Silicon nanowire composite material and preparation method and application thereof

Abstract

The invention provides a silicon nanowire composite material and a preparation method and application thereof. The silicon nanowire composite material comprises an inner core and a coating layer, the inner core comprises silicon nanowires and expanded graphite, the silicon nanowires are embedded in lamellar gaps of the expanded graphite, and the coating layer comprises a conductive polymer. Porous, flexible, high-elasticity and good-conductivity expanded graphite is used as a framework, the cycle life of the silicon nanowire is effectively prolonged, the outer layer of the inner core is coated with a layer of uniform, compact and flexible conductive polymer, the negative influence of low first effect of the expanded graphite is eliminated, and the service life of the silicon nanowire is prolonged. And the capacity and the cycle performance of the silicon nanowire composite material are further improved.

Description

Technical Field

[0001] The present invention belongs to the field of lithium ion batteries and relates to a silicon nanowire composite material and a preparation method and application thereof. Background Art

[0002] Lithium-ion power batteries are typically composed of a current collector, anode, cathode, separator, electrolyte, and casing. Increasing the amount of lithium atoms embedded in the positive and negative electrode materials is the most direct and effective way to boost the overall battery cell's energy storage capacity. Currently, commercially available cathode materials are approaching their theoretical capacity, leaving limited room for further improvements in battery cell energy density.

[0003] For example, the gram capacity of graphite-based anode materials is already very close to the theoretical gram capacity of 372 mAh / g, leaving little room for improvement. Silicon-based anode materials, on the other hand, have a gram capacity of 3600 mAh / g, and their lithium storage capacity is almost 10 times that of graphite-based carbon anode materials. However, the problem is that the silicon added to the battery anode expands during charging, forming an alloy with lithium. During discharge, as lithium is released from the silicon, the silicon contracts, causing its volume to change by over 300%. This drastic expansion / contraction cycle often leads to silicon shattering, causing a sharp decline in battery cell performance.

[0004] Silicon nanowires have excellent electron and hole transport properties. Through microstructure optimization, they can achieve faster electron conduction rates and significantly improve the energy density of batteries. At the same time, unlike general silicon structures, they can release stress well through axial expansion without causing cracking or damage to the nanowires, thereby preventing the powdering of the electrodes. Therefore, compared with other silicon-based materials, they have better capacity retention performance and extend the cycle life of the battery cell.

[0005] However, silicon nanowires still have the problem of radial expansion. During the battery cycle, the connection with the conductive agent and adhesive in the radial direction will still deteriorate, resulting in a gradual increase in internal resistance and a decrease in battery discharge performance. To overcome the defects caused by the radial expansion of silicon nanowires, silicon nanowires need to be modified, for example:

[0006] (1) Composite of silicon and carbon-based materials: Existing technologies combine silicon particles with carbon-based materials, which is an effective method to improve the performance of silicon negative electrodes. For example, silicon-carbon composite materials mainly include silicon / graphene, silicon / carbon nanotubes and core-shell silicon / carbon composite materials. The carbon-based matrix carrier can not only provide a conductive network for the silicon active material, but also alleviate the expansion caused by the silicon during lithium insertion and extraction. Carbon-coated silicon-based negative electrode materials can also isolate the active material silicon from the electrolyte, help form a stable SEI film, and thus enhance its electrochemical performance.

[0007] (2) Alloying of silicon with other metal elements: Metals have high electrical conductivity, excellent ductility, and mechanical strength. Introducing metals into silicon materials to form alloy phases with silicon or heterostructures is a novel approach. This not only forms a surface protective layer to effectively suppress the volume change of silicon, but also plays a role in electron transport, thereby reducing irreversible capacity.

[0008] (3) Silicon and conductive polymer composite: Conductive polymer is a kind of high molecular material with high structural flexibility and high elasticity, so it can effectively suppress the volume expansion produced during the silicon-lithium alloying process. At the same time, these conductive polymers can be composited with silicon-based materials to form a good conductive and strong electronic coating.

