A low-swelling and low-impedance lithium-ion battery negative electrode and a preparation method thereof

Abstract

The application discloses a low-swelling and low-impedance lithium ion battery negative electrode, which comprises a composite current collector and a negative active material coating layer coated on at least one surface of the composite current collector; the negative active material coating layer comprises negative active material, graphene and lithium polyacrylate; the mass ratio of the negative active material, the graphene and the lithium polyacrylate is (90-99):(0.5-5):(0.5-5); the negative active material comprises a silicon-carbon material and a cladding material layer arranged on the outer surface of the silicon-carbon material, and the cladding material layer comprises a polypyrrole layer and single-walled carbon nanotubes uniformly distributed in the polypyrrole layer. Compared with the prior art, the lithium ion battery negative electrode provided by the application has the advantages of low swelling and low impedance, can effectively inhibit the expansion of silicon particles, reduce the internal resistance of the battery, and maintain the stability of the electrode structure.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a low-expansion, low-resistance lithium-ion battery anode and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used in electronic devices and new energy vehicles due to their high energy density and long cycle life. Among the various components of a lithium-ion battery, the negative electrode is crucial for improving battery performance and safety; the negative electrode typically includes a current collector and a layer of negative electrode material disposed on the current collector, and its performance directly affects the battery's capacity, rate performance, and lifespan.

[0003] Currently, graphite is the most commonly used anode material for lithium-ion batteries, widely used due to its low cost and long cycle life. However, graphite's theoretical specific capacity is only 372 mAh / g, which is insufficient to meet the demands of high-energy-density batteries. Therefore, researchers have begun exploring new anode materials, among which silicon-based materials have attracted considerable attention due to their high specific capacity of up to 4200 mAh / g.

[0004] However, silicon-based anodes still face many challenges in practical applications. First, silicon-based materials exhibit significant volume expansion during charge and discharge, with expansion rates exceeding 300%. This drastic volume change leads to the pulverization and shedding of active materials, causing damage to the electrode structure and rapid degradation of electrochemical performance, thus severely limiting the cycle stability of silicon-based materials in practical applications. Second, silicon-based materials themselves have poor conductivity, resulting in severe polarization during high-current charge and discharge, and poor rate performance. Furthermore, silicon particles easily form an unstable solid electrolyte interphase (SEI) film on the surface of the electrolyte, continuously consuming electrolyte and lithium ions, leading to low coulombic efficiency. These problems severely restrict the commercial application of silicon-based anodes.

[0005] In recent years, composite material design has become an important strategy for improving the performance of silicon-based anode materials. By combining silicon with carbon materials, the volume expansion and poor conductivity of silicon can be alleviated to some extent, but it is still difficult to meet the increasingly demanding performance requirements. Therefore, how to effectively alleviate the volume expansion problem of silicon-based materials, reduce the internal resistance of the battery, and improve its cycle performance is a major challenge in current lithium battery anode material research.

[0006] Furthermore, the current collector in the negative electrode, acting as a carrier for active materials and an electron transport channel, significantly impacts the battery's energy density, rate performance, and cycle stability. Currently, copper foil is commonly used as the current collector material in commercial lithium-ion batteries. However, copper foil current collectors suffer from low specific surface area and limited active material loading, hindering further improvements in battery energy density. To address this issue, researchers have proposed a design using copper foam as the negative electrode current collector.

[0007] However, existing foamed copper current collectors generally employ a single-layer design with uniform porosity and density. While increasing specific surface area and conductivity, this often sacrifices the current collector's mechanical strength and structural stability. This makes them prone to deformation or breakage during battery assembly and charge / discharge processes, leading to problems such as active material shedding and increased interfacial impedance, severely impacting battery performance and cycle life. Furthermore, the uneven stress distribution in single-layer foamed copper current collectors makes them susceptible to damage in stress concentration areas, further reducing the current collector's mechanical stability and reliability. Therefore, designing and fabricating a current collector that simultaneously possesses high specific surface area, high conductivity, high mechanical strength, and high structural stability is crucial for improving the performance of the battery's negative electrode.

[0008] In summary, developing a low-expansion, low-resistance lithium-ion battery anode that can effectively suppress silicon particle expansion and maintain electrode structural stability has become an important research topic in lithium-ion battery anodes. Summary of the Invention

[0009] The main objective of this invention is to provide a low-expansion, low-resistance lithium-ion battery anode that addresses the shortcomings of existing technologies, effectively suppressing silicon particle expansion and maintaining electrode structural stability.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] A low-expansion, low-resistance lithium-ion battery anode includes a composite current collector and an anode active material coating coated on at least one surface of the composite current collector; the anode active material coating includes an anode active material, graphene, and lithium polyacrylate; the mass ratio of the anode active material, the graphene, and the lithium polyacrylate is (90~99):(0.5~5):(0.5~5), the anode active material includes a silicon-carbon material and a coating material layer disposed on the outer surface of the silicon-carbon material, the coating material layer includes a polypyrrole layer and single-walled carbon nanotubes uniformly distributed in the polypyrrole layer.

[0012] Preferably, the silicon-carbon material is a mixture of silicon-based particles and graphite particles in a mass ratio of 1:(0.5~1); the particle size of the silicon-based particles is 10~50nm.

[0013] Preferably, the particle size of the negative electrode active material particles is 0.2~2μm.

[0014] Preferably, a multifunctional regulating coating is further provided between the composite current collector and the negative electrode active material coating. The multifunctional regulating coating includes barium titanate, conductive carbon black and lithium polyacrylate, and the mass ratio of barium titanate, conductive carbon black and lithium polyacrylate is 1:1:(0.5~1).

[0015] Preferably, the thickness of the multifunctional control coating is 0.1~1μm.

[0016] Preferably, the preparation method of the negative electrode active material is as follows:

[0017] (1) Mix graphite particles with distilled water at a mass ratio of 1:(10~20) and stir for 1~3 hours until the mixture is homogeneous to form a first mixture; then add silicon-based particles to the first mixture and stir for 1~3 hours until the graphite particles and silicon-based particles are homogeneous to form a second mixture.

[0018] (2) Pyrrole monomer and single-walled carbon nanotubes are mixed at a mass ratio of 1:(0.05~0.1) and dissolved in hydrogen chloride solution and stirred evenly to form a third mixture, wherein the mass ratio of pyrrole monomer to hydrogen chloride solution is 1:(1~5). Then the second mixture is added to the third mixture and stirred for 0.5~2h to make the second mixture and the third mixture evenly mixed to form a fourth mixture.

[0019] (3) Dissolve ammonium persulfate in hydrogen chloride solution to form a fifth mixture, wherein the mass ratio of ammonium persulfate to hydrogen chloride solution is 1:(1~5). Add the fourth mixture to the fifth mixture, stir evenly and react for 2~4 hours. After filtration, washing and drying, the negative electrode active material is obtained.

[0020] In this process, the addition of ammonium persulfate to pyrrole monomers can rapidly polymerize them into chains to form polypyrrole; the addition of hydrogen chloride can provide an acidic environment, enhance the oxidizing properties of ammonium persulfate, and increase the degree of polymerization of pyrrole.

[0021] Preferably, the composite current collector includes a copper foil layer and a composite foamed copper layer disposed on the copper foil layer. The composite foamed copper layer includes a first foamed copper, a second foamed copper, and a third foamed copper arranged sequentially from the inside out. The densities ρ1 of the first foamed copper, ρ2 of the second foamed copper, and ρ3 of the third foamed copper satisfy the relationship: ρ1 > ρ2 > ρ3. The porosities φ1 of the first foamed copper, φ2 of the second foamed copper, and φ3 of the third foamed copper satisfy the relationship: φ1 < φ2 < φ3. Furthermore, the thicknesses h1 of the first foamed copper, h2 of the second foamed copper, and h3 of the third foamed copper satisfy the relationship: h1 < h2 < h3.

