Composite negative electrode current collector and preparation method and application thereof

Abstract

The invention provides a composite negative electrode current collector and a preparation method and application thereof, and belongs to the technical field of lithium battery current collectors. The MXene layer is arranged on the adjacent layer on the surface of the copper foil, so that the effects of blocking, lyophilic and interface protection are achieved, and more excellent interface bonding force and electrochemical stability are achieved. The graphene layer is used as a middle layer, a continuous two-dimensional conductive network can be formed, stable Van der Waals and chemical bond combination can be formed between the MXene layer and the graphene layer, and the thickness-direction conductivity and the mechanical stability are improved. The graphite layer is used as the outermost layer and the micron skeleton layer of the negative electrode current collector, so that the effects of improving the conductivity and adjusting the thickness and the surface porosity of the composite negative electrode current collector are achieved, and the finally obtained composite negative electrode current collector has a multi-layer gradient conductive structure; the electrochemical stability, the ion diffusivity and the interface bonding force of the negative electrode current collector material can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery current collector, in particular to a composite negative electrode current collector and a preparation method and application thereof. BACKGROUND

[0002] The negative electrode current collector is a key component inside the lithium metal battery, which does not participate in the electrochemical reaction, but acts as a physical support for active materials (such as graphite, silicon, etc.) and a conductive channel for electrons. The negative electrode current collector plays a role in electron conduction and collection, mechanical support and current distribution regulation in the lithium metal battery, and has an important influence on the electrochemical performance of the lithium metal battery.

[0003] Patent CN202210551296.0 discloses a layered coated negative electrode sheet structure, the surface of the current collector is sequentially coated with a conductive coating, an artificial graphite coating and a natural graphite coating. The scheme improves the adhesion of the active material to the current collector and the overall conductivity by layered coating. However, the conductive layer usually uses traditional carbon black or conductive graphite film, and the interface bonding force with the graphite layer is poor, the peeling strength is not good, and the electrical conductivity also needs to be further improved.

[0004] Patent CN202010502079.3 discloses an electrode sheet containing a graphite material layer, a silicon-based material layer and a conductive layer, which mainly improves the first efficiency of the negative active layer through the structure setting of active carbon material, conductive agent and binder layer. However, it has the defect of insufficient electrochemical stability. SUMMARY

[0005] Therefore, the purpose of the present application is to provide a composite negative electrode current collector and a preparation method and application thereof. The composite negative electrode current collector provided by the present application has a MXene-graphene-graphite three-dimensional conductive gradient structure, which can improve the electrochemical stability, ion diffusion capacity and interface bonding force of the negative electrode current collector material.

[0006] In order to achieve the above-mentioned purpose of the application, the present application provides the following technical scheme: The present application provides a composite negative electrode current collector, comprising a copper foil, and a MXene layer, a graphene layer and a graphite layer sequentially located on the surface of the copper foil. The thickness of the MXene layer is 0.2-0.35 μm, the thickness of the graphene layer is 0.3-0.6 μm, and the thickness of the graphite layer is 0.9-1.2 μm.

[0007] Preferably, the surface roughness Ra of the copper foil is 0.2-0.6 μm, and the thickness of the copper foil is 5-20 μm; the surface resistance of the composite negative electrode current collector is ≤15 Ω·sq -1 , and the pore volume is 0.05-0.20 cm 3·g -1 , the average pore size is 50-500 nm.

[0008] Preferably, the MXene layer and the graphene layer do not contain a binder; the graphite layer further comprises a conductive agent, and a carboxymethyl cellulose / styrene-butadiene rubber composite binder; Preferably, the graphite layer comprises, in terms of mass percentage: graphite 94-97.5%; conductive agent 0.5-2%; carboxymethyl cellulose 1-2%; styrene-butadiene rubber 1-3%.

[0009] The application provides a preparation method of the composite negative electrode current collector described above, comprising the following steps: Spray the MXene aqueous dispersion on the surface of the copper foil for the first time to obtain a MXene layer on the surface of the copper foil; Spray the reduced graphene oxide aqueous dispersion on the surface of the MXene layer for the second time to obtain a graphene layer on the surface of the MXene layer; Coat the graphite aqueous system slurry on the surface of the graphene layer, and perform roll forming to obtain a graphite layer on the surface of the graphene layer; Perform heat treatment on the copper foil containing the MXene layer, the graphene layer and the graphite layer to obtain a composite negative electrode current collector.

[0010] Preferably, the mass concentration of the MXene aqueous dispersion is 1-10 mg·mL -1 ; The temperature of the substrate during the first spraying is 40-60℃, the nozzle diameter during the first spraying is 0.2-0.5 mm, the atomization pressure is 0.1-0.3 MPa, the nozzle-substrate distance is 50-150 mm, and the walking speed is 0.1-0.5 m·s -1 .

[0011] Preferably, the mass concentration of the reduced graphene oxide aqueous dispersion is 0.2-3 mg·mL -1 ; The temperature of the substrate during the second spraying is 40-60℃, the nozzle diameter during the second spraying is 0.2-0.5 mm, the atomization pressure is 0.1-0.3 MPa, the spraying track interval is 2-5 mm, and the forward and backward interval is 5-15 mm.

