A method of powdering silicon nanowires

Abstract

The application discloses a silicon nanowire powderization method, which comprises the following steps: preparing silicon nanowires into regular-shaped blocks with consistent thickness and bulk density; drying the regular-shaped blocks in a drying box; allowing the mutually intersecting silicon nanowires to be broken by external mechanical force, and becoming blocks with compact appearance; and processing the compacted block-shaped silicon nanowires into powder by external mechanical force. The silicon nanowires are sheared and broken by external pressure; the bulk density of the silicon nanowires is controlled by adjusting the filter pressing pressure, so that the length of the broken silicon nanowires is regulated, and the subsequent carbon coating process is facilitated. The shearing and pulverization do not need any additives, so that the pollution of the additives to the silicon nanowires is avoided, and the good performance of the silicon nanowires, such as high capacity and high initial efficiency, is ensured. The required equipment is common and low in price, the production process is simple and efficient, no additional auxiliary materials are needed, and therefore the processing cost is low.

Description

Technical Field

[0001] This invention relates to a lithium-ion battery material, and more particularly to a method for pulverizing silicon nanowires. Background Technology

[0002] Currently, graphite remains the dominant anode material for lithium-ion batteries. However, graphite anode materials are approaching their theoretical capacity, making it difficult to meet the demands of products requiring long battery life. Silicon possesses a theoretical specific capacity 10 times that of graphite anodes, a lower lithium storage reaction voltage plateau, and is widely distributed in nature, second only to oxygen in abundance in the Earth's crust. Therefore, silicon-based anode materials are a promising new type of high-energy material. Among them, silicon nanowires (SiNWs), due to their one-dimensional effect, exhibit a much smaller volume change during charge and discharge compared to nano-silicon particles, making them more valuable for applications in novel high-capacity lithium-ion anode materials.

[0003] The SiNWs prepared by the VLS method range in length from tens of micrometers to millimeters. After cleaning and purification, they are entangled clumps, which are difficult to use directly for subsequent modification and application.

[0004] The SiNWs prepared by the VLS method have diameters ranging from tens of nanometers to 250 nm. The surface accumulates a large number of positive and negative charges, making silicon nanowires prone to aggregation. The large surface area and high surface energy of silicon nanowires make them energy unstable, and they easily aggregate to reach a stable state. The extremely short distance between silicon nanowires means that the van der Waals attraction between them is much greater than their own gravity, thus attracting each other and causing aggregation. The surface hydrogen bonds and chemical bonds between silicon nanowires also lead to mutual attraction and aggregation.

[0005] There is a need to develop a technology to break silicon nanowires, which are as long as millimeters or hundreds of micrometers, into lengths of 2 to 20 μm, resulting in uniformly dispersed silicon nanowire powder. This powder can be used to modify silicon nanowires with other materials or directly as a negative electrode material in battery slurry production.

[0006] Powdering of silicon nanowires is a technique that physically shears silicon nanowire clumps into shorter nanowires and uniformly disperses them into powder using external mechanical force. There are three main physical dispersion methods: high-energy processing, ultrasonic dispersion, and mechanical dispersion. Chemical dispersion cannot shear long silicon nanowires into short ones and is therefore unsuitable for silicon nanowire pulverization.

[0007] High-energy processing method: Utilizing the action of high-energy particles to enhance the surface activity of nanoparticles, enabling them to react chemically with other substances, thereby achieving a dispersion effect.

[0008] Ultrasonic dispersion method: In order to avoid or reduce the aggregation of nanoparticles, the particle suspension to be treated is placed in an ultrasonic field and treated with scientific ultrasound.

[0009] Mechanical dispersion is the most commonly used physical dispersion method. It involves using external shear or impact forces to disperse nanoparticles in a medium. Common mechanical dispersion methods include grinding dispersion, ball milling dispersion, sand milling dispersion, and high-shear dispersion.

[0010] Grinding and dispersing: Utilizing the different speeds of the rollers in a three-roll or multi-roll mill, the abrasive material is fed into the feeding groove between the feeding roller (rear roller) and the middle roller. The two rollers rotate inward at different speeds. Some of the abrasive material enters the feeding gap and is subjected to strong shearing force. Passing through the feeding gap, the abrasive material is divided into two parts: one part adheres to the feeding roller and returns to the feeding groove, while the other part is carried by the middle roller to the paint scraping gap between the middle and front rollers, where it is subjected to even stronger shearing force again. After passing through the paint scraping gap, the abrasive material is divided into two parts again: one part is carried by the front roller to the scraper and falls into the scraper disc, while the other part returns to the feeding groove. This process is repeated several times to achieve dispersion. However, using a three-roll or multi-roll mill results in low efficiency and high energy consumption, which cannot meet the needs of large-scale production.

