Silicon composite negative electrode material and its preparation method and application

Abstract

The present invention provides a silicon composite negative electrode material, a preparation method thereof, and an application thereof. The silicon composite negative electrode material includes carbon nanotubes, a binder, and silicon nanoparticles. The carbon nanotubes and the binder are intertwined to form a spider web-like structure, and the silicon nanoparticles are wrapped at the nodes of the spider web-like structure. The present invention provides a silicon composite negative electrode material with a special structure. The spider web-like structure formed by carbon nanotubes and a binder has high strength, elasticity, and flexibility. The carbon tubes have high intrinsic conductivity and can form a good conductive network in the electrode. The intertwined network structure will not easily slide during the expansion and contraction of silicon, causing the conductive network to be destroyed, thereby effectively suppressing the volume expansion of the silicon material and improving its energy density, conductivity, and cycle stability.

Description

Technical Field

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

[0002] In order to solve the range anxiety problem of electric vehicle users, it is necessary to improve the battery energy density. There are many ways to improve the battery energy density, such as optimizing the battery structure, increasing the energy density of active materials, reducing the proportion of inactive materials, etc. As for the negative electrode material, the capacity of the graphite negative electrode widely used in commercial lithium batteries is about 360mAh / g, which is very close to its theoretical specific capacity (372mAh / g). Therefore, it is difficult to make a breakthrough in the graphite negative electrode. So people turn their attention to silicon materials. In lithium batteries, silicon materials form Li at high temperatures. 22 Si5, the corresponding specific capacity is 4200mAh / g, and Li is formed at room temperature. 15 Si4 has a corresponding specific capacity of 3579mAh / g, and the delithiation potential of silicon is relatively low. Therefore, using silicon as a negative electrode can reduce material usage and increase battery energy density. It is an important development direction for power battery negative electrode materials in the future.

[0003] But silicon material has poor conductivity, and at the same time, Li + There is a serious volume effect during the embedding and ejection process, with a volume expansion rate of up to 400%, which causes the material to pulverize and even separate from the current collector, resulting in a rapid and severe drop in cycle attenuation. In addition, the expansion during the charge and discharge process causes the active material to continuously repair the SEI film, consuming active Li and also causing poor cycle performance. To solve the above problems, research has mainly focused on silicon nanosizing and special structures such as core-shell and porous. Nanosizing can slow down volume expansion, but the specific surface area of ​​nano-sized silicon is large, the battery coulombic efficiency is low, and it may re-aggregate into large particles during cycling. The synthesis process of special core-shell and porous structures is complex and costly.

[0004] CN111933919A discloses a nano-silicon powder, a silicon-based negative electrode, a lithium-ion battery containing the silicon-based negative electrode, and a method for manufacturing the same. The silicon-based negative electrode exhibits cycling performance comparable to that of graphite-based negative electrodes, with an initial discharge efficiency exceeding 89% and a discharge capacity greater than 3000 mAh / g. The invention utilizes nano-metal oxides, nano-silicon particles, and a lithium source to react in situ at high temperatures on the surface of the nano-silicon particles to produce the lithium ion conductor Li2SiO3 and the conductive nano-metal. Low-melting-point tin also binds the nano-silicon particles. An organic titanium source and / or zirconium source is pyrolyzed at high temperatures to produce TiO2 and / or ZrO2, minimizing side reactions between the nano-silicon and the electrolyte. The organic aluminum source is pyrolyzed and reacts with a lithium source to produce the lithium ion conductor LiAlO2. The organic carbon source is pyrolyzed to produce conductive carbon. The silicon nanoparticles obtained in this document, while nano-sizing can slow volume expansion, have a large specific surface area, resulting in low battery coulombic efficiency, and may reaggregate into larger particles during cycling.

[0005] CN111755677A discloses a core-shell porous silicon negative electrode material for lithium-ion batteries and a preparation method thereof. The porous silicon negative electrode material has a core-shell structure, with the core comprising nanoporous silicon, graphite, and amorphous carbon, and the shell being amorphous carbon. The nanoporous silicon, graphite, and amorphous carbon account for 30% to 70% by weight, 20% to 45% by weight, and 10% to 40% by weight of the amorphous carbon in the negative electrode material. The microporous silicon raw material contains 1% to 10% by weight of oxygen, and the oxygen content of the nanoporous silicon obtained by wet grinding is 12% to 35% by weight. However, the synthesis process of the porous structure is complex and costly.