[0012] Preferably, the solid content of the graphite aqueous system slurry is 48-55 wt%, and the viscosity at 25℃ is 3000-5000 mPa·s; The coating of the graphite aqueous system slurry is gravure coating, the line number of the gravure coating is 150-300 lpi, and the mesh volume is 2-8 cm 3 ·m -2The scraper angle is 35~55°, the scraper pressure is 0.2~0.6 MPa, and the conveyor speed is 0.5~3 m·min. -1 ; The rolling temperature is 25~100℃, and the linear pressure is 50~200 N·cm. -1 ; The heat treatment is performed at a temperature of 80~120℃ for a time of 0.5~2 h.

[0013] This invention provides the application of the above-mentioned composite negative electrode current collector in a negative electrode-free lithium metal battery.

[0014] The present invention provides a negative electrode-free lithium metal battery, comprising a positive electrode, a negative electrode current collector, a separator, and an electrolyte; wherein the negative electrode current collector is the aforementioned composite negative electrode current collector.

[0015] This invention provides a composite negative electrode current collector, comprising a copper foil, and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil. The thickness of the MXene layer is 0.2~0.35 μm, the thickness of the graphene layer is 0.3~0.6 μm, and the thickness of the graphite layer is 0.9~1.2 μm. This invention uses the MXene layer as an enrichment layer, placing it adjacent to the copper foil surface to provide barrier, hydrophilic, and interfacial protection functions, replacing traditional carbon black layers and achieving superior interfacial bonding and electrochemical stability. This invention uses the graphene layer as an intermediate layer, forming a continuous two-dimensional conductive network (i.e., a mesh bridging layer), and the MXene layer and graphene layer can form stable van der Waals and chemical bonds, improving thickness conductivity and mechanical stability. This invention uses the graphite layer as the outermost layer, serving as a micron-sized framework layer for the negative electrode current collector, enhancing the conductivity of the composite negative electrode current collector and adjusting its thickness and surface porosity. The composite current collector comprises an outermost graphite layer, an intermediate graphene layer, and an inner MXene layer, forming a three-dimensional conductive gradient structure from the inside out. The inner MXene layer adheres closely to the copper foil, providing high lithium affinity and interfacial protection, inhibiting electrolyte corrosion of the copper foil, and reducing interfacial contact resistance. The intermediate graphene layer forms a continuous two-dimensional conductive network, acting as a "bridge" between MXene sheets and between MXene and graphite layers, significantly improving thickness-to-thickness electron conduction. The outer graphite layer serves as a micron-scale framework and carrier; its controllable pore structure provides buffer space for lithium deposition and stripping, and also facilitates electrolyte wetting and ion transport. The synergistic effect of these three elements ensures high conductivity while also exhibiting excellent interfacial bonding, anti-pulverization capability, and polarization suppression at high rates. This invention, by setting a three-layer structure of MXene-graphene-graphite with a clear thickness gradient and functional partitioning, achieves synergistic optimization of electron and ion channels. This overcomes the shortcomings of existing negative electrode current collectors, such as poor adhesion of the conductive layer, insufficient electrochemical stability, and limited ion diffusion. The resulting composite negative electrode current collector is suitable for negative electrode-less lithium metal batteries and is suitable for 3-5C high-rate charge-discharge cells. Example results show that the 180° peel strength of the composite negative electrode current collector of this invention is ≥1.0 N·cm. -1 When used in negative electrode-free lithium metal batteries, the initial assembly coulombic efficiency is ≥90%, and at a rate of ≥3C, the impedance is reduced by ≥30% compared to unmodified copper foil.

[0016] Furthermore, this invention utilizes the synergistic effect of MXene and graphene, which has high chemical bonding strength, to significantly reduce the amount of traditional binders used, lower interfacial impedance, and increase energy density.

[0017] This invention provides a method for preparing the above-mentioned composite negative electrode current collector. The present invention uses a spraying method to prepare the MXene layer and the graphene layer, which can precisely control the interlayer spacing of the layered materials MXene and graphene; and uses a coating-rolling method to prepare the graphite layer, which can control the porosity of the graphite layer. The resulting composite negative electrode current collector has the advantages of high interfacial bonding force and strong anti-powdering properties.

[0018] Furthermore, this invention achieves precise control over the thickness and pore structure of each functional layer by controlling the concentration or viscosity of each slurry during the preparation of the composite current collector. When the concentration or viscosity is too low, the coating is prone to local undercoating and discontinuity of the conductive network, leading to increased sheet resistance and decreased peel strength. When the concentration or viscosity is too high, excessive interlayer stacking and severe densification occur, significantly reducing pore volume and average pore size, limiting ion diffusion, and deteriorating rate performance and cycle stability. By controlling the slurry concentration and viscosity within the range defined by this invention, an optimal balance can be achieved between conductivity, adhesion, and ion transport capability, thereby obtaining a composite current collector with excellent comprehensive electrochemical performance. Attached Figure Description

[0019] Figure 1 This is a microstructure diagram of the composite negative electrode current collector obtained in Example 1; Figure 2 Electrochemical impedance spectroscopy of lithium metal batteries assembled with composite negative electrode current collectors in Examples 1-8; Figure 3 The peel strength of the composite negative electrode current collectors in Examples 1-8; Figure 4 The capacity retention rate of lithium metal batteries assembled with composite negative electrode current collectors in Examples 1-8; Figure 5 The initial DC internal resistance and 25-turn DC internal resistance of the lithium metal battery assembled with composite negative electrode current collectors in Examples 1-8 are shown. Detailed Implementation

[0020] The present invention provides a composite negative electrode current collector, comprising a copper foil, and an MXene layer, a graphene layer and a graphite layer sequentially located on the surface of the copper foil.