[0022] In this invention, the negative electrode employs a composite current collector consisting of copper foil and composite copper foam. Firstly, the composite copper foam layer possesses a high specific surface area and a three-dimensional network structure, which not only facilitates electrolyte wetting but also allows for the loading of more active materials and promotes their uniform distribution. This shortens the electron and ion transport paths, reduces interfacial resistance, and improves the battery's energy density and rate performance. Secondly, the composite copper foam with gradient density, porosity, and thickness distribution enables layer-by-layer stress transfer and buffering, preventing stress concentration that could lead to localized deformation, fracture, or damage of the current collector, thus improving its mechanical stability and reliability. Thirdly, the composite copper foam with gradient density, porosity, and thickness distribution significantly increases the current collector's specific surface area and conductivity while maintaining mechanical strength and structural stability, effectively addressing the problem of deformation and fracture common in existing single-layer copper foam current collectors. In addition, the second and third layers of low-density, high-porosity copper foam can provide a larger specific surface area, which is beneficial for the loading of active materials and the wetting of electrolyte, thereby improving the energy density and rate performance of the battery. The first layer of copper foam has a relatively high density and low porosity, which can provide sufficient mechanical support and resistance to deformation, ensuring the structural integrity and stability of the composite current collector.

[0023] Preferably, the density ρ1 of the first copper foam is 0.5-0.9 g / cm³, the density ρ2 of the second copper foam is 0.3-0.5 g / cm³, and the density ρ3 of the third copper foam is 0.1-0.3 g / cm³; the porosity φ1 of the first copper foam is 50-70%, the porosity φ2 of the second copper foam is 70-85%, and the porosity φ3 of the third copper foam is 85-95%; the thickness h1 of the first copper foam is 0.5-1 μm, the thickness h2 of the second copper foam is 1-2 μm, and the thickness h3 of the third copper foam is 2-4 μm; the thickness h4 of the copper foil layer is 0.5-1 μm; and the total thickness of the composite current collector is 4-8 μm.

[0024] Preferably, the pore diameters d1 of the first foamed copper, d2 of the second foamed copper, and d3 of the third foamed copper satisfy the following relationship: d1 < d2 < d3; the pore diameter d1 of the first foamed copper is 1~5 μm, the pore diameter d2 of the second foamed copper is 5~10 μm, and the pore diameter d3 of the third foamed copper is 10~15 μm.

[0025] Preferably, the surface of the composite foam copper layer is further provided with a carbon nanotube coating, the thickness of which is 10-20 nm.

[0026] Furthermore, the present invention also provides a method for preparing the above-mentioned low-expansion, low-resistance lithium-ion battery negative electrode, comprising the following steps:

[0027] 1) Prepare the first foamed copper layer slurry, the second foamed copper layer slurry, and the third foamed copper layer slurry respectively;

[0028] a. The first foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:(15~25):1, adding anhydrous ethanol, and adjusting the solid content to 30-50 wt%; wherein the particle size of the copper powder is 0.1~0.4 μm, and the particle size of the urea is 0.2~0.5 μm;

[0029] b. The second foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:(25~35):1, adding anhydrous ethanol, and adjusting the solid content to 15-30wt%; wherein the particle size of the copper powder is 0.2~0.8μm, and the particle size of the urea is 0.4~1μm.

[0030] c. The third foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:(45~55):1, adding anhydrous ethanol, and adjusting the solid content to 5-15wt%; wherein the particle size of the copper powder is 0.5~1.5μm, and the particle size of the urea is 0.8~1.8μm.

[0031] 2) Preheat the copper foil substrate, and then coat the first foamed copper layer paste, the second foamed copper layer paste and the third foamed copper layer paste onto the copper foil substrate in sequence. During the coating process, spray a small amount of anhydrous ethanol onto the surface of each layer of paste.

[0032] 3) Place the coated copper foil into a vacuum sintering furnace and heat it to 650-750℃ at a heating rate of 2-5℃ / min under argon or nitrogen protection. Hold it at this temperature for 0.5-1h to sinter the copper powder into a gradient porous structure. At the same time, urea decomposes to produce gas and form a gradient foam copper layer.

[0033] 4) After cooling, the composite current collector is obtained. The composite current collector is ultrasonically cleaned in anhydrous ethanol for 3-5 minutes to remove residual urea decomposition products and impurities on the surface, and then dried.

[0034] 5) Mix the negative electrode active material, graphene and lithium polyacrylate in deionized water at a mass ratio of (90~99):(0.5~5):(0.5~5) and stir evenly to obtain the negative electrode active material coating slurry.

[0035] 6) Coat at least one surface of the composite current collector with the negative electrode active material coating slurry, and dry it to obtain the lithium-ion battery negative electrode.

[0036] Preferably, step 4) further includes surface modification of the cooled composite current collector, depositing a 10-20 nm thick carbon nanotube coating, and hot rolling the surface-modified composite current collector on a precision rolling mill at a rolling temperature of 100-150 °C and a rolling rate of 5-10%.

[0037] Compared with the prior art, the present invention has at least the following beneficial effects:

[0038] 1) This invention forms a negative electrode active material by introducing a coating layer of polypyrrole and single-walled carbon nanotubes on the outer surface of silicon-carbon material. This effectively prevents structural damage caused by volume changes of silicon particles during charging and discharging. Polypyrrole has high electrical conductivity, good environmental stability, and strong charge storage capacity. Therefore, the polypyrrole layer, as a flexible coating layer, can buffer the volume changes of silicon particles during charging and discharging, reduce stress accumulation, and improve electrode conductivity and structural stability. Meanwhile, single-walled carbon nanotubes, with their high aspect ratio and flexibility, can continuously and effectively contact the silicon material during cycling, preventing the breakage of the conductive network. The synergistic effect of the two can effectively suppress the expansion and pulverization of silicon particles, maintain the integrity of the electrode structure, and improve the cycle life and stability of lithium batteries.

[0039] 2) The single-walled carbon nanotubes added in this invention have excellent electronic conductivity and together with graphene form a line-to-surface conductive network, which significantly improves the conductivity of the electrode and reduces the battery interface impedance. Moreover, the three-dimensional conductive network not only establishes good conductive channels between silicon-carbon anode particles, but also wraps, entangles or binds the anode active material, further suppressing anode expansion. In addition, the high adhesion of single-walled carbon nanotubes (twice as high as that of multi-walled carbon nanotubes) further enhances the stability of the electrode structure.

[0040] 3) The negative electrode active material coating of this invention further optimizes the conductive network and structural stability of the material by adding graphene and lithium polyacrylate to the negative electrode active material. The introduction of graphene as a conductive agent allows it to synergistically work with single-walled carbon nanotubes to form a three-dimensional line-plane conductive network between the active material particles, providing efficient electron transport channels, reducing contact resistance between particles, and improving the electrode's conductivity. Simultaneously, lithium polyacrylate acts as a binder, firmly bonding the active material particles together, preventing particle pulverization and detachment caused by volume changes during charging and discharging, and improving the electrode's structural stability and mechanical strength. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the structure of the negative electrode active material in one embodiment of the present invention;

[0042] Figure 2 This is a schematic diagram of the negative electrode structure in one embodiment of the present invention;

[0043] Figure 3 This is a schematic diagram of the negative electrode structure in another embodiment of the present invention;

[0044] Figure 4 This is a schematic diagram of the composite current collector for the negative electrode in one embodiment of the present invention;

[0045] Figure 5 This is a schematic diagram of the composite current collector for the negative electrode in another embodiment of the present invention.

