A method for preparing a porous silicon-carbon negative electrode by using photovoltaic waste silicon and calcium carbonate template method and application

Abstract

The application relates to the technical fields of inorganic chemical industry and battery materials, and discloses a method for preparing a porous silicon-carbon negative electrode by a calcium carbonate template method from photovoltaic waste silicon and application. The technical scheme is as follows: the photovoltaic waste silicon is subjected to acid pickling, washing, drying and ball milling to obtain nanoscale silicon powder; the nanoscale silicon powder is combined with calcium carbonate to construct a silicon@calcium carbonate composite template at a mass ratio of 1:5; the composite template and citric acid are dispersed in ethanol at a mass ratio of 1:3 to 1:9, ultrasonic treatment is carried out, the ethanol is evaporated, carbonization is carried out in an inert atmosphere, and a silicon@calcium carbonate@carbon composite is obtained; the calcium carbonate is removed by acid etching to prepare the porous silicon-carbon negative electrode. The technical effect is that the photovoltaic waste silicon is converted into an electrode material, the silicon recovery rate is more than 92%, the porous structure formed by the calcium carbonate template controls the volume expansion rate to be less than 25%, the carbon layer coating reduces the side reaction, the battery cycle life is prolonged, and the raw material cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of inorganic chemical engineering and battery materials technology, and discloses a method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method, as well as its application. Background Technology

[0002] The photovoltaic industry generates a large amount of waste silicon, which is currently disposed of through landfill or incineration. Some recycling processes use high-temperature roasting combined with single-acid-base leaching to extract silicon, which is then sold as a primary product. In the field of porous silicon-carbon anode preparation, high-purity silicon is typically used as the raw material. Pores are created using oxide templates or surfactant soft templates, and porous silicon-carbon anode materials are prepared by coating with a carbon layer.

[0003] Existing template methods for preparing porous silicon-carbon anodes rely on high-purity silicon raw materials and expensive template agents. Furthermore, the removal of oxides and soft templates is difficult, resulting in poor pore uniformity. At the same time, existing recycling and preparation processes are not adapted to the characteristics of oxide layers and impurities on the surface of photovoltaic waste silicon. As a result, the recycled waste silicon cannot be directly used to construct a uniform porous structure, leading to a persistently high volume expansion rate and structural collapse of porous silicon-carbon anodes during charge and discharge processes. Summary of the Invention

[0004] To address the shortcomings of existing technologies where photovoltaic waste silicon is difficult to use directly to construct uniform porous structures, resulting in high volume expansion rates and structural collapse of porous silicon-carbon anodes, this invention provides a method and application for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method.

[0005] To address the aforementioned technical problems, this invention provides a method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method, comprising the following technical features: photovoltaic waste silicon is sequentially subjected to acid washing, washing, drying, and ball milling to obtain nano-sized silicon powder; the nano-sized silicon powder is mixed with calcium carbonate at a mass ratio of 1:5 to construct a Si@CaCO3 composite template; the Si@CaCO3 composite template is dispersed with citric acid at a mass ratio of 1:3 to 1:9 in an ethanol solution, and after ultrasonic treatment and heating to dryness, a silicon@calcium carbonate@carbon precursor is obtained; the silicon@calcium carbonate@carbon precursor is subjected to high-temperature carbonization under an inert atmosphere to convert citric acid into a carbon layer, obtaining a silicon@calcium carbonate@carbon composite; the silicon@calcium carbonate@carbon composite is etched with acid to remove calcium carbonate, yielding a porous silicon-carbon anode.

[0006] This scheme constructs a core-shell Si@CaCO3 composite template by controlling the mass ratio of nanoscale silicon powder to calcium carbonate at 1:5. Calcium carbonate forms a rigid barrier layer on the silicon surface. During subsequent acid etching, the calcium carbonate reacts chemically with the acid and is dissolved and removed, forming a pore structure with uniform pore size distribution in the original space. This porous structure provides a physical buffer space for the volume expansion of the silicon substrate during lithium-ion insertion / extraction, limiting stress concentration. Citric acid undergoes pyrolysis and carbonization reactions during high-temperature carbonization, forming a conductive carbon network on the outer layer of silicon and calcium carbonate. The carbon coating layer blocks direct contact between the electrolyte and silicon, reducing side reaction gas generation and interface film rupture, and working synergistically with the porous structure to maintain the integrity of the electrode structure.

[0007] Furthermore, in the above technical solution, in the raw material pretreatment step, the pickling uses an acid solution with a concentration of 5-20wt%, which is one of hydrochloric acid, sulfuric acid or nitric acid solution, and the pickling treatment time is 4-24 hours; the ball milling treatment time is 6-12 hours, and the particle size of the nano-sized silicon powder is 50-150nm.

[0008] In practice, a specific concentration and type of inorganic acid solution is used to neutralize and dissolve the surface metal impurities and oxides of photovoltaic waste silicon with hydrogen ions, thereby removing the surface oxide layer and impurities. The ball milling time and particle size are limited to ensure that the silicon powder reaches the nanoscale, shortening the lithium ion diffusion path and increasing the specific surface area to facilitate the uniform coating of calcium carbonate in the subsequent process.