[0021] In this invention, the thickness of the copper foil is preferably 5~20 μm, more preferably 8~15 μm, and even more preferably 10 μm; the surface roughness of the copper foil is preferably 0.2~0.6 μm, more preferably 0.3~0.5 μm.

[0022] In this invention, the thickness of the MXene layer is 0.2~0.35 μm, preferably 0.25~0.30 μm; the composition of the MXene layer is few-layer MXene, the sheet diameter of the few-layer MXene is preferably 0.5~2 μm, more preferably 0.8~1.5 μm, and the number of layers is preferably 1~5. In this invention, the MXene layer preferably does not contain a binder to ensure that the conductivity and interfacial lithiophilicity of the MXene layer are not affected by the organic phase.

[0023] In this invention, the thickness of the graphene layer is 0.3~0.6 μm, preferably 0.35~0.55 μm, more preferably 0.40~0.50 μm; the raw material of the graphene layer is preferably reduced graphene oxide (rGO), and its sheet diameter is preferably 2~10 μm, more preferably 3~8 μm; the thickness is preferably 1~3 nm. In this invention, the graphene layer preferably does not contain a binder to further improve the continuity of the current-carrying channel and the interfacial electron transport efficiency.

[0024] In this invention, the thickness of the graphite layer is 0.9~1.2 μm, preferably 1.0~1.1 μm. In this invention, the graphite layer has a porous structure, and the pore volume of the graphite layer is preferably 0.20~0.45 cm³. 3 ·g -1 More preferably, it is 0.25~0.40 cm. 3 ·g -1 The pore size is preferably 20-80 nm, more preferably 30-60 nm. In this invention, the graphite layer preferably also includes a conductive agent and a carboxymethyl cellulose (CMC) / styrene-butadiene rubber (SBR) composite binder, wherein the conductive agent is preferably acetylene black (SP). In this invention, by mass percentage, the composition of the graphite layer preferably includes 94-97.5% graphite, more preferably 95-96%; 0.5-2% conductive agent, more preferably 1-1.5%; 1-2% carboxymethyl cellulose, more preferably 1.5%; and 1-3% styrene-butadiene rubber, more preferably 1.5-2.5%. In this invention, the CMC / SBR composite binder system helps the graphite layer obtain sufficient interlayer adhesion and anti-powdering properties.

[0025] In this invention, the total thickness of the MXene-graphene-graphite three-dimensional conductive gradient structure is preferably ≤2.0 μm. In this invention, the surface resistivity (four-probe method) of the composite negative electrode current collector is preferably ≤15 Ω·sq. -1 The preferred pore volume is 0.05~0.20 cm³. 3 ·g -1 More preferably, it is 0.1~0.15 cm. 3 ·g -1The average pore size is preferably 50~500 nm, more preferably 100~300 nm. In this invention, the 180° peel strength of the composite negative electrode current collector is ≥1.0 N·cm. -1 In this invention, the contact angle of the composite negative electrode current collector in an ether electrolyte (such as DME, DEE, or THF) is ≤ 30°.

[0026] This invention provides a method for preparing the aforementioned composite negative electrode current collector, comprising the following steps: The MXene aqueous dispersion is first sprayed onto the surface of the copper foil to obtain an MXene layer on the surface of the copper foil; A second aqueous dispersion of reduced graphene oxide is sprayed onto the surface of the MXene layer to obtain a graphene layer on the surface of the MXene layer. A graphite-based ink slurry is coated onto the surface of the graphene layer and then rolled to form a graphite layer on the surface of the graphene layer. A composite negative electrode current collector was obtained by heat-treating copper foil containing MXene, graphene and graphite layers.

[0027] Unless otherwise specified, all raw materials used in this invention are commercially available.

[0028] In this invention, an MXene aqueous dispersion is first sprayed onto the surface of a copper foil to obtain an MXene layer on the copper foil surface.

[0029] Before spraying, the copper foil is preferably pretreated, and the pretreatment preferably includes cleaning and / or plasma treatment. Through this pretreatment, the surface roughness of the copper foil is reduced to 0.2~0.6 μm, more preferably 0.3~0.5 μm.

[0030] In this invention, the preferred mass concentration of the MXene aqueous dispersion is 1~10 mg·mL. -1 More preferably 2~6 mg·mL -1 More preferably 3~5 mg·mL -1 This invention, by controlling the concentration of the MXene aqueous dispersion, can obtain a uniform and dense MXene coating with a certain degree of porosity, thereby achieving optimal capacity retention and coulombic efficiency in electrodeless lithium metal batteries. When the MXene concentration is too low, the MXene sheets do not cover the copper foil surface sufficiently, easily forming local exposed areas, leading to increased current collector surface resistance and uneven interfacial current distribution, resulting in a significant decrease in capacity retention and coulombic efficiency. When the concentration is too high, the MXene sheet stacking intensifies, causing blockage of interlayer channels, and a significant increase in interfacial polarization and DC internal resistance.

[0031] In this invention, the method for preparing the MXene aqueous dispersion includes the following steps: The MAX phase was mixed with an etchant, etched, and then washed with water to obtain layered Ti3C2T. x Precursor; For the layered Ti3C2T x The precursor was subjected to ultrasonic exfoliation and water dispersion to obtain an MXene aqueous dispersion.