[0046] In the figure: 1. Composite current collector; 11. Copper foil layer; 12. Composite foamed copper layer; 121. First foamed copper; 122. Second foamed copper; 123. Third foamed copper; 13. Carbon nanotube coating; 2. Negative electrode active material coating; 3. Multifunctional control coating; 100. Negative electrode active material; 101. Graphite particles; 102. Silicon-based particles; 103. Polypyrrole layer; 104. Single-walled carbon nanotubes. Detailed Implementation

[0047] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0048] Please see the appendix Figures 1-2 and Figure 4 This application provides a low-expansion, low-resistance lithium-ion battery negative electrode, comprising a composite current collector 1 and a negative electrode active material coating 2 coated on at least one surface of the composite current collector 1; the negative electrode active material coating 2 comprises a negative electrode active material 100, graphene, and lithium polyacrylate; the mass ratio of the negative electrode active material 100, graphene, and lithium polyacrylate is (90~99):(0.5~5):(0.5~5), the negative electrode active material 100 comprises silicon-carbon material and a coating material layer disposed on the outer surface of the silicon-carbon material, the coating material layer comprising a polypyrrole layer 103 and single-walled carbon nanotubes 104 uniformly distributed in the polypyrrole layer 103.

[0049] In this invention, a silicon-carbon composite anode material coated with polypyrrole and single-walled carbon nanotubes 104 is used, which can effectively suppress the expansion, pulverization and shedding of silicon-based particles 102. At the same time, the single-walled carbon nanotubes 104 have a high length-to-diameter ratio, thus having high flexibility. During cycling, the single-walled carbon nanotubes 104 can effectively contact the silicon material and avoid detachment failure, thereby improving the cycle performance of the lithium battery.

[0050] Polypyrrole, with its high electrical conductivity, good environmental stability, reversible electrochemical redox properties, and strong charge storage capacity, is an ideal electrode material and also acts as a coating agent. One-dimensional single-walled carbon nanotubes (SUVs) 104, cylindrical and hollow, possess excellent electronic conductivity. Their fibrous structure allows for point-to-line contact with the electrode active material, forming a continuous conductive network that acts as a "conductor," thus improving battery capacity, rate performance, cycle life, and reducing interfacial impedance. Compared to multi-walled carbon nanotubes, SUVs 104 offer the following performance advantages: 1) Enhanced conductivity: Due to its higher aspect ratio, SUVs 104 exhibit superior conductivity, requiring less material to form a highly efficient conductive network and achieve the same or even better conductivity. 2) Improved safety: SUVs 104 have up to twice the adhesion of multi-walled carbon nanotubes or conductive carbon black, resulting in better safety. 3) Improve the cycle life of silicon-based anode batteries: Silicon-based anodes have weak conductivity, which can be perfectly compensated for by combining them with single-walled carbon nanotubes, thus significantly improving cycle life.

[0051] In this invention, the silicon-carbon anode material system uses single-walled carbon nanotubes 104 and graphene in combination to form a line-to-surface conductive network between silicon-carbon anode particles, thus establishing a good conductive channel between the silicon-carbon anode particles. At the same time, this line-to-surface three-dimensional conductive network can wrap, entangle, or bind the anode active material 100, thereby suppressing the expansion of the anode, greatly reducing the electrode expansion rate, and improving the stability of battery cycle.

[0052] Furthermore, the negative electrode active material coating 2 of the present invention further optimizes the conductive network and structural stability of the material by adding graphene and lithium polyacrylate to the negative electrode active material 100. The introduction of graphene as a conductive agent allows it to synergistically work with single-walled carbon nanotubes 104 to form a three-dimensional line-plane conductive network between the active material particles, providing efficient electron transport channels, reducing contact resistance between particles, and improving the conductivity of the electrode. Simultaneously, lithium polyacrylate acts as a binder, firmly bonding the active material particles together, preventing particle pulverization and detachment caused by volume changes during charging and discharging, and improving the structural stability and mechanical strength of the electrode.

[0053] In summary, this invention achieves a lithium-ion battery anode with low expansion, low impedance, and high stability through multiple structural designs and material selections.

[0054] In one embodiment of this application, the silicon-carbon material is a mixture of silicon-based particles 102 and graphite particles 101 in a mass ratio of 1:(0.5~1); the particle size of the silicon-based particles 102 is 10~50 nm. Preferably, the silicon-based particles 102 are silicon suboxide. By combining the nano-silicon particles with the graphite particles 101, high specific capacity can be ensured while buffering the volume expansion of the silicon particles. Furthermore, by optimizing the ratio of silicon-based particles 102 to graphite particles 101 and the particle size of the silicon-based particles 102, high capacity can be ensured while minimizing the negative impact of silicon expansion. The 10~50 nm particle size of the silicon-based particles 102 helps alleviate volume expansion, while the mixture formed with graphite can improve overall conductivity and stability.

[0055] In one embodiment of this application, the particle size of the negative electrode active material 100 is 0.2~2μm. By controlling the particle size of the negative electrode active material 100 to 0.2~2μm, this invention can significantly increase the specific surface area of ​​the material, shorten the ion diffusion path, and thus improve the rate performance of the material.

[0056] In one embodiment according to this application, such as Figure 3 As shown, a multifunctional control coating 3 is also provided between the composite current collector 1 and the negative electrode active material coating 2. The multifunctional control coating 3 includes barium titanate, conductive carbon black and lithium polyacrylate, and the mass ratio of barium titanate, conductive carbon black and lithium polyacrylate is 1:1:(0.5~1).

[0057] The introduced barium titanate, as a ferroelectric material, can induce the directional alignment of its internal dipoles under the action of an external electric field, forming a uniform negative charge layer on the surface of the composite current collector 1. This negative charge layer can promote the uniform deposition of Li+ on the negative electrode surface, inhibit the growth of lithium dendrites, thereby alleviating local stress concentration, reducing the degree of silicon particle breakage, and extending the battery cycle life. On the other hand, barium titanate also has piezoelectric properties. During cycling, the volume expansion of the composite material causes deformation of the barium titanate lattice, resulting in dynamic changes in the local built-in electric field. This dynamic electric field can guide lithium ions to migrate to areas with lower stress, achieving uniform lithium ion distribution and avoiding electrode pulverization failure caused by stress concentration. In addition, conductive carbon black can improve the conductivity of the coating, and lithium polyacrylate acts as a binder. Therefore, the setting of the multifunctional control coating 3 can further suppress the expansion of the silicon-based negative electrode, improve lithium ion distribution, inhibit lithium dendrite growth, and improve battery performance.

[0058] In one embodiment according to this application, the thickness of the multifunctional control coating 3 is 0.1~1 μm. This thickness range ensures the effective functioning of the coating without significantly increasing the total thickness and weight of the negative electrode, thus reducing the battery energy density.

[0059] In one embodiment of this application, the negative electrode active material 100 is prepared as follows:

[0060] (1) Mix graphite particles 101 with distilled water at a mass ratio of 1:(10~20), stir for 1~3 hours, and form a first mixture after mixing evenly; then add silicon-based particles 102 to the first mixture, stir for 1~3 hours, and form a second mixture after mixing graphite particles 101 and silicon-based particles 102 evenly.

[0061] (2) Pyrrole monomer and single-walled carbon nanotubes 104 are mixed at a mass ratio of 1:(0.05~0.1) and dissolved in hydrogen chloride solution and stirred evenly to form a third mixture, wherein the mass ratio of pyrrole monomer to hydrogen chloride solution is 1:(1~5). Then the second mixture is added to the third mixture and stirred for 0.5~2h to make the second mixture and the third mixture evenly mixed to form a fourth mixture.

[0062] (3) Dissolve ammonium persulfate in hydrogen chloride solution to form a fifth mixture, wherein the mass ratio of ammonium persulfate to hydrogen chloride solution is 1:(1~5). Add the fourth mixture to the fifth mixture, stir evenly and react for 2~4 hours. After filtration, washing and drying, the negative electrode active material 100 is obtained.

