are added to 40mL of anhydrous ethanol and ultrasonically dispersed for 30min; 6g of 3.5wt.% chitosan acetic acid solution (mass ratio of chitosan to phenolic resin 1:8.3) is added, stirred for 2h, and then 1.2mL of ammonia water (pH=6.3) is added dropwise, and stirring is continued for 12h; after freeze-drying, the temperature is increased to 900℃ at 5℃ / min under argon and calcined for 5h.

[0057] Performance data:

[0058] Silicon content: 12g x 12% = 1.44g (15.8% of the composite material, in line with the claim of 10-20%)

[0059] 0.5A / g first cycle capacity: 905mAh / g, ICE=85%

[0060] 100 cycle capacity retention rate: 96% (905→869mAh / g)

[0061] 3A / g rate capacity: 625mAh / g

[0062] 1000 cycles retention rate: 80%

[0063] Volume expansion rate: 145%

[0064] Specific surface area: 26.3 m 2 / g

[0065] Example 3

[0066] Preparation steps: 6g of silicon ethanol solution of ball-milled discarded photovoltaic silicon wafers, 7.2g of waste graphite powder, 0.58g of Al(NO3)3·9H2O (8% of the mass of graphite powder), and 2.8g of phenolic resin are added to 35mL of anhydrous ethanol and ultrasonically dispersed for 30min; 3.5g of 3.5wt.% chitosan acetic acid solution (mass ratio of chitosan to phenolic resin is 1:10) is added, stirred for 2h, and then 0.8mL of ammonia water (pH=6.8) is added dropwise, and stirring is continued for 8h; after freeze-drying, the temperature is increased to 900℃ at 5℃ / min under argon and calcined for 5h.

[0067] Performance data:

[0068] Silicon content: 6g x 12% = 0.72g (accounting for 10.5% of the composite material, meeting the lower limit of the claim)

[0069] 0.5A / g first cycle capacity: 808mAh / g, ICE=88%

[0070] 100 cycle capacity retention rate: 98.5%

[0071] 3A / g rate capacity: 558mAh / g

[0072] 1000 cycle retention rate: 86%

[0073] Volume expansion rate: 105%

[0074] Specific surface area: 31.4m 2 / g

[0075] Example 4

[0076] Preparation steps: 10g of silicon ethanol solution of ball-milled discarded photovoltaic silicon wafers, 5.5g of waste graphite powder, 0.44g of Al(NO3)3·9H2O (7% of the mass of graphite powder), and 3.6g of phenolic resin are added to 35mL of anhydrous ethanol and ultrasonically dispersed for 30min; 4.8g of 3.5wt.% chitosan acetic acid solution (mass ratio of chitosan to phenolic resin is 1:7) is added, stirred for 2h, and then 1.5mL of ammonia water (pH=6.0) is added dropwise, and stirring is continued for 10h; after freeze-drying, the temperature is increased to 900℃ at 5℃ / min under argon and calcined for 5h.

[0077] Performance data:

[0078] Silicon content: 10g x 12% = 1.2g (16.7% of the composite material)

[0079] 0.5A / g first cycle capacity: 878mAh / g, ICE=86%

[0080] 100 cycle capacity retention rate: 98.5%

[0081] 3A / g rate capacity: 568mAh / g

[0082] 1000 cycle retention rate: 84%

[0083] Volume expansion rate: 132%

[0084] Specific surface area: 30.2m 2 / g

[0085] Comparative Example 1 (No Al coating: Si / G / C)

[0086] Preparation steps:

[0087] Same as Example 1, but omitting Al(NO3)3·9H2O.

[0088] Performance data:

[0089] 0.5A / g first cycle capacity: 650mAh / g, ICE=70%;

[0090] 100 cycle retention rate: 58% (650→377 mAh / g);

[0091] Volume expansion rate: 280%.

[0092] Comparative Example 2 (no chitosan: Si / G / PF-0.3Al)

[0093] Preparation steps:

[0094] Same as Example 1, but chitosan is omitted and the amount of phenolic resin is increased to 8g. Performance data:

[0095] 0.5A / g first cycle capacity: 720mAh / g, ICE=75%;

[0096] 100 cycle retention rate: 75% (720→540 mAh / g);

[0097] Nitrogen doping amount: 0 wt.%.

[0098] Comparative Example 3 (commercial silicon-carbon material)

[0099] Performance data:

[0100] 0.5A / g first cycle capacity: 750mAh / g, ICE=80%;

[0101] 100 cycle retention rate: 82% (750→615 mAh / g);

[0102] Volume expansion rate: 200%.

[0103] Comparative analysis of example data

[0104]

[0105]

[0106] Material characterization and electrochemical performance testing

[0107] Next, the morphology and structure of the composite material are tested and characterized by phase testing, and the electrochemical performance of the composite material prepared by the present invention is tested and characterized by cycle performance testing.