[0032] In this invention, the MAX phase is Ti3AlC2, with a preferred particle size of 5-20 μm, more preferably 10-15 μm. In this invention, the etchant is preferably a LiF / HCl composite etching system, wherein the concentration of LiF in the composite etching system is preferably 5-10 mol / L, more preferably 6-8 mol / L; the concentration of LiF is preferably 6-12 mol / L, more preferably 8-10 mol / L. In this invention, the etching reaction is preferably carried out under stirring conditions, the etching reaction temperature is preferably 20-45℃, more preferably 30-40℃, and the time is preferably 12-48 h, more preferably 24-36 h. In this invention, the water washing is preferably carried out under centrifugation conditions, and the water used for washing is preferably deionized water. In this invention, the water washing is preferably performed until the pH value is 6-7.

[0033] In this invention, the ultrasonic ablation is preferably performed under N2 protection, and the ultrasonic ablation time is preferably 30-90 min, more preferably 40-60 min.

[0034] The present invention preferably performs the first spraying by electric or pneumatic means. In this invention, the temperature of the substrate (copper foil) during the first spraying is preferably 40~60℃, more preferably 50℃; the diameter of the nozzle for the first spraying is preferably 0.2~0.5 mm, more preferably 0.3~0.4 mm; the atomization pressure is preferably 0.1~0.3 MPa, more preferably 0.2 MPa; the nozzle-substrate distance is preferably 50~150 mm, more preferably 80~120 mm; and the travel speed is preferably 0.1~0.5 m·s. -1 More preferably, it is 0.2~0.4 m·s -1 After the first spraying, the present invention preferably performs drying, and the drying temperature is preferably 60~100℃, more preferably 70~80℃.

[0035] In this invention, a reduced graphene oxide aqueous dispersion is second-sprayed onto the surface of the MXene layer to obtain a graphene layer on the surface of the MXene layer. In this invention, the mass concentration of the reduced graphene oxide aqueous dispersion is preferably 0.2~3 mg·mL. -1 More preferably 0.5~2 mg·mL -1 More preferably 1~1.5 mg·mL -1This invention achieves an optimal balance between conductive network connectivity and pore structure by controlling the concentration of the reduced graphene oxide aqueous dispersion. When the concentration of the reduced graphene oxide aqueous dispersion is too low, the two-dimensional conductive network formed by the graphene layer is discontinuous, and the electron conduction path in the thickness direction is interrupted, leading to an increase in the DC internal resistance of the electrode and a decrease in rate performance. When the concentration is too high, the reduced graphene oxide sheets severely agglomerate, the interlayer pores collapse, and the electrolyte is difficult to fully wet, resulting in a decrease in capacity retention and coulombic efficiency. In this invention, the preparation method of the reduced graphene oxide aqueous dispersion preferably includes the following steps: A water dispersion of graphene oxide was mixed with a reducing agent to carry out a reduction reaction, and then washed with water to obtain a reduced water dispersion of graphene oxide.

[0036] This invention does not impose any special requirements on the preparation method of the graphene oxide; any graphene oxide preparation method well-known in the art can be used. In this invention, the preferred mass concentration of the graphene oxide aqueous dispersion is 1-10 mg / mL. -1 More preferably 3~6 mg·mL -1 The reducing agent is preferably sodium borohydride (NaBH4), and the amount of the reducing agent added to the graphene oxide aqueous dispersion is preferably 0.1~0.5 g·L. -1 More preferably, it is 0.2~0.4 g·L. -1 In this invention, the temperature of the reduction reaction is preferably 60-95°C, more preferably 70-80°C, and the time is preferably 0.5-6 h, more preferably 1-4 h. In this invention, the water washing is preferably deionized water washing, and preferably washing until the pH value is 6-7.

[0037] The second spraying is preferably performed electrically or pneumatically. In this invention, the temperature of the substrate (copper foil containing the MXene layer) during the second spraying is preferably 40-60°C, more preferably 50°C; the nozzle diameter during the second spraying is preferably 0.2-0.5 mm, more preferably 0.3-0.4 mm; the atomization pressure is preferably 0.1-0.3 MPa, more preferably 0.2 MPa; the spray trajectory spacing is preferably 2-5 mm, more preferably 3-4 mm; and the round-trip interval is preferably 5-15 mm, more preferably 8-12 mm. After the second drying, the invention preferably performs further drying at a temperature of 60-100°C, more preferably 70-80°C.

[0038] In this invention, a graphite-based ink slurry is coated onto the surface of a graphene layer and then rolled to obtain a graphite layer on the surface of the graphene layer. In this invention, the graphite is preferably artificial graphite, and the conductive agent is preferably acetylene black. In this invention, the solid phase composition of the graphite-based ink slurry, by weight percentage, preferably includes: graphite 94-97.5%, more preferably 95-96%; conductive agent 0.5-2%, more preferably 1-1.5%; carboxymethyl cellulose 1-2%, more preferably 1.5%; and styrene-butadiene rubber 1-3%, more preferably 1.5-2.5%. In this invention, the styrene-butadiene rubber is preferably added in the form of an emulsion, and the solid content of the styrene-butadiene rubber emulsion is preferably 30-60 wt%, more preferably 40-50 wt%.