[0009] However, the above modification methods also have some problems. For example, after multiple cycles, the carbon layer on the carbon-coated surface will be expanded and cracked by the silicon core encapsulated therein; the silicon nanoparticles encapsulated in the porous carbon layer will still crack the hard porous carbon during the expansion process; the modified silicon-based negative electrode alloyed with other metals will still expand in volume and gradually pulverize during the cycle; after the conductive polymer is combined with silicon, it can well adapt to the volume changes of the silicon-based negative electrode material, but its generally high price makes it difficult to apply industrially. Moreover, the above modification technologies in the existing technology all focus on silicon nanoparticles, and most of them have failed to achieve satisfactory charge-discharge cycle performance. For the one-dimensional characteristics of silicon nanowires, a single composite technology cannot effectively achieve the goal of a long-cycle negative electrode material.

[0010] Therefore, how to prepare silicon nanowires with high capacity and high cycle performance is an important research direction in this field. Summary of the Invention

[0011] The purpose of the present invention is to provide a silicon nanowire composite material with high capacity and cycle performance, and a preparation method and application thereof.

[0012] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0013] One of the purposes of the present invention is to provide a silicon nanowire composite material, which includes a core and a coating layer, wherein the core includes silicon nanowires and expanded graphite, the silicon nanowires are embedded in the interlamellar gaps of the expanded graphite, and the coating layer includes a conductive polymer.

[0014] The expanded graphite of the present invention has the characteristics of high conductivity and long cycle life, and has sufficient interlamellar gaps to support silicon nanowires. It maintains a stable volume when the silicon nanowires expand during charging and discharging. Even if the silicon nanowires expand to the limit of the internal interlamellar gap space, the expanded graphite has good flexibility and resilience and will not break, ensuring that the internal silicon nanowire material is not corroded by the electrolyte. Therefore, the present invention uses the porous, flexible, highly elastic and highly conductive expanded graphite as the skeleton to effectively improve the cycle life of the silicon nanowires. The outer layer of the core is coated with a uniform, dense and flexible conductive polymer, eliminating the negative impact of the low initial efficiency of the expanded graphite and further improving the capacity and cycle performance of the silicon nanowire composite material.

[0015] As a preferred technical solution of the present invention, calculated with the total mass as 100, the mass ratio of the coating layer and the core is (1-10): (90-99), wherein the mass ratio can be 1:99, 2:98, 3:97, 4:96, 5:95, 6:94, 7:93, 8:92, 9:91 or 10:90, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0016] Preferably, the mass ratio of the silicon nanowires to the expanded graphite is (5-75): (25-95) calculated based on the total mass of 100, wherein the mass ratio may be 5:95, 10:90, 15:85, 20:80, 25:75, 30:70, 35:65, 40:60, 45:55, 50:50, 55:45, 60:40, 65:35, 70:30 or 75:25, etc., but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable, preferably (20-55): (45-80).

[0017] As a preferred technical solution of the present invention, the average length of the silicon nanowires is ≤20μm, wherein the average length is 2μm, 4μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm or 20μm, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0018] Preferably, the diameter of the silicon nanowire is ≤250 nm, wherein the diameter may be 10 nm, 50 nm, 100 nm, 150 nm, 200 nm or 250 nm, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0019] Preferably, the conductive polymer includes any one of polyaniline, polypyrrole, polythiophene or polyacetylene.

[0020] A second object of the present invention is to provide a method for preparing the silicon nanowire composite material as described in the first object, the preparation method comprising the following steps:

[0021] (1) adding expanded graphite to a silicon nanowire slurry and mixing the mixture to obtain a composite of the silicon nanowire and the expanded graphite;

[0022] (2) In situ polymerizing the composite material of step (1) with a conductive polymer to obtain the silicon nanowire composite material.

[0023] The present invention uses expanded graphite in step (1), which not only retains the high conductivity and long cycle life characteristics of graphite materials, but also has sufficient interlamellar gaps to support silicon nanowires. This maintains the stability of the volume of the silicon nanowires as they expand and contract during the charge and discharge process. Even when the silicon nanowires expand to the limit of the internal interlamellar gap space, the expanded graphite has good flexibility and resilience and will not break, thus ensuring that the internal silicon nanowire material is not corroded by the electrolyte. The porous, soft, elastic, and highly conductive expanded graphite adsorption skeleton effectively improves the cycle performance of the silicon nanowires.

