Preparation method and device of lithium ion battery silicon-based negative electrode precursor silicon material

Abstract

The invention discloses a preparation method and device of a lithium ion battery silicon-based negative electrode precursor silicon material. The method is carried out in a polycrystalline silicon ingot furnace with a modified heating and heat insulation system. In the heating and heat insulation system, a plurality of quartz crucibles with the same size are uniformly arranged on a heat exchange table to replace original standard large-size quartz crucibles, the adjacent quartz crucibles are separated by heat insulation plates which are not lower than the tops of the quartz crucibles, and water-cooling cold plates are embedded in the upper surface of a heat insulation bottom plate to serve as active cooling units; the preparation method comprises the following steps: (1) charging and vacuumizing; (2) melting and homogenizing; (3) directional solidification: when the temperature of the silicon material is reduced to the initial solidification temperature of 1410-1420 DEG C, starting an active cooling unit, and introducing cooling liquid into a cooling flow channel in a water-cooling cold plate; the instantaneous cooling rate in the process of controlling the temperature of the solid-phase silicon material to be reduced from 1410-1420 DEG C to 1240-1250 DEG C is maintained at 3.5-5.5 DEG C / min; finally solidifying the silicon material into a silicon ingot; (4) annealing; and (5) cooling.

Description

Technical Field

[0001] This invention specifically relates to a method and a dedicated apparatus for preparing silicon-based anode precursor silicon materials for high-energy-density lithium-ion batteries, wherein the lithium-ion batteries include, but are not limited to, solid-state and liquid electrolyte systems. Background Technology

[0002] Silicon is used because of its extremely high theoretical specific capacity (approximately 4200 mAh / g) and suitable lithium intercalation potential (0.1~0.4 V vs. Li). + Silicon (Li₂O₃) and silicon resources are abundant, making it an ideal anode material to replace graphite (372 mAh / g). However, silicon's volume expansion rate exceeds 300% during charge and discharge, easily leading to electrode pulverization and cycle failure. To alleviate this problem, the industry, especially for solid-state lithium-ion batteries, often uses micron-sized silicon particles (D50 = 1~2μm) and controls their narrow particle size distribution to improve cycle stability. Mechanical pulverization is the main method to obtain particles of this size. Studies have shown that if the grain size of the precursor polycrystalline silicon is controlled within the range of 20~50μm, it is easier to break along the grain boundaries during subsequent pulverization, obtaining silicon powder with uniform morphology and fewer defects, while significantly reducing energy consumption.

[0003] Currently, the mainstream preparation methods for silicon-based anode precursors in lithium-ion batteries include: chemical reduction (such as Mg reduction of SiO2), plasma / vapor deposition, and ball milling. However, these mainstream preparation methods each have the following problems:

[0004] (1) Chemical reduction method: Although submicron to micron-sized powders can be obtained, the product has high oxygen and carbon impurity content (O>2wt%, C>0.2wt%), resulting in the first coulombic efficiency of lithium-ion batteries generally being less than 75%; and the post-processing process of this method is complicated.

[0005] (2) Plasma / vapor deposition: High-purity nano-silicon can be produced, but the equipment investment is large and the energy consumption is high, which makes the production cost high and not economically feasible, and it is difficult to achieve large-scale production.

[0006] (3) Ball milling crushing method: Ball milling crushing of photovoltaic polycrystalline silicon ingots is currently the closest route to mass production. However, it is difficult to control the particle size of silicon particles, resulting in an extremely wide particle size distribution. The inventors found that after crushing, the particle size distribution of silicon particles is extremely wide, specifically D10 is 5μm, D90 is 150μm, and RSD>60% (Relative Standard Deviation, RSD, which is the ratio of standard deviation to mean (expressed as a percentage), used to characterize the dispersion of data). Moreover, the ball milling process makes the silicon surface prone to severe oxidation (O>2 wt%), which requires acid pickling and passivation, leading to further increased costs and the introduction of pollution.

[0007] In other words, existing methods for preparing silicon-based anode precursors for lithium-ion batteries cannot simultaneously achieve high purity, uniform particle size, and controllable cost. Therefore, how to prepare high-purity silicon materials with uniform particle size at low cost has become a key technological bottleneck that the lithium-ion battery industry urgently needs to solve.

[0008] In existing technologies, the mainstream method for producing polycrystalline silicon in the photovoltaic industry is directional solidification, where the entire production process can be completed within a single polycrystalline silicon ingot furnace. Therefore, this method offers advantages such as simple process, low equipment cost, and scalability, as well as stable growth, large grain size, and high purity. However, due to its coarse grains and severe doping segregation, it struggles to meet the core requirements of lithium-ion batteries for fine-grained and uniform silicon materials used as anode precursors. Therefore, there are currently no reports on using this method to prepare battery-grade silicon materials.

[0009] Existing polycrystalline silicon ingot casting furnaces typically include a furnace body, a heating and insulation system, a vacuum and gas supply system, a cooling system, and a power supply and control system. Among these, the JJL series polycrystalline silicon ingot casting furnaces produced by Zhejiang Jinggong Technology Co., Ltd. are common equipment in the domestic photovoltaic industry. The melting, directional solidification, annealing, and cooling processes of the directional solidification method are all carried out within the heating and insulation system inside the furnace body. Existing heating and insulation systems in polycrystalline silicon ingot casting furnaces, such as... Figure 1 As shown, the furnace includes an insulated cage, an insulated base plate, a heating assembly, a heat exchange platform, and a quartz crucible. A driving mechanism is located at the bottom of the insulated base plate to raise and lower it. When the insulated base plate reaches its highest position, it cooperates with the insulated cage above it to form a sealed cavity. The heat exchange platform is fixed above the insulated base plate, and a quartz crucible is mounted on the platform. The heating assembly is located inside the insulated cage. When the insulated base plate and the insulated cage are closed, the heating assembly surrounds the top and sides of the quartz crucible. Polycrystalline silicon ingot furnaces are typically equipped with a single large-sized quartz crucible to meet the needs of large-scale production. For example, the quartz crucible in the JJL800D polycrystalline silicon ingot furnace has dimensions of 1040 mm × 1040 mm × 520 mm and a rated ingot weight of 800 kg.

[0010] Existing methods for preparing photovoltaic polycrystalline silicon using directional solidification specifically include the following steps:

[0011] (1) Loading and vacuuming: Load the high-purity polycrystalline silicon material into the quartz crucible of the polycrystalline silicon ingot furnace, raise the heat insulation bottom plate and heat insulation cage to close, close the furnace body, and vacuum.

[0012] (2) Melting: Heating causes the silicon raw material to completely melt into liquid silicon;

[0013] (3) Directional solidification (crystal growth): By slowly lowering the heat-insulating base plate to fully expose the heat exchange platform supporting the quartz crucible, the heat of the heat exchange platform is radiated to the furnace wall through which the coolant is introduced, so that the melt in the crucible forms a temperature gradient from top to bottom. The crystal begins to nucleate from the bottom of the crucible and grows slowly in a vertically upward columnar crystal manner. The solidification rate is usually 0.8~1.5 cm / h (i.e., the cooling rate is about 0.13~0.25℃ / min), which aims to obtain columnar silicon ingots with large grain size and low defects.