[0039] In this invention, the solid content of the graphite-based slurry is preferably 48-55 wt%, more preferably 50-52 wt%, and the viscosity at 25°C is preferably 3000-5000 mPa·s, more preferably 4000 mPa·s. When the solid content and viscosity are too low, the slurry has excessive fluidity during coating, making graphite particles prone to migration and delamination. After drying, the coating thickness is insufficient and the porosity is too high, leading to increased sheet resistance and initial DC internal resistance. When the solid content and viscosity are too high, the coating is too dense, the pore volume is significantly reduced, the lithium-ion diffusion path is restricted, and polarization is intensified and capacity decay occurs during cycling. By controlling the solid content and viscosity within the above ranges, a graphite layer with balanced pore volume and average pore size can be obtained, thereby balancing the conductivity and ion transport capability of the current collector.

[0040] In this invention, the method for preparing the graphite-based slurry preferably includes the following steps: Water and carboxymethyl cellulose are mixed and swollen to obtain a transparent mother liquor; Graphite and a conductive agent are added to the transparent mother liquor and dispersed at high speed. Then, styrene-butadiene rubber latex is added and sheared at low speed. After degassing, a graphite-based slurry is obtained.

[0041] In this invention, the swelling is preferably room temperature swelling or heated swelling, the room temperature swelling time is preferably 2~12 h, more preferably 6~10 h; the heated swelling temperature is preferably 40~60℃, more preferably 50℃, and the time is preferably 0.5~2 h, more preferably 1~1.5 h.

[0042] In this invention, the high-speed dispersion rate is preferably 1000~3000 r·min. -1 More preferably 2000 r·min -1 The preferred time is 30-90 min, more preferably 50-70 min. In this invention, the rate of low-speed shearing is preferably 300-800 r·min. -1More preferably, 400~600 r·min -1 In this invention, the degassing is preferably performed under vacuum, and the degassing time is preferably 15 minutes.

[0043] In this invention, the graphite ink slurry is preferably coated by gravure printing, and the line count of the gravure printing is preferably 150-300 lpi, more preferably 200-250 lpi; the cell volume is preferably 2-8 cm³. 3 ·m -2 More preferably 3~5cm 3 ·m -2 The scraper angle is preferably 35~55°, more preferably 40~50°; the scraper pressure is preferably 0.2~0.6 MPa, more preferably 0.3~0.5 MPa; and the conveyor speed is preferably 0.5~3 m·min. -1 More preferably 1~2 m·min -1 After coating, the present invention preferably performs drying, which is preferably segmented drying, specifically drying at 60°C for 2-4 min, drying at 80°C for 2-4 min, and drying at 100°C for 2-4 min.

[0044] In this invention, the rolling temperature is preferably 25~100℃, more preferably 50~80℃, and the linear pressure is preferably 50~200 N·cm. -1 More preferably 100~150 N·cm -1 .

[0045] This invention controls the thickness and porosity of the graphite layer by controlling the coating and rolling parameters. This allows for the optimization of the pore volume and average pore size of the graphite layer while ensuring sufficient mechanical strength and interfacial bonding, thereby further reducing the current collector surface resistance and improving the battery's capacity retention and high-rate performance.

[0046] This invention involves heat-treating copper foil containing MXene, graphene, and graphite layers to obtain a composite negative electrode current collector. In this invention, the heat treatment temperature is preferably 80-120°C, more preferably 90-100°C, and the time is preferably 0.5-2 hours, more preferably 1-1.5 hours. Through this heat treatment, this invention achieves stress relief and optimized interlayer re-bonding.

[0047] This invention provides the application of the aforementioned composite negative electrode current collector in a negative electrode-free lithium metal battery. In this invention, the rate capability of the negative electrode-free lithium metal battery is preferably 3-5C.

[0048] The present invention provides a negative electrode-free lithium metal battery, comprising a positive electrode, a negative electrode current collector, a separator, and an electrolyte; wherein the negative electrode current collector is the aforementioned composite negative electrode current collector.

[0049] This invention does not have any special requirements on the specific types of the positive electrode, the separator, and the electrolyte. As a specific embodiment of this invention, the positive electrode material used is NCM ternary material, the separator is a 10μm PP / PE / PP composite separator, and the electrolyte is 6M DME / LIFSI / TFEO.

[0050] The following detailed description, in conjunction with embodiments, illustrates the composite negative electrode current collector, its preparation method, and its applications provided by the present invention. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0051] In various embodiments of the present invention, the thickness of the MXene layer and the graphene layer is mainly achieved by adjusting the number of spray passes and the spraying speed: under the condition that the dispersion concentration, nozzle diameter, atomization pressure, and nozzle-substrate distance are consistent, increasing the number of spray passes or decreasing the spraying speed can increase the deposition per unit area, thereby obtaining a larger layer thickness; conversely, a thinner coating is obtained. Therefore, within the concentration range, the different layer thicknesses listed in Table 1 can be obtained by fine-tuning the process parameters.

[0052] Example 1 The MXene-graphene-graphite three-dimensional conductive gradient structure composite negative electrode current collector includes a copper foil and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil; the thickness of the MXene layer is 0.3 μm, the thickness of the graphene layer is 0.5 μm, the thickness of the graphite layer is 1.1 μm, and the total thickness of the three layers is 1.90 μm.