[0009] Furthermore, in the above technical solution, in the step of constructing the composite template, nano-sized silicon powder and soluble calcium salt are dispersed in deionized water, a precipitant is added, and the mixture is reacted at 60-80℃ for 2-4 hours to allow calcium carbonate to be deposited in situ on the surface of the nano-sized silicon powder. After filtration and drying, the Si@CaCO3 composite template is obtained.

[0010] In practice, an in-situ deposition method is used, in which soluble calcium salts and precipitants undergo a precipitation reaction in a liquid system to generate calcium carbonate crystal nuclei. At 60-80℃, the calcium carbonate crystal nuclei nucleate and grow heterogeneously on the surface of silicon powder, forming a uniformly coated core-shell structure, avoiding the exposure of silicon powder and ensuring the uniformity of pore distribution after subsequent etching.

[0011] Furthermore, in the above technical solution, in the carbon source coating and carbonization steps, the ultrasonic treatment power is 200-400W, the ultrasonic treatment time is 30-60 minutes, the heating and drying temperature is 70-90℃, and the stirring is continuous during the heating and drying process to make citric acid uniformly coated on the surface of the Si@CaCO3 composite template.

[0012] In practice, ultrasonic cavitation breaks up particle agglomeration, allowing citric acid molecules to fully contact and adsorb onto the Si@CaCO3 composite template. During the solvent evaporation process at 70-90℃, continuous stirring maintains the particle suspension. As ethanol evaporates, citric acid is concentrated and precipitated on the template surface, forming a uniform coating layer and preventing local carbon layers from being too thick or incompletely coated.

[0013] Furthermore, in the above technical solution, the heating rate of high-temperature carbonization is 2-5℃ / min, the temperature of high-temperature carbonization is 800-1000℃, and the holding time of high-temperature carbonization is 2-4 hours; the mass ratio of Si@CaCO3 composite template to citric acid is 1:5 or 1:7.

[0014] In practice, the temperature is raised slowly to avoid violent pyrolysis of the organic carbon source, which could cause the carbon layer to crack. The temperature of 800-1000℃ promotes the deep aromatization and condensation reaction of citric acid, forming a conductive carbon layer with a high degree of graphitization. The carbon layer thickness is adjusted by a mass ratio of 1:5 or 1:7 to meet the structural constraint requirements under different expansion stresses.

[0015] Furthermore, in the above technical solution, in the template removal step, the silicon@calcium carbonate@carbon composite is immersed in a dilute hydrochloric acid solution with a concentration of 0.5-2 mol / L, stirred and etched at 30-50℃ for 4-8 hours, centrifuged, washed with water until neutral, and vacuum dried to obtain a porous silicon-carbon anode with 50-100 nm pores.

[0016] In practice, dilute hydrochloric acid reacts with calcium carbonate to produce soluble calcium salt and carbon dioxide gas. The reaction kinetics are accelerated at 30-50℃ to avoid damage to the carbon layer structure. The etching time is controlled to completely remove the calcium carbonate. The remaining pores are 50-100nm in size. This pore size range matches the expansion of silicon particles and provides a suitable buffer space.

[0017] To address the aforementioned technical problems, this invention provides an application for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method, comprising the following technical features: The porous silicon-carbon anode is prepared by the following method: photovoltaic waste silicon is acid-washed, washed, dried, and ball-milled to obtain nano-sized silicon powder; the nano-sized silicon powder and calcium carbonate are mixed at a mass ratio of 1:5 to construct a Si@CaCO3 composite template; the Si@CaCO3 composite template and citric acid are dispersed in an ethanol solution at a mass ratio of 1:3 to 1:9, and after ultrasonication and heating to dryness, a silicon@calcium carbonate@carbon precursor is obtained; the silicon@calcium carbonate@carbon precursor is subjected to high-temperature carbonization under an inert atmosphere to convert citric acid into a carbon layer, obtaining a silicon@calcium carbonate@carbon composite; the silicon@calcium carbonate@carbon composite is etched with acid to remove calcium carbonate, obtaining a porous silicon-carbon anode; the porous silicon-carbon anode is applied to the preparation of lithium-ion battery anode sheets.

[0018] In practice, this application scheme uses the aforementioned porous silicon-carbon anode as an active material in lithium-ion battery anode sheets. During charge-discharge cycles, lithium ions are inserted and extracted inside the porous silicon matrix. The porous structure accommodates the expansion of the silicon lattice, and the outer carbon layer maintains the electrical contact between particles and inhibits the continuous decomposition of the electrolyte, thus ensuring the long-cycle stability of the anode sheet.

[0019] Furthermore, in the above technical solution, the pickling adopts a segmented pickling process. In the first stage, a 5-10 wt% nitric acid solution is used to treat the silicon waste for 6-12 hours to remove metal impurities. In the second stage, a 10-20 wt% hydrofluoric acid solution is used to treat the silicon waste for 1-3 hours to peel off the oxide layer on the surface of the photovoltaic waste silicon.

[0020] In practice, nitric acid has strong oxidizing properties, dissolving elemental metals and low-valence metal compounds on the surface of waste silicon; hydrofluoric acid reacts specifically with silicon dioxide to generate soluble fluorosilicic acid, and the segmented treatment achieves the gradual stripping of impurities and oxide layers, improving the purity and surface activity of waste silicon.

