C-Si anode for lithium-ion batteries and its manufacturing method

Abstract

The present invention aims to improve the performance of lithium-ion batteries by suppressing the volume expansion of Si in C-Si negative electrodes during charge and discharge. [Solution] In a C-Si negative electrode, a honeycomb structure of CNTs formed on the surface of a Cu substrate is filled with Si microparticles and a composite material of graphene and Si microparticles, and the surface is coated with a thin film of CNTs to suppress the volume expansion of Si during charging and discharging, thereby improving the performance of lithium-ion batteries.

Description

[Technical Field]

[0001] The present invention relates to a C-Si negative electrode for a lithium ion battery and a method for producing the same. [Background technology]

[0002] In recent years, there has been a growing demand for increased battery capacity for automotive applications. Because battery capacity directly affects driving range, the higher the capacity density per volume or weight, the better. Currently, graphite is widely used as the negative electrode active material in lithium-ion batteries, but existing graphite negative electrodes have a theoretical gram capacity of only 372 mAh / g, which is insufficient to meet future energy density demands.

[0003] In contrast, Si has desirable properties such as a low operating potential, abundant content, and a high theoretical specific capacity of 4200 mAh / g at room temperature. + It is attracting attention as an anode material that can store more hydrogen, and is expected to have a capacity approximately 10 times greater by weight than carbon materials. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-239369 [Patent Document 2] Special Publication No. 2024-509237 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when C-Si materials are used as the anode of a lithium-ion battery, the volume of the Si typically expands by 100% to 300% during charging and discharging. The constant contraction and expansion causes the Si material to pulverize, seriously affecting the lifespan of the lithium-ion battery. Furthermore, the expansion of Si generates significant stress inside the lithium-ion battery, leading to the extrusion or fracture of the electrode pieces, and further reducing the porosity inside the lithium-ion battery, promoting the precipitation of Li metal and affecting the safety of the battery.

[0006] The present invention proposes the following as a result of research conducted to suppress the volumetric contraction and expansion of Si in the C-Si negative electrode described above during charge and discharge, thereby improving the safety and performance of lithium-ion batteries. [Means for solving the problem]

[0007] This invention proposes a C-Si anode for lithium-ion batteries, in which a composite material of Si particles and graphene is filled into the honeycomb of carbon nanotubes (CNTs) formed on the surface of a Cu substrate, and the surface is further coated with a thin film of CNTs.

[0008] The present invention proposes a method for manufacturing a C-Si negative electrode for a lithium-ion battery, which involves growing vertically aligned CNTs on a Cu substrate surface, spraying hot pure water onto the CNTs to transform them into honeycomb-structured CNTs, filling the honeycomb of the CNTs with a composite material of Si microparticles and graphene, and coating the surface with a thin film of CNTs.

[0009] In addition, when manufacturing the C-Si negative electrode of the lithium ion battery, a series of manufacturing steps can be performed using a vertical thermal CVD apparatus.

[0010] Honeycomb-structured CNTs are nanoscale honeycomb structures arranged in a unique pattern. They are carbon-based materials similar to carbon nanotubes. The honeycomb structure is formed by uniformly growing vertically aligned CNTs on the surface of a Cu substrate and then subjecting them to steam treatment, which causes the individual CNTs to locally converge due to the surface tension of the aqueous solution, resulting in the formation of an overall continuous honeycomb structure.

[0011] Honeycomb-structured CNTs have very high electrical conductivity, making them important in electronic devices and sensors. Their extremely large surface area also makes them useful as catalyst supports and electrode materials in energy conversion and battery technology.

[0012] Due to its conductive and structural properties, it is considered a promising anode material for lithium-ion batteries, which are expected to offer higher energy density and safety than conventional lithium-ion batteries.

[0013] In the present invention, the "honeycomb" of honeycomb-structured CNTs is used as a "container" for the composite material of Si microparticles and graphene.

[0014] The "honeycomb" of honeycomb-structured CNTs can be filled with Si particles and graphene composites as anode materials. Utilizing this property, it is possible to create lithium-ion batteries. By filling the CNTs with a composite of Si particles and graphene, the honeycomb structure allows ions to pass through, resulting in high-performance lithium-ion batteries.

[0015] The use of graphene to improve the performance of C-Si electrodes has the following advantages: (i) the graphene structure has a certain degree of mechanical strength and flexibility, which can mitigate the volume expansion of Si during lithiation; (ii) graphene has a high specific surface area and high electrical conductivity; and (iii) graphene can disperse Si particles more uniformly, which helps improve the cycling performance and specific capacity of the material.

[0016] Furthermore, in the composite of Si microparticles and graphene, graphene forms "point-surface" contact with the Si microparticles, which is superior to the "point-line" contact between CNTs and Si microparticles or the "point-point" contact between carbon black and Si microparticles.

