Flexible binder and method for preparing the same, silicon negative electrode sheet and method for preparing the same, lithium ion battery

Abstract

The application provides a flexible adhesive and a preparation method thereof, a silicon negative electrode sheet and a preparation method thereof, and a lithium ion battery. The flexible adhesive is a crosslinked polymer with a three-dimensional network structure, and the three-dimensional network structure is formed by chemical crosslinking of a poly(urethane-urea) segment and a linear polymer segment; wherein the linear polymer segment is selected from any one or more of a polyacrylonitrile segment, a polyacrylic acid segment and a polyacrylamide segment. The above technical scheme solves the technical problem that the existing adhesive for a silicon negative electrode cannot simultaneously have high bonding strength, excellent elasticity and deformation recovery capability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a flexible binder and its preparation method, a silicon anode sheet and its preparation method, and a lithium-ion battery. Background Technology

[0002] Against the backdrop of the rapid rise of the new energy industry, lithium-ion batteries, as core energy storage devices, have seen their energy density and cycle life become key indicators for technological breakthroughs. Silicon-based anode materials, with a theoretical specific capacity of 4200 mAh / g (approximately 11 times that of traditional graphite anodes), are widely recognized as the core material for next-generation high-energy-density batteries, possessing irreplaceable application potential in electric vehicles, energy storage power stations, and other fields. However, the commercialization of silicon-based anodes is still limited by multiple technological bottlenecks, which urgently need to be overcome.

[0003] Silicon anode materials have inherent performance defects. Firstly, silicon's electronic conductivity is only 1.56 × 10⁻⁶. -3 S / m, much lower than 10 of graphite 4 The S / m ratio (solid-to-m²) obstructs electron transport within the electrode, leading to severe polarization during charging and discharging, directly impacting the battery's rate performance and charge / discharge efficiency. Secondly, during lithium-ion insertion / extraction, silicon undergoes a dramatic volume change of 300% to 400%. This drastic expansion and contraction causes silicon particles to shatter and the electrode structure to disintegrate, disrupting electron conduction pathways and drastically reducing cycle stability. Simultaneously, volume changes trigger repeated rupture and reconstruction of the solid electrolyte interphase (SEI), resulting in continuous electrolyte consumption. This not only reduces the initial coulombic efficiency (typically below 80%) but also leads to rapid capacity decay, severely shortening battery life. Furthermore, the weak interfacial bonding between silicon particles and the current collector and conductive agent makes them susceptible to interfacial separation due to volume changes, further exacerbating electrical contact failure of the active materials.

[0004] Furthermore, the performance defects of binders are particularly prominent in existing silicon anode fabrication technologies. Currently mainstream binders, such as polyacrylic acid (PAA) and polyacrylonitrile (PAN), are linear polymers lacking an effective cross-linking network between molecular chains, resulting in insufficient flexibility and elasticity. During repeated volume changes of silicon particles, these binders cannot buffer expansion stress and are prone to irreversible deformation, causing slippage and delamination between the active material and the binder. Simultaneously, the adhesive strength of linear structures relies on intermolecular van der Waals forces, and the adhesive strength significantly decreases after long-term cycling, leading to the detachment of the active material from the current collector. In addition, existing binders have poor interfacial compatibility with the silicon particle surface, making it difficult to form a stable coating structure, further exacerbating the instability of the electrode structure.

[0005] In summary, existing technologies suffer from the following core problems: First, the linear structure of existing binders results in insufficient stress dissipation, failing to buffer the enormous stress generated by the expansion of silicon particles, easily leading to electrode structure damage; second, the adhesive strength decreases with the number of cycles, causing active material to detach; and third, the elastic recovery performance is poor, making it difficult to adapt to repeated volume deformation of silicon particles. These problems collectively restrict the practical application of silicon-based anodes. Therefore, developing novel binders that combine high adhesive strength, excellent elasticity, and deformation recovery capabilities has become a core requirement for driving breakthroughs in silicon-based battery technology. Summary of the Invention

[0006] The main objective of this invention is to provide a flexible binder and its preparation method, a silicon anode sheet and its preparation method, and a lithium-ion battery, so as to solve the technical problem that the binders used for silicon anodes in the prior art are difficult to simultaneously possess high bonding strength, excellent elasticity and deformation recovery ability.

[0007] To achieve the above objectives, according to one aspect of the present invention, a flexible adhesive is provided, which is a cross-linked polymer having a three-dimensional network structure formed by chemical cross-linking of poly(urethane-urea) segments and linear polymer segments; wherein the linear polymer segments are selected from any one or more of polyacrylonitrile segments, polyacrylic acid segments, and polyacrylamide segments.

[0008] Furthermore, the degree of crosslinking of the above-mentioned flexible adhesive is 5% to 22%.

[0009] Furthermore, the aforementioned poly(urethane-urea) segments constitute 20% to 60% of the mass of the flexible adhesive.

[0010] Furthermore, the above-mentioned linear polymer segments are a combination of polyacrylonitrile segments, polyacrylic acid segments and polyacrylamide segments, wherein the mass ratio of polyacrylonitrile segments, polyacrylic acid segments and polyacrylamide segments is (2~4):(3~5):(1~3).

