A functionally graded silicon-based electrode and a multi-layer composite coating method thereof

Abstract

The application provides a functional gradient silicon-based electrode and a multilayer composite coating method thereof. The functional gradient silicon-based electrode is obtained by the multilayer composite coating method thereof. The multilayer composite coating method of the functional gradient silicon-based electrode is as follows: a binder is uniformly mixed with a solvent to prepare a glue solution, then an active substance and a conductive agent are uniformly dry-mixed and mixed with the glue solution to prepare a slurry, the slurry is divided into five kinds of slurries according to the proportion of the active substance, and the slurries are respectively recorded as slurry layer one, slurry layer two, slurry layer three, slurry layer four and slurry layer five; the slurry layer one is coated on a copper foil; the slurry layer two is coated on the basis of the slurry layer one; the slurry layer three is coated on the basis of the slurry layer two; the slurry layer four is coated on the basis of the slurry layer three; the slurry layer five is coated on the basis of the slurry layer four; the copper foil coated with the five layers of slurry is vacuum-dried at 70-90 DEG C for 6-8 hours, and then is placed in an indoor environment for 46-50 hours for curing and standby.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a functionally graded silicon-based electrode and its multilayer composite coating method. Background Technology

[0002] Lithium-ion batteries represent a revolutionary innovation in the energy sector, boasting advantages such as light weight, small size, high capacity, long lifespan, and low environmental pollution. They have propelled the leapfrog development of emerging industries like new energy vehicles and are applied in strategic fields such as satellites and drones, becoming a key focus of technological development. Currently, with the gradual development of pure electric aircraft, the construction of a low-altitude economic industrial system, and the rapid development of pure electric vehicles, the requirements for the energy density of single-cell power batteries are increasingly higher. Existing lithium-ion batteries are no longer sufficient to meet the technological demands for high-energy-density, novel, and high-performance batteries.

[0003] Among the components of lithium-ion batteries, the anode material is a crucial factor affecting battery capacity and lifespan. Currently, commercially available lithium-ion battery anode materials (mainly carbon-based materials such as graphite) are nearing their theoretical capacity limits and cannot meet the demands of high-energy-density battery development. Therefore, adopting new materials is key to improving battery energy density. Compared to graphite anodes, silicon-based anodes can increase battery energy density by 4 to 10 times and improve charging performance. According to predictions from battery energy research company SNE Research, the global penetration rate of silicon anodes will increase from 1% in 2023 to 5% in 2025. By 2030 and 2035, the market share of silicon-based anodes is expected to reach 7% and 10%, respectively. With electric vehicle manufacturers and battery manufacturers demanding increased range and fast-charging performance for electric vehicles, competition in silicon-based anodes is expected to gradually intensify starting in 2024.

[0004] The reaction mechanism of silicon and lithium is through intermetallic alloying, which can provide almost ten times the capacity of graphite, thereby increasing the energy density of the battery. However, silicon undergoes volume expansion (>300%) during alloying with lithium, resulting in significant circumferential tensile stress on silicon particles, making them prone to pulverization and ionization. Simultaneously, the volume effect of silicon disrupts the entire conductive network; shear stress caused by uneven deformation leads to the peeling of the active layer at the current collector interface. Furthermore, volume changes cause dynamic changes at the electrode-electrolyte interface, further deteriorating and thickening the solid electrolyte interface. The performance degradation of silicon-based electrode lithium-ion batteries is mainly caused by the aforementioned mechanical degradation, chemical degradation, and the loss of active material, active lithium, and polarization losses resulting from their coupling.

[0005] To address these issues, technologies such as coating structures, hollow structures, and altering the surface morphology of the current collector have been proposed. However, these methods have limitations and drawbacks. For example, carbon coating cannot suppress silicon particle fracture; the manufacturing process of porous silicon or core-shell structures is complex and not suitable for industrial production; and the manufacturing process for altering the surface morphology of the current collector is complex and costly. Using nano-sized silicon particles can effectively alleviate the volume effect. For example, silicon particles with a particle size of less than 150 nm do not have sufficient stored strain energy from the electrochemical reaction to drive crack propagation, thus avoiding silicon expansion fracture caused by lithiation. However, continuously reducing particle size means higher production costs and also negatively impacts battery performance. For instance, high specific surface area leads to lower first-cycle coulombic efficiency (50%–85%), hindering its commercial application. Furthermore, current lithium-ion batteries primarily use a single-layer homogeneous electrode formed by a one-time coating and drying process. Changes in porosity / torsion caused by silicon lithiation expansion result in changes in ionic conductivity / diffusion performance, leading to low utilization of active material on the current collector side. Moreover, the excessive shear stress caused by silicon lithiation expansion can easily cause stress mismatch at the coating / current collector interface. Therefore, there is currently a lack of an economical and effective method to improve the performance of monolayer homogeneous silicon-based electrode lithium-ion batteries.