[0017] Compared with activated carbon and graphite anodes, the honeycomb-structured CNT anode not only has a wall-like structure, but also a paved bottom structure, which improves the flow of electrons and the diffusion of ions within the battery, thereby improving battery performance.

[0018] To prevent the composite material of graphene and Si particles from falling off from the "honeycomb" of the honeycomb-structured CNTs, a CNT thin film was fabricated using a vertical thermal CVD apparatus and coated on the surface of the honeycomb-structured CNTs. This CNT thin film serves to seal the composite material of graphene and Si particles covered in the "honeycomb."

[0019] The anode of the lithium-ion battery according to the present invention is composed of Si nanoparticles, graphene composite material, and honeycomb-structured CNTs. The honeycomb-structured CNTs have a fine structure similar to CNTs, which increases the surface area of ​​the anode, improving the capacity of the lithium-ion battery and providing high power output. [Effects of the Invention]

[0020] (1) The use of graphene can suppress the expansion of Si nanoparticles and effectively prevent their pulverization. This improves the stability of lithium-ion batteries. (2) Reducing Si nanoparticle size to 150 nm or less is an effective method for suppressing Si expansion. This significantly reduces the expansion rate from 300% to approximately 30%. (3) Furthermore, wrapping the outer layer of the Si nanoparticles with graphene acts as a buffer layer, further suppressing the volumetric expansion of the Si nanoparticles. Furthermore, to prevent the composite material of graphene and Si nanoparticles from falling off the "honeycomb" of the honeycomb-structured CNTs, the surface of the honeycomb-structured CNTs can be covered with a thin film of CNTs. This can be done using chemical vapor deposition (thermal CVD).

[0021] Furthermore, the porous honeycomb-structured CNTs ensure that the negative electrode of the present invention maintains pores even when the C-Si negative electrode material expands in volume, and no obvious changes occur to the entire electrode. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a cross-sectional view showing the charge and discharge states of the honeycomb-structured CNTs negative electrode according to the present invention. [Figure 2] Figure 2 shows an image of a lithium-ion battery with a honeycomb-structured CNT anode. [Figure 3] Figure 3 shows the structure of a vertical thermal CVD device. [Figure 4] Figure 4 shows an SEM image of vertically aligned carbon nanotubes. [Figure 5] Figure 5 shows an SEM image of honeycomb-structured carbon nanotubes. [Figure 6] Figure 6 shows (a) SEM and (b) TEM images of Si particles. [Figure 7] Figure 7 shows a TEM image of graphene. [Figure 8] Figure 8 shows (a) SEM and (b) TEM images of a mixture of graphene and Si particles. [Figure 9] Figure 9 shows (a) SEM and (b) TEM images of a thin film of carbon nanotubes (CNTs). [Figure 10] Figure 10 shows the process of coating a mixture of graphene and Si particles and creating a thin film of coated CNTs. [Figure 11] Figure 11 shows the fabrication access diagram for a honeycomb-structured CNTs anode for a lithium-ion battery. [Figure 12] Figure 12 shows the Raman spectroscopy of the honeycomb-structured CNTs anode. DETAILED DESCRIPTION OF THE INVENTION

[0023] A C-Si anode for lithium-ion batteries, characterized by a honeycomb structure of CNTs formed on the surface of a Cu substrate, filled with a composite material of Si particles and graphene, and the surface of which is coated with a thin film of CNTs. [Example]

[0024] The present invention employs a pouch-type design. The pouch-type design has a relatively simple internal structure, making it easy to measure and operate. This makes the product easy to use and maintain. Furthermore, a compact design is possible, and the pouch-type design helps keep the product compact. This is particularly suitable for space-constrained applications such as mobile phones, electronic devices, and electric vehicles. Effective stacking of the electrodes and electrolyte allows for the storage of a large amount of energy in a small space, achieving high energy density and contributing to improved battery and cell performance.

[0025] Figure 1 shows a simulation of the operating cross section of the honeycomb-structured CNTs anode according to the present invention. This unique structure offers two advantages: (1) Because there are voids between the Si particles and the carbon (graphene, CNTs), even if the Si particles expand during lithiation, the graphene and CNTs are not destroyed. (2) The electronic and ionic conductivity of the carbon (graphene, CNTs) improves the intercalation rate and prevents the electrolyte from coming into contact with the Si particles. In addition, carbon (CNTs, graphene) has excellent ionic conductivity and lithium storage capacity, and is characterized by its ability to flexibly adapt to large volume fluctuations in the Si particles.