[0014] (3) Annealing: After solidification, the silicon ingot is kept at a temperature below the melting point to eliminate internal thermal stress and reduce lattice defects;

[0015] (4) Cooling: The temperature is slowly reduced to a safe discharge temperature under program control, and polycrystalline silicon ingots are obtained after discharge.

[0016] However, existing directional solidification methods, based on the photovoltaic industry's requirements for polycrystalline silicon and limited by furnace heat capacity and radiation efficiency, result in slow crystal solidification rates (low cooling rates), leading to coarse silicon ingot grains (>500μm, even up to 8 mm) and severe doping segregation (phosphorus doping axial concentration RSD≥25%). Furthermore, the coarse grains and dense structure result in high crushing energy consumption of 1.8~2.2 kWh / kg. Moreover, the use of a single standard large-size crucible for production leads to long heat durations, large edge / center temperature differences, and uneven cooling, resulting in poor consistency of the produced silicon powder. Therefore, silicon materials produced by existing directional solidification methods cannot meet the requirements of lithium-ion battery silicon-based anodes for fine grains, uniform doping, and ease of processing.

[0017] In summary, the development of a method for preparing silicon-based anode precursors for lithium-ion batteries based on directional solidification technology not only has the advantages of existing directional solidification technologies but also yields silicon materials with quality that meets or exceeds requirements. This has significant innovative and practical value for the field of lithium-ion batteries. Summary of the Invention

[0018] The purpose of this invention is to provide a method and apparatus for preparing silicon-based anode precursor materials for lithium-ion batteries based on directional solidification, so as to solve the problem that the existing technology cannot simultaneously achieve purity, particle size uniformity and cost controllability.

[0019] To achieve the above objectives, the present invention adopts the following technical solution.

[0020] A method for preparing silicon-based anode precursor materials for lithium-ion batteries based on directional solidification is characterized by preparing solar-grade silicon material in a polycrystalline silicon ingot furnace with a modified heating and insulation system; wherein...

[0021] The modification method of the heating and insulation system of the polycrystalline silicon ingot furnace is as follows: 4 to 9 quartz crucibles of the same size are evenly arranged on the heat exchange platform to replace the original standard large-size quartz crucibles, and adjacent quartz crucibles are separated by heat insulation plates with a height not lower than their tops to form independent melting units that are thermally isolated from each other; a water-cooled plate is embedded on the upper surface of the heat insulation base plate as an active cooling unit, and the area of ​​the water-cooled plate is not less than the bottom projection range of the quartz crucible.

[0022] The preparation method includes the following steps:

[0023] (1) Loading and vacuuming: Load the solar-grade silicon material into each quartz crucible; lift the heat insulation base plate and heat insulation cage to close, close the furnace body, and vacuum;

[0024] (2) Melting and homogenization: Heat to 1420~1450℃ to completely melt the silicon material; keep it at this temperature for a period of time to ensure uniform composition;

[0025] (3) Directional solidification: When the temperature of the silicon material drops to the initial solidification temperature of 1410-1420℃, the active cooling unit is started and coolant is introduced into the cooling channel in the water-cooled plate; the instantaneous cooling rate of the solid silicon material during the process of dropping from 1410-1420℃ to 1240-1250℃ is maintained within 3.5-5.5℃ / min; finally, the silicon material solidifies into silicon ingot;

[0026] (4) Annealing: After solidification, the obtained silicon ingot is annealed;

[0027] (5) Cooling: After annealing, the silicon ingot is cooled to a safe discharge temperature and the silicon ingot is removed to obtain silicon material, which is the precursor of silicon-based negative electrode for lithium-ion batteries.

[0028] This invention improves the heating and insulation system of existing polycrystalline silicon ingot furnaces in the photovoltaic field, making it suitable for preparing silicon materials as silicon-based anode precursors for lithium-ion batteries. The polycrystalline silicon ingot furnace can be a JJL800D type polycrystalline silicon ingot furnace or a resistance heating ingot furnace with a similar thermal field structure.

[0029] The silicon material prepared by this invention has advantages such as small and controllable grain size, uniform doping height, easy crushing, high batch consistency, and good electrochemical performance, and the cost of this invention is controllable. The specific principle of this invention is as follows:

[0030] ① Grain Refinement Mechanism and Size Control: By adding an active cooling unit, the instantaneous cooling rate of the solid silicon material is precisely controlled at 3.5~5.5℃ / min during the directional solidification stage, achieving rapid solidification, significantly improving the nucleation rate, and effectively inhibiting grain growth, thereby controlling the grain size of the silicon ingot within the range of 20~50μm. This size range is based on the synergistic optimization of subsequent grinding efficiency and electrode electrochemical performance.

[0031] This instantaneous cooling rate window is the optimal balance point between silicon solidification kinetics and thermal stress determined by numerous experiments in this invention: below this rate, nucleation is insufficient and grains are coarse; above this rate (>5.5℃ / min), thermal stress is too high, and severe lattice distortion or micropore aggregation occurs in local areas due to thermal stress concentration, as shown in Comparative Example 5 below, where XRD shows broadened diffraction peaks and a significant decrease in crystallinity.

[0032] Furthermore, the grain size (20~50μm) obtained by this invention is based on the synergistic optimization of fracture mechanics and powder engineering. At this scale, a high-density and uniformly distributed grain boundary network is formed inside the material. As natural weak surfaces, grain boundaries preferentially become crack propagation paths during subsequent mechanical pulverization, promoting controlled dissociation of particles along the grain boundaries. This not only significantly reduces pulverization energy consumption but also effectively suppresses the formation of irregularly shaped particles caused by transgranular fracture, resulting in pulverized products that retain good monocrystalline properties as micron-sized silicon particles (D50=1~2μm, RSD≤15%). This size has been proven by electrode engineering practice to be the optimal balance window between cycle stability, initial coulombic efficiency, and tap density for silicon-based anode materials.

[0033] ② Achieve high uniformity doping: Under the above-mentioned controlled rapid solidification, the solidification time is significantly shortened, thereby effectively suppressing the diffusion and segregation of dopants such as boron and phosphorus at the solid-liquid interface, and achieving a relative standard deviation (RSD) of axial concentration of dopants ≤ 15%.

[0034] ③ Forming a controllable microcrack network to reduce crushing energy consumption: By controlling the instantaneous cooling rate within the range of 3.5~5.5℃ / min, the silicon ingot generates controllable thermal stress due to rapid cooling. This stress reaches the critical value for grain boundary weakening but does not exceed the material's fracture toughness, thus preferentially inducing the formation of non-penetrating closed microcracks along the grain boundaries (due to the higher energy and weaker bonding at grain boundaries). These non-penetrating closed microcracks refer to microcracks distributed along grain boundaries, with a depth less than 1 / 3 of the silicon ingot thickness, a width of 0.5~5μm, and that are not interconnected; their distribution along grain boundaries is confirmed by scanning electron microscopy (SEM). This microcrack network can serve as a preset fracture path during subsequent crushing, reducing crushing energy consumption by approximately 40% compared to traditional photovoltaic silicon, without damaging the crystal integrity of individual grains, thus maintaining a high initial coulombic efficiency.