[0006] Therefore, how to give full play to the high capacity advantage of silicon negative electrode materials while reducing their own volume expansion and improving their conductivity is a technical problem that needs to be solved urgently. Summary of the Invention

[0007] The present invention aims to provide a silicon composite negative electrode material, its preparation method, and its application. The present invention provides a silicon composite negative electrode material with a special structure. The spiderweb-like structure formed by carbon nanotubes and a binder has high strength, elasticity, and flexibility. Furthermore, the carbon nanotubes have high intrinsic conductivity, forming a good conductive network in the electrode. Furthermore, this interlaced network structure prevents slippage during silicon expansion and contraction, which could lead to damage to the conductive network. This effectively suppresses the volume expansion of the silicon material and improves its conductivity.

[0008] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a silicon composite negative electrode material, which includes carbon nanotubes, a binder and silicon nanoparticles. The carbon nanotubes and the binder are intertwined to form a spider web-like structure, and the silicon nanoparticles are wrapped at the nodes of the spider web-like structure.

[0010] The present invention provides a silicon composite negative electrode material with a special structure. The spider-web-like structure formed by carbon nanotubes and a binder has high strength, elasticity and flexibility. In addition, the carbon nanotubes have high intrinsic conductivity and can form a good conductive network in the electrode. The interlaced network structure will not easily slide during the expansion and contraction of silicon, causing the conductive network to be destroyed, thereby effectively suppressing the volume expansion of the silicon material and improving its conductivity.

[0011] Preferably, the diameter of the carbon nanotubes is 10 to 100 nm, for example, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm or 100 nm.

[0012] Preferably, the length of the carbon nanotubes is 10 to 100 μm, for example, 10 μm, 15 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 95 μm or 100 μm.

[0013] In the present invention, if the length of the carbon nanotubes is too long, the carbon nanotubes will be easily folded and entangled, while if the length of the carbon nanotubes is too short, the stability of the spider web structure will be affected.

[0014] Preferably, the average particle size of the silicon nanoparticles is 10 to 100 nm, for example, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm or 100 nm.

[0015] Preferably, the binder includes any one of carboxymethyl cellulose, polymethyl methacrylate, polyacrylic acid, sodium alginate, polyurethane, polyimide or polyacrylamide, or a combination of at least two thereof.

[0016] Preferably, the mass ratio of the silicon nanoparticles to the carbon nanotubes is (4-8):1, for example, 4:1, 4.5:1, 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1 or 8:1.

[0017] In the present invention, if the mass ratio of silicon nanoparticles to carbon nanotubes is too small, that is, there are too few silicon nanoparticles, it is not conducive to improving the energy density of the battery. If the mass ratio is too large, that is, there are too many silicon nanoparticles, it will affect the effect of inhibiting silicon expansion.

[0018] Preferably, the mass ratio of the binder to the carbon nanotubes is (0.5-5):1, for example, 0.1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1 or 5:1.

[0019] In the present invention, if the mass ratio of the binder to the carbon nanotubes is too large, that is, there is too much binder, it will affect the silicon capacity and thus affect the battery performance of the negative electrode material. If the mass ratio is too small, that is, there is too little binder, the silicon will expand during the cycle and the negative electrode material structure will be unstable.

[0020] In a second aspect, the present invention provides a method for preparing the silicon composite negative electrode material as described in the first aspect, the preparation method comprising the following steps:

[0021] (1) In an external magnetic field, a metal catalyst and a carbon source are deposited by chemical vapor deposition to obtain carbon nanotubes;

[0022] (2) After obtaining the carbon nanotubes described in step (1), a gaseous silicon source is introduced to obtain a precursor by chemical vapor deposition, wherein silicon nanoparticles in the precursor are deposited on the carbon nanotubes;

[0023] (3) Mixing and cross-linking the binder solution with the precursor described in step (2) to obtain the silicon composite negative electrode material.

[0024] The preparation method provided by the present invention comprises the following steps: in step (1), a divergent arrangement of carbon nanotubes extending outward from the center is obtained under the action of an external magnetic field; nano-silicon is then in situ deposited on the carbon nanotubes by chemical vapor deposition, and the chemical vapor deposition method can achieve uniform deposition of nano-silicon; and then, in step (3), a mixed cross-linking of the binder and the precursor is performed to obtain an interlaced spider-web-like structure, with the nano-silicon being in a coated state at the nodes.

[0025] In the present invention, if an external magnetic field is not added in step (1), the spider web-like structure morphology of the present invention cannot be obtained, and the carbon nanotubes and silicon particles cannot be evenly distributed.

[0026] Preferably, the magnetic field strength of the external magnetic field in step (1) is 0.5 to 5 T, for example, 0.5 T, 1 T, 1.5 T, 2 T, 2.5 T, 3 T, 3.5 T, 4 T, 4.5 T or 5 T.