[0006] Traditional electrodes, due to their monolayer structure, suffer from surface and internal structural instability and severe electrode polarization, which to some extent limits the performance of high-load, high-capacity electrodes. Improving battery performance while maintaining the overall component content is a common challenge faced by academia and industry. Inspired by the gradient distribution of reinforcing phases in the matrix of many natural biocomposite materials, which aligns with the principle of maximizing efficiency with minimal materials and simplest structures, the concept of functionally graded electrodes was proposed. Currently, academia and industry have designed and manufactured electrodes with bilayer and even trilayer gradients. Functionally graded electrodes combine the design concept of functionally graded materials into the electrode, offering a new solution to electrode performance degradation. They can improve the cycle performance of silicon-based electrodes without complex material modification. Furthermore, through multilayer gradient optimized electrode structures, they can alleviate stress mismatch during charge and discharge processes, thereby promoting the application of high-energy-density silicon-based lithium-ion batteries.

[0007] Therefore, addressing the problems of current single-layer homogeneous electrodes, developing a multilayer composite coating process and designing functionally graded silicon-based electrodes with superior performance is crucial for promoting the healthy development of high-energy-density batteries. This invention proposes a multilayer composite coating process, particularly a five-layer coating process, and designs several functionally graded silicon-based electrodes. Implementation and testing revealed that, with the same active material mass percentage (wt%), the functionally graded design significantly improves the capacity decay problem of silicon-based electrodes, demonstrating the feasibility of the multilayer composite coating process and the superiority of the designed functionally graded silicon-based electrodes.

[0008] Battery electrodes consist of a metal current collector and an active coating. The active coating is a porous composite material, and its microstructure includes active material particles, regions where conductive agents and binders are mixed (carbon gel phase), and microscopic pore spaces between the solid phases, which need to be filled with electrolyte. During the charge and discharge process of lithium-ion batteries, the concentration gradient generated by the lithium-ion diffusion process and the deformation caused by the lithiation expansion of the active material lead to diffusion stress. Excessive diffusion stress can cause various mechanical failure modes, such as cracking of the active material, separation between active materials, fracture of the active layer, and delamination between the active layer and the current collector, ultimately leading to a series of failure phenomena such as capacity decay, increased impedance, and shortened lifespan. This phenomenon is particularly severe in silicon-based electrodes. However, the volume deformation of silicon materials and its unstable SEI film are inherent problems; at the same time, the instability of the upper and lower surfaces of the electrode active coating and the overall instability of the electrode structure caused by internal polarization are inherent problems of traditional single-layer electrode structures. Therefore, for single-layer silicon electrodes, with the progress of charge and discharge cycles, the result is a significant deterioration of the electrode structure and a significant capacity decay, thereby shortening the battery's lifespan.

[0009] However, graphite anode lithium-ion batteries cannot meet the high energy density requirements, and the performance degradation of monolayer homogeneous silicon-based electrodes is severe due to their volume effect, mainly due to the following three technical problems:

[0010] 1. Inherent problems of homogeneous silicon-based electrodes: insufficient utilization of active materials, poor structural stability, rapid mechanical and chemical degradation, and fast capacity decay.

[0011] 2. Functional graded electrode design issues: The key technical issues are how to regulate the composition and design silicon-based electrodes that can improve the mechanical and electrochemical performance, taking into account the influence of each component on the electrode performance and the competition between multiple components.

[0012] 3. Cracking problem in multilayer electrode coatings: To achieve a more optimized and continuous gradient design, the existing 2-3 layer coatings are no longer sufficient. However, when applying more layers, especially five or more, cracking and peeling of the coating are unavoidable due to stress and strain accumulation during drying. Considering both feasibility and economy, how to prepare composite electrodes with multiple active layers and implement high-quality 4-5 layer composite coatings is a key technical issue. Summary of the Invention

[0013] This invention is made to solve the above-mentioned problems, and its purpose is to provide a functionally graded silicon-based electrode and a multilayer composite coating method thereof.

