Method for preparing silicon-carbon composite negative electrode material by electrostatic self-assembly technology

Abstract

The invention discloses a method for preparing a silicon-carbon composite negative electrode material through an electrostatic self-assembly technology, and belongs to the technical field of lithium ion battery negative electrode materials. According to the invention, lignin is used as a biomass carbon source and lauryl sodium sulfate (SDS) and poly (diallyldimethylammonium chloride) (PDDA) are used as activators through a simple electrostatic self-assembly technology; a proper amount of nano silicon and PDDA are subjected to in-situ adsorption, and lignin and SDS are subjected to in-situ adsorption; a self-assembly process is carried out through the positive and negative charge attraction effect among colloidal particles, so that effective compounding of the nano silicon particles and the biomass carbon is realized; and performing vacuum drying, and performing one-stage sintering in a tubular furnace to obtain the silicon-carbon composite negative electrode material. Based on the problems of non-uniform particle size, easy agglomeration and great abrasion of a silicon-carbon composite material in a traditional mechanical synthesis technology, the invention improves the compounding efficiency of the silicon-carbon material and improves the problems of volume expansion of a silicon negative electrode, slow migration rate of lithium ions, poor cycle performance and the like.

Description

Technical Field

[0001] The invention relates to a method for preparing a silicon-carbon composite negative electrode material by using an electrostatic self-assembly technology, belonging to the technical field of negative electrode materials for lithium ion batteries. Background Art

[0002] With the rapid promotion of new energy vehicles and electronic devices, higher requirements are placed on the energy density and cycle stability of energy storage devices. Compared with traditional graphite materials (372mAh / g), alloy materials (such as Si, Ge, Sn) have the advantages of high energy density and low lithium insertion potential, and are widely used as negative electrode materials for lithium-ion batteries. Due to its high lithium insertion amount, the silicon-based negative electrode can reach a theoretical specific capacity of 4200mAh / g. Therefore, silicon-based materials are the most potential for large-scale industrialization of lithium-ion battery negative electrode materials. However, the frequent volume changes of silicon particles lead to crushing and shedding, unstable solid + The formation of the solid electrolyte interface layer and the generation of irreversible capacity are the main challenges. The repeated insertion / extraction movement of lithium ions will cause the silicon particles to frequently expand and shrink in volume and then shatter and fall off, forming powder that blocks the internal channel and compresses the space of the electrolyte. The direct contact between silicon and the electrolyte and the volume effect also lead to the repeated formation of the solid electrolyte interface (SEI) layer, which will lead to a decrease in conductivity due to the thickening of the film.

[0003] In order to achieve the commercial application of silicon-based anodes, common modification methods and performance improvement strategies include: morphology and micro-nano junction regulation, conductive polymer coating, carbon material composite, etc. Due to their high mechanical strength and conductivity, carbon-based materials have been widely used to enhance the conductivity of silicon and as a protective layer to reduce the adverse effects of silicon expansion. Carbon materials have high specific surface area and adjustable microstructure morphology, which can effectively improve the volume expansion of silicon-based materials and improve the overall conductivity and structural stability of the electrode. The traditional methods for synthesizing silicon-carbon composite negative electrode materials are mainly: ① Mechanical ball milling: The non-uniform wetting and uncontrollable aggregation of silicon particles make it difficult to ensure the required uniformity of the mixture composition. The uniformity of the particle size after ball milling is poor, which may destroy the particle structure, introduce impurities, and have low efficiency. ② Chemical vapor deposition (CVD): The process is complex, the cost is high, and high-temperature equipment is required. ③ Combustion synthesis method: The reaction is violent, impurities remain, and the repeatability is poor. Summary of the invention

[0004] In view of the problems in the traditional synthesis process of silicon-carbon materials, such as poor composite effect between silicon particles and carbon materials and unstable structural homogeneity, and combined with the problems of high initial irreversible capacity and poor cycling performance of silicon anodes, the present invention uses a simple electrostatic self-assembly technique, with lignin as the biomass carbon source, sodium dodecyl sulfate (SDS) and poly(diallyldimethylammonium chloride) (PDDA) as activators; by charging nano-silicon and lignin, the self-assembly process occurs due to the attraction between positive and negative charges of colloidal particles, realizing the effective composite of nano-silicon particles and biomass carbon.