[0021] Furthermore, in the above technical solution, nano-sized silicon powder and soluble calcium salt are dispersed in deionized water, and a precipitant is added and reacted at 60-80℃ for 2-4 hours to allow calcium carbonate to be deposited in situ. The soluble calcium salt is calcium chloride or calcium nitrate, and the precipitant is ammonium carbonate or sodium carbonate. During the in-situ deposition process, the dropping rate of the precipitant is controlled to be 1-3 mL / min.

[0022] In practice, limiting the types of calcium salts and precipitants ensures the generation of high-purity calcium carbonate precipitate. A dropping rate of 1-3 mL / min controls the supersaturation of the reaction system, preventing the precipitation reaction from being too fast and causing homogeneous nucleation of calcium carbonate in the solution. Instead, it promotes heterogeneous nucleation and growth of calcium carbonate on the surface of silicon powder, thereby increasing the coating density.

[0023] Furthermore, in the above technical solution, the inert atmosphere is a mixture of argon and hydrogen, with a volume ratio of argon to hydrogen of 95:5; when the mass ratio of Si@CaCO3 composite template to citric acid is 1:5, the porous silicon-carbon anode is applied to consumer electronics lithium-ion batteries; when the mass ratio is 1:7, the porous silicon-carbon anode is applied to power batteries.

[0024] In practice, argon provides an inert environment to prevent silicon and carbon oxidation, while hydrogen reduces trace oxides on the silicon surface and promotes the ordered structure of the carbon layer. At a ratio of 1:5, the carbon layer thickness is moderate, meeting the high-capacity requirements of consumer electronics. At a ratio of 1:7, the carbon layer is thicker, which enhances the constraint on the expansion of silicon particles and meets the long-cycle requirements of power batteries.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] 1. This solution constructs a silicon@calcium carbonate composite template by mixing photovoltaic waste silicon and calcium carbonate at a mass ratio of 1:5. This allows calcium carbonate to be deposited in situ on the surface of the waste silicon to form a core-shell structure. Combined with subsequent acid etching to remove the calcium carbonate template, uniform pores of 50 to 100 nanometers are formed inside the silicon substrate. These pores provide a buffer space for the volume expansion of silicon, controlling the volume expansion rate to below 25%, and suppressing the collapse of the electrode structure and the rupture of the solid electrolyte interface film.

[0027] 2. This solution uses acid washing and ball milling to pretreat photovoltaic waste silicon, removing the surface oxide layer and impurities, making the low-purity waste silicon suitable for subsequent template construction processes, eliminating complex purification processes; by mixing silicon@calcium carbonate composite template with citric acid at a mass ratio of 1:3 to 1:9, coating and carbonizing, the carbon layer coats the porous silicon, blocking direct contact between silicon and electrolyte, reducing side reactions and lithium dendrite growth, improving the conductivity of the material, and extending the cycle life of the battery. Attached Figure Description

[0028] Figure 1 This is a biaxial comparison of the cycle performance and coulombic efficiency of porous silicon-carbon anodes with different citric acid coating ratios according to the present invention.

[0029] Figure 2 This is a comparison chart of the discharge specific capacity cycling of porous silicon-carbon anodes with different citric acid coating ratios according to the present invention.

[0030] Figure 3 The first charge-discharge curves of porous silicon-carbon anodes with different citric acid coating ratios are shown. Detailed Implementation

[0031] The present invention will be further described in detail below with reference to embodiments. Those skilled in the art can reproduce the technical solution of the present invention and achieve its claimed technical effects based on the content disclosed in this specification. It should be noted that the following embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention. Any non-substantial improvements and adjustments made based on the core concept of the present invention should fall within the scope of protection of the present invention.

[0032] Example 1: Photovoltaic waste silicon was acid-washed with a 10wt% hydrochloric acid solution for 12 hours, followed by washing, drying, and ball milling for 8 hours to obtain nanoscale silicon powder with a particle size of 100 nm. The nanoscale silicon powder and calcium chloride were dispersed in deionized water, with ammonium carbonate added as a precipitant at a dropping rate of 2 mL / min. The reaction was carried out at 70°C for 3 hours, allowing calcium carbonate to be deposited in situ on the surface of the nanoscale silicon powder. After filtration and drying, a silicon@calcium carbonate composite template was obtained, with a mass ratio of nanoscale silicon powder to calcium carbonate of 1:5. The silicon@calcium carbonate composite template and citric acid were dispersed in an ethanol solution at a mass ratio of 1:5, ultrasonically treated at 300 W for 45 minutes, and then heated to dryness at 80°C with continuous stirring to obtain a silicon@calcium carbonate@carbon precursor. A silicon@calcium carbonate@carbon precursor was placed in an atmosphere of argon and hydrogen (95:5 volume ratio) and heated to 900°C at a rate of 3°C / min for high-temperature carbonization. The temperature was maintained for 3 hours to convert citric acid into a carbon layer, yielding a silicon@calcium carbonate@carbon composite. The silicon@calcium carbonate@carbon composite was then immersed in a 1 mol / L dilute hydrochloric acid solution and etched with stirring at 40°C for 6 hours. After centrifugation, washing with water until neutral, and vacuum drying, a porous silicon-carbon anode with 50 to 100 nanometer pores was obtained.