[0014] This invention provides a multilayer composite coating method for functionally graded silicon-based electrodes, characterized by the following features:

[0015] S1. Weigh the binder, conductive agent, and active material. Mix the weighed binder with the solvent to form a glue solution. Then, dry mix the active material and conductive agent evenly and mix them with the glue solution. Stir for 2-4 hours to obtain a slurry. The slurry is divided into five types according to the proportion of active material, which are respectively called slurry layer one, slurry layer two, slurry layer three, slurry layer four, and slurry layer five.

[0016] S2. The slurry of slurry layer one is coated onto a copper foil with a thickness of 9-15 μm, the single-layer coating thickness is 40-100 μm, the coating speed is 2-6 mm / s, and after coating, it is dried at a temperature of 70-100℃ for 10-30 minutes to obtain slurry layer one.

[0017] S3. Apply slurry layer two on top of slurry layer one. The thickness of a single layer is 40-100 μm. The coating speed is 2-6 mm / s. The coating width should be at least 2 mm wider than the width of slurry layer one. After coating, dry the slurry layer two at a temperature of 70-100℃ for 10-30 minutes to obtain slurry layer two.

[0018] S4. Apply slurry layer three to slurry layer two. The thickness of a single layer is 40-100 μm. The coating speed is 2-6 mm / s. The coating width should be at least 2 mm wider than the width of slurry layer two. After coating, dry the slurry layer three at a temperature of 70-100℃ for 10-30 minutes to obtain slurry layer three.

[0019] S5. Apply slurry layer four to slurry layer three. The thickness of a single layer is 40-100 μm. The coating speed is 2-6 mm / s. The coating width should be at least 2 mm wider than the width of slurry layer three. After coating, dry the slurry layer four at a temperature of 70-100℃ for 10-30 minutes to obtain slurry layer four.

[0020] S6. Apply slurry layer five to slurry layer four. The thickness of a single layer is 40-100 μm. The coating speed is 2-6 mm / s. The coating width should be at least 2 mm wider than the width of slurry layer four. After coating, dry the slurry layer five at a temperature of 70-100℃ for 10-30 minutes to obtain slurry layer five.

[0021] S7. The copper foil with the final five layers of slurry is vacuum dried at 70-90℃ for 6-8 hours, and then left to stand in an indoor environment for 46-50 hours to mature before use.

[0022] The multilayer composite coating method for functionally graded silicon-based electrodes provided by this invention may also have the following characteristics: the viscosity of the slurry is 1200-2400 mPa·s, the viscosity of the first slurry layer is 2000-2400 mPa·s, and the viscosity of the fifth slurry layer is 1200-1500 mPa·s.

[0023] The multilayer composite coating method for functionally graded silicon-based electrodes provided by this invention may also have the following characteristics: the viscosity of the second, third, and fourth slurry layers is 1500–2000 mPa·s.

[0024] The multilayer composite coating method for functionally graded silicon-based electrodes provided by this invention may also have the following features: wherein the binder is a polyacrylic acid binder or a sodium alginate binder, the solvent is deionized water, the active material is nano-silicon particles, and the conductive agent is conductive carbon black, single-walled carbon nanotubes, or a mixture of conductive carbon black and single-walled carbon nanotubes.

[0025] The multilayer composite coating method for functionally graded silicon-based electrodes provided by this invention may also have the following characteristics: wherein, the slurry of slurry layer one contains 10 wt% nano-silicon particles, the slurry of slurry layer two contains 20 wt% nano-silicon particles, the slurry of slurry layer three contains 30 wt% nano-silicon particles, the slurry of slurry layer four contains 40 wt% nano-silicon particles, and the slurry of slurry layer five contains 50 wt% nano-silicon particles.

[0026] The multilayer composite coating method for functionally graded silicon-based electrodes provided by this invention may also have the following characteristics: wherein the average particle size of the nano-silicon particles in the slurry of slurry layer one is 50 nm, the average particle size of the nano-silicon particles in the slurry of slurry layer two is 100 nm, the average particle size of the nano-silicon particles in the slurry of slurry layer three is 100 nm, the average particle size of the nano-silicon particles in the slurry of slurry layer four is 150 nm, and the average particle size of the nano-silicon particles in the slurry of slurry layer five is 150 nm.

[0027] The multilayer composite coating method for functionally graded silicon-based electrodes provided by this invention may also have the following characteristics: wherein, the slurry of slurry layer one contains 10 wt% nano-silicon particles, the slurry of slurry layer two contains 40 wt% nano-silicon particles, the slurry of slurry layer three contains 50 wt% nano-silicon particles, the slurry of slurry layer four contains 40 wt% nano-silicon particles, and the slurry of slurry layer five contains 10 wt% nano-silicon particles.