[0035] ④ Improve uniformity and batch consistency: The design of replacing large-size quartz crucibles with multiple small quartz crucibles in parallel and the use of heat insulation plates to separate them shortens the heat conduction path and eliminates the inherent edge-center temperature difference of large-size crucibles. This makes each melting unit independently controllable, thereby significantly improving the radial uniformity of single ingots and the consistency of products between batches.

[0036] ⑤ Equipment compatibility and low cost: This invention only requires integrating a water-cooled plate into the heat-insulating base plate of the existing polycrystalline silicon ingot furnace and replacing it with a multi-crucible array, without changing the main structure of the furnace body, thus realizing the high-value and low-cost transformation and reuse of mature photovoltaic industrial equipment.

[0037] ⑥ Excellent overall product performance: The obtained silicon material has high purity (metal impurities <10 ppm, oxygen content <100 ppm, carbon content <1000 ppm), and the lithium-ion battery anode made from the silicon particles (D50=1~2μm) processed by it has significantly improved initial coulombic efficiency and cycle stability.

[0038] Preferably, in step (3), when the temperature of the solid silicon material drops to the initial solidification temperature of 1410-1420℃, the active cooling unit is started simultaneously, and coolant is introduced into the cooling channel in the water-cooled plate and the furnace jacket. The heat insulation base plate is controlled to descend from the closed position to the heat dissipation working position 200-250 mm away from the heat exchange table. At the same time, the control system of the polycrystalline silicon ingot furnace collects silicon material temperature data at a cycle frequency of 10-30 seconds, calculates the instantaneous cooling rate, dynamically adjusts the coolant flow rate / velocity and the power of the heating components, and controls the instantaneous cooling rate of the solid silicon material to be precisely stabilized at 3.5-5.5℃ / min during the process of dropping from 1410-1420℃ to 1240-1250℃.

[0039] In this invention, the heat-insulating base plate descends to a heat dissipation working position 200-250 mm away from the heat exchange platform. This allows the thermal radiation energy of the heat exchange platform to act simultaneously on the water-cooled cold plate and the furnace wall, forming synergistic heat dissipation and maximizing the thermal radiation energy of the heat exchange platform. This is beneficial for achieving uniform and controllable directional solidification. Preferably, the heat-insulating base plate adopts a variable speed strategy of initially fast and then slowing down during its descent from the closed position to the heat dissipation working position. That is, the thermal radiation channel is opened at a higher speed in the initial stage, and then the speed is gradually reduced, with the position converged in the early stage of solidification, in order to moderate the thermal field transition and avoid sudden temperature changes.

[0040] Preferably, the instantaneous cooling rate of the solid silicon material during the process of decreasing from 1410 to 1420°C to 1240 to 1250°C is maintained at 5.0 to 5.5°C / min.

[0041] Preferably, on the original heat exchange platform, four quartz crucibles of the same size are arranged in a 2×2 array, or eight quartz crucibles of the same size are arranged in a 3×3 de-centered array, replacing the original standard large-sized quartz crucibles and reducing the volume of the quartz crucibles. More preferably, eight quartz crucibles of the same size are arranged in a 3×3 de-centered array to avoid the problem of large differences in heating between the central structure and the four sides, resulting in better thermal uniformity at the edges.

[0042] Furthermore, at the bottom of the heat exchange platform corresponding to the center of the bottom of each quartz crucible, blind holes are opened and high-temperature thermocouples are embedded, with their measuring ends in close contact with the heat exchange platform. Simultaneously, both the heat-insulating base plate and the water-cooled plate have through holes coaxially aligned with the blind holes, allowing the signal leads of the high-temperature thermocouples to pass through and accommodating the vertical movement of the heat-insulating base plate. The signal leads of the high-temperature thermocouples are led out of the furnace body via a high-temperature vacuum-sealed joint and connected to the control system of the polycrystalline silicon ingot casting furnace. The control system collects the data detected by the high-temperature thermocouples, calculates the instantaneous cooling rate of the solid silicon material, and dynamically adjusts the coolant flow rate / velocity and the heating component power accordingly, controlling it to remain stable at 3.5~5.5℃ / min. This design ensures direct and rapid sensing of the temperature at the solidification front of the silicon ingot, providing the necessary conditions for achieving precise closed-loop control of the instantaneous cooling rate of 3.5~5.5℃ / min. Specifically, the blind holes of the heat exchange platform contain pre-set high-temperature thermocouple channels with ceramic sheaths.

[0043] Furthermore, an infrared thermometer is installed above each quartz crucible, and its signal lead is connected to the control system of the polycrystalline silicon ingot furnace as an auxiliary means of thermocouple measurement for monitoring.

[0044] Preferably, the water-cooled plate is made of a high thermal conductivity metal and has a spiral cooling channel ≥5mm from the upper surface inside; the upper surface of the water-cooled plate is precision polished (Ra≤1.6μm) and then sprayed with a high emissivity ceramic coating. The inlet and outlet pipes of the cooling channel are connected to a coolant source outside the polycrystalline silicon ingot casting furnace, and the flow rate of the coolant in the cooling channel is ≥180L / min. It should be noted that the coolant flow rate is only an exemplary parameter for achieving the target solidification rate under the experimental conditions; those skilled in the art can maintain an instantaneous cooling rate of 3.5~5.5 ℃ / min by adjusting the flow rate, temperature, or heat exchange area according to the equipment structure, the type of cooling medium (such as water, ethylene glycol, etc.) and environmental conditions, all of which are equivalent embodiments of the present invention. The high-emissivity ceramic coating has a full-band infrared emissivity ε≥0.90 in an oxidizing atmosphere of 500~1000℃, which is beneficial for the present invention to maintain an instantaneous cooling rate of 3.5~5.5℃ / min, and its thickness is 20~50μm.

[0045] Preferably, the water-cooled plate is configured as a plurality of small water-cooled plates, each corresponding to a quartz crucible, so that each quartz crucible can be cooled independently; more preferably, the quartz crucible and the corresponding small water-cooled plate are coaxially aligned.

[0046] Preferably, in step (3), after the solid silicon material is cooled to 1240-1250°C, it is not necessary to control the instantaneous cooling rate of the solid silicon material, but the active cooling unit is still started to maintain the fast cooling mode; when the solid silicon material is cooled to 1100-1150°C, the annealing process of the next step is started.

[0047] Preferably, in step (2), the heat preservation time is 30~60 min.

[0048] Preferably, the heat insulation board is made of graphite or carbon fiber composite material, and its surface is machined to ensure flatness.

[0049] Furthermore, the heating components located on the side of the quartz crucible are divided into three sections: upper, middle, and lower. Each section is independently powered and its power is adjustable, which facilitates dynamic control of the axial temperature gradient during solidification and optimizes the crystal growth quality.

[0050] Preferably, in step (3), the power of the upper, middle, and lower heating components is kept to increase from bottom to top, such as: the power of the upper, middle, and lower heating components are 80%, 60%, and 0%, respectively. During the directional solidification stage of silicon material, an axial gradient of "upper heat preservation, middle stable heating, and lower strong cooling" is dynamically constructed. Under rapid cooling conditions, the curvature of the solid-liquid interface is significantly reduced, approaching a straight shape, effectively suppressing the enrichment of dopants at the top, and further reducing the axial concentration RSD of dopants.