[0026] Figure 2 shows the cell structure and operating principle. Honeycomb-structured CNTs (5) are synthesized on a Cu substrate (1) that functions as the negative electrode. Carbon nanotubes (CNTs) have high conductivity and surface area, which contribute to improving battery performance. By filling the honeycomb of honeycomb-structured CNTs with a mixture of graphene and Si particles (7), the energy density and charge / discharge efficiency of the lithium-ion battery are improved. The liquid electrolyte is LiPF6 and ethylene carbonate (9), and the positive electrode material is lithium cobalt oxide (LiCoO2) (11). In a lithium-ion secondary battery, the positive electrode (11) is made of lithium ions (Li + ), and polyvinylidene fluoride is used for the diaphragm (10). Gold (12), which has good electrical conductivity, is used for the positive electrode current collector to collect the current generated in the positive electrode (lithium cobalt oxide) (11) and transmit it to the wiring. Electrode wiring (13) refers to the conductors and wires that connect each component, and is important for efficiently transmitting current. The charge and discharge reactions of lithium-ion secondary batteries involve the movement of lithium ions back and forth between the active materials, and irreversible reactions rarely occur.

[0027] Figure 3 shows an image of a vertical thermal CVD apparatus. This thermal CVD apparatus is composed of a main electric furnace (14), an ultrasonic nebulizer (15), a carrier gas (N2 gas) (16), a reaction quartz tube (19), and an exhaust quartz tube (18).

[0028] To fabricate a lithium-ion battery anode using honeycomb-structured CNTs, as shown in Figure 3, the Cu substrate (1) is placed in two locations, one in the center and one on the outside of the electric furnace (14) and the quartz reaction tube (19). First, the Cu substrate (1) is placed in the center of the quartz reaction tube (19), the synthesis temperature of the electric furnace (14) is set to 800°C, and a mixture of ethanol and Fe (2) is placed in the ultrasonic nebulizer (15). The atomized mixture of ethanol and Fe (2) is sent into the quartz reaction tube (19) together with nitrogen (16), and the exhaust gas is discharged through the quartz exhaust tube (18). Vertically aligned CNTs (3) are synthesized on the Cu substrate (1).

[0029] The temperature of the electric furnace (14) is set to 300°C, and the Cu substrate on which the vertically aligned CNTs (3) were placed is left in the center of the electric furnace (14) reaction quartz tube (19). Next, the ethanol and Fe (2) are replaced with ultrapure water (4), atomized with an ultrasonic nebulizer (15), and sent through N2 gas (16) into the reaction quartz tube (19) of the electric furnace (14). The vertically aligned CNTs (3) on the Cu substrate (1) are transformed into honeycomb-structured CNTs (5). Then, while maintaining the temperature at 300°C, the container of the ultrasonic nebulizer (15) is replaced with a mixture of solvent (20), Si microparticles, and graphene (7). This mixture is sent through N2 gas (16) into the reaction quartz tube (19) of the electric furnace (14) and packed into the "honeycomb" of the honeycomb-structured CNTs.

[0030] Finally, the Cu substrate with the "honeycomb" of the honeycomb-structured CNTs (5) filled with Si microparticles and graphene (7) is moved to a position outside the electric furnace (14) reaction quartz tube (19), the container of the nebulizer (15) is changed again to the mixture of ethanol and Fe (2), and the reaction temperature of the electric furnace (14) is adjusted to 1000°C. This allows the honeycomb-structured CNTs (5) containing the Si microparticles and graphene (7) to be covered with a thin film of CNTs (8), completing the production of a lithium-ion battery anode.

[0031] Figure 4 shows vertically aligned CNTs (3). Scanning electron microscope images show that the CNT film is vertically aligned with a thickness of 5–10 μm on the Cu substrate (1).

[0032] Figure 5 shows a typical scanning electron microscope (SEM) image of honeycomb-like CNTs (5) formed on a Cu substrate (1). This image shows the Cu substrate (1) from above, revealing a thin layer of material deposited on the surface of the Cu substrate, with the honeycomb structure protruding vertically from the substrate.

[0033] As shown in Figure 6, uniformly shaped Si particles can be clearly observed. The outer diameter of these Si particles is in the range of 100 to 150 nm.

[0034] As shown in Figure 7, graphene is in the form of a sheet.

[0035] As shown in Figures 8(a) and (b), Si nanoparticles with diameters of 150–200 nm are tightly wrapped in graphene. This composite structure is very robust, and the Si nanoparticles do not detach from the graphene. This structure significantly improves the electrochemical properties and structural stability of the nanosized Si nanoparticles, even under deep constant current cycling and high current density conditions.

[0036] As shown in Figures 9(a) and (b), a CNT thin film (8) is coated on the honeycomb-structured CNTs (5) to prevent the graphene-Si nanoparticle mixture (7) from falling off the honeycomb of the honeycomb-structured CNTs (5). This CNT thin film (8) was synthesized using a vertical thermal CVD apparatus.