[0027] In the present invention, if the magnetic field strength of the external magnetic field is too large, the network structure will be dense, which is not conducive to the performance of silicon capacity. If the magnetic field strength is too small, the network structure will be relatively loose, the material structure will be unstable, and the performance of the negative electrode material will be affected.

[0028] Preferably, the metal catalyst in step (1) includes ferrocene.

[0029] Preferably, the carbon source in step (1) includes xylene.

[0030] Preferably, the chemical vapor deposition process in step (1) comprises:

[0031] In an environment of an external magnetic field, a metal catalyst is mixed with a carbon source to obtain a catalyst precursor solution, which is then introduced into a reaction chamber using a carrier gas and chemical vapor deposition is performed on a substrate surface to obtain carbon nanotubes.

[0032] Preferably, the carrier gas comprises a mixture of hydrogen and argon.

[0033] Preferably, the temperature of the chemical vapor deposition is 400-1000° C., for example, 400° C., 500° C., 600° C., 700° C., 800° C., 900° C. or 1000° C.

[0034] Preferably, the gas-phase silicon source in step (2) comprises silane.

[0035] Preferably, in step (2), the carrier gas is introduced at the same time as the gas-phase silicon source is introduced.

[0036] Preferably, the temperature of the chemical vapor deposition in step (2) is 300-800°C, for example, 300°C, 400°C, 500°C, 600°C, 700°C or 800°C.

[0037] Preferably, the mixing and cross-linking process in step (3) also includes a cross-linking agent solution.

[0038] Preferably, the cross-linking agent comprises epichlorohydrin.

[0039] In the present invention, epichlorohydrin is selected as the cross-linking agent, which is more conducive to increasing the hydrogen bonding effect between silicon and the binder.

[0040] Preferably, the mass fraction of the binder in the binder solution in step (3) is 0.5-2.5%, for example, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3% or 2.5%.

[0041] Preferably, the binder solution further comprises a dispersant.

[0042] Preferably, the mass ratio of the binder to the dispersant is (1-3):1, such as 1:1, 2:1 or 3:1.

[0043] Preferably, the mixing and cross-linking process in step (3) includes:

[0044] The binder solution is applied to the surface of the precursor in step (2), and then mixed with the crosslinker solution for crosslinking.

[0045] Preferably, the mixed and cross-linked material in step (3) is soaked in water and dried.

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

[0047] (1) In an external magnetic field environment with a magnetic field strength of 0.55 T, a metal catalyst and a carbon source are mixed to obtain a catalyst precursor solution, the catalyst precursor solution is introduced into a reaction chamber using a mixture of hydrogen and argon, and chemical vapor deposition is performed on a substrate surface at a temperature of 400 to 1000° C. to obtain carbon nanotubes;

[0048] (2) After obtaining the carbon nanotubes described in step (1), a gas-phase silicon source is introduced and chemical vapor deposition is performed at a temperature of 300 to 800° C. to obtain a precursor, in which silicon nanoparticles are deposited on the carbon nanotubes;

[0049] (3) applying a binder solution with a mass fraction of 0.5 to 2.5% to the surface of the precursor in step (2), and then mixing it with a crosslinker solution for crosslinking. The mixed and cross-linked material is soaked in water and dried to obtain the silicon composite negative electrode material.

[0050] In a third aspect, the present invention provides a negative electrode plate, which comprises the silicon composite negative electrode material as described in the first aspect and a binder.

[0051] In the present invention, when the negative electrode material described in the first aspect is selected for the negative electrode plate, no additional conductive agent is required. The carbon nanotubes not only form a spider web-like structural support but also serve as a conductive network, and the carbon nanotubes have high electrical conductivity.

[0052] In a fourth aspect, the present invention provides a method for preparing the negative electrode sheet according to the third aspect, the preparation method comprising:

[0053] The silicon composite negative electrode material is mixed with a binder in a dry process and coated on the surface of the negative electrode current collector to obtain the negative electrode plate.

[0054] In the present invention, a solvent-free method can be used to prepare the negative electrode sheet, which does not require a solvent and avoids organic pollution. At the same time, it also improves the compaction density of the electrode, which is more conducive to the performance of the negative electrode capacity and the improvement of the battery energy density.

[0055] In a fifth aspect, the present invention further provides a lithium-ion battery, comprising the silicon composite negative electrode material as described in the first aspect or the negative electrode plate as described in the third aspect.