[0053] The preparation method of the composite negative electrode current collector includes the following steps: (1) Preparation of MXene aqueous dispersion Take 1.00 g of Ti3AlC2 (MAX phase, particle size 5~20 μm) as a precursor and prepare 20 mL of composite etching solution (containing 1.00 g LiF and 9 mol·L⁻¹). -1 Ti3AlC2 powder was added to the etching solution, and the etching reaction was carried out under stirring at 35°C for 24 h. After the etching reaction, the supernatant was washed with deionized water until the pH of the supernatant was 7, and centrifuged at 4500 r / min to remove residual impurities. The resulting MXene aqueous dispersion was transferred to a N2 atmosphere and ultrasonically exfoliated for 60 min to ensure the stability of the MXene few-layer structure. The final volume was adjusted to obtain 4.5 mg·mL⁻¹. -1 MXene aqueous dispersion.

[0054] (2) Preparation of rGO aqueous dispersion Take 50 mL of solution with a concentration of 2 mg / mL -10.3 g·L⁻¹ sodium borohydride (NaBH₄) was added to the GO aqueous dispersion. -1 The GO was reduced by stirring continuously in an 85°C water bath for 2 hours. After reduction, the sample was washed with deionized water, centrifuged to pH 7 to remove residual reducing agent, and finally diluted to a final volume of 1.0 mg / mL. -1 rGO aqueous dispersion.

[0055] (3) Preparation of graphite layer aqueous slurry 1.5 g of CMC was added to deionized water and allowed to swell at room temperature for 6 h to obtain a transparent mother liquor. 95.0 g of artificial graphite and 1.0 g of conductive agent Super P were added to the mother liquor, and the mixture was dispersed at 2000 r / min for 60 min to ensure thorough and uniform dispersion of the solids. Then, 2.5 g of SBR aqueous emulsion with a solid content of 50 wt% was slowly added, and the mixture was stirred for another 20 min under low shear conditions to homogenize the slurry. The slurry was then degassed under vacuum for 15 min until no obvious bubbles remained. Finally, the total solid content of the slurry was adjusted to 50 wt%, and the viscosity was adjusted to 4000 mPa·s to obtain an aqueous graphite-layered slurry.

[0056] (4) Copper foil pretreatment A 10 μm thick copper foil was immersed in a beaker containing a 5 wt% dilute sulfuric acid solution and ultrasonically cleaned for 5 min at room temperature to remove the surface anti-oxidation layer and oil. It was then rinsed thoroughly with deionized water and dried with nitrogen. 800 mL of deionized water and 150 mL of concentrated sulfuric acid (98%) were added sequentially to a plastic bath. After thorough stirring, 50 mL of hydrogen peroxide (30%) was slowly added under continuous mechanical stirring, maintaining the bath temperature at 30°C. The cleaned copper foil was then completely immersed in the micro-etching solution for 60 ± 10 s. During the treatment, the solution was moderately stirred to ensure uniform etching. The micro-etched copper foil was quickly removed and immediately immersed in a bath containing a large amount of deionized water, rinsing three times at room temperature for 1 min each time to completely terminate the reaction and remove any residual chemicals. The copper foil was then immersed in a 5 wt% sodium bicarbonate aqueous solution at room temperature for 30 s to neutralize. Finally, it was rinsed again with deionized water and dried with nitrogen. The surface roughness of the copper foil was randomly measured at 3 to 5 different locations using a white light interferometer to ensure that the surface roughness Ra was 0.3 to 0.5 μm, in order to improve the adhesion of the coating.

[0057] (5) Preparation of composite negative electrode current collector ①MXene spraying: Load MXene aqueous dispersion into the spray gun, set the nozzle diameter to 0.3 mm, spray pressure to 0.20 MPa, spray distance to 100 mm, and travel speed to 0.25 m·s. -1After spraying, the coating is dried in stages at 60 / 80 / 100℃ (2 / 3 / 2 min) to ensure uniform coating of the MXene layer, with a final thickness of 0.30 μm.

[0058] ②rGO spraying: Load the rGO aqueous dispersion into the spray gun, set the nozzle diameter to 0.3 mm, the atomization pressure to 0.2 MPa, the spray trajectory spacing to 3 mm, the round trip interval to 10 mm, spray 3 times to achieve a thickness of 0.50 μm, and the substrate temperature to 50℃ during spraying.

[0059] ③ Graphite Coating: A gravure roller coating process is used to coat the copper foil surface with an aqueous graphite slurry. The gravure roller has a mesh size of 220 lpi and a cell volume of 4.5 cm². 3 ·m -2 The scraper pressure is 0.4 MPa, the scraper angle is 45°, and the conveyor speed is 1.5 m·min. -1 The coated copper foil was dried in stages at 60 / 80 / 100℃ (3 / 3 / 3 min) to ensure that the graphite layer thickness was 1.10 μm.

[0060] ④ Rolling process: The copper foil containing MXene, graphene, and graphite layers is compacted using a hot-rolling process. The rolling temperature is 100℃ and the linear pressure is 120 N·cm. -1 To ensure the stability of the coating, check the coating thickness after each coating and rolling operation to ensure that the final total thickness of the three layers is 1.90 μm.

[0061] ⑤ Heat treatment: The composite copper foil is subjected to heat setting / stress relief treatment at 120℃ for 2 hours to eliminate stress and stabilize the coating structure.

[0062] Example 2 The MXene-graphene-graphite three-dimensional conductive gradient structure composite negative electrode current collector includes a copper foil and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil; the thickness of the MXene layer is 0.25 μm, the thickness of the graphene layer is 0.5 μm, the thickness of the graphite layer is 1.15 μm, and the total thickness of the three layers is 1.90 μm.