[0051] Preferably, in step (2), the power of the upper heating component is slightly higher than that of the lower heating component to ensure that the melt is fully melted and there is no local overheating, for example, 90% for the upper section, 85% for the middle section, and 80% for the lower section.

[0052] Specifically, the annealing step described in this invention can be performed using conventional silicon ingot annealing operations. As a specific embodiment of this invention, the annealing operation involves holding the silicon ingot at 900~1100℃ for 20~60 minutes, preferably 30 minutes.

[0053] This invention also provides a polycrystalline silicon ingot furnace for preparing silicon materials as precursors for lithium-ion battery silicon-based anodes, comprising a furnace body and a heating and insulation system within the furnace body; the heating and insulation system includes an insulation cage, an insulation base plate, a heating assembly, a heat exchange platform, and a quartz crucible; the lower part of the insulation base plate is provided with a driving mechanism to enable it to lift and lower, and when the insulation base plate is in its highest position, it cooperates with the insulation cage located above it to form a sealed cavity; the heat exchange platform is fixed to the upper part of the insulation base plate; the heating assembly is located inside the insulation cage, surrounding the top and sides of the quartz crucible when the insulation base plate and the insulation cage are closed; characterized in that 4 to 9 quartz crucibles of the same size are evenly arranged on the heat exchange platform, replacing the original standard large-size quartz crucibles, and adjacent quartz crucibles are separated by insulation plates with a height not lower than their tops, forming independent melting units that are thermally isolated from each other; a water-cooled plate is embedded on the upper surface of the insulation base plate as an active cooling unit, and the area of ​​the water-cooled plate is not less than the projected area of ​​the bottom of the quartz crucible.

[0054] Preferably, on the heat exchange platform, four quartz crucibles of the same size are arranged in a 2×2 array, or eight quartz crucibles of the same size are arranged in a 3×3 de-centered array, replacing the original standard large-sized quartz crucibles. More preferably, eight quartz crucibles of the same size are arranged in a 3×3 de-centered array to avoid the problem of large differences in heating between the central structure and the four sides, resulting in better thermal uniformity at the edges.

[0055] Furthermore, at the bottom of the heat exchange platform corresponding to the center of the bottom of each quartz crucible, a blind hole is opened and a high-temperature thermocouple is embedded therein, with its measuring end tightly fitted to the heat exchange platform. Simultaneously, both the heat-insulating base plate and the water-cooled plate have through holes coaxially aligned with the blind holes, allowing the signal leads of the high-temperature thermocouples to pass through and accommodating the vertical movement of the heat-insulating base plate. The signal leads of the high-temperature thermocouples are led out of the furnace body via a high-temperature vacuum-sealed joint and connected to the control system of the polycrystalline silicon ingot casting furnace. Specifically, a high-temperature thermocouple channel with a ceramic sheath is pre-set in the blind hole of the heat exchange platform.

[0056] Furthermore, an infrared thermometer is installed above each quartz crucible, and its signal lead is connected to the control system of the polycrystalline silicon ingot furnace.

[0057] Preferably, the heat insulation board is made of graphite or carbon fiber composite material, and its surface is machined to ensure flatness.

[0058] Preferably, the water-cooled plate is made of a high thermal conductivity metal and has a spiral cooling channel ≥5mm from the upper surface inside; the inlet and outlet pipes of the cooling channel are connected to a cooling liquid source outside the polycrystalline silicon ingot casting furnace. Preferably, the upper surface of the water-cooled plate is coated with a high emissivity ceramic coating; the high emissivity ceramic coating has a full-band infrared emissivity ε≥0.90 in an oxidizing atmosphere of 500~1000℃ to facilitate maintaining the instantaneous cooling rate described in this invention, and its thickness is 20~50μm.

[0059] Preferably, the upper surface of the water-cooled plate is flush with or slightly higher than the upper surface of the heat-insulating base plate by 0.1 to 0.5 mm.

[0060] Preferably, the water-cooled plate is configured as a plurality of small water-cooled plates, each corresponding to a quartz crucible, so that each plate independently cools its respective quartz crucible, and the quartz crucible and its corresponding small water-cooled plate are coaxially aligned. A through hole is pre-reserved at the geometric center of the water-cooled plate, which is coaxially aligned with the blind hole of the heat exchange platform.

[0061] Furthermore, the heating components located on the side of the quartz crucible are configured into three sections: upper, middle, and lower, each with independent power supply and adjustable power.

[0062] Compared with the prior art, the present invention has the following beneficial effects:

[0063] (1) Controlling grain size: In the directional solidification stage, the instantaneous cooling rate of the solid silicon material is precisely controlled at 3.5-5.5℃ / min, which effectively improves the nucleation density of the silicon material and inhibits excessive grain growth, thereby obtaining a fine-grained structure with a grain size of 20-50μm. This grain size not only meets the requirements of high crystal quality (such as low defect density) of silicon materials for silicon-based anodes in lithium-ion batteries, but also provides an ideal structural basis for subsequent pulverization processes. Specifically, with a grain size of 20-50μm, silicon ingots tend to undergo cleavage fracture along grain boundaries during mechanical pulverization, and the resulting powder has a near-spherical morphology. Most of the micron-sized particles are composed of single silicon grains (which can be regarded as single-crystal silicon particles); their particle size distribution is highly concentrated, D50 = 1-2μm, and the relative standard deviation (RSD) is ≤15%, that is, the present invention improves the particle size uniformity of silicon materials after pulverization.

[0064] (2) Improve the uniformity of doped elements: Integrate an active cooling unit at the bottom of each crucible to achieve rapid solidification at 3.5 to 5.5 °C / min, significantly shorten the solidification time, suppress the diffusion and segregation of doped elements (such as B and P), and make the axial concentration RSD ≤ 15%.

[0065] (3) Reduced energy consumption during crushing: In the directional solidification stage, the instantaneous cooling rate is controlled at 3.5 to 5.5℃ / min. In the later stage of solidification, due to thermal stress, a non-penetrating, closed microcrack network is formed at the grain boundary, which reduces the energy consumption of crushing by about 40% compared with traditional photovoltaic silicon. It does not damage the crystal integrity of individual grains and can still maintain a high initial coulombic efficiency.

[0066] (4) Improve material consistency: The multi-crucible + heat insulation plate design avoids cross-contamination of the melt and eliminates the temperature difference between the edge and center of the large crucible, which greatly improves the performance consistency between batches and is conducive to obtaining products with a high pass rate.

[0067] (5) High product purity: The present invention retains the advantage of high product purity of the existing directional solidification method. The resulting silicon material has metal impurities <10 ppm, oxygen content <100 ppm, and carbon content <1000 ppm, which improves the electrical performance of the lithium-ion battery anode.

[0068] (6) Shorten the production cycle: A rapid cooling mode is adopted in the directional solidification stage, which shortens the entire production cycle from 10 hours in the traditional preparation of photovoltaic polycrystalline silicon to 4.5 hours.