[0063] In this process, the addition of ammonium persulfate to pyrrole monomers can rapidly polymerize them into chains to form polypyrrole; the addition of hydrogen chloride can provide an acidic environment, enhance the oxidizing properties of ammonium persulfate, and increase the degree of polymerization of pyrrole.

[0064] The preparation of the negative electrode active material 100 of this invention involves first thoroughly premixing silicon-based particles 102 and graphite particles 101, then mixing them with pyrrole monomers and single-walled carbon nanotubes 104; subsequently, in-situ polymerization is carried out under conditions where ammonium persulfate acts as an initiator and hydrogen chloride provides an acidic environment. Premixing the silicon-based particles 102 and graphite particles 101 ensures uniform dispersion of the two materials at the nanoscale, preventing silicon particle agglomeration. The subsequent introduction of pyrrole monomers and single-walled carbon nanotubes 104 allows for simultaneous and uniform coating of silicon and carbon particles by polypyrrole and carbon nanotubes during the in-situ polymerization process, resulting in a composite material with a uniform structure and good dispersibility. The ammonium persulfate initiator and the acidic environment of hydrogen chloride accelerate the polymerization rate of pyrrole, improving the degree of polymerization and electrical conductivity of polypyrrole.

[0065] In one embodiment of this application, the composite current collector 1 includes a copper foil layer 11 and a composite foamed copper layer 12 disposed on the copper foil layer 11. The composite foamed copper layer 12 includes a first foamed copper 121, a second foamed copper 122, and a third foamed copper 123 disposed sequentially from the inside out. The densities ρ1 of the first foamed copper 121, ρ2 of the second foamed copper 122, and ρ3 of the third foamed copper 123 satisfy the relationship: ρ1 > ρ2 > ρ3. The porosities φ1 of the first foamed copper 121, φ2 of the second foamed copper 122, and φ3 of the third foamed copper 123 satisfy the relationship: φ1 < φ2 < φ3. The thicknesses h1 of the first foamed copper 121, h2 of the second foamed copper 122, and h3 of the third foamed copper 123 satisfy the relationship: h1 < h2 < h3.

[0066] This invention employs a hierarchical structure design for a composite foamed copper current collector, which significantly improves the mechanical strength and structural stability of the current collector while ensuring high specific surface area and conductivity. The presence of the inner high-density foamed copper effectively bears the mechanical load of the current collector, preventing deformation and breakage; while the outer low-density foamed copper provides a larger specific surface area and porosity, promoting electrolyte wetting and ion transport. Therefore, the hierarchical structure design effectively balances multiple performance requirements of the current collector, significantly improving the overall performance of the battery anode.

[0067] Traditional single-layer foamed copper current collectors, while possessing high specific surface area and porosity, suffer from poor mechanical strength and structural stability. They are unable to withstand the stress impacts during battery assembly and charge / discharge processes, and are prone to deformation or breakage, leading to problems such as active material detachment and increased interfacial impedance. This invention employs a hierarchical structure design. The inner high-density foamed copper acts as a mechanical support layer, providing sufficient mechanical strength and rigidity to prevent deformation and breakage of the current collector. The middle layer, with moderate density, serves as a transition and connection. The outer low-density foamed copper maximizes specific surface area and porosity, promoting electrolyte wetting and ion transport, and reducing interfacial impedance. Simultaneously, this hierarchical structure allows for a gradient distribution of stress, avoiding localized damage caused by stress concentration. Therefore, the hierarchical structure foamed copper current collector can ensure high specific surface area and conductivity while maintaining mechanical strength and structural stability, improving the applicability and reliability of the current collector in the battery negative electrode.

[0068] In summary, the composite current collector 1 of the present invention includes a copper foil layer 11 and a composite foamed copper layer 12, wherein the composite foamed copper layer 12 is composed of three layers of foamed copper with different densities, different porosities and different thicknesses, arranged in a gradient distribution. This structure has the following outstanding advantages:

[0069] 1) High specific surface area: The gradient-distributed composite foam copper layer 12 has a high specific surface area and a three-dimensional porous structure. The outer layer of low-density, high-porosity, and thick foam copper can load more active materials, improve the utilization rate of active materials, and increase battery capacity.

[0070] 2) High conductivity: The outer layer of copper foam is in direct contact with the active material. Its loose and porous structure is conducive to electrolyte wetting and rapid ion transport, reducing interfacial resistance. The inner layer of high-density, low-porosity copper foam can provide an effective electron transport channel, ensuring the high conductivity of the current collector.

[0071] 3) High mechanical strength: The inner layer of high-density, low-porosity, and thin-thickness copper foam can provide sufficient mechanical support to ensure the structural integrity of the current collector; the middle layer has moderate density and porosity, which plays a transition and buffering role, improving the mechanical stability of the current collector.

[0072] 4) High structural stability: The gradient structure design can achieve the layer-by-layer transmission and uniform distribution of stress. The outer layer of low-density, high-porosity, and thick foamed copper can effectively absorb and buffer the stress generated during battery charging and discharging, avoid stress concentration that could lead to local deformation or breakage of the current collector, and extend the cycle life of the battery.

[0073] In one embodiment according to this application, the density ρ1 of the first copper foam 121 is 0.5-0.9 g / cm³, preferably 0.7 g / cm³; the density ρ2 of the second copper foam 122 is 0.3-0.5 g / cm³, preferably 0.4 g / cm³; and the density ρ3 of the third copper foam 123 is 0.1-0.3 g / cm³, preferably 0.2 g / cm³. The porosity φ1 of the first copper foam 121 is 50-70%, preferably 60%. The porosity φ2 of the second foam copper 122 is 70-85%, preferably 80%, and the porosity φ3 of the third foam copper 123 is 85-95%, preferably 90%; the thickness h1 of the first foam copper 121 is 0.5-1μm, the thickness h2 of the second foam copper 122 is 1-2μm, and the thickness h3 of the third foam copper 123 is 2-4μm; the thickness h4 of the copper foil layer 11 is 0.5-1μm; and the total thickness of the composite current collector 1 is 4-8μm.

[0074] By rationally designing the above parameters, the specific surface area and conductivity of the current collector can be maximized while ensuring its mechanical strength and structural stability. For example, appropriately reducing the density of the inner layer of copper foam and increasing its porosity can increase the specific surface area while ensuring mechanical strength, promoting electrolyte wetting and ion transport; while appropriately reducing the thickness and pore size of the outer layer of copper foam can achieve a higher specific surface area and smaller pore size, increasing the loading of active materials and shortening the ion diffusion path. At the same time, optimizing the thickness ratio of each layer of copper foam can achieve the best balance between mechanical strength and specific surface area. Therefore, by rationally designing the structural parameters of the composite copper foam current collector, the conductivity and specific surface area of ​​the current collector can be maximized while ensuring mechanical stability, thereby improving the rate performance and cycle performance of the battery anode. In addition, the total thickness of the anode composite current collector 1 of the present invention is controlled within the ultra-thin range of 4-8 μm. Compared with traditional copper foil current collectors, this can significantly reduce the thickness and mass of the current collector, further improving the energy density and specific power of the battery, which is beneficial for the miniaturization and lightweight design of the battery. Furthermore, the thickness of each copper foam layer directly affects battery performance. Insufficient thickness can cause active material particles to be squeezed out of the pores or damaged under pressure, reducing the overall performance of the electrode and consequently decreasing the battery's rate performance and cycle life. Conversely, excessive thickness increases the proportion of inactive materials, thus reducing the battery's energy density. Therefore, controlling the appropriate thickness of each copper foam layer is crucial for optimizing lithium-ion battery performance.