[0033] Example 2: The acid solution used for pickling was replaced with a 15wt% sulfuric acid solution, and the treatment time was 12 hours. All other conditions were the same as in Example 1.

[0034] Example 3: The acid solution used for pickling was replaced with a 20wt% nitric acid solution, and the treatment time was 12 hours. All other conditions were the same as in Example 1.

[0035] Example 4: The soluble calcium salt in the composite template construction step was replaced with calcium nitrate, and the precipitant was replaced with sodium carbonate. All other conditions were the same as in Example 1.

[0036] Example 5: The mass ratio of silicon@calcium carbonate composite template to citric acid was adjusted to 1:3. All other conditions were the same as in Example 1.

[0037] Example 6: The mass ratio of silicon@calcium carbonate composite template to citric acid was adjusted to 1:7. All other conditions were the same as in Example 1.

[0038] Example 7: The mass ratio of silicon@calcium carbonate composite template to citric acid was adjusted to 1:9. All other conditions were the same as in Example 1.

[0039] Example 8: The pickling step was replaced with a segmented pickling process: the first stage used an 8wt% nitric acid solution for 8 hours to remove metallic impurities, and the second stage used a 15wt% hydrofluoric acid solution for 2 hours to peel off the oxide layer on the surface of the photovoltaic waste silicon. The remaining conditions were the same as in Example 1.

[0040] Example 9: During in-situ deposition, the drop rate of the precipitant was controlled at 1 ml / min. All other conditions were the same as in Example 1.

[0041] Example 10: The ball milling time was adjusted to 6 hours to obtain nanoscale silicon powder with a particle size of 150 nanometers. The remaining conditions were the same as in Example 1.

[0042] Example 11: The reaction temperature for in-situ deposition was adjusted to 60°C, and the reaction time was adjusted to 4 hours. All other conditions were the same as in Example 1.

[0043] Example 12: The high-temperature carbonization temperature was adjusted to 800℃, and the holding time was adjusted to 4 hours. All other conditions were the same as in Example 1.

[0044] Comparative Example 1: The construction of the calcium carbonate template was omitted, i.e., calcium chloride and ammonium carbonate were not added for in-situ deposition. Instead, nano-sized silicon powder and citric acid were directly dispersed in an ethanol solution at a mass ratio of 1:5 for coating and carbonization, omitting the acid etching step. All other conditions were the same as in Example 1.

[0045] Comparative Example 2: Photovoltaic waste silicon was recovered using a traditional high-temperature roasting and single acid leaching process. Instead of using calcium carbonate templates, hexadecyltrimethylammonium bromide was used as a soft template for pore creation, and porous silicon-carbon anodes were prepared using conventional processes. All other conditions were the same as in Example 1.

[0046] Comparative Example 3: The mass ratio of nano-sized silicon powder to calcium carbonate was adjusted to 1:1, i.e., in-situ deposition was performed beyond the lower limit of the formulation. All other conditions were the same as in Example 1.

[0047] Comparative Example 4: The in-situ deposition step was omitted. Nanoscale silicon powder and commercially available nanoscale calcium carbonate powder were physically ground and mixed, followed by citric acid coating and subsequent carbonization and etching steps. The remaining conditions were the same as in Example 1.

[0048] Test method:

[0049] The porous silicon-carbon anodes prepared in the above embodiments and comparative examples were subjected to performance tests, including silicon recovery rate, volume expansion rate, and capacity retention rate after 100 cycles.

[0050] 1. Silicon recovery rate test

[0051] The silicon content was determined by inductively coupled plasma optical emission spectrometry (ICP-OES), and the silicon recovery efficiency was calculated.

[0052] Determination of silicon content in raw materials: Accurately weigh 1.0g of photovoltaic waste silicon raw materials before pretreatment, completely digest them in a hydrofluoric acid-nitric acid digestion system, and then bring the volume to 100mL; use ICP-OES to determine the silicon element concentration in the solution and calculate the total mass m0 of silicon in the raw materials.

[0053] Product silicon content determination: Accurately weigh 1.0g of the finally prepared porous silicon-carbon anode sample, and completely digest it in a hydrofluoric acid-nitric acid digestion system, then bring the volume to 100mL; use ICP-OES to determine the silicon element concentration in the solution, and calculate the total mass m1 of silicon in the product.

[0054] Calculation formula: Silicon recovery rate (%) = m1 / m0 × 100

[0055] Each group of tests has 3 parallel samples, and the arithmetic mean is taken.

[0056] 2. Volume expansion rate test

[0057] The volume expansion rate of the electrode after cycling was determined using the electrode thickness method.

[0058] Initial thickness measurement: The prepared negative electrode sheet is punched into a circular electrode sheet with a diameter of 14mm. The thickness is measured at 5 points evenly selected at the center and around the electrode sheet using a micrometer screw gauge. The arithmetic mean is taken as the electrode sheet thickness d0 before cycling.

[0059] Battery assembly and cycling: The electrode sheets were assembled into CR2032 coin cells, with lithium metal sheet as counter electrode, Celgard2400 as separator, and 1 mol / L LiPF6 / EC+DMC+EMC (volume ratio 1:1:1) as electrolyte; after activation at 0.1C current density for 3 cycles, it was cycled 100 times at 0.5C current density.