[0028] The multilayer composite coating method for functionally graded silicon-based electrodes provided by this invention may also have the following characteristics: the average particle size of the nano-silicon particles in the slurry of slurry layer one is 50 nm, the average particle size of the nano-silicon particles in the slurry of slurry layer two is 100 nm, the average particle size of the nano-silicon particles in the slurry of slurry layer three is 150 nm, the average particle size of the nano-silicon particles in the slurry of slurry layer four is 100 nm, and the average particle size of the nano-silicon particles in the slurry of slurry layer five is 50 nm.

[0029] The present invention also provides a functionally graded silicon-based electrode, which is obtained by the above-described multilayer composite coating method for functionally graded silicon-based electrodes.

[0030] The functionally graded silicon-based electrode provided by the present invention may also have the following features: wherein the functionally graded silicon-based electrode includes: a metal current collector and an active coating, wherein the metal current collector is a copper foil, and the active coating is a slurry layer one, a slurry layer two, a slurry layer three, a slurry layer four, and a slurry layer five.

[0031] The role and effect of invention

[0032] The functionally graded silicon-based electrode and its multilayer composite coating method according to the present invention can achieve:

[0033] 1. Avoiding cracking in multilayer coatings: Traditional multilayer coating processes have fewer coating layers (2-3 layers) and may crack due to slurry characteristics and coating parameters. They also require the development of new coating equipment. The multilayer composite coating process proposed in this invention provides a process that can avoid cracking by controlling slurry characteristics and coating parameters. It can also perform coatings of up to 6 layers, improving the possibility of functional graded electrode layer optimization design.

[0034] 2. Optimizing the performance of monolayer homogeneous silicon-based electrodes: Traditional monolayer homogeneous silicon-based electrodes suffer from instability in their surface and internal structure, leading to rapid degradation of mechano-coupling and limiting their high load and high capacity performance. Inhomogeneous utilization of active coating components and electrode polarization are inherent problems in the operation of traditional electrodes. These problems can be alleviated by rationally designing the internal structure of the active coating. The two types of functionally graded silicon-based electrodes involved in this invention have been experimentally verified to have excellent cycle performance, demonstrating that functionally graded electrodes are an economical and effective solution for improving homogeneous electrodes.

[0035] 3. Improving the energy density of lithium-ion batteries: Due to material limitations and structural design challenges, existing lithium-ion batteries struggle to achieve higher energy densities. The functionally graded silicon-based electrode implemented using the multilayer composite coating process proposed in this invention can improve the utilization rate of electrode active materials, more effectively utilize the high-capacity characteristics of silicon, and simultaneously address the inherent problems of homogeneous silicon-based electrodes, thereby enhancing the performance of high-energy-density silicon-based electrode lithium-ion batteries. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the structure of a functionally graded silicon-based electrode according to Embodiment 1 of the present invention;

[0037] Figure 2 This is a schematic diagram of the structure of a functionally graded silicon-based electrode according to Embodiment 2 of the present invention;

[0038] Figure 3 This is a schematic diagram of the structure of a functionally graded silicon-based electrode according to Embodiment 3 of the present invention;

[0039] Figure 4 This is a conceptual diagram of the gradient composition of a functionally graded silicon-based electrode according to Embodiment 1 of the present invention;

[0040] Figure 5 This is a conceptual diagram of the gradient composition of a functionally graded silicon-based electrode according to Embodiment 2 of the present invention;

[0041] Figure 6 This is a conceptual diagram of the gradient composition of a functionally graded silicon-based electrode according to Embodiment 3 of the present invention;

[0042] Figure 7 This is a comparison chart of the cycling capacity of three embodiments of the present invention at a 0.2C rate;

[0043] Figure 8 This is a capacity-voltage comparison chart of the fifth cycle stage embodiment of the present invention; and

[0044] Figure 9 This is the actual coating effect of the multilayer composite coating process of the present invention. Detailed Implementation

[0045] To make the technical means, creative features, objectives and effects of the present invention easy to understand, the following embodiments, in conjunction with the accompanying drawings, specifically illustrate the functionally graded silicon-based electrode and its multilayer composite coating method of the present invention.

[0046] Example 1

[0047] The functionally graded silicon-based electrode 100 of Example 1 is implemented according to the following multilayer composite coating method:

[0048] S1. Weigh 10wt% of nano-silicon particles 11 (average particle size 50nm), 68wt% of conductive carbon black, and 22wt% of sodium alginate. Mix the weighed sodium alginate with a certain proportion of deionized water to make a glue solution. Then, dry mix the nano-silicon particles and conductive carbon black evenly and mix them with the glue solution to make a slurry. Stir the slurry for 3 hours. Adjust the viscosity of the slurry by adding deionized water to make the adjusted slurry viscosity between 2200mPa·s, thus making the first layer of slurry.

