Preparation method of high-performance silicon-carbon negative electrode

Abstract

The invention discloses a preparation method of a high-performance silicon / carbon negative electrode. The preparation method comprises the steps of selecting a high-toughness carbon nanotube film, constructing a mesoporous structure and filling a negative electrode material. The preparation method comprises the following steps: soaking a carbon nanotube film twice with chlorosulfonic acid, carrying out tensile deformation of 80-300%, carrying out vacuum annealing strengthening and toughening treatment, packaging the carbon nanotube film in a PET film, processing mesopores with femtosecond laser, filling a silicon-based negative electrode material in modes of PVD, CVD and the like, and unpackaging to obtain the silicon / carbon negative electrode. According to the method, the mechanical and electrical properties of the carbon nanotube film are improved through strengthening and toughening treatment, the mesoporous structure provides buffer for silicon volume expansion, and the lithium ion diffusion path is shortened. The capacity retention ratio of the prepared negative electrode material after 100 cycles at 0.5 C exceeds 90%, the first charge-discharge efficiency is nearly 90%, and the negative electrode material is suitable for a high-energy-density lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a method for preparing a high-performance silicon-carbon negative electrode. Background Art

[0002] In the research of anode materials, silicon has shown significant advantages in terms of reserves, cost-effectiveness, and eco-friendliness. Therefore, it is considered an extremely attractive material and has gradually become a hot topic in anode material research in recent years. However, the volume expansion problem often accompanied by silicon during the lithium insertion and deintercalation process can lead to the destruction and shattering of the material structure, seriously affecting the performance and life of the battery. In addition, the volume expansion can also trigger the continuous growth of the solid electrolyte interface (SEI), which not only consumes active lithium but also reduces the coulombic efficiency of the battery.

[0003] The current methods for preparing silicon negative electrodes include: chemical vapor deposition, physical vapor deposition, high-temperature solid-phase synthesis, mechanical alloying, and electrospinning. In these preparation processes of silicon materials, porous carbon or amorphous carbon is often required to support and stabilize silicon nanomaterials. However, compared with carbon nanotubes, these forms of carbon have poor conductivity and mechanical properties, unstable structure, and the final performance of the prepared negative electrode materials is limited.

[0004] In order to solve the above problems, a method for preparing a high-performance silicon-carbon negative electrode is now provided. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing a high-performance silicon-carbon negative electrode to solve the technical problems existing in the prior art.

[0006] This invention, based on the inherent ability of carbon nanotubes to unfold their graphite layers, which inherit the unique aspect ratio and mesoporous cavity structure of carbon nanotubes, has developed a unique key technology for preparing a "silicon / carbon" anode. Using an ultra-fine laser cutting machine, a mesoporous structure is constructed within and on the surface of the carbon nanotube film, and the coaxial layers of carbon nanotubes are unfolded into graphite layers. Subsequently, silicon nanoparticles are deposited within the mesoporous structure of the carbon nanotube film, achieving the preparation of a carbon nanotube film-based "silicon / carbon" anode.

[0007] A method for preparing a high-performance silicon-carbon negative electrode comprises the following steps:

[0008] Step 1: The carbon nanotube film is subjected to a toughening treatment, wherein the toughening treatment is achieved by soaking in chlorosulfonic acid, as follows:

[0009] a. Soak the carbon nanotube film in chlorosulfonic acid for 12-15 hours, place it in moist air for 3 hours, and then soak it in chlorosulfonic acid for 12-15 hours;

[0010] b. The soaked carbon nanotube film is stretched by a tensile testing machine, and the tensile deformation is controlled at 80%-300%;

[0011] c. Dry and anneal the stretched film by heating it to 150-300°C at a vacuum of less than 10-2Pa and a heating rate of 2-5°C / min and maintaining it for 12 hours, then cooling it to room temperature at a rate of 2-5°C / min;

[0012] Step 2: Construction of the internal mesoporous structure of the high-strength and tough film and filling of the electromechanical negative electrode material:

[0013] d. Before constructing the mesoporous structure, the carbon nanotube film is flattened on the PET film to make the film surface flat. The other side is covered with a PET film of the same size and sealed around;

[0014] e. Using a femtosecond laser device to construct mesopores in the encapsulated film, achieving an open design of the carbon nanotube graphite layer inside the film;

[0015] f. After the mesopores are constructed, the mesopores are filled with negative electrode materials;

[0016] g. After the negative electrode material is filled, cut the packaging around the PET film, open the PET film, remove the carbon nanotube film, and obtain a high-performance "silicon / carbon" negative electrode material.