[0011] According to another aspect of the present invention, a method for preparing a flexible adhesive is provided, comprising: mixing raw materials including poly(urethane-urea), a linear polymer and a solvent, and then carrying out a crosslinking reaction in the presence of a crosslinking agent to obtain a flexible adhesive; wherein the linear polymer is selected from any one or more of polyacrylonitrile, polyacrylic acid and polyacrylamide.

[0012] Further, the above preparation method includes: mixing the linear polymer with a solvent to obtain a mixed solution; adding poly(urethane-urea) and a crosslinking agent to the mixed solution to obtain a mixed liquid; stirring the mixed liquid at 70~90°C to obtain a primary product containing a flexible binder; and sequentially washing, separating solids and liquids, and drying the primary product to obtain a flexible binder; wherein the preparation method satisfies at least one of the following conditions: (1) the mass ratio of poly(urethane-urea) to the linear polymer is (2~6):(4~8); (2) the solvent is selected from N,N-dimethylformamide. (3) The mass concentration of the linear polymer in the mixed solution is 5~10wt%; (4) The stirring speed is 500~800rpm, the stirring temperature is 60~80℃, and the stirring time is 2~4h; (5) The solid content of the mixed solution is 1~3wt%; (6) The stirring temperature is 70~90℃, and the stirring time is 3~5h; (7) The weight average molecular weight of the poly(urethane-urea) segments is 80000~150000; (8) The weight average molecular weight of the linear polymer segments is 100000~250000.

[0013] According to another aspect of the present invention, a silicon negative electrode sheet is provided, comprising a negative electrode active material, a conductive agent and a binder, wherein the binder is the aforementioned flexible binder.

[0014] According to another aspect of the present invention, a method for preparing the aforementioned silicon negative electrode sheet is provided, comprising: mixing a binder with water to obtain a binder solution; mixing raw materials including the binder solution, silicon powder, a conductive agent and water to obtain a mixed slurry; coating the mixed slurry onto a current collector to obtain a wet film; and drying the wet film to obtain a silicon negative electrode sheet; wherein the binder is the aforementioned flexible binder.

[0015] Furthermore, the above preparation method satisfies at least one of the following conditions: (1) the mass concentration of the binder solution is 8~15wt%, and the viscosity of the flexible binder is 3000~5000 mPa·s; (2) the particle size of the silicon powder is 1~3μm; (3) the mass ratio of the flexible binder, silicon powder and conductive agent is (10~15):(75~85):(3~10); (4) the viscosity of the mixed slurry is 4000~6000mPa·s; (5) the thickness of the current collector is 8~12μm; (6) the thickness of the wet film is 50~80μm; (7) the drying temperature is 80~100℃, and the drying time is 2~4 hours.

[0016] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode and a negative electrode, wherein the negative electrode is the aforementioned silicon negative electrode.

[0017] By applying the technical solution of this invention, the aforementioned flexible binder effectively solves the problems of insufficient stress buffering capacity, decreased adhesion, and poor elastic recovery performance of binders in lithium-ion battery applications of silicon-based anodes by constructing a three-dimensional network structure composed of poly(urethane-urea) segments, polyacrylonitrile segments, polyacrylic acid segments, or polyacrylamide segments. Specifically, the flexible segments of poly(urethane-urea) endow the flexible binder with excellent deformation recovery capability, which can recover its initial shape even under strains as high as 1900%, with an elastic recovery rate >84%. It can recover its initial shape when silicon particles shrink, avoiding irreversible deformation, effectively buffering cyclic stress, and significantly improving the binder's adaptability to the volume expansion of silicon particles, thus preventing electrode structure damage. The introduction of linear polymer segments such as polyacrylonitrile segments enhances the interfacial bonding between the flexible binder and the silicon particle surface. Through the interaction between its polar groups and the hydroxyl groups on the silicon particle surface, the bonding strength is improved (bonding strength for copper foil >18 N / m), effectively preventing the detachment of active materials and ensuring the integrity of the electrode. Meanwhile, the three-dimensional cross-linked network structure reduces the swelling rate of the binder in the electrolyte (<15%, far lower than that of linear binders (typically >50%)), ensuring structural stability during long-term cycling, reducing adhesion attenuation caused by swelling, and suppressing repeated rupture of the solid electrolyte interphase (SEI) film, thereby reducing electrolyte consumption and significantly improving the structural stability of the battery during long-term cycling. Therefore, the flexible binder of this invention significantly improves the cycling stability of silicon-based anodes through a synergistic design of high elasticity, strong adhesion, and swelling resistance. Detailed Implementation

[0018] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0019] As analyzed in the background section, commonly used binders for silicon anodes, such as polyacrylic acid and polyacrylonitrile, have fundamental defects in their linear molecular structures. The molecular chains of these binders are predominantly linearly arranged, lacking cross-linked network support, resulting in insufficient flexibility and elasticity, and an inability to adapt to the more than 300% volume changes of silicon particles during lithiation / delithiation. Under the enormous stress generated by the expansion of silicon particles, the linear molecular chains are prone to irreversible stretching or even breakage, failing to achieve effective stress dissipation, thus leading to slippage and separation between the active material and the binder. Simultaneously, the bonding strength of the linear structure mainly relies on intermolecular forces; with increasing cycle count, the bonding strength continuously decays, causing silicon particles to detach from the current collector surface, compromising the integrity of the electrode. Furthermore, existing binders have limited affinity for the silicon particle surface, making it difficult to form a stable interfacial bond, further exacerbating the collapse of the electrode structure, ultimately leading to rapid capacity decay and shortened cycle life, severely hindering the practical application of silicon-based anodes. The existing technology for silicon anode binders has the technical problem that it is difficult to simultaneously achieve high bonding strength, excellent elasticity and deformation recovery ability. In order to solve this problem, the present invention provides a flexible binder and its preparation method, a silicon anode sheet and its preparation method, and a lithium-ion battery.