[0049] Weigh 20 wt% of nano-silicon particles 21 (average particle size 100 nm), 60 wt% of conductive carbon black, and 20 wt% of sodium alginate. Mix the weighed sodium alginate with a certain proportion of deionized water to prepare a glue solution. Then, dry mix the nano-silicon particles and conductive carbon black evenly and mix them with the glue solution to prepare a slurry. Stir the slurry for 3 hours. Adjust the slurry viscosity by adding deionized water to achieve a viscosity of 1500 mPa·s, thus preparing the second layer of slurry.

[0050] Weigh 30 wt% of nano-silicon particles 31 (average particle size 100 nm), 50 wt% of conductive carbon black, and 20 wt% of sodium alginate. Mix the weighed sodium alginate with a certain proportion of deionized water to prepare a glue solution. Then, dry mix the nano-silicon particles and conductive carbon black evenly and mix them with the glue solution to prepare a slurry. Stir the slurry for 3 hours. Adjust the slurry viscosity by adding deionized water to achieve a viscosity of 1700 mPa·s, thus preparing a three-layer slurry.

[0051] Weigh 40 wt% of nano-silicon particles 41 (average particle size 150 nm), 40 wt% of conductive carbon black, and 20 wt% of sodium alginate. Mix the weighed sodium alginate with a certain proportion of deionized water to prepare a glue solution. Then, dry mix the nano-silicon particles and conductive carbon black evenly and mix them with the glue solution to prepare a slurry. Stir the slurry for 3 hours. Adjust the slurry viscosity by adding deionized water to achieve a viscosity of 2000 mPa·s, thus preparing the fourth layer of slurry.

[0052] Weigh 50 wt% of nano-silicon particles 51 (average particle size 150 nm), 30 wt% of conductive carbon black, and 20 wt% of sodium alginate. Mix the weighed sodium alginate with a certain proportion of deionized water to prepare a slurry. Then, dry-mix the nano-silicon particles and conductive carbon black evenly and mix them with the slurry to prepare a slurry. Simultaneously, add 0.05 wt% of single-walled carbon nanotubes. Stir the slurry for 3 hours. Adjust the slurry viscosity by adding deionized water to achieve a viscosity between 1300 mPa·s, thus preparing a slurry with five layers.

[0053] S2. Coat the slurry layer 10 onto the 9μm copper foil 60, with a single layer coating thickness of 65μm and a coating speed of 2mm / s. Dry the coating immediately after coating, with a drying temperature of 80℃ and a drying time of 15 minutes.

[0054] S3. Apply slurry layer 2 on top of slurry layer 10. The thickness of a single layer is 70μm, the coating speed is 3mm / s, and the coating width is 2mm wider than the previous layer. After coating, dry immediately at 80℃ for 15 minutes to obtain slurry layer 20.

[0055] S4. Apply slurry layer three on top of slurry layer two 20. The thickness of a single layer is 70μm, the coating speed is 3mm / s, and the coating width is 2mm wider than the previous layer. After coating, dry immediately at 80℃ for 15 minutes to obtain slurry layer three 30.

[0056] S5. Apply slurry layer four on top of slurry layer three 30. The thickness of a single layer is 70μm, the coating speed is 3mm / s, and the coating width is 2mm wider than the previous layer. After coating, dry immediately at 80℃ for 15 minutes to obtain slurry layer four 40.

[0057] S6. Apply slurry layer five on top of slurry layer four 40. The thickness of a single layer is 75μm, the coating speed is 3mm / s, and the coating width is 2mm wider than the previous layer. After coating, dry immediately at 80℃ for 15 minutes to obtain slurry layer five 50.

[0058] S7. The electrode with the final five layers of slurry is vacuum dried at 80°C for 6 hours, and then left to stand in an indoor environment for 48 hours for curing.

[0059] Figure 1 This is a schematic diagram of the structure of a functionally graded silicon-based electrode in Embodiment 1 of the present invention. Figure 4 This is a conceptual diagram of the gradient composition of a functionally graded silicon-based electrode according to Embodiment 1 of the present invention.