[0024] Step (2) utilizes an in-situ polymerization method to coat the surface of the silicon nanowire composite with a uniform, dense, and flexible conductive polymer shell, thereby eliminating the negative impact of the low initial efficiency of the expanded graphite and further improving the cycle life of the silicon nanowire composite. Compared to directly adding a conductive polymer, the in-situ polymerization method used in step (2) means that the conductive polymer is simultaneously coated during the generation and growth process. By forming a uniform, dense, and flexible high-molecular conductive polymer layer on the surface of the composite formed by the expanded graphite embedded with the silicon nanowires, it can effectively block the electrolyte from eroding the core expanded graphite and silicon nanowires, thereby effectively improving the initial charge and discharge efficiency and cycle life of the battery.

[0025] As a preferred technical solution of the present invention, the solvent of the silicon nanowire slurry in step (1) includes an organic solvent and / or deionized water, preferably an organic solvent.

[0026] Preferably, the organic solvent includes any one of N-methylpyrrolidone, N-dimethylformamide, anhydrous ethanol, acetone, tetrahydrofuran or dimethyl sulfoxide.

[0027] Preferably, the solid content of the silicon nanowire slurry is 1 to 10 wt%, wherein the solid content can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt% or 10 wt%, etc., but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable, preferably 2 to 5 wt%.

[0028] As a preferred technical solution of the present invention, the expanded graphite in step (1) is pretreated, and the pretreatment method includes: heat-treating the expanded graphite.

[0029] Step (1) of the present invention uses expanded graphite as a raw material. Expanded graphite is a loose, porous, worm-like substance obtained by intercalating, washing, drying, and high-temperature expansion of natural flake graphite. Expanded graphite is used in lithium-ion batteries. In addition to possessing the excellent electrical conductivity and excellent charge-discharge cycle life of natural graphite itself, it also has properties that natural graphite does not have, such as softness, compression resilience, and adsorption. When exposed to high temperatures, expanded graphite can instantly expand in volume by 150 to 300 times, changing from a flake shape to a worm-like shape. This results in a loose, porous, and curved structure, an expanded surface area, and increased surface energy. The worm-like graphite can self-interlock, thereby increasing its softness, resilience, and plasticity. Studies have shown that the interlamellar spacing ratio between expanded graphite particles is over 85%, the interlamellar spacing ratio of open pores is over 13%, and the interlamellar spacing ratio of closed pores is less than 1%. The maximum pore size inside the expanded graphite can reach 600 μm.

[0030] The present invention heat-treats expanded graphite to form a porous graphite material with an interlamellar interstitial ratio exceeding 85%. This material retains the high conductivity and long cycle life of graphite while providing sufficient interlamellar interstitial space to support silicon nanowires. This material maintains volume stability during the expansion and contraction of the silicon nanowires during charge and discharge. Even when the silicon nanowires expand to the internal interlamellar interstitial space limit, the expanded graphite's excellent flexibility and resilience prevent them from breaking, protecting the internal silicon nanowires from electrolyte corrosion. The porous, soft, elastic, and highly conductive expanded graphite adsorption framework effectively improves the cycle life of the silicon nanowires.

[0031] Preferably, the particle size of the expanded graphite is 100 to 3000 mesh, wherein the particle size can be 100 mesh, 200 mesh, 300 mesh, 500 mesh, 1000 mesh, 1500 mesh, 2000 mesh, 2500 mesh or 3000 mesh, etc., but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable, preferably 300 to 1000 mesh.

[0032] Preferably, the temperature of the heat treatment is 80-800°C, wherein the temperature can be 80°C, 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C or 800°C, etc., but is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable, preferably 300-500°C.

[0033] If the heat treatment temperature is too high during the present invention, the expansion rate of the expanded graphite may be too high, resulting in an excessively large specific surface area, affecting the initial charge and discharge efficiency and continuously consuming lithium sources during subsequent cycles, shortening the material's cycle life. If the heat treatment temperature is too low, the expansion rate may be insufficient, preventing the silicon nanowires from being effectively embedded between the graphite sheets, causing the silicon nanowires to aggregate on the surface and reducing the material's cycle life. A temperature range of 300-500°C and a moderate expansion rate are preferred, which allows for effective embedding of the silicon nanowires while achieving an appropriate specific surface area, resulting in ideal initial charge and discharge efficiency and cycle life.

[0034] Preferably, the heat treatment time is 5 to 60 min, wherein the time can be 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min, etc., but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable, preferably 15 to 30 min.