[0020] In a typical embodiment of this application, a flexible adhesive is provided, which is a cross-linked polymer with a three-dimensional network structure. The three-dimensional network structure is formed by chemical cross-linking of poly(urethane-urea) segments and linear polymer segments; wherein the linear polymer segments are selected from any one or more of polyacrylonitrile segments, polyacrylic acid segments, and polyacrylamide segments.

[0021] The aforementioned flexible binder, by constructing a three-dimensional network structure composed of poly(urethane-urea) segments and polyacrylonitrile, polyacrylic acid, or polyacrylamide segments, effectively solves the problems of insufficient stress buffering capacity, decreased adhesion, and poor elastic recovery performance of binders in lithium-ion battery applications of silicon-based anodes. Specifically, the flexible segments of poly(urethane-urea) endow the flexible binder with excellent deformation recovery capability, able to recover its initial shape even under strains as high as 1900%, with an elastic recovery rate >84%. It can recover its initial shape when silicon particles shrink, avoiding irreversible deformation, effectively buffering cyclic stress, and significantly improving the binder's adaptability to the volume expansion of silicon particles, thus preventing electrode structure damage. Furthermore, the introduction of linear polymer segments such as polyacrylonitrile segments enhances the interfacial bonding between the flexible binder and the silicon particle surface. Through the interaction of its polar groups with the hydroxyl groups on the silicon particle surface, it improves the adhesion strength (adhesion strength to copper foil >18 N / m), effectively preventing the detachment of active materials and ensuring electrode integrity. Meanwhile, the three-dimensional cross-linked network structure reduces the swelling rate of the binder in the electrolyte (<15%, far lower than that of linear binders (typically >50%)), ensuring structural stability during long-term cycling, reducing adhesion attenuation caused by swelling, and suppressing repeated rupture of the solid electrolyte interphase (SEI) film, thereby reducing electrolyte consumption and significantly improving the structural stability of the battery during long-term cycling. Therefore, the flexible binder of this invention significantly improves the cycling stability of silicon-based anodes through a synergistic design of high elasticity, strong adhesion, and swelling resistance.

[0022] In one embodiment of this application, the degree of crosslinking of the above-mentioned flexible adhesive is 5% to 22%.

[0023] In one embodiment, the degree of crosslinking of the above-mentioned flexible adhesive can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, or 22%, or the degree of crosslinking of the flexible adhesive can be between any two of the above values, which will not be elaborated further here.

[0024] The optimized crosslinking degree of the flexible binder enhances its mechanical properties, maintaining sufficient elasticity to accommodate the volume expansion and contraction of silicon particles during charging and discharging, while also providing high adhesion to effectively lock the connection between the active material and the current collector, reducing the shedding of active material during long-term cycling. Precise control of the crosslinking degree is achieved through the appropriate addition of the crosslinking agent and suitable reaction conditions, ensuring the stability and processability of the binder's three-dimensional network structure.

[0025] In one embodiment of this application, the poly(urethane-urea) segments account for 20% to 60% of the mass of the flexible adhesive.

[0026] The optimal mass ratio of poly(urethane-urea) segments in the flexible adhesive imparts both elasticity and bonding properties. This precise control of the mass ratio balances the flexibility and resilience provided by the PUU segments with the bonding strength imparted by the linear polymer segments, ensuring that the adhesive, when subjected to stresses caused by the volume expansion of silicon particles, neither softens excessively and loses its morphological stability nor becomes too stiff and brittle.

[0027] In one embodiment, the mass percentage of the poly(urethane-urea) segments in the flexible adhesive can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or 60%, or the mass percentage of the poly(urethane-urea) segments in the flexible adhesive can be between any two of the above values, which will not be elaborated further here.

[0028] In one embodiment of this application, the linear polymer chain segment is a combination of polyacrylonitrile chain segment, polyacrylic acid chain segment and polyacrylamide chain segment, wherein the mass ratio of polyacrylonitrile chain segment, polyacrylic acid chain segment and polyacrylamide chain segment is (2~4):(3~5):(1~3).