[0075] In one embodiment according to this application, the pore sizes d1 of the first copper foam 121, d2 of the second copper foam 122, and d3 of the third copper foam 123 satisfy the relationship: d1 < d2 < d3; the pore size d1 of the first copper foam 121 is 1~5 μm, preferably 3 μm, the pore size d2 of the second copper foam 122 is 5~10 μm, preferably 8 μm, and the pore size d3 of the third copper foam 123 is 10~15 μm, preferably 13 μm. The funnel-shaped pore size gradient distribution can form a multi-level porous structure that facilitates lithium-ion diffusion and active material embedding. The large pore size in the outer layer facilitates the rapid transport of electrolyte and lithium ions, while the small pore size in the inner layer can suppress the volume expansion of the active material, maintain the stability of the electrode structure, and improve the rate performance and cycle stability of the battery. Furthermore, the pore size has a decisive impact on the permeability of the slurry. If the pore size is too small, the slurry may not be able to effectively fill the pores due to surface tension, which will hinder the uniform distribution of the active material and affect the performance of the electrode. Conversely, if the pore size is too large, the slurry will easily overfill the pores, leading to an increase in areal density. During the drying process, this overfilling may cause local cracks, further affecting the rate performance of the battery. Therefore, selecting an appropriate pore size is crucial for ensuring electrode quality and battery performance.

[0076] In one embodiment according to this application, such as Figure 5 As shown, a carbon nanotube coating 13, with a thickness of 10-20 nm, is also disposed on the surface of the composite copper foam layer 12. Coating the composite copper foam layer with carbon nanotube 13 further improves the conductivity and corrosion resistance of the current collector. The carbon nanotube coating 13 is tightly bonded to the copper foam substrate, forming a dense and uniform conductive protective layer on the current collector surface, effectively reducing interfacial resistance and improving the chemical stability of the current collector in the electrolyte environment, thus extending the battery's cycle life. Furthermore, an appropriate thickness of the carbon nanotube coating 13 can prevent excessive coating thickness from causing pore blockage or increased interfacial impedance.

[0077] While copper foam possesses excellent conductivity and specific surface area, its surface is susceptible to corrosion by the electrolyte, forming an insulating copper oxide passivation layer. This leads to increased interfacial resistance and decreased battery performance. Coating the surface of copper foam with a carbon nanotube coating 13 effectively isolates the copper foam from direct contact with the electrolyte, preventing corrosion. Simultaneously, the carbon nanotube coating 13 itself exhibits excellent conductivity, forming a uniform and dense conductive network on the copper foam surface, reducing interfacial resistance and improving the conductivity of the current collector. Furthermore, the carbon nanotube coating 13 demonstrates excellent chemical stability, allowing it to remain stable in the electrolyte environment for extended periods, delaying current collector performance degradation and improving battery cycle life. Therefore, the introduction of the carbon nanotube coating 13 further optimizes the performance of the composite copper foam current collector in terms of both conductivity and corrosion resistance, providing a superior current collector material for high-performance lithium-ion battery anodes.

[0078] Furthermore, this application also provides a method for preparing the above-mentioned low-expansion, low-resistance lithium-ion battery negative electrode, comprising the following steps:

[0079] 1) Prepare the first foamed copper layer slurry, the second foamed copper layer slurry, and the third foamed copper layer slurry respectively;

[0080] a. The first foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:(15~25):1, adding anhydrous ethanol, and adjusting the solid content to 30-50wt%; wherein the particle size of copper powder is 0.1~0.4μm and the particle size of urea is 0.2~0.5μm.

[0081] b. The second foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:(25~35):1, adding anhydrous ethanol, and adjusting the solid content to 15-30wt%; wherein the particle size of copper powder is 0.2~0.8μm and the particle size of urea is 0.4~1μm.

[0082] c. The third foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:(45~55):1, adding anhydrous ethanol, and adjusting the solid content to 5-15wt%; wherein the particle size of copper powder is 0.5~1.5μm and the particle size of urea is 0.8~1.8μm.

[0083] 2) Preheat the copper foil substrate, and then coat the first foamed copper layer paste, the second foamed copper layer paste and the third foamed copper layer paste onto the copper foil substrate in sequence. During the coating process, spray a small amount of anhydrous ethanol onto the surface of each layer of paste.

[0084] 3) Place the coated copper foil into a vacuum sintering furnace and heat it to 650-750℃ at a heating rate of 2-5℃ / min under argon or nitrogen protection. Hold it at this temperature for 0.5-1h to sinter the copper powder into a gradient porous structure. At the same time, urea decomposes to produce gas and form a gradient foam copper layer.

[0085] 4) After cooling, the composite current collector is obtained. The composite current collector is ultrasonically cleaned in anhydrous ethanol for 3-5 minutes to remove residual urea decomposition products and impurities on the surface, and then dried.

[0086] 5) Mix the negative electrode active material, graphene and lithium polyacrylate in deionized water at a mass ratio of (90~99):(0.5~5):(0.5~5) and stir evenly to obtain the negative electrode active material coating slurry.

[0087] 6) Coat the negative electrode active material coating slurry onto at least one surface of the composite current collector and dry it to obtain the lithium-ion battery negative electrode.

[0088] The current collector preparation method for the negative electrode of this invention employs a gradient slurry formulation and layer-by-layer coating and sintering process, which is simple to operate and easy to control. By adjusting the slurry composition, solid content, particle size, and coating process parameters, the microstructural parameters of the composite copper foam, such as density, porosity, thickness, and pore size, can be precisely controlled, ensuring the consistency and repeatability of the current collector performance. Furthermore, controlling the particle size of copper powder and urea pore-forming agent in different slurry layers can effectively control the pore size and porosity of the copper foam, ensuring the formation of a gradient structure while avoiding problems such as incomplete sintering or pore collapse caused by excessively large or small particle sizes. Therefore, the negative electrode preparation process proposed in this invention is simple and controllable, and easy to promote and apply industrially.

[0089] In one embodiment of this application, step 4) further includes surface modification of the cooled composite current collector 1, depositing a 10-20 nm thick carbon nanotube coating 13, and hot rolling the surface-modified composite current collector 1 on a precision rolling mill at a rolling temperature of 100-150°C and a rolling rate of 5-10%. Introducing surface modification and hot rolling processes can further improve the conductivity and bonding strength of the composite current collector 1, and improve the interfacial compatibility and mechanical properties between the current collector and the active material coating.

[0090] The embodiments of this application are described in detail below. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0091] Example 1

[0092] Preparation of negative electrode sheet:

[0093] The lithium-ion battery negative electrode of this embodiment includes a composite current collector 1 and a negative electrode active material coating 2 coated on one surface of the composite current collector 1. The negative electrode active material coating 2 includes a negative electrode active material 100, graphene, and lithium polyacrylate. The mass ratio of the negative electrode active material 100, graphene, and lithium polyacrylate is 96:2:2. The negative electrode active material 100 includes a silicon-carbon material and a coating material layer disposed on the outer surface of the silicon-carbon material. The coating material layer includes a polypyrrole layer 103 and single-walled carbon nanotubes 104 uniformly distributed in the polypyrrole layer 103.

[0094] The silicon-carbon material is a mixture of silicon suboxide particles and graphite particles 101 in a mass ratio of 1:1; the particle size of the silicon suboxide particles is 25 nm; and the particle size of the negative electrode active material 100 particles is 0.5 μm.

[0095] The preparation method of the negative electrode active material is as follows:

[0096] (1) Mix graphite particles and distilled water at a mass ratio of 1:15 and stir for 2 hours until they are evenly mixed to form a first mixture; then add silica particles to the first mixture and stir for 2 hours until the graphite particles and silica particles are evenly mixed to form a second mixture.

[0097] (2) Pyrrole monomer and single-walled carbon nanotubes are mixed at a mass ratio of 1:0.05 and dissolved in hydrogen chloride solution and stirred evenly to form a third mixture, wherein the mass ratio of pyrrole monomer to hydrogen chloride solution is 1:3. Then the second mixture is added to the third mixture and stirred for 1 hour to make the second mixture and the third mixture evenly mixed to form a fourth mixture.