[0037] Figure 10 shows a simulation of the cross section of a honeycomb-structured CNT electrode. (a) The graphene flakes completely encase the Si particles. The Si particles and graphene (7) are stirred and diluted with an appropriate amount of solvent (20). (b) The mixture was atomized in an ultrasonic nebulizer (15). The electric furnace was heated to 300°C, and the mixture was transported to a reaction quartz tube (19) using N2 gas (16), where it was uniformly dispersed in the honeycomb of the honeycomb-structured CNTs. (c) Finally, the Cu substrate with the honeycomb-structured CNTs is moved from the center of the quartz reaction tube (19) of the electric furnace (14) to the bottom of the quartz reaction tube (19). A mixture of Fe and ethanol (2) is atomized using an ultrasonic nebulizer (15) and sent into the electric furnace (14) together with N2 gas (16), which is then heated to 1000°C. Numerous floating CNTs (17) are synthesized within the quartz reaction tube (19) of the electric furnace (14), and these CNTs (17) continue to cover the surface of the honeycomb-structured CNTs (5) on the Cu substrate below the quartz reaction tube (19), forming a CNT thin film (8).

[0038] The lithium-ion battery manufacturing simulation is shown in Figure 11. A) A Cu substrate (1) is cleaned and placed in the center of the electric furnace (14) of a vertical thermal CVD apparatus. B) The mixture of ethanol and Fe (2) was atomized using a nebulizer (15) and supplied to a Cu substrate placed in an electric furnace (14) to synthesize aligned vertical CNTs (3). C) Aligned vertical CNTs (3) with a thickness of approximately 5–10 μm were synthesized on a Cu substrate (3). D) The reaction temperature of the electric furnace was set to 300°C, and a nebulizer was used to spray atomized pure water (4) onto the vertically aligned CNTs (3) on the copper substrate surface for 15 minutes, followed by drying. E) Many CNTs were bundled together on the walls of the honeycomb structure. It was also found that the shape of the honeycomb structure changed when the exposure time to atomized pure water was changed. F) Since the wall height of the honeycomb structure is preferably 1 to 2 μm, the wall height of the honeycomb structure, which is 5 to 10 μm, is compressed by the weight block (6). The boxes G) to J) are the same accesses as those in the simulation of Figure 10. The frames K) to L) have the same structure as the simulation in Figure 2.

[0039] Figure 12 shows the Raman spectrum of typical carbons (graphene and CNTs). Raman spectra are obtained when a material scatters light, and provide information about the molecular structure and vibrational modes of the material. -1 The peak around 1,350 cm (G band): This peak is due to the graphite structure and indicates the presence of graphene layers within the carbon nanowalls. The G band is a peak caused by the lattice vibration of graphene, and is an important characteristic that indicates the crystallinity of a material and the presence of graphene. -1 Peak around 520 cm (D band): This peak is due to defects and imperfect structures. The D band occurs due to the disorder and defects in the crystalline structure of carbon nanomaterials. Si nanoparticles have a peak around 520 cm. -1There is a sharp peak nearby. [Industrial Applicability]

[0040] According to the present invention, the volume expansion of Si during charge and discharge can be suppressed, and the performance of the lithium ion battery can be improved. [Explanation of symbols]

[0041] 1………………Cu substrate 2. Mixture of ethanol and Fe 3……………… vertically oriented CNTs 4………………pure water 5. Honeycomb structure CNTs 6………………Weight block 7. Si nanoparticle and graphene composite 8………………CNTs thin film 9. Liquid electrolyte (LiPF6 + ethylene carbonate) 10...Separator (polyvinylidene fluoride) 11………………Positive electrode (LiCoO2) 12...................Current collector (gold) 13……………………Electrode wiring 14... Electric furnace 15. Ultrasonic nebulizer device 16………………N2 gas 17...Floating CNTs 18....Quartz tube for exhaust 19....Quartz tube for reaction 20………………Solvent

Claims

1. A C-Si anode for lithium-ion batteries, characterized by filling Si particles and graphene composite material into honeycomb-structured CNTs formed on the surface of a Cu substrate, and coating the surface with a thin film of CNTs.

2. A method for manufacturing a C-Si anode for a lithium-ion battery, in which vertically aligned CNTs are grown on the surface of a Cu substrate, the CNTs are sprayed with hot pure water to transform them into honeycomb-structured CNTs, the honeycomb of the CNTs is filled with a composite material of Si microparticles and graphene, and the surface of the honeycomb-structured CNTs filled with the mixture of Si microparticles and graphene is coated with a thin film of CNTs.

3. The method for producing a C-Si negative electrode for a lithium ion battery according to claim 2, wherein a series of production steps are carried out using a vertical thermal CVD apparatus.

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