[0035] As a preferred technical solution of the present invention, vacuuming is performed during the mixing.

[0036] Preferably, the vacuum degree of the mixing in step (1) is ≤-0.08 MPa, wherein the vacuum degree can be -0.001 MPa, -0.005 MPa, -0.01 MPa, -0.02 MPa, -0.03 MPa, -0.04 MPa, -0.05 MPa, -0.06 MPa, -0.07 MPa, -0.08 MPa, -0.09 MPa or -0.095 MPa, etc., but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable, preferably ≤-0.095 MPa.

[0037] The present invention controls the vacuum degree of the mixture to ensure that the silicon nanowires can be fully embedded in the expanded graphite.

[0038] Preferably, the number of vacuuming is 1 to 5 times, where the number can be 1, 2, 3, 4 or 5 times, etc., but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable, preferably 2 to 3 times.

[0039] Preferably, after the mixing in step (1), grinding, sieving, filtering and drying are sequentially performed to obtain the silicon nanowire composite.

[0040] Preferably, the particle size of the ground product is ≤20 μm, wherein the particle size can be 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm or 20 μm, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0041] Preferably, the mesh number of the sieving is ≥300 mesh, wherein the mesh number can be 300 mesh, 400 mesh, 500 mesh, 600 mesh, 700 mesh, 800 mesh, 900 mesh or 1000 mesh, etc., but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable, preferably ≥800 mesh.

[0042] Preferably, the drying temperature is 35 to 80°C, wherein the temperature can be 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C, etc., but is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable, preferably 45 to 60°C.

[0043] Preferably, the vacuum degree of drying is ≤-0.08 MPa, wherein the vacuum degree may be -0.08 MPa, -0.085 MPa, -0.09 MPa, -0.095 MPa or -0.098 MPa, etc., but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable, preferably ≤-0.095 MPa.

[0044] As a preferred technical solution of the present invention, the temperature of the in-situ polymerization in step (2) is 0 to 30°C, for example, it can be 0°C, 5°C, 10°C, 15°C, 20°C, 25°C or 30°C, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0045] Preferably, the time of the in-situ polymerization in step (2) is 1 to 8 hours, for example, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours or 8 hours, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0046] As a preferred technical solution of the present invention, the preparation method comprises the following steps:

[0047] (1) adding expanded graphite to a silicon nanowire slurry, mixing under a vacuum degree of ≤-0.08 MPa, grinding, sieving, filtering, and drying in sequence to obtain a silicon nanowire composite, and pretreating the expanded graphite, the pretreatment method comprising: heat-treating the expanded graphite at a temperature of 80 to 800° C. for 5 to 60 minutes;

[0048] (2) In-situ polymerizing the silicon nanowire composite and the conductive polymer at 0-30° C. for 1-8 hours to obtain the silicon nanowire composite material.

[0049] A third object of the present invention is to provide an application of the silicon nanowire composite material as described in the first object, wherein the silicon nanowire composite material is applied in the field of lithium-ion batteries.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] (1) The silicon nanowire composite material prepared in this example has good initial discharge capacity, initial charge capacity, initial charge-discharge efficiency, and cycle performance. Specifically, the initial discharge capacity can reach over 1553 mAh / g, the initial charge capacity can reach over 1359 mAh / g, the initial charge-discharge efficiency can reach over 87.51%, and the capacity retention rate after 200 cycles at 0.1C can reach over 93.88%.

[0052] (2) The preparation method of the present invention is simple and can be used for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 3 is a SEM image of the silicon nanowire composite material according to Example 1 of the present invention. DETAILED DESCRIPTION

[0054] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0055] Example 1

[0056] This embodiment provides a silicon nanowire composite material, which includes a coating layer and a core with a mass ratio of 5:95. The core includes silicon nanowires (average length 20 μm and diameter 250 nm) and expanded graphite with a mass ratio of 50:50. The silicon nanowires are embedded in the interlamellar gaps of the expanded graphite, and the coating layer is polyaniline.