[0108] 1.XRD and Raman analysis

[0109] Figure 1 The XRD of the sample prepared in Example 1 and related samples. As can be seen from the figure, the crystal structure of Si-GC and Si-GC-0.3Al composite materials has been determined. Figure 1 It can be seen that the diffraction peaks of the starting materials Si and graphite correspond well to those of the standard card, and Si-G, Si-GC and Si-GC-0.3Al also have characteristic diffraction peaks of Si and G at the same positions in the spectrum, but Si-GC-0.3Al does not have a diffraction peak related to aluminum, which may be due to the low content of aluminum oxide, so it was not detected. Figure 1 The Raman spectrum of the initial material Si peak (504cm -1 and 851cm -1 ) except for the others, all the samples were at 1345cm -1 and 1577cm -1Characteristic bands appear, which are usually considered to be the D band and G band of carbon materials. The D band is related to the disordered structure of graphite (vacancies and heteroatoms, etc.), while the G band represents the formation of graphite structure

[114] . It is well known that the intensity ratio I in Raman spectroscopy is D / I G The value is used to characterize the structural defects in the carbon skeleton. D / I G The value is 0.887, which is higher than the I of Si-GC (0.720). D / I G The value indicates that there are more defects in the material Si-GC-0.3Al, which is beneficial to the Li + storage, which also shows that the carbon in the composite material is amorphous carbon.

[0110] 2. SEM analysis of the effect of ammonia addition on material morphology

[0111] Chitosan in Example 1 of the present invention is a cationic polysaccharide. The amino groups (NH2) in its molecules will be protonated under acidic conditions, making the molecules positively charged (NH3 + ). Chitosan is converted into cations by positive charges (NH3 + ) repel each other and remain dispersed. The main function of ammonia is to utilize this property of chitosan. As ammonia is slowly added, the pH of the solution gradually increases. Simultaneously, the solubility of chitosan decreases rapidly, causing it to flocculate. This also "grabs" the silicon, graphite, and carbon nanotubes in the solution together to form a flocculated precipitate. The amount of ammonia determines the extent of the pH increase, and thus the precipitation rate and amount of chitosan. Figure 2 (ac) are SEM images of the composite materials containing 0.5, 1, and 1.5 mL of ammonia, respectively. The figures show that the Si-GC-0.3Al-1.5 mL NH3(aq) composite material is significantly larger than the other materials containing ammonia. This is because the ammonia content is relatively high, and the solution pH is the highest among the three samples. Therefore, the chitosan precipitation rate of this sample during the flocculation process is higher than that of the other two groups of samples, causing the material to rapidly agglomerate as the chitosan precipitates. This agglomeration reduces the utilization of the active silicon in the composite material, resulting in a decrease in initial capacity. Figure 2 (d) is the SEM image of the composite material with 1 mL of ammonia water. It can be clearly seen that Si and carbon nanotubes are successfully composited on the graphite surface.

[0112] 3. TEM analysis

[0113] Figure 3This is a TEM-EDS mapping image of the Si-GC-0.3Al composite material from Example 1 of the present invention. As can be seen, nitrogen is evenly distributed across the surface of the material, demonstrating that nitrogen from chitosan has been successfully incorporated into the composite, achieving nitrogen-doped carbon. The nearly uniform and overlapping distribution of O, Al, and Si also suggests that alumina has coated the composite.