[0011] Ball milling dispersion: Through the mutual rolling and collision between grinding balls and between grinding balls and cylinder in the ball mill, the powder particles in contact with the steel balls are crushed or ground, and at the same time, the mixture is uniformly dispersed by a highly turbulent mixing effect in the gaps between the balls.

[0012] Sand milling dispersion: Sand milling is an extension of ball milling. The only difference is that the grinding media are fine beads or sand. Sand mills can continuously feed nanoparticles. When the premixed slurry of nanoparticles passes through the cylinder, it is subjected to intense impact and shearing action from the violently agitated sand particles, allowing the nanoparticles to be well dispersed in the slurry. The dispersed slurry leaves the sand grinding zone and overflows through the outlet screen, which traps the sand particles and returns them to the cylinder. Dispersion using ball mills and sand mills can achieve good dispersion effects and material fineness, but both ball mills and sand mills cannot avoid the disadvantages of low processing efficiency and high energy consumption.

[0013] High-shear dispersion: High-shear dispersion is achieved using a high-shear disperser. Its core component is the stator / rotor structure. The high tangential velocity and high-frequency mechanical effect generated by the high-speed rotation of the rotor bring strong kinetic energy, subjecting the material to intense mechanical shearing, hydraulic shearing, centrifugal extrusion, liquid layer friction, impact tearing, and turbulence within the narrow gap between the stator and rotor. This allows immiscible solid, liquid, and gas phases to be instantly, uniformly, and finely dispersed under appropriate mature process conditions. Through high-frequency repetition, a stable, high-quality product is ultimately obtained. Compared with three-roll mills, ball mills, and sand mills, high-shear dispersers have significant advantages such as high efficiency and low energy consumption, making them the preferred choice for dispersion processes.

[0014] Since the silicon nanowires obtained using VLS technology are one-dimensional single-crystal linear products grown along the 111 and 112 crystal planes, they have an oxide layer of 2-5 nm thickness on their surface. Numerous silicon nanowires of varying lengths intertwine and stack together to form a lightweight, soft, and tough flocculent.

[0015] High-energy processing is suitable for surface modification of nanoparticles, but it cannot turn long silicon nanowires into short silicon nanowires, so it is not suitable for pulverizing silicon nanowires.

[0016] Ultrasonic dispersion can disperse low-concentration silicon nanowires in a liquid medium to a certain extent, but it cannot break long silicon nanowires into short ones. After drying, the result is a silicon nanowire cake, not powder.

[0017] The grinding and dispersion method can disperse low-concentration silicon nanowires in liquid media to a certain extent, but it cannot break long silicon nanowires into short silicon nanowires. After drying, what is obtained is a silicon nanowire cake, not powder.

[0018] Mechanical ball milling and sand milling can disperse low-concentration silicon nanowires into short silicon nanowire powder in inert organic liquid media, but they require a large amount of inert organic solvents. Furthermore, they are inefficient and costly because they can only process low-concentration silicon nanowires, making them unsuitable for large-scale production.

[0019] High-shear dispersion using a high-speed blender can break up silicon nanowire clumps into light, fluffy particles. However, these fluffy particles will spin freely in the blender's container with the shear blades and cannot effectively break long silicon nanowires into short ones.

[0020] Existing physical dispersion techniques cannot efficiently and cost-effectively pulverize silicon nanowires. Summary of the Invention

[0021] The purpose of this invention is to provide a method for the efficient and low-cost powdering and dispersion of silicon nanowires.

[0022] To achieve the above objectives, the present invention is implemented according to the following technical solution:

[0023] This invention includes the following steps:

[0024] S1: Silicon nanowires are fabricated into regularly shaped blocks with relatively uniform thickness and packing density. First, the silicon nanowires are placed in a solvent and stirred into a uniformly dispersed slurry using a stirrer. Then, the solvent is removed by filtration to obtain regularly shaped blocks with relatively uniform thickness and packing density. The filtration includes pressure filtration and vacuum filtration.

[0025] Preferably, the solvent is one or more selected from deionized water, NMP, DMF, benzene, toluene, pentane, hexane, methanol, ethanol, diethyl ether, ethyl acetate, acetone, and carbon tetrachloride. The mass concentration of the silicon nanowires and the solvent is between 1% and 30%, preferably between 3% and 5%. The mixer includes a dispersion mixer and a double planetary vacuum mixer, and the mixture is stirred until there are no visible lumps and the mixture is a homogeneous slurry. The pressure filtration adopts a pressure-controlled filtration method with a pressure of 0.1 MPa to 30 MPa, preferably 1 MPa to 3 MPa.