[0056] Compared with the prior art, the present invention has the following beneficial effects:

[0057] (1) The present invention provides a silicon composite negative electrode material with a special structure. The spider-web-like structure formed by carbon nanotubes and a binder has high strength, elasticity and flexibility. In addition, the carbon nanotubes have high intrinsic conductivity and can form a good conductive network in the electrode. The interlaced network structure will not easily slide during the expansion and contraction of silicon, causing the conductive network to be destroyed, thereby effectively suppressing the volume expansion of the silicon material and improving its conductivity and cycle stability.

[0058] (2) The preparation method provided by the present invention comprises the following steps: in step (1), a divergent arrangement of carbon nanotubes extending outward from the center is obtained under the action of an external magnetic field; nanosilicon is then in situ deposited on the carbon nanotubes by chemical vapor deposition, which can achieve uniform deposition of nanosilicon; and in step (3), a cross-linking of the binder and the precursor is performed to obtain an interlaced spider web-like structure, with the nanosilicon being coated at the nodes. When a battery employs the negative electrode material provided by the present invention, the energy density can reach above 330Wh / kg, and the capacity will not decay to below 80% until at least 1,300 cycles are completed at 25°C, and the capacity will not decay to below 80% until at least 1,000 cycles are completed at 45°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 Schematic diagram of the structure of the Si / CNT skeleton provided in Example 1.

[0060] Figure 2 This is a schematic diagram of the structure of the silicon composite negative electrode material provided in Example 1.

[0061] Figure 3 This is the SEM image of the carbon nanotubes provided in Example 1. DETAILED DESCRIPTION

[0062] 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.

[0063] Example 1

[0064] This embodiment provides a silicon composite negative electrode material, which includes carbon nanotubes (diameter 100nm, length 100μm), carboxymethyl cellulose (CMC) and silicon nanoparticles (average particle size 70nm), and the carbon nanotubes and carboxymethyl cellulose are intertwined to form a spider web structure (such as Figure 2 As shown), the silicon nanoparticles are wrapped at the nodes of the spider web-like structure; wherein the mass ratio of silicon nanoparticles to carbon nanotubes is 5:1, and the mass ratio of carboxymethyl cellulose to carbon nanotubes is 3:1.

[0065] The preparation method of the silicon composite negative electrode material is as follows:

[0066] (1) Dissolve 0.1 g of ferrocene (sublimation point 190 ° C) in 10 ml of xylene (boiling point 140 ° C) and ultrasonically vibrate for 15 minutes to obtain a catalyst precursor solution. 2 The cleaned iron-nickel alloy substrate is placed in a quartz tube. After sealing both ends of the furnace, the vacuum pump is slowly adjusted to evacuate the quartz tube to a vacuum degree of less than -0.098 MPa. Ultra-high-purity argon gas with a flow rate of 100 sccm is then introduced to drive out the air in the quartz tube. The catalyst precursor solution is continuously injected into the reaction chamber at a liquid flow rate of 0.11 ml / min. The temperature in the reaction chamber is preheated to maintain 200°C. The injected ferrocene solution will evaporate rapidly and enter the reaction chamber through 100 sccm of ultra-high-purity argon gas and 10% pure hydrogen. After reacting for a period of time at 770°C (the reaction process is carried out in an external magnetic field environment with a magnetic field strength of 3 T), argon gas is introduced to cool the temperature to obtain a divergent carbon nanotube structure (such as Figure 3 shown);

[0067] (2) The quartz tube is heated to 500°C and 20 sccm of SiH4 and 680 sccm of argon are introduced. Silicon is deposited on the carbon nanotubes to form a Si / CNT skeleton (e.g. Figure 1 As shown), the precursor is obtained,

[0068] (3) After the Si / CNT skeleton is formed, epichlorohydrin (ECH) is used as a crosslinking agent, and a 1% CMC solution (the solvent in the solution is water, and PVP dispersant is added, and the mass ratio of dispersant to CMC is 1:2) with a mass fraction of 1% undergoes a self-crosslinking reaction of hydroxyl groups at 60°C, forming a spiral sticky line on the skeleton. Specifically, a 1% CMC solution is evenly applied to the Si / CNT skeleton, and then immersed in a 2% ECH / ethanol solution for crosslinking for a period of time. After being taken out, it is immersed in deionized water for 24 hours, and then dried to obtain a spider web structured silicon composite negative electrode material.

[0069] Example 2

[0070] This embodiment provides a silicon composite negative electrode material, which includes carbon nanotubes (tube diameter 80nm, length 45μm), carboxymethyl cellulose (CMC) and silicon nanoparticles (average particle size 50nm), the carbon nanotubes and carboxymethyl cellulose are intertwined to form a spider web-like structure, and the silicon nanoparticles are wrapped at the nodes of the spider web-like structure; wherein the mass ratio of silicon nanoparticles to carbon nanotubes is 4:1, and the mass ratio of carboxymethyl cellulose to carbon nanotubes is 1:1.