[0060] like Figure 1-2 As shown, this is the slurry layer obtained after coating in Example 1. In Example 1, the high silicon content layer is positioned near the separator to achieve the shortest path for ions to travel to and from the high silicon content layer. Compared with a homogeneous electrode with the same silicon content, this improves the utilization rate of active material. In Example 1, large-diameter silicon nanoparticles are positioned near the separator. Since, under the same active material content, the larger the particles, the larger the pore size and the smaller the tortuosity, the larger the silicon particles near the separator enhance the liquid phase diffusion ability from the separator / electrode interface inward.

[0061] The prepared and aged electrodes were vacuum dried at 90°C for 12 hours, and the water content of the electrodes was controlled to be below 200 ppm. The treated electrodes were then punched into round discs with a diameter of 12 mm.

[0062] The average loading of the active coating was measured after weighing the obtained circular electrode sheet: 2.4 mg / cm³. 2 Since the overall mass percentage of nano-silicon particles is 30 wt%, the silicon loading is 0.72 mg / cm³. 2 Organize all components of the CR2032 battery (including the positive electrode shell, negative electrode shell, circular gasket, spring contact, separator, silicon-based electrode disc, lithium disc, and electrolyte), adjust the gas and pressure inside the glove box (water vapor and oxygen content ≤0.1ppm), and assemble the coin cells in sequence. Use 1M LiPF6 (EC:DEC = 3:7, 10% FEC) as the electrolyte and Celgard 2320 as the separator. Let the assembled batteries rest for 12 hours to allow the electrolyte to fully soak in.

[0063] Cyclic voltammetry tests were performed on the batteries that had been stored. Before the test, formation was performed first, with a formation current of 0.1C and 5 cycles. After formation, cyclic testing was performed, with a test voltage range of 0.01V to 1V and a test current of 0.2C constant current charge and discharge.

[0064] Example 2

[0065] Based on the multilayer composite coating method of the functionally graded silicon-based electrode 100 in Example 1, in Example 2, the slurry layer 20' of the functionally graded silicon-based electrode 100' contains 40 wt% silicon nanoparticles 21' (average particle size 100 nm), 40 wt% conductive carbon black, and 20 wt% sodium alginate; the slurry layer 30' contains 50 wt% silicon nanoparticles 31' (average particle size 150 nm), 30 wt% conductive carbon black, and 20 wt% sodium alginate; the slurry layer 40' contains 40 wt% silicon nanoparticles 41' (average particle size 100 nm), 40 wt% conductive carbon black, and 20 wt% sodium alginate; and the slurry layer 50' contains 10 wt% silicon nanoparticles 51' (average particle size 50 nm), 70 wt% conductive carbon black, and 20 wt% sodium alginate.

[0066] Figure 2 This is a schematic diagram of the structure of a functionally graded silicon-based electrode in Embodiment 2 of the present invention. Figure 5 This is a conceptual diagram of the gradient composition of a functionally graded silicon-based electrode in Embodiment 2 of the present invention.

[0067] like Figure 2 , 5As shown, this is the slurry layer obtained after coating in Example 2. In Example 2, a high silicon content layer and large-diameter silicon nanoparticles are positioned in the middle layer of the electrode, while a low silicon content layer and small-diameter silicon nanoparticles are positioned on the side of the electrode near the metal current collector and the side near the separator, forming a "sandwich" structure with high silicon content in the middle and low silicon content on both sides. Due to the volume effect of silicon electrodes, cracks will appear on the electrode surface after multiple cycles, and the cracks will gradually expand with the increase of the number of cycles, eventually leading to a rapid decline in battery capacity. Example 2 alleviates the cracking caused by the expansion of silicon particles on the electrode surface by coating a low silicon content layer (slurry layer 5) on the middle high silicon content layer (slurry layer 2, slurry layer 3, slurry layer 4). At the same time, the low silicon content layer means a high conductivity layer. With the agglomeration effect of the conductive carbon black itself, a higher porosity will be formed, which is also conducive to the diffusion of electrolyte to the middle high silicon content layer.

[0068] Example 3

[0069] Based on the multilayer composite coating method of the functionally graded silicon electrode 100 in Example 1, in Example 3, the slurry of each slurry layer (10”, 20”, 30”, 40”, 50”) of the functionally graded silicon electrode 100” contains 30wt% silicon nanoparticles (11”, 21”, 31”, 41”, 51”) (average particle size 100nm).

[0070] Figure 3 This is a schematic diagram of the structure of a functionally graded silicon-based electrode in Embodiment 3 of the present invention. Figure 6 This is a conceptual diagram of the gradient composition of a functionally graded silicon-based electrode in Embodiment 3 of the present invention.

[0071] like Figure 3 , 6 As shown, this is the slurry layer obtained after coating in Example 3.