[0069] (7) Compatible with existing equipment and low modification cost: It only requires replacing the multi-crucible design and adding baffles and active cooling units on the basis of the original JJL800D ingot furnace, without changing the main structure, which is easy to promote industrialization; taking the JJL800 ingot furnace as an example, the modification cost of a single unit is about RMB160,000, and the annual production capacity of high-quality silicon materials is 24.7 tons; in other words, this invention not only has low production equipment and modification costs, but also realizes the high-value reuse of mature industrial equipment.

[0070] (8) Green and efficient process: Compared with CVD or ball milling, the method of the present invention has low energy consumption, no chemical waste, and high yield, which meets the needs of large-scale battery material manufacturing.

[0071] (9) For the first time, the polycrystalline silicon ingot furnace in the photovoltaic field has been successfully extended to the field of battery materials, solving the industrialization bottleneck of "high cost and difficulty in scaling up" of battery silicon, and opening up a new path for low-cost, high-performance silicon-based anode materials. Attached Figure Description

[0072] Figure 1 This is a schematic diagram of the heating and insulation system inside an existing polycrystalline silicon ingot casting furnace.

[0073] Figure 2 This is a schematic diagram of the heating and insulation system inside the polycrystalline silicon ingot furnace of the present invention;

[0074] Figure 3 A schematic diagram showing the setup of the quartz crucible and the heat insulation plate.

[0075] Figure 4 This is a cross-sectional optical schematic diagram of the silicon ingots prepared in Example 3 (A) and Comparative Example 1 (B).

[0076] Attached reference numerals: 1-furnace body, 2-insulation cage, 3-insulation base plate, 4-heating component, 5-heat exchange platform, 6-quartz crucible, 7-water-cooled plate, 8-blind hole, 9-insulation plate. Detailed Implementation

[0077] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0078] All the electrochemical performance tests in the following examples and comparative examples used the same solid-state battery configuration, electrolyte batch, and electrode fabrication process; the only variable was the source of the precursor silicon material. Therefore, the differences in initial coulombic efficiency and cycle performance can be clearly attributed to the grain size, purity, and surface condition of the silicon material itself.

[0079] Example 1: Modification of the heating and insulation system inside a polycrystalline silicon ingot casting furnace.

[0080] Taking the JJL800D polycrystalline silicon ingot casting furnace produced by Zhejiang Jinggong Technology Co., Ltd. as an example, the heating and insulation system inside the furnace body was modified. For example... Figure 2 As shown, the heating and insulation system inside the furnace body 1 includes an insulation cage 2, an insulation base plate 3, a heating assembly 4, a heat exchange platform 5, and a quartz crucible 6.

[0081] The heat-insulating base plate 3 serves as a thermal support platform, with a parallel drive screw mechanism at its lower part to enable it to lift. When the heat-insulating base plate 3 reaches its highest position, it cooperates with the heat-insulating cage 2 located above it to form a sealed cavity. The heat exchange table 5 is fixed to the upper part of the heat-insulating base plate 3. The heating component 4 is located inside the heat-insulating cage, surrounding the top and sides of the quartz crucible 6 when the heat-insulating base plate 3 and the heat-insulating cage 2 are closed.

[0082] The specific modification method is as follows: On the heat exchange platform 5, four identical quartz crucibles 6 are arranged in a 2×2 array or eight identical quartz crucibles 6 are arranged in a 3×3 decentered array to replace the original standard large-sized quartz crucibles. The volume of a single small quartz crucible is approximately 1 / 4 or 1 / 9 of the original standard quartz crucible; for example... Figure 3As shown, in a preferred embodiment, eight crucibles of the same size are arranged in a 3×3 matrix with the center position empty (“de-centering” array) to optimize thermal uniformity. A heat insulation plate 9, with a height not lower than its top, is provided between adjacent quartz crucibles to form independent melting units that are thermally isolated from each other; this heat insulation plate is made of graphite or carbon fiber composite material.

[0083] At the bottom of the heat exchange platform 5 corresponding to the center of the bottom of each quartz crucible, a blind hole 8 is made and a high-temperature resistant thermocouple (such as a type B thermocouple) is embedded therein, with its measuring end in close contact with the heat exchange platform. The heat insulation base plate 3 is provided with a through hole coaxially aligned with the blind hole 8. The heat exchange platform is an isostatically pressed part. The invention provides a non-through blind hole at its bottom for installing thermocouples, which has little impact on its structure and will not cause structural failure or significant deterioration of thermal performance of the heat exchange platform during service.

[0084] The upper surface of the heat-insulating base plate 3 is machined with a precision mounting groove, and the water-cooled plate 7 is embedded and fixed in the groove as an active cooling unit. The area of ​​the water-cooled plate 7 is not less than the projected area of ​​the bottom of the quartz crucible, and its upper surface is flush with or slightly higher than the upper surface of the heat-insulating base plate 3 by 0.1 to 0.5 mm. It is provided with a through hole coaxially aligned with the blind hole 8. Preferably, the water-cooled plate 7 can be configured as a plurality of small water-cooled plates, each corresponding to one of the quartz crucibles and coaxially aligned. In this case, a through hole coaxially aligned with the blind hole 8 is reserved at its geometric center.

[0085] The water-cooled plate 7 has a spiral cooling channel ≥5 mm from the upper surface. The inlet and outlet pipes of the cooling channel pass through the side wall of the heat-insulating base plate 3 and lead out to the outside of the furnace body 1, connecting to the cooling liquid source outside the polycrystalline silicon ingot furnace; the cooling liquid source can be an external circulating cooling system. The water-cooled plate 7 is specifically made of oxygen-free copper (OFHC, grade C10200) with a thickness of 20~30 mm. The upper surface is precision polished (Ra≤1.6μm) and then sprayed with a Pyromark® 2500 high emissivity ceramic coating, which has a full-band infrared emissivity ε=0.92 in an oxidizing atmosphere of 500~1000℃. The dry film thickness of the coating after sintering and curing is 20~50μm.

[0086] The signal lead of the thermocouple embedded at the bottom of the heat exchange platform 5 passes through the blind hole 8 of the heat exchange platform and the through holes on the corresponding heat insulation base plate and water cooling plate, and can adapt to the up and down movement of the heat insulation base plate. The signal lead is led out of the furnace body through a high-temperature vacuum sealing joint and connected to the control system of the polycrystalline silicon ingot furnace.

[0087] In this embodiment, an infrared thermometer can also be installed above each quartz crucible 6 to assist in temperature measurement.

[0088] The heating components 4 on the side of the quartz crucible 6, that is, the heating components surrounding the outermost part (encompassing the entire crucible array), are set into three sections: upper, middle and lower, each with independent power supply and adjustable power.