[0071] The preparation method of the silicon composite negative electrode material is as follows:

[0072] (1) Dissolve 0.1 g of ferrocene (sublimation point 190 ° C) in 10 ml of xylene (boiling point 140 ° C) and ultrasonically vibrate for 15 minutes to obtain a catalyst precursor solution. 2 The cleaned iron-nickel alloy substrate is placed in a quartz tube. After sealing both ends of the furnace, the vacuum pump is slowly adjusted to evacuate the quartz tube to a vacuum degree of less than -0.098 MPa. Ultra-high-purity argon gas with a flow rate of 100 sccm is then introduced to expel the air in the quartz tube. The catalyst precursor solution is continuously injected into the reaction chamber at a liquid flow rate of 0.11 ml / min. The temperature in the reaction chamber is preheated to maintain 200°C. The injected ferrocene solution will evaporate rapidly and enter the reaction chamber through 100 sccm of ultra-high-purity argon gas and 10% pure hydrogen. After reacting at 500°C for a period of time (the reaction process is carried out in an external magnetic field environment with a magnetic field strength of 0.5 T), argon gas is introduced to cool down to obtain a divergent carbon nanotube structure.

[0073] (2) The quartz tube is heated to 300°C and 20 sccm of SiH4 and 680 sccm of argon are introduced. Silicon is deposited on the carbon nanotubes to form a Si / CNT skeleton, thereby obtaining a precursor.

[0074] (3) After the Si / CNT skeleton is formed, epichlorohydrin (ECH) is used as a crosslinking agent, and a CMC solution with a mass fraction of 2.5% (the solvent in the solution is water, and PVP dispersant is added, and the mass ratio of dispersant to CMC is 1:3) undergoes a self-crosslinking reaction of hydroxyls at 60°C, forming a spiral sticky line on the skeleton. Specifically, a CMC solution with a mass fraction of 2.5% is evenly applied to the Si / CNT skeleton, and then immersed in a 2% ECH / ethanol solution for crosslinking for a period of time. After being taken out, it is immersed in deionized water for 24 hours, and then dried to obtain a spider web structured silicon composite negative electrode material.

[0075] Example 3

[0076] This embodiment provides a silicon composite negative electrode material, which includes carbon nanotubes (tube diameter 30 nm, length 10 μm), polymethyl methacrylate (PMMA) and silicon nanoparticles (average particle size 10 nm). The carbon nanotubes and polymethyl methacrylate are intertwined to form a spider web-like structure, and the silicon nanoparticles are wrapped at the nodes of the spider web-like structure; wherein the mass ratio of silicon nanoparticles to carbon nanotubes is 8:1, and the mass ratio of polymethyl methacrylate to carbon nanotubes is 5:1.

[0077] The preparation method of the silicon composite negative electrode material is as follows:

[0078] (1) Dissolve 0.1 g of ferrocene (sublimation point 190 ° C) in 10 ml of xylene (boiling point 140 ° C) and ultrasonically vibrate for 15 minutes to obtain a catalyst precursor solution. 2 The cleaned iron-nickel alloy substrate is placed in a quartz tube. After sealing both ends of the furnace, the vacuum pump is slowly adjusted to evacuate the quartz tube to a vacuum degree of less than -0.098 MPa. Ultra-high-purity argon gas with a flow rate of 200 sccm is then introduced to expel the air in the quartz tube. The catalyst precursor solution is continuously injected into the reaction chamber at a liquid flow rate of 0.2 ml / min. The temperature in the reaction chamber is preheated to maintain 200°C. The injected ferrocene solution will evaporate rapidly and enter the reaction chamber through 150 sccm of ultra-high-purity argon gas and 10% pure hydrogen. After reacting at 1000°C for a period of time (the reaction process is carried out in an external magnetic field environment with a magnetic field strength of 5 T), argon gas is introduced to cool down to obtain a divergent carbon nanotube structure.

[0079] (2) The quartz tube is heated to 800°C and 25 sccm of SiH4 and 700 sccm of argon are introduced. Silicon is deposited on the carbon nanotubes to form a Si / CNT skeleton, thereby obtaining a precursor.

[0080] (3) After the Si / CNT skeleton is formed, a 0.5% by mass PMMA solution (the solvent in the solution is water, and PVP dispersant is added, and the mass ratio of the dispersant to CMC is 1:1) with epichlorohydrin (ECH) as a crosslinking agent undergoes a self-crosslinking reaction of hydroxyls at 60°C to form a spiral sticky line on the skeleton. Specifically, a 0.5% by mass PMMA solution is evenly applied to the Si / CNT skeleton, and then immersed in a 2% by mass ECH / ethanol solution for crosslinking for a period of time. After being taken out, it is immersed in deionized water for 24 hours, and then dried to obtain a spider web structured silicon composite negative electrode material.