[0057] This embodiment also provides a method for preparing the above silicon nanowire composite material, the preparation method comprising the following steps:

[0058] (1) Silicon nanowires and nitrogen methyl pyrrolidone are prepared into a silicon nanowire slurry with a solid content of 3.5 wt %, and the expanded graphite is pretreated. The pretreatment method includes: heat treating the expanded graphite with a particle size of 800 mesh at a temperature of 400° C. for 25 minutes;

[0059] Pretreated expanded graphite was added to the silicon nanowire slurry, mixed under a vacuum degree of ≤-0.08 MPa, vacuumed three times, and dried the slurry at 60° C. to obtain a silicon nanowire composite;

[0060] (2) The silicon nanowire composite prepared in step (1) was in situ polymerized with polyaniline at 0° C. for 8 h. The specific process is as follows:

[0061] First, a certain amount of 2 mol / L HCl solution is taken, placed in a 0°C environment and stirred for 30 minutes, and a certain amount of aniline liquid is slowly added and stirred for 2 hours; then a certain amount of (NH4)2S2O8 is slowly added and stirred for 10 minutes; then a certain amount of the composite powder described in step (1) is slowly added and reacted for 8 hours to obtain a dark green slurry, which is then washed multiple times with anhydrous ethanol, deionized water, and acetone, and then dried at 60°C for 24 hours. The final solid product is the polyaniline in-situ coated silicon nanowire expanded graphite composite material.

[0062] The SEM image of the silicon nanowire composite material prepared in this example is as follows: Figure 1 As shown in Figure 2, it can be seen that some silicon nanowires are embedded in the open-porous sheets of expanded graphite.

[0063] Example 2

[0064] This embodiment provides a silicon nanowire composite material, which includes a coating layer and a core with a mass ratio of 1:99. The core includes silicon nanowires (average length 15 μm and diameter 200 nm) and expanded graphite with a mass ratio of 20:80. The silicon nanowires are embedded in the interlamellar gaps of the expanded graphite, and the coating layer is polypyrrole.

[0065] This embodiment also provides a method for preparing the above silicon nanowire composite material, the preparation method comprising the following steps:

[0066] (1) Silicon nanowires and dimethylformamide are prepared into a silicon nanowire slurry with a solid content of 2 wt %, and the expanded graphite is pretreated. The pretreatment method includes: heat treating the expanded graphite with a particle size of 800 mesh at a temperature of 300° C. for 30 minutes;

[0067] Pretreated expanded graphite was added to the silicon nanowire slurry, mixed under a vacuum degree of ≤-0.08 MPa, vacuumed twice, and dried at 45° C. to obtain a silicon nanowire composite;

[0068] (2) The silicon nanowire composite material prepared in step (1) and polypyrrole were subjected to in-situ polymerization at a temperature of 30° C. for 6 h to obtain the silicon nanowire composite material. The specific process is as follows:

[0069] The silicon nanowire composite prepared in step (1) and a certain amount of 50 wt% phytic acid solution were added to a certain amount of isopropanol, stirred evenly, and then the desired pyrrole was added to obtain a mixture A. A certain amount of (NH4)2S2O8 was dissolved in deionized water to form a uniform and stable solution B. A certain amount of B was added to the mixture A, stirred continuously at 30°C for 6 hours, and washed and dried to obtain the polypyrrole in-situ polymerized and coated silicon nanowire expanded graphite composite material.

[0070] Example 3

[0071] This embodiment provides a silicon nanowire composite material, which includes a coating layer and a core in a mass ratio of 10:90. The core includes silicon nanowires (average length 10 μm and diameter 100 nm) and expanded graphite in a mass ratio of 55:45. The silicon nanowires are embedded in the interlamellar gaps of the expanded graphite, and the coating layer is polythiophene.

[0072] This embodiment also provides a method for preparing the above silicon nanowire composite material, the preparation method comprising the following steps:

[0073] (1) Silicon nanowires and anhydrous ethanol are mixed to form a silicon nanowire slurry having a solid content of 5 wt %, and the expanded graphite is pretreated. The pretreatment method comprises: heat treating the expanded graphite having a particle size of 1500 mesh at a temperature of 500° C. for 15 minutes;

[0074] Pretreated expanded graphite was added to the silicon nanowire slurry, mixed under a vacuum degree of ≤-0.08 MPa, vacuumed three times, and dried at 60° C. to obtain a silicon nanowire composite;