[0063] When preparing the composite negative electrode current collector, steps (1) to (4) are the same as in Example 1. In step (5), during the preparation of the composite negative electrode current collector, the parameters of MXene spraying, rGO spraying, graphite coating, and rolling process are finely adjusted based on Example 1 so that the MXene layer, graphene layer, and graphite layer meet the requirements.

[0064] Example 3 The MXene-graphene-graphite three-dimensional conductive gradient structure composite negative electrode current collector includes a copper foil and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil; the thickness of the MXene layer is 0.2 μm, the thickness of the graphene layer is 0.6 μm, the thickness of the graphite layer is 1.1 μm, and the total thickness of the three layers is 1.90 μm.

[0065] When preparing the composite negative electrode current collector, steps (1) to (4) are the same as in Example 1. In step (5), during the preparation of the composite negative electrode current collector, the parameters of MXene spraying, rGO spraying, graphite coating, and rolling process are finely adjusted based on Example 1 so that the MXene layer, graphene layer, and graphite layer meet the requirements.

[0066] Example 4 The MXene-graphene-graphite three-dimensional conductive gradient structure composite negative electrode current collector includes a copper foil and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil; the thickness of the MXene layer is 0.35 μm, the thickness of the graphene layer is 0.45 μm, the thickness of the graphite layer is 1.1 μm, and the total thickness of the three layers is 1.90 μm.

[0067] When preparing the composite negative electrode current collector, steps (1) to (4) are the same as in Example 1. In step (5), during the preparation of the composite negative electrode current collector, the parameters of MXene spraying, rGO spraying, graphite coating, and rolling process are finely adjusted based on Example 1 so that the MXene layer, graphene layer, and graphite layer meet the requirements.

[0068] Example 5 The MXene-graphene-graphite three-dimensional conductive gradient structure composite negative electrode current collector includes a copper foil and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil; the thickness of the MXene layer is 0.25 μm, the thickness of the graphene layer is 0.45 μm, the thickness of the graphite layer is 1.2 μm, and the total thickness of the three layers is 1.90 μm.

[0069] When preparing the composite negative electrode current collector, steps (1) to (4) are the same as in Example 1. In step (5), during the preparation of the composite negative electrode current collector, the parameters of MXene spraying, rGO spraying, graphite coating, and rolling process are finely adjusted based on Example 1 so that the MXene layer, graphene layer, and graphite layer meet the requirements.

[0070] Example 6 The MXene-graphene-graphite three-dimensional conductive gradient structure composite negative electrode current collector includes a copper foil and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil; the thickness of the MXene layer is 0.35 μm, the thickness of the graphene layer is 0.60 μm, the thickness of the graphite layer is 0.95 μm, and the total thickness of the three layers is 1.90 μm.

[0071] When preparing the composite negative electrode current collector, steps (1) to (4) are the same as in Example 1. In step (5), during the preparation of the composite negative electrode current collector, the parameters of MXene spraying, rGO spraying, graphite coating, and rolling process are finely adjusted based on Example 1 so that the MXene layer, graphene layer, and graphite layer meet the requirements.

[0072] Example 7 The MXene-graphene-graphite three-dimensional conductive gradient structure composite negative electrode current collector includes a copper foil and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil; the thickness of the MXene layer is 0.3 μm, the thickness of the graphene layer is 0.5 μm, the thickness of the graphite layer is 1.1 μm, and the total thickness of the three layers is 1.90 μm.

[0073] The preparation method of the composite negative electrode current collector is similar to that in Example 1, except that the concentration of the MXene dispersion is 1.5 mg·mL. -1 .

[0074] Example 8 The MXene-graphene-graphite three-dimensional conductive gradient structure composite negative electrode current collector includes a copper foil and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil; the thickness of the MXene layer is 0.3 μm, the thickness of the graphene layer is 0.5 μm, the thickness of the graphite layer is 1.1 μm, and the total thickness of the three layers is 1.90 μm.

[0075] The preparation method of the composite negative electrode current collector is similar to that in Example 1, except that the concentration of the MXene dispersion is 8 mg·mL. -1 .

[0076] Comparative Example 1 The difference compared to Example 1 is that the thickness of the MXene layer is 0.1 μm.

[0077] Comparative Example 2 The difference compared to Example 1 is that the thickness of the MXene layer is 0.8 μm.

[0078] Comparative Example 3 The difference compared to Example 1 is that the thickness of the graphene layer is 0.2 μm.

[0079] Comparative Example 4 The difference compared to Example 1 is that the thickness of the graphene layer is 0.8 μm.

[0080] Comparative Example 5 The difference compared to Example 1 is that the thickness of the graphite layer is 0.4 μm.

[0081] Comparative Example 6 The difference compared to Example 1 is that the thickness of the graphite layer is 1.3 μm.

[0082] Comparative Example 7 The difference from Example 1 is that the MXene layer is omitted.

[0083] Comparative Example 8 The difference from Example 1 is that the graphene layer is omitted.

[0084] Comparative Example 9 The difference from Example 1 is that the graphite layer is omitted.

[0085] Comparative Example 10 Compared with Example 1, the difference is that the arrangement of MXene-graphene-graphite is changed to graphene-MXene-graphite, that is, a composite negative electrode current collector, which includes a copper foil and a graphene layer, an MXene layer and a graphite layer sequentially located on the surface of the copper foil; the thickness of the graphene layer is 0.5 μm, the thickness of the MXene layer is 0.3 μm, the thickness of the graphite layer is 1.1 μm, and the total thickness of the three layers is 1.90 μm.