[0029] When the linear polymer segments are a combination of polyacrylonitrile (PA) segments, polyacrylic acid (PA) segments, and polyacrylamide (PA) segments, the introduction of PA segments enhances the interfacial bonding between the binder and the silicon particle surface. The interaction between the polar cyano group (-CN) and the hydroxyl groups on the silicon surface significantly improves the interfacial bonding between the binder and silicon particles, reduces interfacial slippage and separation, and improves electrode integrity. The PA segments enhance water solubility, which is beneficial for uniform dispersion of the slurry. The PA segments provide additional elasticity, which helps to alleviate the volume expansion stress of silicon particles during lithiation / delithiation. The combination of these three components and their chemical cross-linking forms a three-dimensional network structure, enabling the binder to withstand ultra-large strain while possessing excellent electronic conductivity and mechanical strength. By precisely controlling the molecular weight and ratio of the linear segments, the synergistic effect of the three components further suppresses the breakage of silicon particles and the collapse of the electrode structure during cycling, thereby significantly improving the cycle stability and overall capacity retention of the battery.

[0030] In one embodiment, the mass ratio of polyacrylonitrile segments, polyacrylic acid segments, and polyacrylamide segments can be 3:4:2, 2:3:5, or 4:3:1, or the mass ratio of polyacrylonitrile segments, polyacrylic acid segments, and polyacrylamide segments can be any two of the above ratios, which will not be elaborated further here.

[0031] According to another aspect of the present invention, a method for preparing a flexible adhesive is provided, comprising: mixing raw materials including poly(urethane-urea), a linear polymer and a solvent, and then carrying out a crosslinking reaction in the presence of a crosslinking agent to obtain a flexible adhesive; wherein the linear polymer is selected from any one or more of polyacrylonitrile, polyacrylic acid and polyacrylamide.

[0032] The technical principle of the above preparation method lies in introducing poly(urethane-urea) segments into polymer chains such as polyacrylonitrile (PAN), polyacrylic acid (PAA), or polyacrylamide (PAM) through a crosslinking reaction to form a composite binder with a three-dimensional network structure. Poly(urethane-urea) possesses excellent elastic properties due to its unique segmental structure. After crosslinking, it significantly improves the binder's deformation recovery ability, perfectly matching the volume changes of silicon particles. Simultaneously, the crosslinked network achieves high elastic recovery force, ensuring the stability of the bonded structure during cycling. This allows it to maintain structural stability even under strains as high as 1900%, effectively alleviating the volume expansion pressure of the silicon-based anode during charging and discharging, and reducing electrode structure damage. Furthermore, the formation of the three-dimensional crosslinked network enhances the mechanical strength of the binder, ensuring its adhesion strength to copper foil exceeds 20 N / m. In addition, the polar groups in linear polymers such as PAN enhance the interfacial bonding with silicon particles, achieving ultra-high adhesion force, significantly improving the adhesion between electrode materials and preventing the active material from detaching during cycling.

[0033] It should be noted that the cross-linked network not only effectively disperses and buffers the stress generated by the expansion of silicon particles, but also avoids irreversible stretching and breakage of molecular chains, thus ensuring the durability of the binder under extreme conditions. More importantly, this cross-linking process does not sacrifice the processability of the binder; it maintains good dispersibility and coating uniformity at appropriate solid content, facilitating practical production applications. The binder prepared by this method exhibits a lower swelling rate during long-term cycling, inhibits repeated rupture of the solid electrolyte interphase (SEI) film, reduces electrolyte consumption, and improves the cycle stability and initial coulombic efficiency of the battery.

[0034] In one embodiment of this application, the preparation method includes: mixing a linear polymer with a solvent to obtain a mixed solution; adding poly(urethane-urea) and a crosslinking agent to the mixed solution to obtain a mixed liquid; stirring the mixed liquid at 70~90°C to obtain a primary product containing a flexible adhesive; and sequentially washing, separating solids and liquids, and drying the primary product to obtain a flexible adhesive; wherein the preparation method satisfies at least one of the following conditions: (1) the mass ratio of poly(urethane-urea) to the linear polymer is (2~6):(4~8); (2) the solvent is selected from N,N-dimethyl... (3) The mass concentration of the linear polymer in the mixed solution is 5~10wt%; (4) The stirring speed is 500~800rpm, the stirring temperature is 60~80℃, and the stirring time is 2~4h; (5) The solid content of the mixed solution is 1~3wt%; (6) The stirring temperature is 70~90℃, and the stirring time is 3~5h; (7) The weight average molecular weight of the poly(urethane-urea) segments is 80000~150000; (8) The weight average molecular weight of the linear polymer segments is 100000~250000.

[0035] Optimal stirring temperature and time, solvent selection, linear polymer concentration, and crosslinking agent addition enhance the formation and stability of the binder network, resulting in excellent processability. Simultaneously, good dispersibility and coating uniformity are maintained at suitable solid content. Through in-situ crosslinking (requiring no additional complex equipment and directly integrating into existing electrode production lines, allowing the crosslinking reaction to be completed during electrode preparation, suitable for large-scale production), the binder's swelling rate is controlled below 15%, reducing the risk of significant adhesion degradation during long-term cycling and providing stable structural support for silicon-based anodes.

[0036] If the PUU ratio is too high, it will reduce the polarity of the binder and weaken the interfacial bonding with silicon particles; if the PUU ratio is too low, it will not provide sufficient elasticity and will be difficult to buffer the expansion stress of silicon particles. A preferred mass ratio of poly(urethane-urea) to linear polymer within the above range helps to give the binder both excellent adhesion and elasticity.