[0089] Examples 2-4: Preparation of silicon materials as precursors for silicon-based anodes in lithium-ion batteries

[0090] The preparation of silicon material as a silicon-based anode precursor for lithium-ion batteries using a polycrystalline silicon ingot furnace modified according to Example 1 includes the following steps:

[0091] (1) Loading and vacuuming: The solar-grade silicon material (doped with boron and phosphorus) is loaded into each small quartz crucible, the furnace is closed, and the vacuum is evacuated to ≤0.01 mbar;

[0092] (2) Melting and homogenization: Heat to 1420~1450℃ to completely melt the silicon material, and hold for 30~60 min to ensure uniform composition;

[0093] (3) Directional solidification: When the temperature of the silicon material drops to the initial solidification temperature of 1410-1420℃, the active cooling unit is activated, and coolant is introduced into the cooling channel of the water-cooled plate and the furnace jacket. The heat insulation base plate is controlled to descend smoothly from the closed position to the heat dissipation working position 200-220 mm away from the heat exchange table by the drive mechanism. During the process of the heat insulation base plate descending from the closed position to the heat dissipation working position, a speed-changing strategy of fast first and slow later is adopted. That is, the heat radiation channel is opened at a higher speed in the initial stage, and then the speed is gradually reduced and the position converges in the early stage of solidification. At the same time, the control system collects the center temperature data of the solid silicon material at a period frequency of 10-30 seconds, calculates the instantaneous cooling rate of the solid silicon material, dynamically adjusts the flow rate of coolant and the power of heating components, and controls the instantaneous cooling rate of the solid silicon material from 1410-1420℃ to 1240-1250℃ to be accurately stabilized at 3.5-5.5℃ / min. Finally, the silicon material solidifies into silicon ingot.

[0094] (4) Annealing: After solidification, the silicon ingot is held at 900~1100℃ for 20~60 min (preferably 30 min) to eliminate thermal stress;

[0095] (5) Cooling: Slowly cool to ≤200℃ under high-purity argon or high vacuum (≤0.01 mbar) and remove the silicon ingot;

[0096] The control process during the aforementioned directional solidification stage is a continuously operating dynamic closed loop, specifically:

[0097] Setting the target: When the temperature of the molten silicon drops to the initial solidification temperature of 1420~1410℃, the rapid cooling mode is activated. The target is that the instantaneous cooling rate of the solid silicon material in the range of 1420~1410℃ → 1240~1250℃ is 3.5~5.5℃ / min, preferably with a monitoring target of controlling the instantaneous cooling rate of the solid silicon material to 5.0℃ / min.

[0098] Real-time calculation: The control system collects temperature data from the bottom thermocouple every 10-30 seconds to calculate the current actual cooling rate (dT / dt).

[0099] Dual-channel adjustment:

[0100] If the measured rate is less than the target value, reduce the heating power of the upper / middle section, increase the flow rate / velocity of the bottom water-cooled plate, and increase the flow rate and velocity of the water in the furnace jacket.

[0101] If the measured rate is greater than the target value, then appropriately increase the insulation power of the lower section, reduce the flow rate / velocity of the bottom water-cooled plate, and increase the flow rate and velocity of the water in the furnace jacket.

[0102] Thermal field shaping: By independently adjusting the power of the upper, middle and lower heating components (in decreasing order), the axial temperature gradient of "upper heat preservation, middle stable heat, and lower strong cooling" is dynamically maintained to ensure a straight solid-liquid interface and suppress segregation.

[0103] Control terminated:

[0104] When the temperature at the center of the silicon ingot (calculated by the bottom thermocouple) drops to 1150℃, the system automatically exits the rapid cooling closed-loop mode and enters the annealing process.

[0105] Comparative Example 1

[0106] Using the existing JJL800D polycrystalline silicon ingot furnace, silicon materials are prepared using traditional photovoltaic polycrystalline silicon preparation methods. The specific steps are as follows:

[0107] (1) Loading and vacuuming: Load the high-purity polycrystalline silicon raw material into the quartz crucible of the polycrystalline silicon ingot furnace, lift the heat insulation bottom plate and heat insulation cage to close, close the furnace body, start the vacuum system, reduce the pressure inside the furnace to ≤0.01 mbar, and complete the leak detection.

[0108] (2) Melting: Heat to about 1420°C to completely melt the silicon raw material into liquid silicon;

[0109] (3) Directional solidification: By slowly lowering the heat insulation base plate to fully expose the heat exchange platform supporting the quartz crucible (exposing the heat exchange platform to the maximum extent to achieve radiation to the furnace wall), coolant is introduced into the furnace jacket of the polycrystalline silicon ingot furnace, so that heat is dissipated from the bottom of the crucible by radiation, and finally the silicon material is completely solidified into silicon ingot.

[0110] (4) Annealing: After solidification, the silicon ingot is kept at 900-1100 ℃ for 30 min;

[0111] (5) Cooling: Slowly cool to a safe discharge temperature (≤200℃) under high-purity argon or high vacuum (≤0.01 mbar), and polycrystalline silicon ingots are obtained after discharge.

[0112] Comparative Examples 2 and 3: Silicon materials were prepared at a rate lower than the instantaneous cooling rate of the present invention.

[0113] Silicon materials were prepared using a polycrystalline silicon ingot furnace modified according to Example 1. The process steps were basically the same as in Example 3, with the only difference being:

[0114] During the process of reducing the temperature of the silicon material from 1410-1420℃ to 1240-1250℃, the instantaneous cooling rate of the solid silicon material is controlled at 2-4.5℃ / min or 3-5℃ / min.

[0115] Comparative Examples 4 and 5: Silicon materials were prepared at a rate exceeding the instantaneous cooling rate of the present invention beyond the acceptable range.

[0116] Silicon materials were prepared using a polycrystalline silicon ingot furnace modified according to Example 1. The process steps were basically the same as in Example 3, with the only difference being:

[0117] During the process of reducing the temperature of the silicon material from 1410-1420℃ to 1240-1250℃, the instantaneous cooling rate of the solid silicon material is set to 3.5-6℃ / min or 4.5-6.5℃ / min.

[0118] Test case

[0119] The silicon ingots obtained in Examples 2-4 and Comparative Examples 1-5 were lightly crushed and sieved to obtain the target silicon material. The grain size, crystal structure, and power consumption required to crush the silicon ingots to a particle size of D50 = 1.8 μm were tested. The initial coulombic efficiency and 100-cycle capacity retention of the silicon material were also tested. The results are shown in Table 1.

[0120] The methods for testing the initial coulombic efficiency and 100-week capacity retention are as follows:

[0121] (1) Preparation of silicon-based anode: The formula is: Si 15 wt% + Li6PS5Cl (LPSCl) 80 wt% + PTFE 5 wt%; The raw materials are dry mixed by a planetary mixer and then cold-pressed into a film to obtain a silicon-based anode sheet with a thickness of about 80 μm;

[0122] (2) Preparation of lithium-ion batteries: The battery configuration is In / Li-In | LPSCl electrolyte sheet (≈300 μm) | silicon-based anode sheet

[0123] (3) The initial coulombic efficiency and 100-cycle capacity retention test conditions were as follows: temperature: 25℃; current density: 0.1C (1C = 2000 mA / g); voltage window: 0.01~0.8 V vs. Li + / Li; EIS: 100 kHz ~ 10 mHz, amplitude 10 mV.