[0098] (3) Dissolve ammonium persulfate in hydrogen chloride solution to form a fifth mixture, wherein the mass ratio of ammonium persulfate to hydrogen chloride solution is 1:3. Add the fourth mixture to the fifth mixture, stir evenly and react for 3 hours. After filtration, washing and drying, the negative electrode active material is obtained.

[0099] The composite current collector 1 includes a copper foil layer 11 and a composite foam copper layer 12 disposed on the copper foil layer 11. The composite foam copper layer 12 is composed of three layers of foam copper with different densities, different porosities and different thicknesses. The composite current collector 1 is connected to a negative electrode tab.

[0100] The composite copper foam layer 12 includes a first copper foam 121, a second copper foam 122, and a third copper foam 123 arranged sequentially from the inside out. The densities ρ1 of the first copper foam 121, ρ2 of the second copper foam 122, and ρ3 of the third copper foam 123 satisfy the relationship: ρ1 > ρ2 > ρ3. The porosities φ1 of the first copper foam 121, φ2 of the second copper foam 122, and φ3 of the third copper foam 123 satisfy the relationship: φ1 < φ2 < φ3. The thicknesses h1 of the first copper foam 121, h2 of the second copper foam 122, and h3 of the third copper foam 123 satisfy the relationship: h1 < h2 < h3.

[0101] Specifically, in this embodiment, the density ρ1 of the first copper foam 121 is 0.7 g / cm³, the density ρ2 of the second copper foam 122 is 0.4 g / cm³, and the density ρ3 of the third copper foam 123 is 0.2 g / cm³. The porosity φ1 of the first copper foam 121 is 60%, the porosity φ2 of the second copper foam 122 is 80%, and the porosity φ3 of the third copper foam 123 is 90%. The thickness h1 of the first copper foam 121 is 0.5 μm, the thickness h2 of the second copper foam 122 is 1.5 μm, and the thickness h3 of the third copper foam 123 is 3 μm; the thickness h4 of the copper foil layer 11 is 1 μm; and the total thickness of the composite current collector 1 is 6 μm.

[0102] The preparation method of the negative electrode sheet is as follows:

[0103] 1) Prepare the first foamed copper layer slurry, the second foamed copper layer slurry, and the third foamed copper layer slurry respectively;

[0104] a. The first foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:20:1, adding anhydrous ethanol, and adjusting the solid content to 40 wt%; wherein the particle size of the copper powder is 0.2 μm and the particle size of the urea is 0.4 μm;

[0105] b. The second foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:30:1, adding anhydrous ethanol, and adjusting the solid content to 25 wt%; wherein the particle size of the copper powder is 0.5 μm and the particle size of the urea is 0.8 μm.

[0106] c. The third foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:50:1, adding anhydrous ethanol, and adjusting the solid content to 10wt%; wherein the particle size of the copper powder is 1μm and the particle size of the urea is 1.5μm.

[0107] 2) Preheat the copper foil substrate, and then coat the first foamed copper layer paste, the second foamed copper layer paste and the third foamed copper layer paste on the copper foil substrate in sequence. During the coating process, spray a small amount of anhydrous ethanol on the surface of each layer of paste to promote the bonding and fusion between the layers.

[0108] 3) Place the coated copper foil into a vacuum sintering furnace and heat it to 700°C at a heating rate of 3°C / min under the protection of argon atmosphere. Hold it at this temperature for 1 hour to sinter the copper powder into a gradient porous structure. At the same time, urea decomposes to produce gas and form a gradient foam copper layer.

[0109] 4) After cooling, the composite current collector is obtained. The composite current collector is ultrasonically cleaned in anhydrous ethanol for 3-5 minutes to remove residual urea decomposition products and impurities on the surface. Then the composite current collector is dried at 70℃ for 0.5 hours.

[0110] 5) Mix the negative electrode active material, graphene and lithium polyacrylate in deionized water at a mass ratio of 96:2:2 and stir until uniform to obtain the negative electrode active material coating slurry.

[0111] 6) Coat the surface of the composite current collector with the negative electrode active material coating slurry and dry it to obtain the battery negative electrode sheet.

[0112] Preparation of the positive electrode sheet:

[0113] The positive electrode active material lithium nickel cobalt manganese oxide, conductive agent conductive carbon black, and binder polyvinylidene fluoride are dispersed in the solvent NMP (N-methylpyrrolidone) at a mass ratio of 95:2:3 to form a uniform slurry. The slurry is uniformly coated on the surface of the current collector aluminum foil and then transferred to a vacuum drying oven for complete drying to obtain the positive electrode sheet.

[0114] Electrolyte preparation:

[0115] Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 1:1, and then fluoroethylene carbonate (FEC) was added at a volume percentage of 5%. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent at a concentration ratio of 1 mol / L to prepare the electrolyte.

[0116] Preparation of the separator: A PET / PP composite film with a thickness of 15 micrometers was selected.

[0117] Preparation of lithium-ion batteries:

[0118] The negative electrode, separator, and positive electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrolyte is then added, and the battery is assembled using conventional processes.

[0119] Example 2

[0120] Unlike Example 1, in this example, a multifunctional control coating 3 is also provided between the composite current collector 1 and the negative electrode active material coating 2. The multifunctional control coating 3 includes barium titanate, conductive carbon black and lithium polyacrylate, and the mass ratio of barium titanate, conductive carbon black and lithium polyacrylate is 1:1:0.5; the thickness of the multifunctional control coating is 0.5 μm.

[0121] The preparation method of the negative electrode sheet is as follows:

[0122] 1) Prepare the first foamed copper layer slurry, the second foamed copper layer slurry, and the third foamed copper layer slurry respectively;

[0123] a. The first foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:20:1, adding anhydrous ethanol, and adjusting the solid content to 40 wt%; wherein the particle size of the copper powder is 0.2 μm and the particle size of the urea is 0.4 μm;

[0124] b. The second foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:30:1, adding anhydrous ethanol, and adjusting the solid content to 25 wt%; wherein the particle size of the copper powder is 0.5 μm and the particle size of the urea is 0.8 μm.

[0125] c. The third foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:50:1, adding anhydrous ethanol, and adjusting the solid content to 10wt%; wherein the particle size of the copper powder is 1μm and the particle size of the urea is 1.5μm.

[0126] 2) Preheat the copper foil substrate, and then coat the first foamed copper layer paste, the second foamed copper layer paste and the third foamed copper layer paste on the copper foil substrate in sequence. During the coating process, spray a small amount of anhydrous ethanol on the surface of each layer of paste to promote the bonding and fusion between the layers.

[0127] 3) Place the coated copper foil into a vacuum sintering furnace and heat it to 700°C at a heating rate of 3°C / min under the protection of argon atmosphere. Hold it at this temperature for 1 hour to sinter the copper powder into a gradient porous structure. At the same time, urea decomposes to produce gas and form a gradient foam copper layer.

[0128] 4) After cooling, the composite current collector is obtained. The composite current collector is ultrasonically cleaned in anhydrous ethanol for 3-5 minutes to remove residual urea decomposition products and impurities on the surface. Then the composite current collector is dried at 70℃ for 0.5 hours.

[0129] 5) Mix barium titanate, conductive carbon black and lithium polyacrylate in deionized water at a mass ratio of 1:1:0.5 and stir until homogeneous to obtain a multifunctional control coating slurry.

[0130] 6) Coat the surface of the composite current collector with the multifunctional regulating coating slurry and dry it to obtain the composite current collector coated with the multifunctional regulating coating.

[0131] 7) Apply an upward external electric field to the composite current collector coated with a multifunctional control coating to induce barium titanate to form directional polarization and construct an internal electric field, wherein the direction of the internal electric field is from the surface of the composite current collector toward the outer surface of the multifunctional control coating.