[0037] The optimized weight-average molecular weight range of PUU further balances its segmental flexibility and elastic recovery capability, enhancing its stress dissipation capacity under extreme volume changes. Through precise setting of these parameters, the binder can effectively adapt to the volume expansion of silicon particles during charging and discharging, reducing separation between the active material and the current collector, and significantly enhancing the cycle stability of the electrode and the reliability of the overall structure.

[0038] In one embodiment, the mass ratio of poly(urethane-urea) to linear polymer can be 2:8, 3:7, 4:6, 5:5, or 6:4, or the mass ratio of poly(urethane-urea) to linear polymer can be any two of the above ratios, which will not be elaborated further here.

[0039] In one embodiment, the mass concentration of the linear polymer in the mixed solution is 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, or 10 wt%, or the mass concentration of the linear polymer in the mixed solution can be between any two of the above values, which will not be elaborated further here.

[0040] In one embodiment, the stirring speed can be 500 rpm, 600 rpm, 700 rpm, or 800 rpm, or the stirring speed can be between any two of the above values, which will not be elaborated further here.

[0041] In one embodiment, the mixing temperature can be 60°C, 70°C, or 80°C, or the mixing temperature can be between any two of the above values, which will not be elaborated further here.

[0042] In one embodiment, the mixing time can be 2h, 3h, or 4h, or the mixing time can be between any two of the above values, which will not be elaborated further here.

[0043] In one embodiment, the solid content of the mixture can be 1wt%, 2wt%, or 3wt%, or the solid content of the mixture can be between any two of the above values, which will not be elaborated further here.

[0044] In one embodiment, the stirring temperature can be 70°C, 80°C, or 90°C, or the stirring temperature can be between any two of the above values, which will not be elaborated further here.

[0045] In one embodiment, the stirring time can be 3h, 4h, or 5h, or the stirring time can be between any two of the above values, which will not be elaborated further here.

[0046] In one embodiment, the weight-average molecular weight of the poly(urethane-urea) segments can be 80,000, 90,000, 100,000, 110,000, 120,000, 130,000, 140,000, or 150,000, or the weight-average molecular weight of the poly(urethane-urea) segments can be between any two of the above values, which will not be elaborated further here.

[0047] In one embodiment, the weight-average molecular weight of the linear polymer segment can be 100,000, 120,000, 140,000, 150,000, 170,000, 190,000, 200,000, 220,000, or 250,000. Alternatively, the weight-average molecular weight of the linear polymer segment can be between any two of the above values, which will not be elaborated further here.

[0048] By controlling the weight-average molecular weight of the linear polymer segments within a suitable range, such as setting the weight-average molecular weight of polyacrylonitrile (PAN) segments to 150,000–250,000 g / mol, polyacrylic acid (PAA) segments to 100,000–150,000 g / mol, and polyacrylamide (PAM) segments to 120,000–180,000 g / mol, the adhesive can possess good electronic conductivity and mechanical strength. Optimizing these parameters helps improve the compatibility between the linear segments and the poly(urethane-urea) (PUU) crosslinking network, thereby enhancing the overall performance of the adhesive.

[0049] According to another aspect of the present invention, a silicon negative electrode sheet is provided, comprising a negative electrode active material, a conductive agent and a binder, wherein the binder is the aforementioned flexible binder.

[0050] This application provides a silicon anode sheet, the core technology of which lies in the use of the above-mentioned composite binder. The three-dimensional network structure of the binder significantly enhances its ability to cope with the huge volume changes of silicon-based anode materials during charging and discharging, thereby enabling the silicon anode sheet using the binder of this invention to exhibit better energy density, cycle life and processing compatibility.

[0051] According to another aspect of the present invention, a method for preparing the aforementioned silicon negative electrode sheet is provided, comprising: mixing a binder with water to obtain a binder solution; mixing raw materials including the binder solution, silicon powder, a conductive agent and water to obtain a mixed slurry; coating the mixed slurry onto a current collector to obtain a wet film; and drying the wet film to obtain a silicon negative electrode sheet; wherein the binder is the aforementioned flexible binder.

[0052] The above-mentioned method for preparing silicon anode sheets, by using the aforementioned flexible binder, results in silicon anode sheets with superior energy density, cycle life, and processing compatibility. Furthermore, the excellent adhesion of the flexible binder improves the processability of the above preparation process.

[0053] In one embodiment of this application, the above preparation method satisfies at least one of the following conditions: (1) the mass concentration of the binder solution is 8~15wt%, and the viscosity of the flexible binder is 3000~5000 mPa·s; (2) the particle size of the silicon powder is 1~3μm; (3) the mass ratio of the flexible binder, silicon powder and conductive agent is (10~15):(75~85):(3~10); (4) the viscosity of the mixed slurry is 4000~6000 mPa·s; (5) the thickness of the current collector is 8~12μm; (6) the thickness of the wet film is 50~80μm; (7) the drying temperature is 80~100℃, and the drying time is 2~4 hours.

[0054] In one embodiment, the mass concentration of the adhesive solution can be 8wt%, 10wt%, 12wt%, 14wt%, or 15wt%, or the mass concentration of the adhesive solution can be between any two of the above values, which will not be elaborated further here.