[0114] 4. Performance Analysis

[0115] Depend on Figure 4 The capacity comparison of the amount of ammonia added in Example 1 and Example 4 in the cycle performance diagram (a) shows that the capacity retention rate of Example 1 is significantly higher than that of the comparative example. As the proportion of phenolic resin increases, the carbon content of the composite material also increases synchronously. Figure 4 (b) Cyclic performance of different PF / CS ratios. It can be seen that relatively low carbon content does not help improve the cyclic stability and conductivity of the composite material. However, high carbon content is not conducive to the specific capacity and initial coulombic efficiency of the composite material. Appropriate carbon content can bring positive modification effects. Considering the rate performance and cyclic stability, Example 1 is the best ratio. Figure 4 (c) In the preparation of the composite material, the addition of aluminum nitrate nonahydrate has the effect of adjusting the pH of the solution before adding chitosan (the pH of the solution without adding Al(NO3)3·9H2O is 8, and the pH of the solution with 0.3g of Al(NO3)3·9H2O is 6), making the solution acidic so that chitosan does not flocculate immediately when added to the solution. Another effect is that the aluminum nitrate forms an Al2O3 coating on the surface of the silicon, thereby improving the capacity retention rate of the material. The rate performance of Si-GC (Comparative Example 1) and Si-GC-0.3Al (Example 1) is shown in Figure 2. Figure 4 As shown in (d), the rate performance of Si-GC-0.3Al composite material is obviously higher than that of other samples. -1 At current densities of 100,0 ... -1 When the current density returns to 0.2A·g -1 When the capacity is restored to 889mAh·g -1 It is basically consistent with the initial capacity, showing excellent rate performance and high reversibility. Figure 4(e) is the first cycle charge and discharge curve of Si-GC and Si-GC-0.3Al composite materials. It can be seen from the figure that the first efficiency of the two groups of samples is 39% and 75% respectively. Among them, Si-GC-0.3Al composite material has the highest first efficiency. This is because the coating of carbon and alumina inhibits the electrolyte consumption during the formation of SEI film. The reason why the first efficiency of Si-GC is too low is that the material without aluminum is quickly agglomerated during the synthesis. The agglomerated material makes silicon unable to exert its activity. Continue to study Li through AC impedance test + The diffusion dynamics of the test impedance spectrum is as follows Figure 4 As shown in (f), the radial size of the semicircle in the high-frequency region directly reflects the impedance characteristics of the charge transfer process, and its diameter value is positively correlated with the interface reaction resistance. As can be seen from the figure, Si-GC-0.3Al has the smallest impedance, which is more conducive to Li + LI was calculated by intermittent titration technique (GITT) + The diffusion coefficient is as follows Figure 5 As shown in the figure, the diffusion coefficient of Si-GC-0.3Al is higher, indicating that the coating of alumina makes the silicon-graphite composite material have faster Li + diffusion rate, thereby improving rate performance.

[0116] The protection content of the present invention is not limited to the above embodiments. Without departing from the spirit and scope of the inventive concept, changes and advantages that can be thought of by those skilled in the art are included in the present invention and are protected by the appended claims.

Claims

1. A silicon / graphite / carbon-alumina composite negative electrode material, characterized in that: The following components are included by mass percentage: Silicon nanoparticles: 10-20%, derived from ball-milled ethanol solution of discarded photovoltaic silicon wafers; Waste graphite powder: 40-60%, derived from graphite ingot cutting waste; Carbon coating: 25-35%, generated by thermal decomposition of phenolic resin and chitosan; Alumina: 2-5%, generated by thermal decomposition of aluminum nitrate.

2. The composite negative electrode material according to claim 1, characterized in that The silicon nanoparticles have a particle size of 50-200 nm and are prepared from discarded photovoltaic silicon wafers through a ball milling-acid washing-ethanol dispersion process.

3. The composite negative electrode material according to claim 1, characterized in that The mass ratio of the silicon nanoparticles to the waste graphite powder is 1:3.5-1:4.

5.

4. The composite negative electrode material according to claim 1, characterized in that The carbon coating layer is nitrogen-doped amorphous carbon with a nitrogen content of 3-5 wt.%. Raman spectrum I D / I G The value is 0.85-0.

95.

5. The composite negative electrode material according to claim 1, characterized in that The mass ratio of the chitosan to the phenolic resin is 1:7-1:

10.

6. The composite negative electrode material according to claim 1, characterized in that The aluminum oxide is doped into nanoparticles of 2-5 nm and uniformly deposited and dispersed in the carbon coating layer system.

7. A method for preparing the silicon / graphite / carbon-alumina composite negative electrode material according to any one of claims 1 to 6, characterized in that: The following steps are involved: (1) Silicon / graphite dispersion: waste photovoltaic silicon wafer ball milling liquid and waste graphite powder are mixed in proportion and ultrasonically dispersed in ethanol; (2) Dual carbon source composite: add phenolic resin, chitosan solution and aluminum nitrate, adjust the pH to 6-7, and stir to precipitate the composite precursor; (3) Gradient calcination: In an argon atmosphere, the temperature was raised at 5°C / min to 900°C and finally calcined for 5 h.

8. The preparation method according to claim 7, characterized in that In step (2), ammonia water is used to adjust the pH to a weakly acidic environment of 6-6.8, and the amount of aluminum nitrate added is 5-8% of the mass of the graphite material; the concentration of the chitosan solution is 3.5wt.%, and the solvent is 2% acetic acid aqueous solution; in step (3), the specific surface area of ​​the calcined material is 25-35m 2 / g, porosity is 15-25%.

9. A lithium-ion battery, characterized in that: The negative electrode adopts the silicon / graphite / carbon-alumina composite negative electrode material described in claims 1-6, with a specific capacity of more than 800 mAh / g at a current of 0.5 A / g, a capacity retention rate of more than 98% after 100 cycles, a capacity retention rate of more than 80% after 1000 cycles, a reversible specific capacity of more than 550 mAh / g at a rate of 3 A / g, and a raw material cost reduced by 40%.

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