[0017] On the basis of the above technical solutions, the present invention also provides the following optional technical solutions:

[0018] In an optional solution: the filling of the negative electrode material in step c is achieved by one or more methods selected from PVD, CVD, ALD, ultrasonic spraying, and plasma spraying.

[0019] In an optional solution: the negative electrode material is a nanowire material or particles.

[0020] In an optional solution: the parameters of the femtosecond laser equipment processing in step b include laser wavelength, laser pulse energy, number of pulses, and laser scanning speed, and the mesoporous morphology and quality can be controlled by adjusting these parameters.

[0021] In one alternative: Selection of high-strength and tough carbon nanotube film:

[0022] A carbon nanotube film of target width was selected and subjected to scanning electron microscopy, optical microscopy and tensile mechanical property analysis, as well as electrical property analysis, to ensure that the carbon nanotube film had excellent mechanical and electrical properties and good tissue uniformity.

[0023] By adopting the above technical solution, the present invention has the following beneficial effects:

[0024] (1) The present invention is based on commercial (industrial-grade) carbon nanotube films. By strengthening and toughening the carbon nanotube films and constructing a mesoporous structure, it achieves the preparation of a new "silicon / carbon" lithium battery negative electrode material, which has a relatively mature market supply foundation. Compared with traditional "silicon / carbon" negative electrode material preparation technology, the silicon-carbon negative electrode preparation process in the present invention is simple, has fewer preparation complications, and has high process controllability.

[0025] (2) The "silicon / carbon" negative electrode prepared by the present invention has a large amount of graphite structure, which is well compatible with the structural system of the lithium battery industry in the current market. Moreover, this type of graphite structure exists in the form of carbon nanotube walls, which can greatly enhance the ability of the graphite structure to resist volume expansion and contraction during the lithium ion insertion and extraction process, thereby improving the cycle stability of the battery.

[0026] (3) The size of the silicon material in the "silicon / carbon" negative electrode of the present invention is at the nanometer level, which can effectively solve the pulverization effect of the silicon material during the lithium insertion and extraction process; furthermore, these silicon nanomaterials are mostly located between the carbon tube walls. Based on the intrinsic high strength characteristics of carbon nanotubes and the confined nature of the mesoporous structure, they can further stabilize and reinforce the nanostructure of the silicon material during the expansion and contraction process, thereby promoting the maintenance of the integrity of the solid electrolyte film at the negative electrode.

[0027] (4) The present invention prepares a high-performance "silicon / carbon" negative electrode based on a high-strength, ultra-thin, flexible carbon nanotube film, providing a new technology and new solution for the application of silicon materials in lithium battery negative electrodes, which can effectively promote the improvement of energy density, charge and discharge power and cycle stability of future lithium batteries.

[0028] (5) The "silicon / carbon" negative electrode material prepared by the present invention does not use any binder. It mainly achieves the binding effect of silicon nanoparticles through the adsorption effect of nanomaterials and the bundling effect of slender carbon nanotubes, thereby achieving the stability of the lithium battery negative electrode during cyclic charge and discharge.

[0029] (6) The "silicon / carbon" negative electrode material prepared by the present invention fully utilizes the high conductivity of carbon nanotubes, so that the negative electrode as a whole has high conductivity, which helps to significantly improve the rapid charge and discharge performance of lithium batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1This is a post-processing flow chart of the present invention: (a) high-strength and tough oriented CNT film, (b) parallel mesopores, (c) vertical mesopores, (d) silicon nanowires, (e) silicon nanoparticles.