[0075] (2) The silicon nanowire composite material prepared in step (1) and polythiophene are subjected to in-situ polymerization at a temperature of 25° C. for 1 hour to obtain the silicon nanowire composite material. The method is as follows:

[0076] The silicon nanowire composite of step (1) and a certain amount of anhydrous ferric chloride are added to chloroform and stirred for 30 minutes. Then, the desired thiophene is dissolved in chloroform and slowly added to the above mixture and fully stirred, and oxidative polymerization is carried out at room temperature for 1 hour. After the reaction, the solution is washed with anhydrous ethanol for several times, then pickled once with 1 mol / L hydrochloric acid, and then washed with a large amount of deionized water for several times until the solution is neutral. Finally, it is vacuum dried at 50°C for 10 hours to obtain the polythiophene in situ polymerization-coated silicon nanowire expanded graphite composite material.

[0077] Example 4

[0078] This embodiment provides a silicon nanowire composite material, which includes a coating layer and a core with a mass ratio of 1:99. The core includes silicon nanowires (average length 20 μm and diameter 250 nm) and expanded graphite with a mass ratio of 5:95. The silicon nanowires are embedded in the interlamellar gaps of the expanded graphite, and the coating layer is polyacetylene.

[0079] This embodiment also provides a method for preparing the above silicon nanowire composite material, the preparation method comprising the following steps:

[0080] (1) Silicon nanowires and tetrahydrofuran are prepared into a silicon nanowire slurry with a solid content of 1 wt%, and the expanded graphite is pretreated. The pretreatment method includes: heat treating the expanded graphite with a particle size of 300 mesh at a temperature of 80° C. for 60 minutes;

[0081] Pretreated expanded graphite was added to the silicon nanowire slurry, mixed under a vacuum degree of ≤-0.08 MPa, vacuumed once, and then ground (particle size 20 μm), sieved (800 mesh), filtered, and dried at 35° C. to obtain a silicon nanowire composite.

[0082] (2) The silicon nanowire composite material prepared in step (1) and polyacetylene are subjected to in-situ polymerization at room temperature (25° C.) for 1 hour to obtain the silicon nanowire composite material. The specific process is as follows:

[0083] The silicon nanowire composite of step (1) is added to a toluene solvent, nitrogen is introduced to replace the air therein, a certain amount of trialkylaluminum, neodymium trichloride and silicon dioxide are added, and the mixture is stirred at room temperature for 30 minutes. Purified acetylene gas is introduced at a certain rate under stirring, and the reaction is continued at room temperature to the desired coating amount (time is about 1 hour), and then treated with 10% HCl-EtOH and washed with deionized water multiple times to obtain the polyacetylene in situ coated silicon nanowire expanded graphite composite material.

[0084] Example 5

[0085] This embodiment provides a silicon nanowire composite material, which includes a coating layer and a core with a mass ratio of 10:90. The core includes silicon nanowires (average length 20 μm and diameter 250 nm) and expanded graphite with a mass ratio of 75:25. The silicon nanowires are embedded in the interlamellar gaps of the expanded graphite, and the coating layer is polyaniline.

[0086] This embodiment also provides a method for preparing the above silicon nanowire composite material, the preparation method comprising the following steps:

[0087] (1) Silicon nanowires and dimethyl sulfoxide are prepared into a silicon nanowire slurry with a solid content of 10 wt%, and the expanded graphite is pretreated. The pretreatment method includes: heat treating the expanded graphite with a particle size of 3000 mesh at a temperature of 800° C. for 5 minutes;

[0088] Pretreated expanded graphite was added to the silicon nanowire slurry, mixed under a vacuum degree of ≤-0.08 MPa, and vacuumed five times. The mixture was then ground (to a particle size of 20 μm), sieved (800 mesh), filtered, and dried at 80° C. to obtain a silicon nanowire composite.

[0089] The silicon nanowire composite and polyaniline in step (1) were subjected to in-situ polymerization at a temperature of 0° C. for 8 h. The specific process is as follows:

[0090] First, a certain amount of 2 mol / L HCl solution is taken, placed in a 0°C environment and stirred for 30 minutes, and a certain amount of aniline liquid is slowly added and stirred for 2 hours; then a certain amount of (NH4)2S2O8 is slowly added and stirred for 10 minutes; then a certain amount of the composite powder described in step (1) is slowly added and reacted for 8 hours to obtain a dark green slurry, which is then washed multiple times with anhydrous ethanol, deionized water, and acetone, and then dried at 60°C for 24 hours. The final solid product is the polyaniline in-situ coated silicon nanowire expanded graphite composite material.