[0124] Table 1

[0125] project Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 2 Example 3 Example 4 Comparative Example 4 Comparative Example 5 Crucible arrangement Single-crate, traditional photovoltaic polycrystalline silicon 8 pots, 3×3 centering 8 pots, 3×3 centering 4 pots, 2×2 8 pots, 3×3 centering 8 pots, 3×3 centering 8 pots, 3×3 centering 8 pots, 3×3 centering Average cooling rate (°C / min) 0.25 3.3 3.4 4.8 5 5 5 5.2 Instantaneous cooling rate (°C / min) — 2~4.5 3~5 3.5~5.5 3.5~5.5 3.5~5.5 3.5~6 4.5~6.5 Grain size (μm) >500 75～100 50～70 25～50 20～35 20～30 15～35 10～25 Crystal structure characteristics Large columnar crystals, dense and crack-free Coarse columnar crystals with a small number of micropores Mixed grains, localized microcracks Fine-grained + non-penetrating microcrack structure Fine-grained + non-penetrating microcrack structure Fine-grained + non-penetrating microcrack structure Localized microcracks connected, with a small number of transgranular cracks. Severe lattice distortion and accumulation of micropores B / P Axial Concentration RSD (%) >25 22 18.5 14 14.2 13.8 13.5 13.7 First Coulomb Efficiency (%) 70～72 72 76 81.2 86.5 83.3 79.5 71.5 100-week capacity retention rate (%) ≈50 ≈53 ≈67 80.5 81 82 79.8 55 Power consumption for grinding (kWh / kg) ~2.1 ~1.7 ~1.4 ~1.2 ~1.2 ~1.2 ~1.2 ~1.0* Compared to Comparative Example 1, the percentage reduction in crushing energy consumption is as follows: — ≈19% ≈33% ≥40% ≥40% ≥40% ≥40% —

[0126] Note:

[0127] 1. All samples were pulverized to silicon particle size D50=1.8 μm using the same air jet milling process and their electrochemical performance was tested in the same solid-state battery system (Si 15 wt% + LPSCl 80 wt% + PTFE 5 wt%) to ensure the fairness and validity of the comparison.

[0128] 2. Although Comparative Example 5 has a lower power consumption for crushing, it cannot obtain a negative electrode material with qualified electrochemical performance due to serious structural defects in the material, and therefore has no practical value.

[0129] 3. The average cooling rate is calculated from the actual temperature measured by the thermocouple and is defined as the average cooling rate of the silicon melt from 1410 to 1420°C to 1240 to 1250°C.

[0130] 4. The instantaneous cooling rate is calculated from the actual temperature data measured by thermocouples, taking the ratio of the temperature change between adjacent sampling points to the time interval (ΔT / Δt); the listed maximum and minimum values ​​are the extreme values ​​of the instantaneous cooling rate of the silicon melt during the cooling process from 1410~1420℃ to 1240~1250℃.

[0131] Analysis and discussion of the results of the above experimental cases:

[0132] As shown in Table 1, in Examples 2 to 4 of the present invention, the instantaneous cooling rate of the solid silicon material in the directional solidification stage from 1410 to 1420°C to 1240 to 1250°C was precisely controlled within a key process window of 3.5 to 5.5°C / min in the modified polycrystalline silicon ingot furnace, thus successfully preparing silicon-based anode precursor materials with excellent performance.

[0133] The resulting silicon material has a uniform fine-grained structure of 20–50 μm, and under controllable thermal stress, a large number of non-penetrating, closed-type microcrack networks (such as...) are selectively formed along the grain boundaries. Figure 4(As shown in A). This microstructure provides an ideal intergranular fracture path for subsequent mechanical pulverization, significantly reducing the energy consumption for pulverizing the material to the target particle size (D50 = 1.8 μm) to approximately 1.2 kWh / kg, a reduction of ≥40% compared to traditional photovoltaic polycrystalline silicon (Comparative Example 1, approximately 2.1 kWh / kg). Simultaneously, thanks to its high purity (O < 100 ppm, C < 1000 ppm), fine grain size, and intact single-crystal morphology, this material exhibits superior electrochemical performance under the same solid-state battery configuration and testing conditions: initial coulombic efficiency reaches 81.2%–86.5%, and 100-cycle capacity retention remains stable at 80.5%–82.0%, fully meeting the application requirements of high-energy-density lithium-ion battery anode materials.

[0134] In contrast, Comparative Example 1 employed a conventional single-crucible slow cooling process (average cooling rate of only about 0.25℃ / min), forming large columnar crystals >500μm in size, with a dense structure and no pre-existing cracks. This not only resulted in difficult crushing and high energy consumption, but also exacerbated the axial segregation of dopants such as boron and phosphorus during the prolonged solidification process (relative standard deviation of concentration RSD > 25%), ultimately leading to poor battery cycle stability (100-cycle retention ≈ 50%) and low initial coulombic efficiency (70%–72%).

[0135] More importantly, the experimental data clearly verified the technical criticality of the cooling rate window of 3.5–5.5 °C / min:

[0136] When the rate is low (Comparative Examples 2-3, average cooling rate is 2.0-3.3℃ / min), although the grains are refined (150-450μm), an effective microcrack network is not formed, and solute segregation is not sufficiently suppressed (RSD ≥ 18.5%). Its electrochemical performance (first coulombic efficiency <80%, 100-cycle retention rate ≤67%) does not meet the practical threshold.

[0137] When the rate is too high, the performance of the resulting silicon material also deteriorates. Although Comparative Example 4 is still in the fine-grained range, local microcracks and a small number of transgranular cracks have appeared, which have damaged the integrity of the particles and led to a significant decrease in cycle stability (79.8%) and first-cycle efficiency (79.5%). Comparative Example 5, due to excessively rapid cooling, caused severe lattice distortion and micropore aggregation. Although the crushing energy consumption was the lowest (about 1.0 kWh / kg), the crystal integrity was significantly damaged, and the electrochemical performance deteriorated sharply (100-cycle retention rate of only 55%, first-cycle coulombic efficiency of 71.5%), completely losing its application value.

[0138] The above results indicate that controlling the instantaneous cooling rate of 3.5–5.5 °C / min is the key process condition for achieving the synergistic optimization of "fine grains (20–50 μm) + intergranular closed microcracks + high structural integrity". This window cannot be easily obtained by those skilled in the art through conventional trial and error, but is a core innovation established based on a deep understanding of silicon solidification kinetics, thermal stress evolution, and fracture behavior.

[0139] Furthermore, the axial concentration RSD of B / P in all embodiments of the present invention (Examples 2-4) was ≤15.0%, significantly better than that of Comparative Example 1 (>25%). This indicates that the multi-crucible array arrangement (4 or 8 crucibles) combined with the thermal isolation design of the heat insulation plate adopted in the present invention effectively weakens the radial temperature gradient and thermal convection in large-size melts. Combined with the rapid solidification process with precise speed control, it jointly suppresses solute redistribution during solidification, thereby significantly improving the axial uniformity of composition.

[0140] In summary, this invention, through multi-crucible thermal field isolation, decentralized arrangement, and synergistic control of active water cooling units and instantaneous cooling rates of 3.5–5.5 °C / min, has for the first time achieved efficient, low-cost, and green preparation of silicon-based anode materials in a mature photovoltaic ingot system. These materials possess high initial coulombic efficiency (≥81.2%), long cycle life (≥80.5%), low crushing energy consumption (↓≥40%), and high compositional uniformity (RSD ≤15.0%). This invention completely breaks through the industrialization technology bottleneck of traditional cast silicon in lithium battery applications, which is characterized by "difficult to crush, low initial efficiency, and rapid degradation".