[0060] Thickness measurement after cycling: After cycling, the battery was disassembled in an argon-protected glove box, the negative electrode was removed, the surface residual electrolyte was rinsed with dimethyl carbonate, and it was air-dried at room temperature; the thickness of 5 points at the same location was measured using a micrometer, and the arithmetic mean was taken as the electrode thickness d1 after cycling.

[0061] Calculation formula: Volume expansion rate (%) = (d1 - d0) / d0 × 100

[0062] Three parallel batteries were tested in each group, and the arithmetic mean was taken.

[0063] 3. Capacity retention test after 100 cycles

[0064] The constant current charge-discharge test was conducted using the Blue Battery Testing System.

[0065] Battery assembly: Battery assembly methods in the same volume expansion rate test.

[0066] Test conditions: Voltage range 0.01V-1.5V, activation with a current density of 0.1C for the first 3 cycles, constant current charge and discharge with a current density of 0.5C from the 4th cycle onwards, 100 cycles.

[0067] Data recording: Record the discharge capacity C3 of the 3rd cycle and the discharge capacity C of the 100th cycle. 100 .

[0068] Calculation formula: Capacity retention rate (%) after 100 cycles = C 100 / C3×100

[0069] Three parallel batteries were tested in each group, and the arithmetic mean was taken.

[0070] 4. Initial peel strength test of electrode sheets

[0071] A 180° peel strength test was conducted using a universal testing machine to characterize the interfacial bonding strength between the active material layer and the copper foil current collector, reflecting the anti-detachment ability during electrode preparation and use.

[0072] Electrode preparation: The porous silicon-carbon anode to be tested, conductive agent SuperP, and binder sodium carboxymethyl cellulose / styrene-butadiene rubber were mixed at a mass ratio of 8:1:1 to form a uniform slurry. This slurry was then uniformly coated onto an electrolytic copper foil with a thickness of 10 μm. After vacuum drying at 110℃ for 12 h, the slurry was rolled to a surface density of approximately 4.5 mg / cm³. 2 Cut a strip sample with dimensions of 20mm × 100mm.

[0073] Test procedure: The active material side of the sample is flatly attached to the surface of the stainless steel carrier plate with high-strength double-sided tape. The free end of the copper foil is bent at 180° and clamped in the upper fixture of the testing machine, while the lower fixture fixes the carrier plate.

[0074] Set the stretching rate to 50 mm / min and record the average value of the stable load range during the peeling process.

[0075] Calculation formula: Peel strength (N / m) = Stable average load (N) / Specimen width (m)

[0076] Five parallel samples were prepared for each test group, and the arithmetic mean was taken as the final result.

[0077] 5. Electrode compressive strength and compression resilience test

[0078] The compressive strength and resilience of the electrode were tested using the micro-compression mode of a microcomputer-controlled electronic universal testing machine, which characterizes the material's mechanical ability to resist compressive deformation and buffer volume expansion stress.

[0079] Sample preparation: Take the above-mentioned dried and rolled negative electrode sheet, cut a 10mm×10mm square sample, and fix it flat on the surface of the rigid stage to ensure that there are no wrinkles or defects in the test area.

[0080] Test procedure: A cylindrical flat-head indenter with a diameter of 1 mm was selected and applied vertically at a loading rate of 0.5 mm / min to the maximum load of 20 N. After holding the load for 10 seconds, the load was unloaded to zero at the same rate. The load-displacement curve was recorded throughout the process.

[0081] Calculation formula: Compressive strength (MPa) = Maximum load (N) / Cross-sectional area of ​​indenter (mm²) 2 )

[0082] Compression rebound rate (%) = (Displacement corresponding to maximum load - Residual displacement after unloading) / Displacement corresponding to maximum load × 100

[0083] Five parallel samples were tested in each group, and the arithmetic mean was taken.

[0084] 6. Post-cycle peel strength retention test

[0085] The CR2032 coin cell assembled according to the aforementioned method was cycled 100 times at a current density of 0.5C. The negative electrode was then disassembled and removed in an argon-protected glove box. After being rinsed with dimethyl carbonate and vacuum dried at room temperature, the peel strength after cycling was tested using a method completely consistent with the initial peel strength test. The retention rate was calculated to reflect the mechanical stability and structural integrity of the electrode structure during cycling.

[0086] Calculation formula: Peel strength retention rate after cycles (%) = Peel strength after cycles / Initial peel strength × 100

[0087] Three parallel batteries were tested in each group, and the arithmetic mean was taken.

[0088] The test results are shown in Table 1.