[0081] Example 4

[0082] The difference between this embodiment and embodiment 1 is that the length of the carbon nanotubes in this embodiment is 110 μm, and the reaction in the preparation method is stopped after the reaction time reaches this length.

[0083] The rest of the preparation methods and parameters were the same as those in Example 1.

[0084] Example 5

[0085] The difference between this embodiment and embodiment 1 is that the average particle size of the silicon nanoparticles in this embodiment is 110 nm.

[0086] The rest of the preparation methods and parameters were the same as those in Example 1.

[0087] Example 6

[0088] The difference between this embodiment and embodiment 1 is that the mass ratio of silicon nanoparticles to carbon nanotubes in this embodiment is 3:1.

[0089] The rest of the preparation methods and parameters were the same as those in Example 1.

[0090] Example 7

[0091] The difference between this embodiment and embodiment 1 is that the mass ratio of silicon nanoparticles to carbon nanotubes in this embodiment is 9:1.

[0092] The rest of the preparation methods and parameters were the same as those in Example 1.

[0093] Example 8

[0094] The difference between this embodiment and embodiment 1 is that the mass ratio of carboxymethyl cellulose to carbon nanotubes in this embodiment is 0.3:1.

[0095] The rest of the preparation methods and parameters were the same as those in Example 1.

[0096] Example 9

[0097] The difference between this embodiment and embodiment 1 is that the mass ratio of carboxymethyl cellulose to carbon nanotubes in this embodiment is 6:1.

[0098] The rest of the preparation methods and parameters were the same as those in Example 1.

[0099] Example 10

[0100] The difference between this embodiment and embodiment 1 is that the magnetic field strength of the external magnetic field in step (1) of this embodiment is 0.2T.

[0101] The rest of the preparation methods and parameters were the same as those in Example 1.

[0102] Example 11

[0103] The difference between this embodiment and embodiment 1 is that the magnetic field strength of the external magnetic field in step (1) of this embodiment is 6T.

[0104] The rest of the preparation methods and parameters were the same as those in Example 1.

[0105] Comparative Example 1

[0106] The difference between this comparative example and Example 1 is that the silicon composite negative electrode material provided in this comparative example is silicon nanoparticles supported on carbon nanotubes, the carbon nanotubes are not interlaced with the carboxymethyl cellulose, and step (3) is not performed in the preparation method.

[0107] The rest of the preparation methods and parameters were the same as those in Example 1.

[0108] Comparative Example 2

[0109] The difference between this comparative example and Example 1 is that no external magnetic field is added in step (1) of this comparative example.

[0110] The rest of the preparation methods and parameters were the same as those in Example 1.

[0111] Comparative Example 3

[0112] The difference between this comparative example and Example 1 is that this comparative example uses a pure silicon negative electrode.

[0113] The silicon composite negative electrode materials provided in Examples 1-11 and Comparative Examples 1-2 were mixed with polyvinylidene fluoride, and negative electrode plates were prepared by a dry process. The negative electrode plates were assembled with NCM811 positive electrode plates to obtain battery cells, and the batteries were obtained after formation and capacity separation.

[0114] The pure silicon negative electrode, polyvinylidene fluoride, conductive carbon black and NMP provided in Comparative Example 3 are mixed to obtain a negative electrode slurry, which is coated on the surface of copper foil, dried and rolled to obtain a negative electrode sheet, and also assembled with the NCM811 positive electrode sheet to obtain a battery cell, which is then formed and divided into different capacities to obtain a battery.

[0115] The negative electrode sheets obtained in Examples 1-11 and Comparative Examples 1-3 were cut into 51 mm by 90 mm long sheets. The positive electrode sheet containing 97.6% NCM811 was cut into 49 mm by 85 mm long sheets. A PE separator was used, each 94 mm wide. The battery was assembled on a laminating machine, with one layer of separator, one layer of negative electrode sheet, one layer of separator, one layer of positive electrode sheet, one layer of separator, and one layer of negative electrode sheet. The number of positive electrode sheet layers was set to 22, the number of negative electrode sheet layers was set to 23, and the number of separator layers was set to 46. The assembled soft-pack battery was encapsulated in 230 x 133 mm aluminum-plastic film and baked to a moisture content below 200 ppm. Then, the battery was filled with silicon electrolyte, allowed to stand at high temperature for 24 hours, and then formed using a low current. The battery was then subjected to 0.33C capacity separation to complete the battery. The provided batteries were subjected to electrochemical performance testing under the following conditions: a test voltage of 4.2-2.5V, a 1C / 1C charge / discharge rate, and cycled at ambient temperatures of 25°C and 45°C until the capacity retention reached 80%. The results are shown in Table 1.