[0005] The preparation method of the silicon-carbon composite anode material by the electrostatic self-assembly method of the present invention is as follows: (1) Weigh nano-silicon powder, add it to deionized water and ultrasonically disperse it for 50 - 200 min, then add PDDA solution and stir at room temperature for 8 - 14 h to obtain a positively charged Si@PDDA solution, where; (2) Weigh lignin and SDS, add them to deionized water, and stir at room temperature for 8 - 14 h to obtain a negatively charged lignin@SDS solution; (3) Slowly add the lignin@SDS solution obtained in step (2) to the Si@PDDA solution obtained in step (1), mix and stir for 3 - 5 h, then wash with deionized water, centrifuge, and dry in a vacuum oven at 60 - 100 °C for 6 - 8 h to obtain a Si / lignin composite material; (4) Mechanically grind the Si / lignin composite material obtained in step (3), under Ar protection, place it in a tubular furnace, control the heating rate at 3 - 15 o °C / min, and sinter at 700 - 850 o °C for one-stage sintering for 3 - 6 h. After complete cooling, take it out, grind and sieve to obtain the silicon-carbon composite anode material.

[0006] In step (1), the mass ratio of nano-silicon powder to PDDA is 1:2 - 1:4.

[0007] In step (1), the size of Si nanoparticles is 20 - 60 nm.

[0008] In step (1), the concentration of PDDA solution is 10 - 30 wt.%.

[0009] In step (1), the mass ratio of nano-silicon powder to deionized water is 1:100 - 1:200.

[0010] In step (1), the mass ratio of lignin to deionized water is 1:100 - 1:200.

[0011] In step (2), the mass ratio of lignin to SDS is 1:1 - 2:1.

[0012] The present invention provides a preparation method of a silicon-carbon composite anode material prepared by electrostatic self-assembly. In this method, SDS and PDDA are used as activators, and are respectively adsorbed in situ on lignin and nano-Si particles. The two activators easily undergo electrostatic interaction. PDDA has significant cationic properties and excellent water solubility, and it contains many positively charged quaternary ammonium ions. SDS is an anionic surfactant, enabling it to be easily assembled with positively charged nano-Si particles through electrostatic self-assembly. After mixing, drying and sintering, lignin carbon is uniformly coated on the surface of nano-silicon particles to form a silicon-carbon composite anode material. The silicon-carbon anode with a uniform coating structure, on the one hand, the carbon material can provide a buffer space for Si, accommodate volume expansion to a certain extent, thereby effectively reducing the cracking and pulverization of silicon during the alloying (dealloying) process, and alleviating the change of structural stress. On the other hand, it can avoid the direct contact between nano-silicon particles and the electrolyte, reduce the loss of active substances during charge and discharge, and reduce the distance of lithium ion insertion channels, improve the conductivity of the electrode material, and thus enhance the electro-chemical reaction ability. Brief Description of the Drawings

[0013] Figure 1 is the scanning electron microscope image of the silicon-carbon composite anode material prepared by the electrostatic self-assembly method in Example 1 of the present invention; Figure 2 is the transmission electron microscope image of the silicon-carbon composite anode material prepared by the electrostatic self-assembly method in Example 1 of the present invention; Figure 3 is the charge-discharge rate graph of the silicon-carbon composite anode material prepared by the electrostatic self-assembly method in Example 2 of the present invention; Figure 4 is the charge-discharge curve graph of the silicon-carbon composite anode material prepared by the electrostatic self-assembly method in Example 3 of the present invention; Figure 5 is the cycle performance graph of the silicon-carbon composite anode material prepared by the electrostatic self-assembly method in Example 4 of the present invention. Detailed Embodiments

[0014] The present invention will be further described in detail below with reference to the drawings and embodiments, but the protection scope of the present invention is not limited to the content described.

[0015] Example 1: The preparation method of the silicon-carbon composite anode material prepared by this electrostatic self-assembly method is as follows: (1) Weigh nano-silicon powder, add it to deionized water and ultrasonically disperse it for 50 min, then add PDDA solution (the mass ratio of nano-silicon powder to PDDA is 1:2 to 1:4), and stir at room temperature for 8 h to obtain a positively charged Si@PDDA solution, where the mass ratio of nano-silicon powder to deionized water is 1:100 to 1:200.

[0016] (2) Weigh lignin and SDS and add them to deionized water (the mass ratio of lignin to SDS is 1:1 - 2:1). Stir at room temperature for 8 h to obtain a negatively charged lignin@SDS solution, where the mass ratio of lignin to deionized water is 1:100 - 1:200.

[0017] (3) Slowly add the lignin@SDS solution obtained in step (2) to the Si@PDDA solution obtained in step (1), and mix and stir for 3 h. Then wash with deionized water, centrifuge, and dry in a vacuum oven at 60 °C for 6 h to obtain a Si / lignin composite material.