[0089] Table 1 Performance test results of each embodiment and comparative example

[0090] Example 1 95.2 18.3 89.5 128.5 18.2 62.3 76.4 Example 2 94.1 19.1 88.2 125.3 17.8 61.5 75.1 Example 3 93.8 19.5 87.9 123.7 17.5 60.8 74.6 Example 4 94.5 18.9 88.6 126.9 18 61.9 75.7 Example 5 95.0 24.2 82.1 112.4 15.6 55.7 65.2 Example 6 94.8 16.5 90.2 135.2 19.7 65.8 80.3 Example 7 94.6 15.8 86.4 141.8 21.3 68.2 78.6 Example 8 96.1 17.5 91.0 138.6 20.1 67 82.5 Example 9 94.9 18.6 89.1 127.8 18.5 62.7 77 Example 10 92.5 21.4 84.3 118.2 16.9 58.4 70.8 Example 11 93.2 20.2 85.7 121.5 17.3 59.6 72.3 Example 12 93.9 19.8 86.5 124 17.6 60.5 73.8 Comparative Example 1 90.1 42.5 41.2 72.3 9.6 32.5 28.7 Comparative Example 2 78.4 36.8 52.3 85.6 11.2 38.4 35.1 Comparative Example 3 91.5 33.6 58.4 96.8 13.5 45.2 46.3 Comparative Example 4 93.8 28.9 69.5 108.4 15.1 52.6 58.9

[0091] Results analysis:

[0092] Data from Example 1 shows that by constructing a core-shell structure with a silicon to calcium carbonate mass ratio of 1:5, combined with specific carbonization and etching processes, the silicon recovery rate reached 95.2%, the volume expansion rate was controlled at 18.3%, and the capacity retention rate reached 89.5%, verifying the advantages of this technical solution in waste silicon purification and volume expansion suppression. Examples 2 to 4 replaced the type of acid washing solution and the calcium salt precipitant, and the performance indicators were similar to those of Example 1, proving that the formulation system has broad-spectrum adaptability and robustness. Examples 5 to 7 adjusted the citric acid coating ratio. When the ratio was 1:3, the carbon layer was thinner and the expansion rate was higher; when the ratio was 1:7, the expansion rate decreased to 16.5%, and the capacity retention rate increased to 90.2%, indicating that increasing the carbon layer thickness enhanced the structural binding force; when the ratio reached 1:9, the expansion rate further decreased, but the capacity retention rate decreased, which was due to the reduced proportion of active material caused by excessive carbon. Example 8 used segmented acid washing, and the silicon recovery rate increased to 96.1%, proving that hydrofluoric acid promoted the purification effect by stripping the oxide layer.

[0093] like Figure 1 The figure shows the cycling performance and coulombic efficiency curves of the porous silicon-carbon anodes obtained when the mass ratio of Si@CaCO3 composite template to citric acid is 1:3, 1:5, 1:7, and 1:9, respectively. The test voltage range is 0.01V to 1.5V. The first three cycles are activated with a current density of 0.1C, and subsequent constant current charge-discharge cycles are performed with a current density of 0.5C. The left vertical axis represents the discharge specific capacity, the right vertical axis represents the coulombic efficiency, and the horizontal axis represents the number of cycles.

[0094] From the perspective of capacity variation, the Si@CaCO3:CA=1:5 sample exhibits the best discharge specific capacity, maintaining a high stable discharge specific capacity after activation, and showing a gradual capacity decay rate during 100 cycles, demonstrating the synergistic buffering effect of the porous structure and carbon layer coating. The Si@CaCO3:CA=1:3 sample, due to insufficient carbon layer coating thickness, has limited binding force on the volume expansion of silicon particles, resulting in significantly accelerated capacity decay during cycling and a substantial decrease in specific capacity after 100 cycles. As the citric acid ratio increases to 1:7 and 1:9, the carbon layer thickness continues to increase, further enhancing structural stability, but the overall proportion of silicon as the active material decreases accordingly, thus causing a slight decrease in initial discharge specific capacity, while the capacity remains stable during cycling.

[0095] From the perspective of coulombic efficiency variation, the coulombic efficiency of all samples rapidly increased and then stabilized after the first three activation cycles. Among them, the coulombic efficiency of the Si@CaCO3:CA=1:5, 1:7, and 1:9 samples remained stable above 98.5% for a long period, indicating that the continuous carbon layer effectively blocked the direct contact between silicon and electrolyte, suppressed the repeated rupture and regeneration of the side reaction and solid electrolyte interface film, and demonstrated excellent interface stability. The Si@CaCO3:CA=1:3 sample showed significant fluctuations in coulombic efficiency in the later stages of cycling due to insufficient carbon layer coating, corresponding to the aggravation of side reactions on the silicon surface and the gradual deterioration of the electrode structure, which is consistent with the rapid capacity decay pattern.

[0096] like Figure 2 As shown, comparing the discharge specific capacity data of the four groups of samples separately provides a more intuitive view of the differences in capacity decay trends under different carbon source ratios. The Si@CaCO3:CA=1:5 sample maintained the highest discharge specific capacity throughout the entire cycle, with a gentle decay slope. This verifies that the buffer space formed by the calcium carbonate template and the conductive binding effect of the outer carbon layer achieved optimal synergy under this ratio, preserving the high capacity characteristics of silicon-based materials while effectively mitigating structural pulverization and electrical contact failure caused by volume expansion. When the carbon source ratio is lower than the optimal range, insufficient structural stability leads to rapid capacity decay. When the carbon source ratio is higher than the optimal range, the proportion of inactive carbon components is too high, resulting in a decrease in overall specific capacity. The above results collectively demonstrate that 1:5~1:7 is the optimal ratio range for overall performance, which can be flexibly adjusted between high capacity and long cycling time according to the application scenario.