[0116] Table 1

[0117]

[0118] It should be noted that the energy density in Table 1 is the rounded numerical result. Although the rounded data results have a slight impact on the final battery performance, the degree is limited and will not affect the presentation of the final data results of other performance.

[0119] From the data results of Example 1 and Example 4, it can be seen that if the carbon nanotubes are too long, they are prone to folding and winding, which is not conducive to the performance of the battery.

[0120] From the data results of Examples 1 and 5, it can be seen that if the average particle size of silicon nanoparticles is too large, it will be difficult for silicon particles to deposit on carbon tubes, making it difficult to control silicon expansion during cycling, resulting in reduced cycling performance.

[0121] From the data results of Examples 1, 6 and 7, it can be seen that if the mass ratio of silicon nanoparticles to carbon nanotubes is too small, the energy density of the battery will be affected. If the mass ratio of silicon nanoparticles to carbon nanotubes is too large, the cyclic expansion of silicon will be difficult to control.

[0122] From the data results of Examples 1, 8 and 9, it can be seen that if the mass ratio of carboxymethyl cellulose to carbon nanotubes is too small, the spider web structure will be unstable and thus affect the battery cycle, while if the mass ratio is too large, it will not be conducive to improving the battery energy density.

[0123] From the data results of Examples 1, 10 and 11, it can be seen that if the magnetic field strength of the external magnetic field is too small, the network structure will be relatively loose, while if the magnetic field strength is too large, the network structure will be too dense, both of which will affect the battery performance.

[0124] From the data results of Example 1 and Comparative Example 1, it can be seen that the silicon composite negative electrode material provided by the present invention does not contain a binder and cannot form a stable spider web structure, thereby failing to achieve silicon cyclic expansion inhibition and ensure a stable structure, and the battery cycle is poor.

[0125] From the data results of Example 1 and Comparative Example 2, it can be seen that if no external magnetic field is applied during the preparation of carbon nanotubes, a uniformly distributed carbon nanotube and silicon composite network structure cannot be obtained, thereby affecting the battery cycle performance.

[0126] It can be seen from the data results of Example 1 and Comparative Example 3 that, compared with conventional pure silicon negative electrode materials, the silicon composite negative electrode material provided by the present invention has higher energy density and significantly improved cycle stability.

[0127] In summary, the spider-web-like structure formed by carbon nanotubes and a binder provided by the present invention has high strength, elasticity, and flexibility. The silicon nanoparticles coated at the nodes will not easily slide during the expansion and contraction of silicon, causing the conductive network to be destroyed, thereby effectively suppressing the volume expansion of the silicon material. In the preparation method, a silicon-deposited carbon nanotube structure is first obtained, and then cross-linked with a binder to obtain a network structure, thereby improving the battery's energy density, conductivity, room temperature cycle stability, and high temperature cycle stability. When a battery uses the negative electrode material provided by the present invention, the energy density can reach more than 330Wh / kg, and the capacity will decay to less than 80% after at least 1300 cycles at 25°C and at least 1000 cycles at 45°C.

[0128] 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 composite negative electrode material, characterized in that: The silicon composite negative electrode material includes carbon nanotubes, a binder, and silicon nanoparticles. The carbon nanotubes and the binder are interwoven to form a spider web-like structure, and the silicon nanoparticles are wrapped at the nodes of the spider web-like structure. The mass ratio of the silicon nanoparticles to the carbon nanotubes is (4-8):1; The silicon composite negative electrode material is prepared by the following preparation method, which comprises: (1) In an external magnetic field, metal catalysts and carbon sources are deposited by chemical vapor deposition to obtain carbon nanotubes; (2) After obtaining the carbon nanotubes described in step (1), a gaseous silicon source is introduced to obtain a precursor by chemical vapor deposition, wherein silicon nanoparticles are deposited on the carbon nanotubes in the precursor; (3) mixing and cross-linking the binder solution with the precursor in step (2) to obtain the silicon composite negative electrode material; the mixing and cross-linking process also includes a cross-linker solution, and the cross-linker includes epichlorohydrin.

2. The silicon composite negative electrode material according to claim 1, characterized in that The diameter of the carbon nanotubes is 10-100 nm.

3. The silicon composite negative electrode material according to claim 1, characterized in that The length of the carbon nanotubes is 10-100 μm.

4. The silicon composite negative electrode material according to claim 1, characterized in that The average particle size of the silicon nanoparticles is 10-100 nm.