[0032] Figure 2 The organizational structure diagram of the carbon nanotube film of the present invention after processing: (a) the original CNT film micro-nano structure, (b) the oriented densified CNT film, and (c) its micro-nano structure.

[0033] Figure 3 Schematic diagram of the degree of orientation of carbon nanotubes in the film of the present invention.

[0034] Figure 4 This is a characteristic diagram of the surface morphology of the flexible carbon nanotubes of the present invention.

[0035] Figure 5 The lithium battery negative electrodes of the present invention have different surface nanolayer thicknesses: (a) Sithickness <150nm, (b) Sithickness ≈ 150nm, and (c) Sithickness > 150nm.

[0036] Figure 6 The lithium battery negative electrode covered with the nano-carbon layer of the present invention: a) Sithickness <150nm, (b) Sithickness ≈150nm, (c) Sithickness >150nm.

[0037] Figure 7 The high-toughness, flexible, ultra-thin CNT film of the present invention: (a) nanostructure inside parallel holes, (b) nanostructure inside vertical holes.

[0038] Figure 8 This is the distribution pattern of nano-scale negative electrode materials such as Li, Si, Cu, and Au along the carbon nanotube wall inside the film of the present invention.

[0039] Figure 9 The ultra-thin, flexible metal current collector nanolayer on the surface of the composite negative electrode film of the present invention: (a) 20nm copper, (b) 20nm gold. DETAILED DESCRIPTION

[0040] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0041] The left, right, up, and down positions of the components shown in the accompanying drawings are merely one arrangement, and the specific positions are set according to specific needs.

[0042] Example 1

[0043] Carbon nanotube film toughening:

[0044] A commercial carbon nanotube film with a width of 10 cm (Shenzhen Nanoport Co., Ltd., model CNT-F100) was selected and immersed in 98% chlorosulfonic acid for 12 hours, taken out and placed in an environment with a humidity of 85% for 3 hours, and then immersed for another 12 hours.

[0045] An electronic universal testing machine (Instron 5967) was used to stretch the sample at a rate of 1 mm / min to a deformation of 200%.

[0046] The film was placed in a vacuum annealing furnace, heated to 200°C at a rate of 3°C / min and maintained at that temperature for 12 hours under a vacuum degree of 10-3 Pa, and then cooled to obtain a toughened film.

[0047] Mesoporous construction and silicon filling:

[0048] The film was encapsulated in a 0.2 mm thick PET film, and mesopores were processed using a femtosecond laser (Coherent Libra). The parameters were set as follows: pulse energy 30 μJ, scanning speed 300 mm / s, and repetition frequency 1 kHz, and a mesopore array with a pore diameter of approximately 200 nm and a pore depth of 3 μm was processed.

[0049] A silicon layer was deposited at 180° C. using an ALD device (Beneq TFS200) with 500 deposition cycles, and the thickness of the silicon layer was about 15 nm.

[0050] After unsealing, a silicon / carbon negative electrode material with a silicon loading of about 45% was obtained.

[0051] Comparative Example 1

[0052] The original carbon nanotube film without toughening treatment was used to directly load silicon nanoparticles through a physical mixing method. Other conditions were the same as those in Example 1.

[0053] Performance Testing

[0054]

[0055]

[0056] In the process of preparing the "silicon / carbon" negative electrode, there are specific requirements for the microstructure of the carbon nanotube film used. Figure 1(a) shows the microstructure of the untreated carbon nanotube film. Its structure is fluffy, with carbon nanotubes bundled and stacked in a disorderly manner. This kind of carbon nanotube film cannot be directly used as the raw base material for the preparation of "silicon / carbon" negative electrodes. After the carbon nanotube film is processed, its structure is significantly densified, and the stacking of carbon nanotubes in the film reaches the molecular level, as shown in Figure 2. Figure 2 As shown in (b) & (c);