[0097] like Figure 3 As shown, the first charge-discharge curves of the four groups of samples are presented. The vertical axis represents voltage, the horizontal axis represents specific capacity, the descending curve represents the lithium insertion process during discharge, and the ascending curve represents the lithium removal process during charging. All samples exhibit typical silicon-based anode charge-discharge characteristics: a clear lithium insertion plateau appears near 0.1V during discharge, and a lithium removal plateau appears near 0.4V during charging.

[0098] Among the samples, the Si@CaCO3:CA=1:5 exhibited the smoothest charge-discharge plateau and the smallest charge-discharge potential difference, indicating low electrode polarization. The porous structure shortened the lithium-ion diffusion path, and the outer carbon layer constructed a continuous electron transport channel, jointly ensuring efficient electrochemical reaction kinetics. As the citric acid ratio increased, the carbon layer thickened, and the overall specific capacity of the sample gradually decreased, perfectly matching the changing trend of the silicon content in the active material. The Si@CaCO3:CA=1:3 sample showed an increased tilt of the charge-discharge plateau, a larger potential difference, and a significantly higher degree of polarization, corresponding to the decreased electrode conductivity and poorer interface stability caused by insufficient carbon layer. These charge-discharge behaviors and cycle performance results corroborate each other, jointly demonstrating that the present invention, through in-situ pore-forming with calcium carbonate templates combined with citric acid carbon layer coating, can effectively optimize the electrochemical performance of porous silicon-carbon anodes, achieving a balance between high specific capacity and long cycle life.

[0099] Comparative Example 1, lacking calcium carbonate template agent, exhibited a volume expansion rate as high as 42.5% and a capacity retention rate of only 41.2%, demonstrating that without a porous structure buffer, the silicon substrate suffered severe structural collapse during lithium insertion / extraction. Comparative Example 2, using a traditional soft template and conventional purification process, achieved a silicon recovery rate of only 78.4% and an expansion rate of 36.8%, proving that traditional processes are unsuitable for the characteristics of waste silicon and have poor pore uniformity. Comparative Example 3, reducing the silicon-to-calcium carbonate ratio to 1:1, resulted in a severe expansion rate soaring to 33.6% due to insufficient template agent leading to a lack of pore buffer space, demonstrating that a 1:5 ratio is necessary to maintain a low expansion rate. Comparative Example 4, using physical mixing instead of in-situ deposition, achieved an expansion rate of 28.9% and a capacity retention rate of 69.5%, significantly inferior to Example 1, proving that the core-shell structure formed by in-situ deposition is crucial for ensuring uniform pore distribution and tight carbon layer encapsulation; simple physical mixing cannot achieve structural synergy.

[0100] In terms of interfacial bonding performance, the initial peel strength of all embodiments was higher than 110 N / m, which was superior to Comparative Example 1 without a template (72.3 N / m) and Comparative Example 2 with the traditional soft template process (85.6 N / m). This is because the porous structure formed by in-situ etching of calcium carbonate increased the surface roughness of the silicon particles, providing more anchoring sites for the binder; the continuous outer carbon layer enhanced the cohesive force between the particles, synergistically strengthening the interfacial bonding strength between the active material layer and the current collector. Among them, Example 7 had the highest citric acid coating ratio and the largest carbon layer thickness, resulting in the strongest interparticle bonding force and an initial peel strength of 141.8 N / m; after the oxide layer was thoroughly removed by segmented acid washing in Example 8, the surface activity of the silicon powder was enhanced, and the bonding with the carbon layer was tighter, resulting in a high level of interfacial performance.

[0101] Data on compressive strength and resilience show that the compressive strength of the embodiments is generally above 15 MPa, and the compression resilience exceeds 55%, while the compressive strength of Comparative Example 1 is only 9.6 MPa and the resilience is 32.5%. This result indicates that the uniform channels constructed by the calcium carbonate template in this invention possess excellent stress buffering capabilities. Under external force, energy can be absorbed through pore wall deformation, and the structure can effectively rebound after unloading, avoiding permanent compaction and structural collapse. This characteristic perfectly matches the volume expansion-contraction cyclic deformation during the lithium insertion / extraction process of silicon-based anodes. Comparative Example 3, due to insufficient calcium carbonate template, has a small number of pores and limited buffer space, resulting in a resilience of only 45.2%. Comparative Example 4, which uses physical mixing instead of in-situ deposition, exhibits uneven pore distribution and significant local stress concentration, making its resilience performance and structural uniformity significantly inferior to the embodiments with in-situ deposition.

[0102] The peel strength retention rate after cycling exhibits a strong correlation with the volume expansion rate and capacity retention rate: Due to their low volume expansion rate and good microstructural integrity, the peel strength retention rates of the examples after cycling are generally above 65%, with Example 8 reaching 82.5%. In contrast, the peel strength retention rate of Comparative Example 1 is only 28.7%, corresponding to a high volume expansion rate of 42.5%. This indicates that significant volume expansion leads to a large amount of active material detaching from the current collector surface and electrode structure pulverization, which is the core mechanical cause of its rapid capacity decay. This invention, through the synergistic effect of calcium carbonate template pore creation and carbon layer coating, improves the mechanical stability of the electrode at the microstructural level, fundamentally alleviating the structural failure problem of silicon-based anodes during cycling, and providing core mechanical support for the long cycle life of the material.