[0026] As another implementation, step S1 is replaced by the following scheme: the cleaned silicon nanowire slurry is dried, and the dried silicon nanowire clumps are first dispersed into loose flocs using a wall-breaking machine. The flocs are then loosely packed into a rectangular mold, and the surface of the mold is flattened and compacted with a scraper to form a regularly shaped block.

[0027] S2: Place the regularly shaped blocks in a drying oven for drying; the drying oven includes circulating air drying and vacuum drying, preferably vacuum drying. The vacuum drying temperature is 30℃~120℃, preferably 45℃~80℃; the drying vacuum degree is -0.06MPa~-0.095Mpa.

[0028] S3: The dried silicon nanowire block is mechanically broken by pressing the intersecting silicon nanowires together to form a dense block; the breaking includes flat pressing and rolling pressing. Rolling pressing is preferred, and the rolling pressing is at a constant pressure, with a breaking pressure of 3MPa to 100MPa, preferably 12MPa to 30MPa.

[0029] S4: The compacted bulk silicon nanowires are processed into powder using mechanical external force. The mechanical external force includes grinding, ball milling, and stirring. Preferably, the stirring is performed using a dual planetary vacuum stirrer.

[0030] The beneficial effects of this invention are:

[0031] This invention relates to a method for pulverizing silicon nanowires, which has the following significant advantages compared to existing technologies:

[0032] 1. High efficiency: This invention utilizes the mutual shearing action of intersecting silicon nanowires under external pressure to quickly cut the silicon nanowires.

[0033] 2. Controllable: By adjusting the filtration pressure of the slurry, the packing density (porosity) of the silicon nanowires is controlled, thereby achieving the goal of regulating the fracture length of the silicon nanowires. The length of the silicon nanowires after compression can be controlled within 2µm ≤ L. 90 ≤20um (more than 90% of silicon nanowires are between 2um and 20um in length), which helps with subsequent processes such as carbon coating.

[0034] 3. Purity: The cutting and pulverization of silicon nanowires does not require the addition of any additives, avoiding contamination of silicon nanowires by additives and ensuring the high capacity and high first-efficiency performance of silicon nanowires.

[0035] 4. Low cost: The required equipment is common and inexpensive industrial equipment. The production process is simple and efficient, and no additional auxiliary materials are required, so the processing cost is low. Attached Figure Description

[0036] Figure 1 It is a cluster of original silicon nanowires;

[0037] Figure 2 The silicon nanowire powder dispersed by this technology is the product of this invention.

[0038] Figure 3 This invention utilizes dispersed silicon nanowire powder to fabricate electrode sheets. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The illustrative embodiments and descriptions herein are used to explain the present invention, but are not intended to limit the present invention.

[0040] This invention is a simple, efficient, and economical silicon nanowire pulverization technology developed after experimental verification showed that existing high-energy processing methods, ultrasonic dispersion methods, and mechanical dispersion methods are all difficult to achieve ideal silicon nanowire pulverization. The principle is to use strong mechanical pressure to break the disordered stacked silicon nanowires into short silicon nanowires of different lengths within the disordered stacked silicon nanowires by utilizing the strong pressure generated at the intersections of the silicon nanowires. The breaking length of the short silicon nanowires is controlled by the degree of their stacking density. The specific technical solution is as follows.

[0041] 1. Shaping: The silicon nanowires are fabricated into regularly shaped blocks with consistent thickness and packing density to ensure uniform pressure across the nanowires during subsequent crushing. The cleaned silicon nanowire slurry is directly added to a solvent and stirred to disperse it into a uniform slurry. The slurry is then pumped into a rectangular cavity mold to filter out water, forming a regularly shaped filter cake.

[0042] The solvent can be deionized water or a commonly used organic solvent, including but not limited to NMP, DMF, benzene, toluene, pentane, hexane, methanol, ethanol, diethyl ether, ethyl acetate, acetone, carbon tetrachloride, etc., with deionized water being preferred.

[0043] The mass concentration of the silicon nanowire slurry is controlled between 1% and 30%, preferably 3% to 5%.

[0044] The mixer can be a conventional dispersion mixer or a double planetary vacuum mixer. To ensure effective dispersion and improve dispersion efficiency, a double planetary vacuum mixer is preferred. No mixing parameters are required; mix until there are no visible lumps and the mixture is a homogeneous slurry.