[0055] In one embodiment, the viscosity of the flexible adhesive can be 3000 mPa·s, 3500 mPa·s, 4000 mPa·s, 4500 mPa·s, or 5000 mPa·s, or the viscosity of the flexible adhesive can be between any two of the above values, which will not be elaborated further here.

[0056] In one embodiment, the particle size of the silicon powder can be 1μm, 2μm, or 3μm, or the particle size of the silicon powder can be between any two of the above values, which will not be elaborated further here.

[0057] In one embodiment, the mass ratio of flexible adhesive, silicon powder and conductive agent can be 10:85:5, 15:75:10, or 12:85:3, or the mass ratio of flexible adhesive, silicon powder and conductive agent can be between any two of the above values, which will not be elaborated further here.

[0058] In one embodiment, the viscosity of the mixed slurry can be 4000 mPa·s, 4500 mPa·s, 5000 mPa·s, 5500 mPa·s, or 6000 mPa·s, or the viscosity of the mixed slurry can be between any two of the above values, which will not be elaborated further here.

[0059] In one embodiment, the thickness of the current collector can be 8μm, 9μm, 10μm, 11μm, or 12μm, or the thickness of the current collector can be between any two of the above values, which will not be elaborated further here.

[0060] In one embodiment, the thickness of the wet film can be 50 μm, 60 μm, 70 μm, or 80 μm, or the thickness of the wet film can be between any two of the above values, which will not be elaborated further here.

[0061] In one embodiment, the drying temperature can be 80°C, 85°C, 90°C, 95°C, or 100°C, or the drying temperature can be any two of the above values, which will not be elaborated further here.

[0062] In one embodiment, the drying time can be 2 hours, 3 hours, or 4 hours, or the drying time can be any two of the above values, which will not be elaborated further here.

[0063] Optimizing the mass concentration range of the binder solution ensures its full dissolution and dispersion in the slurry, forming a stable and uniform solution. This facilitates improved electrode density and consistency during subsequent coating, thereby enhancing overall battery performance. Optimizing the viscosity range of the flexible binder enhances solution flowability and ease of coating operations, while reducing uneven coating caused by excessive viscosity, promoting uniform distribution of active materials. Controlling the silicon powder particle size increases the contact area between silicon particles and the binder, promoting a tighter bond between them, optimizing electron transport paths, and enhancing electrode conductivity. Optimizing the mass ratio of flexible binder to silicon powder and conductive agent balances the effects of each component in the electrode, considering both the high content of active materials to improve energy density and the appropriate addition of binder and conductive agent to ensure electrode structure stability and effective electron transport. Optimizing the viscosity of the mixed slurry promotes uniform coating of the slurry on the current collector, reduces bubbles and cracks during coating, and enhances the density and mechanical strength of the electrode. By controlling the thickness of the current collector, electron conductivity is improved, its weight is reduced, the proportion of inactive materials is decreased, and energy density is increased. A suitable wet film thickness increases the loading of active material while mitigating the lithium-ion diffusion path extension problem caused by excessive electrode thickness, thus optimizing the battery's rate performance. Adjusting the drying temperature and time effectively removes solvent, forming a dense electrode structure, reducing active material detachment or electrode structure damage caused by improper drying, and promoting the full cross-linking reaction, enhancing the binder's performance. Furthermore, its excellent process compatibility simplifies the production process and reduces manufacturing costs.

[0064] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode and a negative electrode, wherein the negative electrode is the aforementioned silicon negative electrode.

[0065] Silicon anode sheets incorporating the aforementioned flexible binder maintain high adhesion even during prolonged electrochemical cycling, effectively preventing the shedding of active materials, which is crucial for maintaining the long lifespan of lithium-ion batteries. Compared to batteries using conventional polyacrylic acid (PAA) as a binder, lithium-ion batteries using the binder of this invention exhibit a capacity retention rate exceeding 80% and an initial coulombic efficiency of 81% after 800 cycles, effectively solving the technical challenge of long-term cycle performance degradation in silicon-based anodes.

[0066] The beneficial effects of this application will be explained below with reference to specific embodiments and comparative examples.

[0067] Example 1:

[0068] Raw material preparation: poly(urethane-urea) (PUU) powder, polyacrylonitrile (PAN) powder, solvent N,N-dimethylformamide (DMF), crosslinking agent hexamethylene diisocyanate.

[0069] Linear polymer dissolution: PAN powder was dissolved in DMF at 60°C to prepare a PAN solution with a mass concentration of 5 wt%, and stirred until completely dissolved.

[0070] Crosslinking reaction: PUU powder was added to the PAN solution, with a PUU to PAN mass ratio of 4:6 (i.e., PUU 40%, PAN 60%), and hexamethylene diisocyanate with a total solid content of 1 wt% was added as a crosslinking agent. The mixed solution was stirred at 80°C for 4 hours to ensure the crosslinking reaction proceeded fully.

[0071] Post-processing: After the reaction is complete, the initial product is washed to remove unreacted raw materials and by-products. Then, solid-liquid separation is performed, and the obtained solid is vacuum dried at a temperature of 90°C for 3 hours to remove residual solvent and obtain a flexible adhesive.