[0072] Figure 7 This is a comparison chart of the cycling capacity of three embodiments of the present invention at a 0.2C rate. Figure 8 This is a comparison chart of capacity and voltage in the fifth cycle stage embodiment of the present invention. Figure 9 This is the actual coating effect of the multilayer composite coating process of the present invention.

[0073] like Figure 1-9 As shown, both Embodiment 1 and Embodiment 2 of the present invention have an adhesive buffer layer designed near the metal current collector side, while Embodiment 3 does not have an adhesive buffer layer designed.

[0074] The bonding buffer layer, namely the low silicon content layer on the metal current collector side, improves the problem that the active material far from the separator cannot fully exert its capacity. At the same time, the low silicon content results in minimal volume expansion, which can effectively avoid the direct impact of silicon particle volume effect on the coating / metal current collector interface. Furthermore, by appropriately increasing the binder content in this layer, it plays an overall bonding buffering role, reducing stress concentration and lowering the possibility of coating peeling. Moreover, the high conductive agent content in the low silicon content layer can form more conductive paths, which is conducive to the formation of a high-speed conductive network.

[0075] Furthermore, in both Embodiments 1 and 2 of this invention, single-walled carbon nanotubes were added to the coating near the diaphragm side. Adding ≤0.1%wt of single-walled carbon nanotubes to the slurry improves the conductivity and mechanical stability of the electrode active layer. In particular, adding single-walled carbon nanotubes to the active layer on the electrode surface near the diaphragm significantly enhances the stability of the coating, forming a protective layer. Nano-silicon is dispersed in an amorphous porous carbon framework composed of single-walled carbon nanotubes and conductive carbon black, preventing agglomeration between silicon nanoparticles. The 3D conductive network formed by the porous carbon framework can also accommodate the volume expansion of the nano-silicon particles and promote electron migration.

[0076] The role and effect of the embodiments

[0077] The functionally graded silicon-based electrode and its multilayer composite coating method according to the present invention can achieve:

[0078] 1. Avoiding cracking in multilayer coatings: Traditional multilayer coating processes have fewer coating layers (2-3 layers) and may crack due to slurry characteristics and coating parameters. They also require the development of new coating equipment. The multilayer composite coating process proposed in this invention provides a process that can avoid cracking by controlling slurry characteristics and coating parameters. It can also perform coatings of up to 6 layers, improving the possibility of functional graded electrode layer optimization design.

[0079] 2. Optimizing the performance of monolayer homogeneous silicon-based electrodes: Traditional monolayer homogeneous silicon-based electrodes suffer from instability in their surface and internal structure, leading to rapid degradation of mechano-coupling and limiting their high load and high capacity performance. Inhomogeneous utilization of active coating components and electrode polarization are inherent problems in the operation of traditional electrodes. These problems can be alleviated by rationally designing the internal structure of the active coating. The two types of functionally graded silicon-based electrodes involved in this invention have been experimentally verified to have excellent cycle performance, demonstrating that functionally graded electrodes are an economical and effective solution for improving homogeneous electrodes.

[0080] 3. Improve the energy density of lithium-ion batteries: Due to material limitations and structural design challenges, existing lithium-ion batteries are unable to achieve higher energy densities. The functionally graded silicon-based electrode implemented by the multilayer composite coating process proposed in this invention can improve the utilization rate of electrode active materials, make more effective use of the high capacity characteristics of silicon, and improve the inherent problems of homogeneous silicon-based electrodes, thereby enhancing the performance of high-energy-density silicon-based electrode lithium-ion batteries.

[0081] The functionally graded electrode implemented in this invention does not require rolling. The multilayer composite coating process proposed in this invention will form a high initial porosity. Due to the high theoretical specific capacity of silicon itself, the volume expansion after lithiation can reach 300%. A low initial porosity may be filled after the silicon lithiation expansion. The closure of the pores will prevent lithium ions from diffusing, and the contact compression between particles will cause a large stress concentration, which can easily lead to a "plummet" in battery capacity. Therefore, a high initial porosity is required to reserve expansion space, so rolling is not necessary.

[0082] The multilayer composite coating process proposed in this invention controls the mixing sequence, time, and solid content of the slurry. It adjusts the viscosity of the slurry near the current collector to form a relatively dense base coating with good surface quality at the bottom, and adjusts the viscosity of the slurry near the diaphragm to ensure good flowability and leveling during the top coating process, preventing the slurry from being too viscous to coat or too diluted to damage the existing coating. It increases the coating width of each layer to avoid the thick edge problem caused by coating with the same width. It does not require the design of additional coating equipment. Multilayer coating can be achieved in the existing production system by simply adjusting process parameters such as slurry viscosity and coating width.