[0057] Figure 3 (a) shows the surface morphology of the original carbon nanotube film. The carbon nanotubes are randomly distributed and obviously fluffy. The film material with this kind of organizational structure cannot withstand laser cutting and cannot achieve the construction of a relatively uniform mesoporous structure inside it. Figure 3 The surface of the carbon nanotube film shown in (b) has a high degree of flatness and the orientation of the carbon nanotubes is high. The tight stacking of the carbon nanotubes and the high orientation of the carbon nanotubes enable it to withstand laser ablation with a high energy density. At the same time, after laser ablation, a large number of carbon nanotube walls are opened, and the stability of the carbon nanotubes is greatly weakened. However, due to the molecular-level densification structure, the strong van der Waals force of macromolecules, and the unique large aspect ratio of carbon nanotubes, the carbon nanotubes can maintain their original position, original shape, and original stacking form almost unchanged after being greatly opened, thereby achieving the good maintenance of the mesoporous structure between carbon nanotubes, the hollow mesoporous structure of the carbon nanotubes themselves, and the layered graphite structure of the carbon nanotube walls, and achieving the stable existence of micro-nanostructures at all levels of the film. In summary, by selecting a densified carbon nanotube film, the construction of a stable mesoporous structure inside the film can be achieved, and a strong and tough carbon nanotube film space support with a stable mesoporous structure can be obtained, preparing for the plating and filling of lithium battery negative electrode materials;

[0058] Figure 4 The figure shows the surface morphology of flexible carbon nanotubes. The surface has a distinct metallic luster, and the film has excellent flexibility, which is conducive to the preparation of new, high-performance lithium battery anode materials and the improvement of the ultimate energy density, charge and discharge power, and cycle stability of lithium batteries.

[0059] The structure of the "silicon / carbon" negative electrode in this invention is unique

[0060] Based on the present invention, the schematic structure of the prepared "silicon / carbon" negative electrode can be realized, such as Figure 5As shown. First, the mesoporous structure inside the carbon nanotube film is filled with negative electrode nanoparticles. These nanoparticles are mainly located in the mesoporous structure between adjacent carbon tubes. Then, in order to further improve the lithium insertion capacity of the negative electrode material, silicon nanolayers of different thicknesses are plated on the surface of the negative electrode film. Based on the critical size of 150nm for easy pulverization of silicon, silicon layers of <150nm, ≈150nm, and >150nm are prepared on the surface of the negative electrode, as shown in the figure. Figure 5 (a) to (c)

[0061] As we all know, during the lithium insertion and extraction process of lithium in lithium batteries, silicon materials will undergo large volume expansion and contraction accompanied by the alloying of silicon and lithium. In order to further improve the fatigue life of the nano-silicon layer, an ultra-thin carbon layer is constructed on the surface of the silicon nano-continuous layer to produce a solid electrolyte membrane layer formed during the charging and discharging process of the lithium battery, thereby helping to improve the ultimate coulombic efficiency of the lithium battery. The schematic structure of the carbon protective layer on the surface of the "silicon / carbon" negative electrode is shown in the figure. Figure 6 (a) to (c) are shown. Based on this structure, the following can be further realized: 1. The role of the continuous carbon nanolayer in forming a stable solid electrolyte membrane; 2. The protective effect of the continuous carbon nanolayer on the fatigue life of the silicon layer; 3. The influence of the silicon layer thickness on the continuous carbon nanolayer;

[0062] The micro-nanostructure characteristics of the "silicon / carbon" negative electrode prepared in the present invention are obviously unique. Figure 7 As shown in the figure, after laser cutting of the high-strength and tough carbon nanotube film, a large number of oriented and tightly stacked graphite layers constructed by the carbon nanotube walls will be formed inside it. The main functions of this characteristic graphite layer are reflected in three aspects: 1. This oriented graphite layer helps to improve the strength and toughness of the lithium battery negative electrode film and greatly improve the mechanical properties of the negative electrode; 2. This oriented graphite layer can inherit the high conductivity of carbon nanotubes, which helps to significantly reduce the resistance of the lithium battery negative electrode and improve the battery's fast charging and discharging capabilities; 3. The combination of highly tightly stacked graphite layers and mesoporous structures can significantly improve the fatigue life of the lithium battery negative electrode, thereby solving the problem of repeated volume expansion and contraction of the negative electrode during the lithium insertion and extraction process;