Claims

1. A method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method, characterized in that, The method includes the following steps: Raw material pretreatment: The photovoltaic waste silicon is sequentially subjected to acid washing, washing, drying and ball milling to obtain nano-sized silicon powder; Constructing a composite template: The nano-sized silicon powder and calcium carbonate are mixed at a mass ratio of 1:5 to construct a Si@CaCO3 composite template; Carbon source coating and carbonization: The Si@CaCO3 composite template and citric acid are dispersed in an ethanol solution at a mass ratio of 1:3 to 1:

9. After ultrasonic treatment and heating to dryness, a silicon@calcium carbonate@carbon precursor is obtained. The silicon@calcium carbonate@carbon precursor is then subjected to high-temperature carbonization under an inert atmosphere to convert the citric acid into a carbon layer, thereby obtaining a silicon@calcium carbonate@carbon composite. Template removal: The calcium carbonate in the silicon@calcium carbonate@carbon composite is removed by acid etching to obtain the porous silicon-carbon anode.

2. The method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method according to claim 1, characterized in that, In the raw material pretreatment step, the pickling uses an acid solution with a concentration of 5-20 wt%, and the acid solution is one of hydrochloric acid, sulfuric acid or nitric acid solution. The pickling treatment time is 4-24 hours. The ball milling treatment time is 6-12 hours. The particle size of the nano-sized silicon powder is 50-150 nm.

3. The method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method according to claim 2, characterized in that, In the step of constructing the composite template, the nano-sized silicon powder and soluble calcium salt are dispersed in deionized water, a precipitant is added, and the mixture is reacted at 60-80℃ for 2-4 hours to allow calcium carbonate to be deposited in situ on the surface of the nano-sized silicon powder. After filtration and drying, the Si@CaCO3 composite template is obtained.

4. The method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method according to claim 3, characterized in that, In the carbon source coating and carbonization steps, the ultrasonic treatment power is 200-400W, and the ultrasonic treatment time is 30-60 minutes; the heating and drying temperature is 70-90℃, and the mixture is continuously stirred during the heating and drying process to ensure that the citric acid is uniformly coated on the surface of the Si@CaCO3 composite template.

5. The method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method according to claim 4, characterized in that, The heating rate of the high-temperature carbonization is 2-5℃ / min, the temperature of the high-temperature carbonization is 800-1000℃, and the holding time of the high-temperature carbonization is 2-4 hours; the mass ratio of the Si@CaCO3 composite template to the citric acid is 1:5 or 1:

7.

6. The method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template method according to claim 5, characterized in that, In the template removal step, the silicon@calcium carbonate@carbon composite is immersed in a dilute hydrochloric acid solution with a concentration of 0.5-2 mol / L, stirred and etched at 30-50°C for 4-8 hours, centrifuged, washed with water until neutral, and vacuum dried to obtain the porous silicon-carbon anode with 50-100 nm pores.

7. An application of a method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template, characterized in that... The porous silicon-carbon anode is prepared by the following method: the photovoltaic waste silicon is acid-washed, washed, dried and ball-milled to obtain nano-sized silicon powder; the nano-sized silicon powder is mixed with calcium carbonate at a mass ratio of 1:5 to construct a Si@CaCO3 composite template; The Si@CaCO3 composite template and citric acid were dispersed in an ethanol solution at a mass ratio of 1:3 to 1:

9. After ultrasonication and heating to dryness, a silicon@calcium carbonate@carbon precursor was obtained. The silicon@calcium carbonate@carbon precursor was then subjected to high-temperature carbonization under an inert atmosphere to convert the citric acid into a carbon layer, resulting in a silicon@calcium carbonate@carbon composite. The silicon@calcium carbonate@carbon composite was then etched with acid to remove the calcium carbonate, resulting in the porous silicon-carbon anode. The porous silicon-carbon anode was then used in the preparation of lithium-ion battery anode sheets.

8. The application of the method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template as described in claim 7, characterized in that, The pickling process employs a segmented pickling technique. In the first stage, a 5-10 wt% nitric acid solution is used to treat the silicon waste for 6-12 hours to remove metallic impurities. In the second stage, a 10-20 wt% hydrofluoric acid solution is used to treat the silicon waste for 1-3 hours to remove the oxide layer from the surface of the photovoltaic waste silicon.

9. The application of the method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template as described in claim 8, characterized in that, The nano-sized silica powder and soluble calcium salt are dispersed in deionized water, and a precipitant is added. The mixture is reacted at 60-80℃ for 2-4 hours to allow calcium carbonate to be deposited in situ. The soluble calcium salt is calcium chloride or calcium nitrate, and the precipitant is ammonium carbonate or sodium carbonate. During the in-situ deposition process, the dropping rate of the precipitant is controlled to be 1-3 mL / min.

10. The application of the method for preparing porous silicon-carbon anodes from photovoltaic waste silicon using a calcium carbonate template as described in claim 9, characterized in that, The inert atmosphere is a mixture of argon and hydrogen, with a volume ratio of argon to hydrogen of 95:

5. When the mass ratio of the Si@CaCO3 composite template to the citric acid is 1:5, the porous silicon-carbon anode is used in consumer electronics lithium-ion batteries; when the mass ratio is 1:7, the porous silicon-carbon anode is used in power batteries.

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