[0083] Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to this invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for multilayer composite coating of functionally graded silicon-based electrodes, characterized in that, The specific method is as follows: S1. Weigh the binder, conductive agent, and active material. Mix the weighed binder with the solvent to form a glue solution. Then, dry mix the active material and conductive agent evenly and mix them with the glue solution. Stir for 2 to 4 hours to obtain a slurry. The slurry is divided into five types according to the proportion of active material, which are respectively denoted as slurry layer one, slurry layer two, slurry layer three, slurry layer four, and slurry layer five. S2. The slurry of the first slurry layer is coated on a copper foil with a thickness of 9-15 μm, the single-layer coating thickness is 40-100 μm, the coating speed is 2-6 mm / s, and after coating, it is dried at a temperature of 70-100℃ for 10-30 minutes to obtain the first slurry layer. S3. Apply slurry layer two to the slurry layer one, with a single layer thickness of 40-100μm, a coating speed of 2-6mm / s, and a coating width that is at least 2mm wider than the width of slurry layer one. After coating, dry the slurry layer two at a temperature of 70-100℃ for 10-30 minutes to obtain slurry layer two. S4. Apply slurry layer three to the slurry layer two. The thickness of a single layer is 40-100 μm. The coating speed is 2-6 mm / s. The coating width should be at least 2 mm wider than the width of slurry layer two. After coating, dry the slurry layer three at a temperature of 70-100℃ for 10-30 minutes to obtain slurry layer three. S5. Apply slurry layer four to the slurry layer three. The thickness of a single layer is 40-100 μm. The coating speed is 2-6 mm / s. The coating width should be at least 2 mm wider than the width of slurry layer three. After coating, dry the slurry layer four at a temperature of 70-100℃ for 10-30 minutes to obtain slurry layer four. S6. Apply slurry layer five to the slurry layer four. The thickness of a single layer is 40-100 μm. The coating speed is 2-6 mm / s. The coating width should be at least 2 mm wider than the width of slurry layer four. After coating, dry the slurry layer five at a temperature of 70-100℃ for 10-30 minutes to obtain slurry layer five. S7. The copper foil with the final five layers of slurry is vacuum dried at 70-90℃ for 6-8 hours, and then left to mature in an indoor environment for 46-50 hours before use. In the slurry layer one, silicon nanoparticles account for 10 wt%; in the slurry layer two, silicon nanoparticles account for 40 wt%; in the slurry layer three, silicon nanoparticles account for 50 wt%; in the slurry layer four, silicon nanoparticles account for 40 wt%; and in the slurry layer five, silicon nanoparticles account for 10 wt%. The average particle size of silicon nanoparticles in the slurry of the first slurry layer is 50 nm, the average particle size of silicon nanoparticles in the slurry of the second slurry layer is 100 nm, the average particle size of silicon nanoparticles in the slurry of the third slurry layer is 150 nm, the average particle size of silicon nanoparticles in the slurry of the fourth slurry layer is 100 nm, and the average particle size of silicon nanoparticles in the slurry of the fifth slurry layer is 50 nm.

2. The multilayer composite coating method for functionally graded silicon-based electrodes according to claim 1, characterized in that: in, The viscosity of the slurry is 1200-2400 mPa·s, the viscosity of the slurry in the first slurry layer is 2000-2400 mPa·s, and the viscosity of the slurry in the fifth slurry layer is 1200-1500 mPa·s.

3. The multilayer composite coating method for functionally graded silicon-based electrodes according to claim 2, characterized in that: in, The viscosity of the slurry in the second, third, and fourth slurry layers is 1500–2000 mPa·s.

4. The multilayer composite coating method for functionally graded silicon-based electrodes according to claim 1, characterized in that: in, The binder is a polyacrylic acid binder or a sodium alginate binder, the solvent is deionized water, the active material is nano-silicon particles, and the conductive agent is conductive carbon black, single-walled carbon nanotubes, or a mixture of conductive carbon black and single-walled carbon nanotubes.

5. A functionally graded silicon-based electrode, characterized in that: It is obtained by the multilayer composite coating method of functionally graded silicon-based electrodes as described in any one of claims 1-4.

6. A functionally graded silicon-based electrode according to claim 5, characterized in that: in, The functionally graded silicon-based electrode comprises: a metal current collector and an active coating, wherein the metal current collector is a copper foil, and the active coating comprises slurry layer one, slurry layer two, slurry layer three, slurry layer four and slurry layer five.

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