[0072] 1) Mix the flexible adhesive with water to obtain an adhesive solution with a mass concentration of 10 wt%;

[0073] 2) Mix the flexible binder with silicon powder (particle size 2μm) and conductive agent (carbon nanotubes) at a mass ratio of 15:80:5, add solvent, adjust the slurry viscosity to 5000 mPa·s, and stir for 1.5 hours until uniformly dispersed;

[0074] 3) The mixed slurry was coated onto a copper current collector with a thickness of 8 μm to obtain a wet film with a thickness of 60 μm;

[0075] 4) Place the wet film under vacuum at 90°C for 3 hours to remove residual solvent and ensure that the crosslinking reaction proceeds fully to obtain a silicon anode sheet.

[0076] Example 2

[0077] The difference from Example 1 is that polyacrylonitrile powder, polyacrylic acid and polyacrylamide are replaced with PAN powder in a mass ratio of 3:4:2 to obtain a flexible binder, and finally a silicon negative electrode sheet is obtained.

[0078] Example 3

[0079] The difference from Example 1 is that polyacrylonitrile, polyacrylic acid and polyacrylamide are replaced with PAN powder in a mass ratio of 4:5:0.5 to obtain a flexible binder, and finally a silicon negative electrode sheet is obtained.

[0080] Example 4

[0081] The difference from Example 1 is that the mass ratio of PUU to PAN is set to 2:8 (i.e., PUU 20%, PAN 80%) to obtain a flexible binder, and finally obtain a silicon negative electrode sheet.

[0082] Example 5

[0083] The difference from Example 1 is that the mass ratio of PUU to PAN is set to 6:4 (i.e., PUU 60%, PAN 40%) to obtain a flexible binder, and finally obtain a silicon negative electrode sheet.

[0084] Example 6

[0085] The difference from Example 1 is that the mass ratio of PUU to PAN is set to 7:3 (i.e., PUU 70%, PAN 30%) to obtain a flexible binder, and finally obtain a silicon negative electrode sheet.

[0086] Example 7

[0087] The difference from Example 1 is that the weight-average molecular weight of poly(urethane-urea) is 80,000, resulting in a flexible binder, which ultimately yields a silicon anode sheet.

[0088] Example 8

[0089] The difference from Example 1 is that the weight-average molecular weight of poly(urethane-urea) is 150,000, resulting in a flexible binder, which ultimately yields a silicon anode sheet.

[0090] Example 9

[0091] The difference from Example 1 is that the weight-average molecular weight of poly(urethane-urea) is 160,000, resulting in a flexible binder, which ultimately yields a silicon anode sheet.

[0092] Example 10

[0093] The difference from Example 1 is that the mass ratio of flexible binder, silicon powder and conductive agent is 10:85:5, and a silicon negative electrode sheet is finally obtained.

[0094] Example 11

[0095] The difference from Example 1 is that the mass ratio of flexible binder, silicon powder and conductive agent is 15:75:10, and a silicon negative electrode sheet is finally obtained.

[0096] Example 12

[0097] The difference from Example 1 is that the mass ratio of flexible binder, silicon powder and conductive agent is 8:80:12, and a silicon negative electrode sheet is finally obtained.

[0098] Example 13

[0099] The difference from Example 1 is that, in the crosslinking reaction, the mixed solution is stirred at 60°C for 4 hours to obtain a flexible binder, and finally a silicon negative electrode sheet is obtained.

[0100] Comparative Example 1

[0101] The difference from Example 1 is that the binder is polyacrylonitrile, and a silicon negative electrode sheet is finally obtained.

[0102] Comparative Example 2

[0103] The difference from Example 1 is that the binder is polyacrylic acid, and a silicon negative electrode sheet is finally obtained.

[0104] Comparative Example 3

[0105] The difference from Example 1 is that the binder is polyacrylamide, and a silicon negative electrode sheet is finally obtained.

[0106] Performance testing;

[0107] Adhesive performance parameters:

[0108] Mass percentage of poly(urethane-urea) segments in flexible adhesives: determined by elemental analysis.

[0109] Crosslinking degree: The gel content was obtained by gel permeation chromatography (GPC).

[0110] Deformation recovery rate: The percentage of a material that recovers to its initial length after a 1900% tensile test using a universal testing machine.

[0111] Bond strength: Bond strength to copper foil using a 180° peel test.

[0112] The test results are listed in Table 1.

[0113] Electrochemical performance testing:

[0114] The silicon anode sheets from the above examples and comparative examples were assembled into coin cells with NCM811 cathodes, separators, and electrolytes, respectively. Electrochemical performance was tested at 25°C. Capacity retention, initial coulombic efficiency, and rate performance were tested after 800 cycles under 0.5C charge-discharge conditions, and the test results are listed in Table 2.