[0018] (4) Mechanically grind the Si / lignin composite material obtained in step (3). Under Ar protection, place it in a tube furnace and control the heating rate at 3 o °C / min, and sinter at 700 o °C for 3 h in one step. Wait until it cools completely, take it out, grind and sieve to obtain a silicon-carbon composite anode material.

[0019] The scanning electron microscope and transmission electron microscope of the silicon-carbon composite anode material prepared by the electrostatic self-assembly method in this example are as Figure 1 、 2 shown. It can be clearly observed that the morphology has an obvious spherical appearance, with large and small spheres connected to each other and in close contact. The morphology of this material is spherical particles with uniform size. Solid spheres with diameters of 148.73 nm and 169.57 nm can be clearly seen. The outermost layer is uniformly wrapped by lignin porous carbon to form a thin carbon layer (Si@C), with a diameter of about 3.01 nm, which can avoid direct contact between nano-silicon particles and the electrolyte, reduce the loss of active substances during charge and discharge, and reduce the distance of lithium-ion insertion channels, thereby improving the conductivity. From the lattice fringes, it can be seen that the interplanar spacing of the nano-silicon particles is 0.31 nm, corresponding to the (111) crystal plane of silicon. The ordered arrangement of nano-Si particles around the lignin porous carbon helps to limit the volume expansion of nano-silicon particles and further improves the conductivity.

[0020] Example 2: The preparation method of the silicon-carbon composite anode material prepared by this electrostatic self-assembly method is as follows: (1) Weigh nano-silicon powder and add it to deionized water for ultrasonic dispersion for 100 min. Then add PDDA solution (the mass ratio of nano-silicon powder to PDDA is 1:2 - 1:4). Stir at room temperature for 8 h to obtain a positively charged Si@PDDA solution, where the mass ratio of nano-silicon powder to deionized water is 1:100 - 1:200.

[0021] (2) Weigh lignin and SDS and add them to deionized water (the mass ratio of lignin to SDS is 1:1~2:1). Stir at room temperature for 10 h to obtain a negatively charged lignin@SDS solution, in which the mass ratio of lignin to deionized water is 1:100~1:200.

[0022] (3) The lignin@SDS solution obtained in step (2) was slowly added to the Si@PDDA solution obtained in step (1), and the mixture was stirred for 4 h. The mixture was then washed with deionized water, centrifuged, and dried in a vacuum oven at 80 °C for 6 h to obtain a Si / lignin composite material.

[0023] (4) The Si / lignin composite material obtained in step (3) was mechanically ground and placed in a tubular furnace under Ar protection with a controlled heating rate of 5 o C / min, 750 o C is sintered in one step for 4 hours, and after being completely cooled, taken out, ground and sieved to obtain a silicon-carbon composite negative electrode material.

[0024] Weigh 0.14 g of the composite material prepared in this example, 0.04 g of acetylene black, and 0.02 g of sodium alginate (SA), put them into a mortar, mix and grind for 30 min, then add 2 mL of deionized water, continue grinding for 30 min, evenly coat the viscous mixture on the copper foil, and then heat at 80 o C for 15 min, and then at 60 o C in a vacuum oven for 12 h, and the coated copper foil was cut into disks with a diameter of 14 mm as working electrodes (1.3 ± 0.1 mg).

[0025] In a glove box filled with argon (O2 content <1ppm, water content <1ppm), the electrode, diaphragm, lithium sheet and nickel foam mesh were assembled into button cells by conventional methods, and the battery electrochemical performance was tested on a constant current charge and discharge system. The rate cycle results are shown in the figure below. Figure 3 As shown in the figure, it can be seen that at current densities of 0.1, 0.2, 0.5, 1, 2 and 0.1 A / g, the silicon-carbon composite negative electrode material prepared by the electrostatic self-assembly method exhibits excellent rate performance, and its discharge specific capacity is 1981.01, 1335.12, 1011.92, 843.17, 644.16 and 929.77 mAh / g, respectively.

[0026] Example 3: The preparation method of the silicon-carbon composite negative electrode material prepared by the electrostatic self-assembly method is as follows: (1) Weigh the nano-silicon powder and add it to deionized water, then ultrasonically disperse it for 150 min. After that, add the PDDA solution (the mass ratio of nano-silicon powder to PDDA is 1:2 - 1:4), and stir at room temperature for 12 h to obtain a positively charged Si@PDDA solution, where the mass ratio of nano-silicon powder to deionized water is 1:100 - 1:200.

[0027] (2) Weigh lignin and SDS and add them to deionized water (the mass ratio of lignin to SDS is 1:1 - 2:1), and stir at room temperature for 12 h to obtain a negatively charged lignin@SDS solution, where the mass ratio of lignin to deionized water is 1:100 - 1:200.