[0091] Example 6

[0092] In this embodiment, the mass ratio of silicon nanowires to expanded graphite is changed to 15:85, and other conditions are the same as those in embodiment 1.

[0093] Example 7

[0094] In this embodiment, the mass ratio of silicon nanowires to expanded graphite is changed to 85:15, and other conditions are the same as those in embodiment 1.

[0095] Example 8

[0096] In this embodiment, except that the expanded graphite is not pretreated and commercial expanded graphite with a particle size of 1200 mesh is directly used for mixing, other conditions are the same as those in Example 1.

[0097] Example 9

[0098] In this embodiment, except that the temperature of the heat treatment of the expanded graphite in step (1) is replaced with 50° C., other conditions are the same as those in Example 1.

[0099] Example 10

[0100] In this embodiment, except that the temperature of the heat treatment of the expanded graphite in step (1) is replaced with 900° C., other conditions are the same as those in Example 1.

[0101] Example 11

[0102] Except that the mixing in step (1) was carried out under normal pressure, other conditions in this embodiment were the same as those in Example 1.

[0103] Comparative Example 1

[0104] In this comparative example, except that the expanded graphite was replaced by porous carbon, other conditions were the same as those in Example 1.

[0105] Comparative Example 2

[0106] In this comparative example, the in-situ polymerization method in step (2) was replaced by a vapor phase coating method to coat a carbon shell on the surface of the silicon nanowire composite. Other conditions were the same as those in Example 1.

[0107] Aqueous slurries were prepared using the nanowire composite materials of Examples 1-11 and Comparative Examples 1-2: SWCNTs (single-walled carbon nanotubes): GF (graphene): PAA: SBR in a ratio of 95:1:0.2:1.8:2. Electrodes were fabricated and assembled using a conventional button-type half-cell manufacturing process. The resulting button cells were then tested for initial discharge capacity, initial charge capacity, initial charge-discharge efficiency, and cycling performance at 0.1C. The test results are shown in Table 1. The capacity retention was based on the fourth discharge capacity.

[0108] Table 1

[0109]

[0110] As can be seen from Table 1:

[0111] As can be seen from Examples 1-5, the silicon nanowire composite material prepared by the present invention has high initial discharge capacity, high initial charge capacity, high initial charge and discharge efficiency, and good cycle performance;

[0112] It can be seen from Examples 1 and 6-7 that as the loading amount of silicon nanowires increases, the specific capacity of the silicon nanowire composite material is effectively improved, but the cycle performance decreases;

[0113] As shown in Example 8, when the expanded graphite is not heat-treated, the silicon nanowires cannot be supported and protected by simply mechanically mixing with the expanded graphite, and the volume change during the cycle cannot be buffered. The cycle life improvement effect is significantly worse than that of the heat-treated expanded graphite.

[0114] The heat treatment temperature of Example 9 is too low, the expanded graphite cannot be effectively expanded, and the silicon nanowires cannot be embedded between the graphite sheets. The performance is similar to that of Example 8 which has not been heat treated.

[0115] In Example 10, the heat treatment temperature is too high, the expanded graphite expands excessively, the specific surface area increases, and more lithium is consumed to form the SEI film, resulting in a decrease in the initial charge and discharge efficiency.

[0116] In Example 11, the mixing was carried out under normal pressure. It was difficult for the silicon nanowires to be effectively embedded in the expanded graphite sheets. Similar to simple mechanical mixing, the cycle performance could not be effectively improved.

[0117] Comparative Example 1: Porous carbon replaces expanded graphite as an adsorption scaffold for silicon nanowires. However, its interlamellar porosity is much lower than that of expanded graphite, and its pore size is difficult to reach the micron level. The amount of silicon nanowires adsorbed / loaded is much lower than that of expanded graphite, and it cannot support and protect most silicon nanowires, resulting in a decrease in the cycling performance of the silicon nanowire composite material.