5. The silicon composite negative electrode material according to claim 1, characterized in that: The binder includes any one of carboxymethyl cellulose, polymethyl methacrylate, polyacrylic acid, sodium alginate, polyurethane, polyimide or polyacrylamide, or a combination of at least two thereof.

6. The silicon composite negative electrode material according to claim 1, characterized in that The mass ratio of the binder to the carbon nanotubes is (0.5-5):

1.

7. A method for preparing the silicon composite negative electrode material according to any one of claims 1 to 6, characterized in that: The preparation method comprises the following steps: (1) In an external magnetic field, metal catalysts and carbon sources are deposited by chemical vapor deposition to obtain carbon nanotubes; (2) After obtaining the carbon nanotubes described in step (1), a gaseous silicon source is introduced to obtain a precursor by chemical vapor deposition, wherein silicon nanoparticles are deposited on the carbon nanotubes in the precursor; (3) mixing and cross-linking the binder solution with the precursor in step (2) to obtain the silicon composite negative electrode material; the mixing and cross-linking process also includes a cross-linking agent solution, and the cross-linking agent includes epichlorohydrin; In the silicon composite negative electrode material, the mass ratio of silicon nanoparticles to carbon nanotubes is (4-8):

1.

8. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The magnetic field strength of the external magnetic field in step (1) is 0.5~5T.

9. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The metal catalyst in step (1) includes ferrocene.

10. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The carbon source in step (1) includes xylene.

11. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: Step (1) Chemical Vapor Deposition process includes: In an environment of an external magnetic field, a metal catalyst is mixed with a carbon source to obtain a catalyst precursor solution, which is then introduced into a reaction chamber using a carrier gas and chemical vapor deposition is performed on a substrate surface to obtain carbon nanotubes.

12. The method for preparing a silicon composite negative electrode material according to claim 11, wherein: The carrier gas includes a mixed gas of hydrogen and argon.

13. The method for preparing a silicon composite negative electrode material according to claim 11, wherein: The temperature of the chemical vapor deposition is 400-1000°C.

14. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The gas-phase silicon source in step (2) includes silane.

15. The method for preparing a silicon composite negative electrode material according to claim 7, characterized in that: In step (2), a carrier gas is introduced at the same time as the gas phase silicon source is introduced.

16. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The temperature of the chemical vapor deposition in step (2) is 300-800°C.

17. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The mass fraction of the binder in the binder solution in step (3) is 0.5-2.5%.

18. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The binder solution also includes a dispersant.

19. The method for preparing a silicon composite negative electrode material according to claim 18, wherein: The mass ratio of the binder to the dispersant is (1-3):

1.

20. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The mixing and cross-linking process in step (3) includes: The binder solution is applied to the surface of the precursor in step (2), and then mixed with the crosslinker solution for crosslinking.

21. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The mixed and cross-linked material in step (3) is soaked in water and dried.

22. The method for preparing a silicon composite negative electrode material according to claim 7, wherein: The preparation method comprises the following steps: (1) In an external magnetic field environment with a magnetic field strength of 0.55 T, a metal catalyst and a carbon source are mixed to obtain a catalyst precursor solution, the catalyst precursor solution is introduced into a reaction chamber using a mixture of hydrogen and argon, and chemical vapor deposition is performed on a substrate surface at a temperature of 400-1000° C. to obtain carbon nanotubes; (2) After obtaining the carbon nanotubes described in step (1), a gas-phase silicon source is introduced and chemical vapor deposition is performed at a temperature of 300-800° C. to obtain a precursor, in which silicon nanoparticles are deposited on the carbon nanotubes; (3) applying a binder solution with a mass fraction of 0.5-2.5% to the surface of the precursor in step (2), and then mixing it with a crosslinker solution for crosslinking. The mixed and cross-linked material is soaked in water and dried to obtain the silicon composite negative electrode material; the crosslinker includes epichlorohydrin; in the silicon composite negative electrode material, the mass ratio of silicon nanoparticles to carbon nanotubes is (4-8):

1.

23. A negative electrode plate, characterized in that: The negative electrode plate comprises the silicon composite negative electrode material according to any one of claims 1 to 6 and a binder.

24. A method for preparing a negative electrode sheet according to claim 23, characterized in that: The preparation method comprises: The silicon composite negative electrode material is mixed with a binder in a dry process and coated on the surface of the negative electrode current collector to obtain the negative electrode plate.

25. A lithium ion battery, characterized in that: The lithium-ion battery comprises the silicon composite negative electrode material according to any one of claims 1 to 6 or the negative electrode plate according to claim 23.

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