[0028] (3) Slowly add the lignin@SDS solution obtained in step (2) to the Si@PDDA solution obtained in step (1), and mix and stir for 5 h. Then wash with deionized water, centrifuge, and dry in a vacuum oven at 100 °C for 10 h to obtain the Si / lignin composite material.

[0029] (4) Mechanically grind the Si / lignin composite material obtained in step (3). Under Ar protection, place it in a tube furnace and control the heating rate at 8 o °C / min, sinter at 800 o °C for 5 h in one step. After complete cooling, take it out, grind and sieve to obtain the silicon-carbon composite anode material.

[0030] Weigh 0.14 g of the composite material prepared in this example, 0.04 g of acetylene black, and 0.02 g of sodium alginate (SA), put them into a mortar, mix and grind for 30 min. Then add 2 mL of deionized water and continue to grind for 30 min. Coat the viscous mixture evenly on the copper foil, and then preliminarily dry it at 80 o °C for 15 min, and then dry it in a vacuum oven at 60 °C for 12 h. Cut the coated copper foil into disks with a diameter of 14 mm as the working electrode (1.3 ± 0.1 mg).

[0031] In a glove box filled with argon (O2 content < 1 ppm, water content < 1 ppm), assemble the electrode sheet, separator, lithium sheet, and nickel foam mesh into a button cell in a conventional method, and conduct battery electrochemical performance tests on a constant current charge-discharge system. The charge-discharge curve is as Figure 4 shown. It can be seen from the figure that the material prepared in this example shows good lithium storage capacity as the anode material of a lithium-ion battery. At a current density of 1 A / g, a constant current charge-discharge test is carried out between 0.01 - 2.0 V. Compared with the first and second charge-discharge cycles, there is a large energy loss in the tenth charge-discharge cycle. However, the energy loss is less in the fiftieth and one-hundredth cycles, and the material is relatively stable.

[0032] Example 4: The preparation method of the silicon-carbon composite anode material prepared by this electrostatic self-assembly method is as follows: (1) Weigh nano-silicon powder, add it to deionized water and disperse it by ultrasonic wave for 200 min. Then add PDDA solution (the mass ratio of nano-silicon powder to PDDA is 1:2 - 1:4), and stir at room temperature for 14 h to obtain a positively charged Si@PDDA solution, where the mass ratio of nano-silicon powder to deionized water is 1:100 - 1:200.

[0033] (2) Weigh lignin and SDS, add them to deionized water (the mass ratio of lignin to SDS is 1:1 - 2:1), and stir at room temperature for 14 h to obtain a negatively charged lignin@SDS solution, where the mass ratio of lignin to deionized water is 1:100 - 1:200.

[0034] (3) Slowly add the lignin@SDS solution obtained in step (2) to the Si@PDDA solution obtained in step (1), mix and stir for 5 h, then wash with deionized water, centrifuge, and dry in a vacuum oven at 100 °C for 6 h to obtain a Si / lignin composite material.

[0035] (4) Mechanically grind the Si / lignin composite material obtained in step (3). Under Ar protection, place it in a tube furnace and control the heating rate at 5 o °C / min, sinter at 850 o °C for one-stage sintering for 6 h. Wait until it cools completely, take it out, grind and sieve to obtain the silicon-carbon composite anode material.

[0036] Weigh 0.14 g of the composite material prepared in this example, 0.04 g of acetylene black, and 0.02 g of sodium alginate (SA), put them into a mortar, mix and grind for 30 min, then add 2 mL of deionized water and continue to grind for 30 min. Spread the viscous mixture evenly on the copper foil, then preliminarily dry it at 80 °C for 15 min, and then dry it in a vacuum oven at 60 °C for 12 h. Cut the copper foil with the coating into discs with a diameter of 14 mm as the working electrode (1.3 ± 0.1 mg).

[0037] In a glove box filled with argon (O2 content < 1 ppm, water content < 1 ppm), assemble the electrode sheet, separator, lithium sheet, and nickel foam mesh into a button cell in a conventional method, and conduct battery electrochemical performance tests on a constant current charge and discharge system. The cycle performance diagram is as Figure 5As shown in the figure, it can be seen that the Si / lignin@C material prepared by electrostatic self-assembly method exhibits high and stable electrochemical performance. The cycle performance of the negative electrode material stabilizes after 200 cycles and still has a discharge capacity of 1200mAh / g. Under the same conditions, the silicon-carbon material prepared without PDDA and SDS activators has a rate performance of 375mAh / g after 200 cycles, and the silicon-carbon material prepared by mechanical ball milling has a discharge capacity of only 360mAh / g after 200 charge and discharge cycles, and both have poor cycle performance.