[0118] Comparative Example 2 employed vapor-phase coating, coating the silicon nanowire composite with a carbon shell instead of a polymer shell. This carbon shell exhibited superior density and initial performance compared to the polymer shell. However, the carbon shell lacked flexibility and was less able to resist silicon nanowire expansion than the polymer shell, resulting in inferior cycling performance.

[0119] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A silicon nanowire composite material, characterized in that: The silicon nanowire composite material comprises a core and a coating layer, wherein the core comprises silicon nanowires and expanded graphite, the silicon nanowires are embedded in interlamellar gaps of the expanded graphite, and the coating layer comprises a conductive polymer.

2. The silicon nanowire composite material according to claim 1, characterized in that The mass ratio of the coating layer to the core is (1-10):(90-99); Preferably, the mass ratio of the silicon nanowires to the expanded graphite is (5-75):(25-95), preferably (20-55):(45-80).

3. The silicon nanowire composite material according to claim 1 or 2, characterized in that: The average length of the silicon nanowires is ≤20 μm; Preferably, the diameter of the silicon nanowire is ≤250 nm; Preferably, the conductive polymer includes any one of polyaniline, polypyrrole, polythiophene or polyacetylene, or a combination of at least two thereof.

4. A method for preparing the silicon nanowire composite material according to any one of claims 1 to 3, characterized in that: The preparation method comprises the following steps: (1) adding expanded graphite to a silicon nanowire slurry and mixing the mixture to obtain a composite of the silicon nanowire and the expanded graphite; (2) In situ polymerizing the composite material of step (1) with a conductive polymer to obtain the silicon nanowire composite material.

5. The preparation method according to claim 4, characterized in that The solvent of the silicon nanowire slurry in step (1) includes an organic solvent and / or deionized water, preferably an organic solvent; Preferably, the organic solvent comprises any one or a combination of at least two of nitrogen-methyl pyrrolidone, nitrogen-nitrogen dimethylformamide, anhydrous ethanol, acetone, tetrahydrofuran or dimethyl sulfoxide; Preferably, the solid content of the silicon nanowire slurry is 1 to 10 wt %, preferably 2 to 5 wt %.

6. The preparation method according to claim 4 or 5, characterized in that The expanded graphite in step (1) is pretreated, and the pretreatment method includes: heat treating the expanded graphite; Preferably, the particle size of the expanded graphite is 100 to 3000 mesh, preferably 800 to 1500 mesh; Preferably, the temperature of the heat treatment is 80-800°C, preferably 300-500°C; Preferably, the heat treatment time is 5 to 60 minutes, preferably 15 to 30 minutes.

7. The preparation method according to any one of claims 4 to 6, characterized in that During the mixing, vacuuming is performed; Preferably, the vacuum degree of the mixing in step (1) is ≤-0.08 MPa, preferably ≤-0.095 MPa; Preferably, the vacuuming is performed 1 to 5 times, preferably 2 to 3 times; Preferably, after the mixing in step (1), grinding, sieving, filtering and drying are sequentially performed to obtain the silicon nanowire composite; Preferably, the particle size of the ground product is ≤20 μm; Preferably, the mesh size of the sieve is ≥300 mesh, preferably ≥800 mesh; Preferably, the drying temperature is 35 to 80°C, preferably 45 to 60°C; Preferably, the vacuum degree of the drying is ≤-0.08 MPa, preferably ≤-0.095 MPa.

8. The preparation method according to any one of claims 4 to 7, characterized in that The temperature of the in-situ polymerization in step (2) is 0-30° C.; Preferably, the time of the in-situ polymerization in step (2) is 1 to 8 hours.

9. The preparation method according to any one of claims 4 to 8, characterized in that The preparation method comprises the following steps: (1) adding expanded graphite to a silicon nanowire slurry, mixing the mixture under a vacuum degree of ≤-0.08 MPa, and sequentially grinding, sieving, filtering, and drying to obtain a silicon nanowire composite; pretreating the expanded graphite, wherein the pretreatment method comprises: heat-treating the expanded graphite at a temperature of 80 to 800° C. for 5 to 60 minutes; (2) In-situ polymerizing the composite material of step (1) and the conductive polymer at 0-30° C. for 8 h to obtain the silicon nanowire composite material.

10. Use of the silicon nanowire composite material according to any one of claims 1 to 3, characterized in that: The silicon nanowire composite material is applied in the field of lithium ion batteries.

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