[0132] 8) Mix the negative electrode active material, graphene and lithium polyacrylate in deionized water at a mass ratio of 96:2:2 and stir until uniform to obtain the negative electrode active material coating slurry.

[0133] 9) Coat the outer surface of the multifunctional control coating with the negative electrode active material coating slurry and dry it to obtain the battery negative electrode sheet.

[0134] The rest is the same as in Example 1, and will not be repeated here.

[0135] Example 3

[0136] Unlike Example 1, in this example, the surface of the composite foam copper layer 12 in the composite current collector 1 of the negative electrode is also provided with a carbon nanotube coating 13, the thickness of which is 15 nm.

[0137] In addition, step 4) of the method for preparing the negative electrode sheet includes surface modification of the composite current collector 1 obtained after cooling, depositing a 15nm thick carbon nanotube coating 13, and hot rolling the surface-modified composite current collector 1 on a precision rolling mill at a rolling temperature of 120℃ and a rolling rate of 8%.

[0138] The rest is the same as in Example 1, and will not be repeated here.

[0139] Example 4

[0140] Unlike Example 1, in this example, the pore diameters d1 of the first foamed copper 121, d2 of the second foamed copper 122, and d3 of the third foamed copper 123 in the composite current collector 1 of the negative electrode satisfy the following relationship: d1 < d2 < d3.

[0141] Specifically, in this embodiment, the pore size d1 of the first copper foam 121 is 3μm, the pore size d2 of the second copper foam 122 is 8μm, and the pore size d3 of the third copper foam 123 is 13μm.

[0142] Furthermore, in step 1) of the preparation method of the negative electrode sheet, the copper powder in the first foamed copper layer slurry has a particle size of 0.1 μm and the urea has a particle size of 0.3 μm; the copper powder in the second foamed copper layer slurry has a particle size of 0.5 μm and the urea has a particle size of 0.8 μm; and the copper powder in the third foamed copper layer slurry has a particle size of 1 μm and the urea has a particle size of 1.3 μm.

[0143] The rest is the same as in Example 1, and will not be repeated here.

[0144] Comparative Example 1

[0145] Unlike Example 1, the negative electrode active material in this comparative example is silicon-carbon material, without any modification or coating.

[0146] Everything else is the same as in Example 1, and will not be repeated here.

[0147] Comparative Example 2

[0148] Unlike Example 1, the negative electrode active material in this comparative example is silicon-carbon material and a single-walled carbon nanotube coating layer disposed on the outer surface of the silicon-carbon material.

[0149] Everything else is the same as in Example 1, and will not be repeated here.

[0150] Comparative Example 3

[0151] Unlike Example 1, the negative electrode active material in this comparative example is silicon-carbon material and a polypyrrole coating layer disposed on the outer surface of the silicon-carbon material.

[0152] Everything else is the same as in Example 1, and will not be repeated here.

[0153] Comparative Example 4

[0154] Unlike Example 1, the current collector for the negative electrode in this comparative example is a copper foil current collector with a thickness of 10 μm.

[0155] Everything else is the same as in Example 1, and will not be repeated here.

[0156] Comparative Example 5

[0157] Unlike Example 1, the negative electrode current collector in this comparative example uses a single-layer copper foam current collector with a density of 0.5 g / cm³, a porosity of 90%, and a thickness of 10 μm.

[0158] Everything else is the same as in Example 1, and will not be repeated here.

[0159] The ductility, stability, and expansion rate of the negative electrode sheets in the above embodiments and comparative examples were tested respectively. The internal resistance and cycle performance of the 1Ah lithium-ion batteries made from each negative electrode sheet were tested. The test results are shown in Table 1.

[0160] Ductility test: Under the same pressure (20 tons of pressure), the negative electrode sheets prepared in the examples and comparative examples were rolled and observed to see if there was any brittle fracture or cracking on the surface of the electrode sheets. If any of the above phenomena occurred, it indicates that the ductility was poor.

[0161] Stability test: Ten negative electrode sheets prepared in the examples and comparative examples were folded and twisted at 90 degrees. The condition of the active material at the folded or twisted point was observed. If the active material fell off or broke, it indicated that the structure was not stable.

[0162] Table 1

[0163]

[0164] The test results in Table 1 above are analyzed as follows:

[0165] Comparing Example 1 and Comparative Examples 1-3, it can be seen that, compared with existing methods that directly use silicon-carbon materials as negative electrode active materials, or only coat silicon-carbon materials with single-walled carbon nanotubes as negative electrode active materials, or only coat silicon-carbon materials with a polypyrrole layer as negative electrode active materials, the present invention forms a negative electrode active material by introducing a coating layer of polypyrrole and single-walled carbon nanotubes on the outer surface of silicon-carbon materials. This can effectively prevent the structural damage of silicon particles caused by volume changes during charging and discharging, suppress negative electrode expansion, reduce battery internal resistance, and improve battery cycle life. This is because polypyrrole has high electrical conductivity, good environmental stability, and strong charge storage capacity. Therefore, the polypyrrole layer, as a flexible coating layer, can buffer the volume change of silicon particles during charging and discharging, reduce stress accumulation, and improve electrode conductivity and structural stability. Meanwhile, single-walled carbon nanotubes, with their high aspect ratio and flexibility, can continuously and effectively contact the silicon material during cycling, preventing the breakage of the conductive network. The synergistic effect of the two can effectively suppress the expansion and pulverization of silicon particles, maintain the integrity of the electrode structure, and improve the cycle life and stability of lithium batteries. Moreover, single-walled carbon nanotubes and graphene together form a line-to-surface conductive network, which significantly improves the conductivity of the electrode and reduces the battery interface impedance. The three-dimensional conductive network not only establishes a good conductive channel between silicon-carbon anode particles, but also wraps, entangles, or binds the anode active material, further suppressing anode expansion.

[0166] Comparing Example 1 and Comparative Examples 4-5, it can be seen that compared with the negative electrodes prepared by existing copper foil current collectors or single-layer foamed copper current collectors, the negative electrode of the present invention has better ductility, structural stability, lower expansion rate and battery internal resistance. This is because the active material of the negative electrode of the present invention has been infiltrated into the three-dimensional structure of the composite foamed copper. The gradient density composite foamed copper itself has good flexibility, so it has a better retention effect on the infiltrated active material. This ensures that the negative electrode prepared by the composite foamed copper has better resistance to deformation and structural stability during the processing and battery use. It can effectively prevent the negative electrode from becoming brittle or cracked, and effectively reduce the expansion rate of the negative electrode. Therefore, the lithium-ion battery prepared by the negative electrode of the present invention has a higher cycle life.

[0167] Comparing Examples 1 and 2, it can be seen that when a multifunctional regulating coating is provided between the composite current collector and the active material coating of the negative electrode, the negative electrode of the present invention has better ductility and structural stability, and can further reduce the negative electrode expansion rate and battery internal resistance. This is because the introduced barium titanate, as a ferroelectric material, can induce the directional alignment of its internal dipoles under the action of an external electric field, so that a uniform negative charge layer is formed on the surface of the composite current collector; this negative charge layer can promote the Li +Uniform deposition on the negative electrode surface suppresses lithium dendrite growth, thereby alleviating local stress concentration, reducing silicon particle breakage, and extending battery cycle life. On the other hand, barium titanate also possesses piezoelectric properties. During cycling, the volume expansion of the composite material causes deformation of the barium titanate lattice, leading to dynamic changes in the localized internal electric field. This dynamic electric field guides lithium ions to migrate to areas of lower stress, achieving uniform lithium ion distribution and preventing electrode pulverization failure caused by stress concentration. Therefore, the multifunctional control coating can further suppress the expansion of the silicon-based negative electrode, improve lithium ion distribution, inhibit lithium dendrite growth, and enhance battery cycle performance.