[0038] The present invention prepares a lithium-ion battery silicon-carbon negative electrode material with a core-shell structure through a simple electrostatic self-assembly technology, and in situ adsorbs anions and cations that are prone to electrostatic reactions on lignin and nano-silicon particles respectively, and a self-assembly process occurs through the attraction of positive and negative charges between colloidal particles. After sufficient mixing, lignin carbon and nano-silicon are composited and coated with a carbon shell, and the porous carbon structure of biomass is used to provide more and more stable diffusion channels for the migration of lithium ions between material particles; the carbon layer provides a buffer space for nano-silicon, effectively reducing the cracking and pulverization of silicon during the alloying (de-alloying) process; the uniform carbon layer provides abundant binding sites for nano-silicon particles. In addition, the presence of the carbon layer avoids direct contact between silicon nano-particles and electrolytes, and promotes the formation of a stable SEI and the lithiation process, thereby significantly improving the cycle stability of the composite material.

[0039] As the second largest natural polymer material, lignin has a complex structure and functional groups, which can form a porous carbon framework with rich mesopores in composite materials. + Provide a fast transfer channel. Li et al. reported that the negative electrode material composed of lignin and metal organic framework (MOF) had a capacity retention rate of 99% after 300 cycles at 0.2A / g, indicating that lignin has good application value in composite negative electrode materials for lithium-ion batteries. Starting from the design of the composite structure of silicon negative electrode and carbon material, the present invention uses electrostatic self-assembly technology to assemble silicon and carbon materials with opposite charges, and efficiently synthesizes silicon-carbon materials with uniform particle size, high bonding strength and no wear.

[0040] The specific implementation modes of the present invention are described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above implementation modes, and various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention.

Claims

1. A method for preparing a silicon-carbon composite anode material by an electrostatic self-assembly technique, characterized in that The nano-Si particles and lignin particles are in-situ adsorbed by an activator, and electrostatic interaction occurs to form a silicon-carbon composite anode material. The specific steps are as follows: (1) Weigh nano-silicon powder, add it to deionized water, and ultrasonically disperse it for 50 - 200 min. Then add PDDA solution and stir at room temperature for 8 - 14 h to obtain a positively charged Si@PDDA solution, where; (2) Weigh lignin and SDS, add them to deionized water, and stir at room temperature for 8 - 14 h to obtain a negatively charged lignin@SDS solution; (3) Slowly add the lignin@SDS solution obtained in step (2) to the Si@PDDA solution obtained in step (1), mix and stir for 3 - 5 h. Then wash with deionized water, centrifuge, and dry in a vacuum oven at 60 - 100 °C for 6 - 8 h to obtain a Si / lignin composite material; (4) Mechanically grind the Si / lignin composite material obtained in step (3), place it in a tube furnace under Ar protection, and control the heating rate at 3-15 o °C / min, and sinter it in one step at 700-850 o °C for 3-6 h. After it is completely cooled, take it out, grind it, and sieve it to obtain the silicon-carbon composite negative electrode material.

2. The preparation method of the silicon-carbon composite anode material prepared by the electrostatic self-assembly technique according to claim 1, characterized in that: In step (1), the mass ratio of nano-silicon powder to PDDA is 1:2 - 1:

4.

3. The preparation method of the silicon-carbon composite anode material prepared by the electrostatic self-assembly technique according to claim 1, characterized in that: In step (1), the size of Si nanoparticles is 20 - 60 nm.

4. The preparation method of the silicon-carbon composite anode material prepared by the electrostatic self-assembly technique according to claim 1, characterized in that: In step (1), the concentration of PDDA solution is 10 - 30 wt.%.

5. The preparation method of the silicon-carbon composite anode material prepared by the electrostatic self-assembly technique according to claim 1, wherein: In step (2), the mass ratio of lignin to SDS is 1:1 - 2:

1.

6. The preparation method of the silicon-carbon composite anode material prepared by the electrostatic self-assembly technique according to claim 1, wherein: In step (1), the mass ratio of nano-silicon powder to deionized water is 1:100 - 1:

200.

7. The preparation method of the silicon-carbon composite anode material prepared by the electrostatic self-assembly technology according to claim 1, characterized in that: In step (1), the mass ratio of lignin to deionized water is 1:100 - 1:

200.

8. A silicon-carbon composite anode material prepared by the method of preparing a silicon-carbon composite anode material using the electrostatic self-assembly technology described in any one of claims 1 - 5.

Images