Claims

1. A method for preparing silicon materials as silicon-based anode precursors for lithium-ion batteries based on directionally solidified silicon, characterized in that, Solar-grade silicon is prepared in a polycrystalline silicon ingot furnace with a modified heating and insulation system; among which... The modification method of the heating and insulation system of the polycrystalline silicon ingot furnace is as follows: 4 to 9 quartz crucibles of the same size are evenly arranged on the heat exchange platform to replace the original standard large-size quartz crucibles, and adjacent quartz crucibles are separated by heat insulation plates with a height not lower than their tops to form independent melting units that are thermally isolated from each other; a water-cooled plate is embedded on the upper surface of the heat insulation base plate as an active cooling unit, and the area of ​​the water-cooled plate is not less than the bottom projection range of the quartz crucible. The preparation method includes the following steps: (1) Loading and vacuuming: Load the solar-grade silicon material into each small quartz crucible; lift the heat insulation base plate and heat insulation cage to close, close the furnace body, and vacuum; (2) Melting and homogenization: Heat to 1420~1450℃ to completely melt the silicon material; keep it at this temperature for a period of time to ensure uniform composition; (3) Directional solidification: When the temperature of the silicon material drops to the initial solidification temperature of 1410-1420℃, the active cooling unit is started and coolant is introduced into the cooling channel in the water-cooled plate; the instantaneous cooling rate of the solid silicon material during the process of dropping from 1410-1420℃ to 1240-1250℃ is maintained within 3.5-5.5℃ / min; finally, the silicon material solidifies into silicon ingot; (4) Annealing: After solidification, the obtained silicon ingot is annealed; (5) Cooling: After annealing, the silicon ingot is cooled to a safe discharge temperature and the silicon ingot is removed to obtain silicon material, which is the precursor of silicon-based negative electrode for lithium-ion batteries.

2. The preparation method according to claim 1, characterized in that, in In step (3), when the temperature of the silicon material drops to the initial solidification temperature of 1410-1420℃, the active cooling unit is started simultaneously, and coolant is introduced into the cooling channel in the water-cooled plate and the furnace jacket. The heat insulation base plate is controlled to descend from the closed position to the heat dissipation working position 200-250 mm away from the heat exchange table. At the same time, the control system of the polycrystalline silicon ingot furnace collects silicon material temperature data at a frequency of 10-30 seconds, calculates the instantaneous cooling rate, and dynamically adjusts the coolant flow rate / velocity and the power of the heating components so that the instantaneous cooling rate of the silicon material during the process of dropping from 1410-1420℃ to 1240-1250℃ is accurately stabilized at 3.5-5.5℃ / min.

3. The preparation method according to claim 2, characterized in that, On the original heat exchange platform, four quartz crucibles of the same size are arranged in a 2×2 array, or eight quartz crucibles of the same size are arranged in a 3×3 decentered array.

4. The preparation method according to claim 3, characterized in that, The water-cooled plate is made of high thermal conductivity metal and has a spiral cooling channel inside. Its upper surface is precision polished and then coated with a high emissivity ceramic coating. The inlet and outlet pipes of the cooling channel are connected to the cooling liquid source outside the polycrystalline silicon ingot furnace. The high emissivity ceramic coating has a full-band infrared emissivity ε≥0.90 in an oxidizing atmosphere of 500~1000℃ and its thickness is 20~50μm.

5. The preparation method according to claim 4, characterized in that, The heating components on the side of the quartz crucible are divided into three sections: upper, middle, and lower. Each section is powered independently and the power is adjustable. In step (3), the power of the heating components in the upper, middle, and lower sections is kept to increase from bottom to top.

6. A polycrystalline silicon ingot casting furnace for preparing silicon-based anode precursor materials for lithium-ion batteries, comprising a furnace body and a heating and insulation system within the furnace body; the heating and insulation system includes an insulation cage, an insulation base plate, a heating assembly, a heat exchange platform, and a quartz crucible; the lower part of the insulation base plate is provided with a parallel drive screw system to enable it to lift and lower, and when the insulation base plate is in its highest position, it cooperates with the insulation cage located above it to form a sealed cavity; the heat exchange platform is fixed to the upper part of the insulation base plate; the heating assembly is located inside the insulation cage, and surrounds the top and sides of the quartz crucible when the insulation base plate and the insulation cage are closed; characterized in that... Four to nine quartz crucibles of the same size are evenly arranged on the heat exchange platform to replace the original standard large-sized quartz crucibles. Adjacent quartz crucibles are separated by heat insulation plates with a height not lower than their tops to form independent melting units that are thermally isolated from each other. A water-cooled plate is embedded on the upper surface of the heat insulation base plate as an active cooling unit. The area of ​​the water-cooled plate is not less than the bottom projection range of the quartz crucible.

7. The polycrystalline silicon ingot casting furnace according to claim 6, characterized in that, in On the heat exchange platform, four quartz crucibles of the same size are arranged in a 2×2 array, or eight quartz crucibles of the same size are arranged in a 3×3 decentered array.

8. The polycrystalline silicon ingot casting furnace according to claim 7, characterized in that, At the bottom of the heat exchange platform corresponding to the center of the bottom of each quartz crucible, a blind hole is opened and a high-temperature resistant thermocouple is embedded therein, with its temperature measuring end in close contact with the heat exchange platform; at the same time, the heat insulation base plate and the water-cooled plate are respectively provided with through holes coaxially aligned with the blind holes, for the signal leads of the high-temperature resistant thermocouples to pass through and to accommodate the up and down movement of the heat insulation base plate. The signal leads of the high-temperature resistant thermocouples are led out of the furnace body through a high-temperature vacuum sealing joint and connected to the control system of the polycrystalline silicon ingot casting furnace.

9. The polycrystalline silicon ingot casting furnace according to claim 8, characterized in that, The heat insulation plate is made of graphite or carbon fiber composite material; the water-cooled plate is made of high thermal conductivity metal and has a spiral cooling channel inside; the inlet and outlet pipes of the cooling channel are connected to the cooling liquid source outside the polycrystalline silicon ingot furnace; the upper surface of the water-cooled plate is coated with a high emissivity ceramic coating, which has a full-band infrared emissivity ε≥0.90 in an oxidizing atmosphere of 500~1000℃ and a thickness of 20~50μm.

10. The polycrystalline silicon ingot casting furnace according to claim 9, characterized in that, The upper surface of the water-cooled plate is flush with or slightly higher than the upper surface of the heat-insulating base plate by 0.1 to 0.5 mm; the water-cooled plate is configured as a plurality of small water-cooled plates, each corresponding to a quartz crucible, so that it independently cools its respective quartz crucible, and the quartz crucible and the corresponding water-cooled plate are coaxially aligned; a through hole is reserved in the geometric center of the water-cooled plate, which is coaxially aligned with the blind hole of the heat exchange platform.

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