[0045] The filtration pressure determines the packing density of silicon nanowires. The higher the packing density, the smaller the gaps between the silicon nanowires, and the shorter the length after compression. To ensure the stability of the compression breakage length, a pressure filtration method with controllable filtration pressure is preferred, with a filtration pressure of 0.1 MPa to 30 MPa, preferably 1 MPa to 3 MPa.

[0046] Another shaping method can be adopted: dry the cleaned silicon nanowire slurry, then disperse the dried silicon nanowire clumps into loose flocs using a high-speed blender, then loosely pack the flocs into a rectangular mold, and use a scraper to smooth the surface of the mold.

[0047] 2. Filter Block Drying: Place the elongated filter blocks in a vacuum drying oven for drying. Drying can be done using either circulating air drying or vacuum drying, but vacuum drying is preferred to prevent oxidation of the silicon nanowires and ensure high initial charge / discharge efficiency. Vacuum drying also effectively removes the moisture adsorbed by the high specific surface area silicon nanowires, preventing agglomeration during subsequent powdering.

[0048] Drying temperature range: 30℃~120℃, preferably 45℃~60℃.

[0049] Drying vacuum degree: -0.06MPa to -0.095MPa; the vacuum degree affects the dispersion effect and the degree of oxidation, but there is no significant change in the dispersion effect and the degree of oxidation within this range.

[0050] 3. Compression breaking: Remove the filter cloth from the dried silicon nanowire block, or use mechanical force to break the intersecting silicon nanowires into a dense block.

[0051] There are two methods of pressing: flat pressing and roller pressing. Constant pressure roller pressing is preferred.

[0052] Break-off pressure: 3MPa~100MPa, preferably 12MPa~30MPa.

[0053] 4. Dispersion: The compacted bulk silicon nanowires are dispersed into powder through mechanical stirring and shearing. Dispersion can be achieved by grinding, ball milling, or stirring. To prevent oxidation of the silicon nanowires and ensure effective dispersion, dual planetary vacuum stirring is preferred.

[0054] The stirring speed has no significant effect on the dispersion effect. High-speed stirring has a short time, while low-speed stirring has a long time. High-speed stirring is preferred.

[0055] like Figure 1-3 As shown in the figure, after the silicon nanowires are broken and dispersed, most of the silicon nanowires are about 5 μm in length. The electrode made by using powdered silicon nanowires has a uniform and smooth surface, achieving the purpose of powdering.

[0056] The technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made in accordance with the technical solutions of the present invention fall within the protection scope of the present invention.

Claims

1. A method for pulverizing silicon nanowires, characterized in that, Includes the following steps: S1: The silicon nanowires are fabricated into regularly shaped blocks with relatively uniform thickness and packing density; specifically, the silicon nanowires are first placed in a solvent and stirred into a uniformly dispersed slurry using a stirrer, and then the solvent is removed by filtration to obtain regularly shaped blocks with relatively uniform thickness and packing density; the filtration is either pressure filtration or vacuum filtration. The mass concentration of the silicon nanowires and solvent is 1% to 30%. The pressure filtration adopts a pressure-controlled pressure filtration method with a pressure of 0.1 MPa to 30 MPa. S2: Dry the block; the drying includes either circulating air drying or vacuum drying; the vacuum drying temperature is 30℃~120℃, and the drying vacuum degree is -0.06MPa~-0.095Mpa; S3: After drying, the silicon nanowire block is broken by mechanical force, and the intersecting silicon nanowires are crushed to form a dense block. S4: Compacted bulk silicon nanowires are processed into powder using mechanical external force.

2. The silicon nanowire powdering method according to claim 1, characterized in that: The solvent is one or more of the following: deionized water, NMP, DMF, benzene, toluene, pentane, hexane, methanol, ethanol, diethyl ether, ethyl acetate, acetone, and carbon tetrachloride.

3. The silicon nanowire powdering method according to claim 1, characterized in that: The mixer is a dispersion mixer or a double planetary vacuum mixer, and is used to mix until there are no visible lumps and the mixture is a uniform slurry.

4. The silicon nanowire powdering method according to claim 1, characterized in that: Step S1 specifically involves: adding the cleaned silicon nanowires directly into a solvent, stirring and dispersing them into a uniform slurry, and then pumping the slurry into a rectangular cavity mold to filter out the water, forming a filter cake with a regular shape.

5. The silicon nanowire powdering method according to claim 1, characterized in that: In step S3, the pressure breaking can be either flat pressure or roller pressure; the roller pressure is constant pressure, and the pressure breaking pressure is 3MPa~100MPa.

6. The silicon nanowire powdering method according to claim 1, characterized in that: The mechanical force in step S4 includes one of grinding, ball milling, or stirring; the stirring is carried out using a double planetary vacuum stirring device.

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