[0168] Comparing Examples 1 and 3, it can be seen that when the surface of the composite foam copper layer of the negative electrode is further modified with a carbon nanotube coating, and the surface-modified composite current collector is further hot-rolled, the conductivity of the electrode can be further improved, the battery resistance can be reduced, and the rate performance and cycle life of the battery can be further improved.

[0169] Comparing Examples 1 and 4, it can be seen that when the composite foam copper layer of the negative electrode adopts a pore size gradient distribution, a multi-level porous structure that is conducive to the embedding of active materials can be formed. The large pore size of the outer layer is conducive to the rapid transport of electrolyte and lithium ions, while the small pore size of the inner layer can suppress the volume expansion of active materials and maintain the stability of the electrode structure. This can ensure that the negative electrode has a high specific surface area and high conductivity, while also taking into account good mechanical strength and structural stability, further improving the rate performance and cycle life of the battery.

[0170] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0171] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A low-expansion, low-resistance lithium-ion battery negative electrode, characterized in that, The present invention includes a composite current collector and a negative electrode active material coating coated on at least one surface of the composite current collector; the negative electrode active material coating includes a negative electrode active material, graphene, and lithium polyacrylate; the mass ratio of the negative electrode active material, the graphene, and the lithium polyacrylate is (90~99):(0.5~5):(0.5~5), the negative electrode active material includes a silicon-carbon material and a coating material layer disposed on the outer surface of the silicon-carbon material, the coating material layer includes a polypyrrole layer and single-walled carbon nanotubes uniformly distributed in the polypyrrole layer; The composite current collector includes a copper foil layer and a composite foamed copper layer disposed on the copper foil layer. The composite foamed copper layer includes a first foamed copper, a second foamed copper, and a third foamed copper arranged sequentially from the inside out. The densities ρ1 of the first foamed copper, ρ2 of the second foamed copper, and ρ3 of the third foamed copper satisfy the relationship: ρ1 > ρ2 > ρ3. The porosities φ1 of the first foamed copper, φ2 of the second foamed copper, and φ3 of the third foamed copper satisfy the relationship: φ1 < φ2 < φ3. The thicknesses h1 of the first foamed copper, h2 of the second foamed copper, and h3 of the third foamed copper satisfy the relationship: h1 < h2 < h3.

2. The low-expansion, low-resistance lithium-ion battery negative electrode according to claim 1, characterized in that: The silicon-carbon material is a mixture of silicon-based particles and graphite particles in a mass ratio of 1:(0.5~1); the particle size of the silicon-based particles is 10~50nm.

3. The low-expansion, low-resistance lithium-ion battery negative electrode according to claim 1, characterized in that: The particle size of the negative electrode active material particles is 0.2~2μm.

4. The low-expansion, low-resistance lithium-ion battery negative electrode according to claim 1, characterized in that: A multifunctional regulating coating is also provided between the composite current collector and the negative electrode active material coating. The multifunctional regulating coating includes barium titanate, conductive carbon black and lithium polyacrylate, and the mass ratio of barium titanate, conductive carbon black and lithium polyacrylate is 1:1:(0.5~1).

5. The low-expansion, low-resistance lithium-ion battery negative electrode according to claim 4, characterized in that: The thickness of the multifunctional control coating is 0.1~1μm.

6. The low-expansion, low-resistance lithium-ion battery negative electrode according to claim 1, characterized in that, The preparation method of the negative electrode active material is as follows: (1) Mix graphite particles with distilled water at a mass ratio of 1:(10~20) and stir for 1~3 hours until the mixture is homogeneous to form a first mixture; then add silicon-based particles to the first mixture and stir for 1~3 hours until the graphite particles and silicon-based particles are homogeneous to form a second mixture. (2) Pyrrole monomer and single-walled carbon nanotubes are mixed at a mass ratio of 1:(0.05~0.1) and dissolved in hydrogen chloride solution and stirred evenly to form a third mixture, wherein the mass ratio of pyrrole monomer to hydrogen chloride solution is 1:(1~5). Then the second mixture is added to the third mixture and stirred for 0.5~2h to make the second mixture and the third mixture evenly mixed to form a fourth mixture. (3) Dissolve ammonium persulfate in hydrogen chloride solution to form a fifth mixture, wherein the mass ratio of ammonium persulfate to hydrogen chloride solution is 1:(1~5). Add the fourth mixture to the fifth mixture, stir evenly and react for 2~4 hours. After filtration, washing and drying, the negative electrode active material is obtained.

7. The low-expansion, low-resistance lithium-ion battery negative electrode according to claim 1, characterized in that: The density ρ1 of the first copper foam is 0.5-0.9 g / cm³, the density ρ2 of the second copper foam is 0.3-0.5 g / cm³, and the density ρ3 of the third copper foam is 0.1-0.3 g / cm³; the porosity φ1 of the first copper foam is 50-70%, the porosity φ2 of the second copper foam is 70-85%, and the porosity φ3 of the third copper foam is 85-95%; the thickness h1 of the first copper foam is 0.5-1 μm, the thickness h2 of the second copper foam is 1-2 μm, and the thickness h3 of the third copper foam is 2-4 μm; the thickness h4 of the copper foil layer is 0.5-1 μm; and the total thickness of the composite current collector is 4-8 μm.

8. The low-expansion, low-resistance lithium-ion battery negative electrode according to claim 1 or 7, characterized in that: The pore diameters d1 of the first foamed copper, d2 of the second foamed copper, and d3 of the third foamed copper satisfy the following relationship: d1 < d2 < d3; the pore diameter d1 of the first foamed copper is 1~5 μm, the pore diameter d2 of the second foamed copper is 5~10 μm, and the pore diameter d3 of the third foamed copper is 10~15 μm.

9. A method for preparing a low-expansion, low-resistance lithium-ion battery negative electrode according to any one of claims 1 to 8, characterized in that, Includes the following steps: 1) Prepare the first foamed copper layer slurry, the second foamed copper layer slurry, and the third foamed copper layer slurry respectively; a. The first foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:(15~25):1, adding anhydrous ethanol, and adjusting the solid content to 30-50 wt%; wherein the particle size of the copper powder is 0.1~0.4 μm, and the particle size of the urea is 0.2~0.5 μm; b. The second foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:(25~35):1, adding anhydrous ethanol, and adjusting the solid content to 15-30wt%; wherein the particle size of the copper powder is 0.2~0.8μm, and the particle size of the urea is 0.4~1μm. c. The third foamed copper layer slurry is made by mixing copper powder, urea and polyvinylpyrrolidone in a mass ratio of 100:(45~55):1, adding anhydrous ethanol, and adjusting the solid content to 5-15wt%; wherein the particle size of the copper powder is 0.5~1.5μm, and the particle size of the urea is 0.8~1.8μm. 2) Preheat the copper foil substrate, and then coat the first foamed copper layer paste, the second foamed copper layer paste and the third foamed copper layer paste onto the copper foil substrate in sequence. During the coating process, spray a small amount of anhydrous ethanol onto the surface of each layer of paste. 3) Place the coated copper foil into a vacuum sintering furnace and heat it to 650-750℃ at a heating rate of 2-5℃ / min under argon or nitrogen protection. Hold it at this temperature for 0.5-1h to sinter the copper powder into a gradient porous structure. At the same time, urea decomposes to produce gas and form a gradient foam copper layer. 4) After cooling, the composite current collector is obtained. The composite current collector is ultrasonically cleaned in anhydrous ethanol for 3-5 minutes to remove residual urea decomposition products and impurities on the surface, and then dried. 5) Mix the negative electrode active material, graphene and lithium polyacrylate in deionized water at a mass ratio of (90~99):(0.5~5):(0.5~5) and stir evenly to obtain the negative electrode active material coating slurry. 6) Coat at least one surface of the composite current collector with the negative electrode active material coating slurry, and dry it to obtain the lithium-ion battery negative electrode.

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