[0063] In the present invention, nanoparticles such as silicon in the "silicon / carbon" negative electrode are tightly attached to the mesoporous structure and the graphite layer of the carbon nanotube wall. The presence of the mesoporous structure and the graphite layer of the tube wall can provide stable attachment sites for nanoparticles such as silicon. As is known to all, nanoparticles such as silicon are highly active, and it is difficult to stably exist and stably exert negative electrode performance. However, due to the restrictions of the graphite layer of the tube wall and the mesoporous structure, these nanoparticles such as silicon can stably exist and exert stable performance, such as Figure 8 (a) to (b)

[0064] The surface of the "silicon / carbon" negative electrode prepared in the present invention can also realize the construction of a nano-continuous metal layer, which in turn helps to realize the design and development of the overall structural parts of the lithium battery negative electrode. The prepared lithium battery negative electrode structural parts contain very little metal, which helps to reduce the weight of the lithium battery negative electrode and improve the ultimate energy density of the lithium battery.

[0065] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a high-performance silicon-carbon negative electrode, characterized in that: The following steps are involved: The following steps are involved: Step 1: The carbon nanotube film is subjected to a toughening treatment, wherein the toughening treatment is achieved by soaking in chlorosulfonic acid, as follows: a. Soak the carbon nanotube film in chlorosulfonic acid for 12-15 hours, place it in moist air for 3 hours, and then soak it in chlorosulfonic acid for 12-15 hours; b. The soaked carbon nanotube film is stretched by a tensile testing machine, and the tensile deformation is controlled at 80%-300%; c. Dry and anneal the stretched film at a vacuum degree of less than 10 -2 Pa, heating rate 2-5 ° C / min to 150-300 ° C and maintain for 12 hours, and then cool to room temperature at a rate of 2-5 ° C / min; Step 2: Construction of the internal mesoporous structure of the high-strength and tough film and filling of the electromechanical negative electrode material d. Before constructing the mesoporous structure, the carbon nanotube film is flattened on the PET film to make the film surface flat. The other side is covered with a PET film of the same size and sealed around; e. Using a femtosecond laser device to construct mesopores in the encapsulated film, achieving an open design of the carbon nanotube graphite layer inside the film; f. After the mesopores are constructed, the mesopores are filled with negative electrode materials; g. After the negative electrode material is filled, cut the packaging around the PET film, open the PET film, remove the carbon nanotube film, and obtain a high-performance "silicon / carbon" negative electrode material.

2. The method for preparing a high-performance silicon-carbon negative electrode according to claim 1, wherein: The filling of the negative electrode material in step c is achieved by one or more methods selected from PVD, CVD, ALD, ultrasonic spraying, and plasma spraying.

3. The method for preparing a high-performance silicon-carbon negative electrode according to claim 1, wherein: The negative electrode material is a nanowire material or particles.

4. The method for preparing a high-performance silicon-carbon negative electrode according to claim 1, wherein: The parameters of the femtosecond laser equipment processing in step b include laser wavelength, laser pulse energy, pulse number, and laser scanning speed. By adjusting these parameters, the mesoporous morphology and quality can be controlled.

5. The method for preparing a high-performance silicon-carbon negative electrode according to claim 1, wherein: Selection of high-strength and tough carbon nanotube films: A carbon nanotube film of target width was selected and subjected to scanning electron microscopy, optical microscopy and tensile mechanical property analysis, as well as electrical property analysis, to ensure that the carbon nanotube film had excellent mechanical and electrical properties and good tissue uniformity.

6. A high-performance silicon-carbon negative electrode material, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 5.

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