[0115] Table 1

[0116]

[0117] Table 2

[0118]

[0119] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0120] The aforementioned flexible binder, by constructing a three-dimensional network structure composed of poly(urethane-urea) segments and polyacrylonitrile, polyacrylic acid, or polyacrylamide segments, effectively solves the problems of insufficient stress buffering capacity, decreased adhesion, and poor elastic recovery performance of binders in lithium-ion battery applications of silicon-based anodes. Specifically, the flexible segments of poly(urethane-urea) endow the flexible binder with excellent deformation recovery capability, able to recover its initial shape even under strains as high as 1900%, with an elastic recovery rate >84%. It can recover its initial shape when silicon particles shrink, avoiding irreversible deformation, effectively buffering cyclic stress, and significantly improving the binder's adaptability to the volume expansion of silicon particles, thus preventing electrode structure damage. Furthermore, the introduction of linear polymer segments such as polyacrylonitrile segments enhances the interfacial bonding between the flexible binder and the silicon particle surface. Through the interaction of its polar groups with the hydroxyl groups on the silicon particle surface, it improves the adhesion strength (adhesion strength to copper foil >18 N / m), effectively preventing the detachment of active materials and ensuring electrode integrity. Meanwhile, the three-dimensional cross-linked network structure reduces the swelling rate of the binder in the electrolyte (<15%, far lower than that of linear binders (typically >50%)), ensuring structural stability during long-term cycling, reducing adhesion attenuation caused by swelling, and suppressing repeated rupture of the solid electrolyte interphase (SEI) film, thereby reducing electrolyte consumption and significantly improving the structural stability of the battery during long-term cycling. Therefore, the flexible binder of this invention significantly improves the cycling stability of silicon-based anodes through a synergistic design of high elasticity, strong adhesion, and swelling resistance.

[0121] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A flexible adhesive, characterized in that, The flexible adhesive is a cross-linked polymer with a three-dimensional network structure, which is formed by chemical cross-linking of poly(urethane-urea) segments and linear polymer segments. The linear polymer segment is selected from any one or more of polyacrylonitrile segments, polyacrylic acid segments, and polyacrylamide segments.

2. The flexible adhesive according to claim 1, characterized in that, The degree of crosslinking of the flexible adhesive is 5% to 22%.

3. The flexible adhesive according to claim 1 or 2, characterized in that, The poly(urethane-urea) segments in the flexible adhesive account for 20% to 60% by mass.

4. The flexible adhesive according to any one of claims 1 to 3, characterized in that, The linear polymer chain segment is a combination of polyacrylonitrile chain segment, polyacrylic acid chain segment and polyacrylamide chain segment, wherein the mass ratio of the polyacrylonitrile chain segment, the polyacrylic acid chain segment and the polyacrylamide chain segment is (2~4):(3~5):(1~3).

5. A method for preparing the flexible adhesive according to any one of claims 1 to 4, characterized in that, The preparation method includes: The flexible adhesive is obtained by mixing raw materials including poly(urethane-urea), linear polymer and solvent, and then carrying out a crosslinking reaction in the presence of a crosslinking agent. The linear polymer is selected from any one or more of polyacrylonitrile, polyacrylic acid, and polyacrylamide.

6. The preparation method according to claim 5, characterized in that, The preparation method includes: The linear polymer and the solvent are stirred and mixed to obtain a mixed solution; The poly(urethane-urea) and the crosslinking agent are added to the mixed solution to obtain a mixed liquid. The mixed liquid is stirred at 70~90°C to obtain a primary product containing the flexible adhesive. The initial product is sequentially washed, separated into solid and liquid components, and dried to obtain the flexible adhesive. The preparation method satisfies at least one of the following conditions: (1) The mass ratio of the poly(urethane-urea) to the linear polymer is (2~6):(4~8); (2) The solvent is selected from N,N-dimethylformamide and / or N-methylpyrrolidone; (3) The mass concentration of the linear polymer in the mixed solution is 5~10 wt%; (4) The stirring speed is 500~800 rpm, the stirring temperature is 60~80℃, and the stirring time is 2~4h; (5) The solid content of the mixture is 1~3 wt%; (6) The stirring temperature is 70~90℃, and the stirring time is 3~5h; (7) The weight-average molecular weight of the poly(urethane-urea) segments is 80,000 to 150,000; (8) The weight-average molecular weight of the linear polymer segments is 100,000 to 250,000.

7. A silicon negative electrode sheet, comprising a negative electrode active material, a conductive agent, and a binder, characterized in that, The adhesive is the flexible adhesive according to any one of claims 1 to 4.

8. A method for preparing the silicon negative electrode sheet according to claim 7, characterized in that, The preparation method includes: The adhesive is mixed with water to obtain an adhesive solution; The raw materials, including the binder solution, silicon powder, conductive agent and water, are mixed to obtain a mixed slurry; The mixed slurry is coated onto a current collector to obtain a wet film, and the wet film is dried to obtain a silicon negative electrode sheet; The adhesive is the flexible adhesive described in any one of claims 1 to 4.

9. The preparation method according to claim 8, characterized in that, The preparation method satisfies at least one of the following conditions: (1) The mass concentration of the adhesive solution is 8~15wt%, and the viscosity of the flexible adhesive is 3000~5000mPa·s; (2) The particle size of the silicon powder is 1~3μm; (3) The mass ratio of the flexible adhesive, the silicon powder and the conductive agent is (10~15):(75~85):(3~10); (4) The viscosity of the mixed slurry is 4000~6000 mPa·s; (5) The thickness of the current collector is 8~12μm; (6) The thickness of the wet film is 50~80μm; (7) The drying temperature is 80~100℃ and the drying time is 2~4 hours.

10. A lithium-ion battery, comprising a positive electrode and a negative electrode, characterized in that, The negative electrode is the silicon negative electrode as described in claim 7.

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