[0086] Comparative Example 11 The difference from Example 1 is that the concentration of the MXene aqueous dispersion is 0.3 mg·mL. -1 .

[0087] Comparative Example 12 The difference from Example 1 is that the concentration of the MXene aqueous dispersion is 25 mg·mL. -1 .

[0088] Comparative Example 13 The difference from Example 1 is that the concentration of the rGO aqueous dispersion is 0.3 mg·mL. -1 .

[0089] Comparative Example 14 The difference from Example 1 is that the concentration of the rGO aqueous dispersion is 25 mg·mL. -1 .

[0090] Comparative Example 15 The difference from Example 1 is that the solid content of the graphite layer aqueous slurry is 30 wt%.

[0091] Comparative Example 16 The difference from Example 1 is that the solid content of the graphite layer aqueous slurry is 65 wt%.

[0092] Structural characterization A scanning electron microscope (SEM) schematic diagram of the obtained MXene-graphene-graphite three-dimensional conductive gradient structure composite negative electrode current collector, as shown below. Figure 1 As shown. By Figure 1 As can be seen, the composite structure consists of a graphite layer, a graphene layer, and an MXene layer from the inside out, with clear interlayer interfaces and a uniform structure. The innermost graphite layer exhibits a typical granular stacking morphology, with certain pores between the particles providing channels for ion transport. The graphene layer on top of the graphite layer is composed of interwoven wrinkled sheets, forming a continuous three-dimensional conductive network, which is beneficial to improving the overall electron mobility efficiency. The outermost MXene layer is composed of multiple thin, layered sheets stacked in parallel, with a smooth and densely distributed interface, which can construct an efficient ion / electron transport interface at the interface and further improve the overall conductivity and interface stability of the current collector. The three-layer structure has a clear thickness gradient and a natural transition, presenting a uniform and continuous three-dimensional conductive gradient structure.

[0093] Performance testing (1) The peel strength of the composite negative electrode current collectors obtained in Examples 1-8 and Comparative Examples 1-16 was tested, and the results are listed in Table 1.

[0094] (2) The composite negative electrode current collectors obtained in Examples 1-8 and Comparative Examples 1-16 were assembled into a negative electrode-free lithium metal battery. The positive electrode material used was NCM ternary material, the separator was a 10μm PP / PE / PP composite separator, and the electrolyte was 6M DME / LIFSI / TFEO. The assembly method was as follows: First, the composite negative electrode current collector was cut into the specified size and its surface was subjected to conventional rolling shaping treatment to make its surface flat and its thickness uniform. Then, the composite negative electrode current collector and the separator were stacked, and the electrolyte was added to it in sequence to fully wet the three-dimensional conductive gradient structure constructed by the composite negative electrode current collector. Then, the pre-made positive electrode sheet and the separator were aligned and stacked, and thermally bonded under vacuum conditions to ensure that the stacked layers were tightly bonded. The resulting stack was placed in an aluminum-plastic film for primary encapsulation, and then an electrolyte containing lithium salt was injected. After the electrolyte injection was completed, a secondary sealing was performed in a vacuum environment to completely seal the cell. After being sealed, the soft-pack battery cell is left to stand and soak, allowing the electrolyte to fully penetrate into the electrode pores and interface area. Then, a formation process is carried out in a constant temperature environment. After formation, the corresponding negative electrode-free lithium metal soft-pack battery cell is obtained through a conventional capacity testing and matching process. This cell is then used for subsequent cycle performance testing, impedance testing, and rate performance evaluation.

[0095] The capacity retention (3C, 25 cycles), coulombic efficiency (25 cycles), initial DC internal resistance, and DC internal resistance after 25 cycles of the electrodeless lithium metal battery were tested and are listed in Table 1. The electrochemical impedance spectra of Examples 1-8 are shown below. Figure 2 As shown, the peel strength is as follows Figure 3 As shown, the capacity retention rate at different numbers of revolutions is as follows: Figure 4 As shown, the initial DC internal resistance and the 25-turn DC internal resistance are as follows: Figure 5 As shown.

[0096] Table 1 Performance test results of the examples and comparative examples

[0097] As shown in Table 1, compared with the comparative examples, the capacity retention of Examples 1-8 of this invention generally remained above 90% after 25 cycles under 3C conditions, reaching a maximum of nearly 98%, and the coulombic efficiency remained stable at approximately 98.7%~99.3%. Simultaneously, their initial DC internal resistance was significantly lower, with only a slight increase in DCIR after 25 cycles, and the peel strength was generally between 1.1 and 1.4 N / cm, significantly better than most comparative examples. This indicates that when the MXene layer thickness is controlled at 0.2~0.35 μm, the graphene layer thickness at 0.3~0.6 μm, and the graphite layer thickness at 0.9~1.2 μm, and combined with a reasonable MXene / rGO dispersion concentration and graphite slurry solid content / viscosity, the constructed MXene-graphene-graphite three-dimensional conductive gradient structure can simultaneously achieve interface protection, electronic conduction, and ion diffusion, enabling the battery to maintain low polarization and stable electrochemical performance during cycling. Conversely, when the structural or slurry parameters were specifically altered in the comparative analysis, the electrochemical performance decreased to varying degrees: thinning or thickening the MXene, graphene, or graphite layers individually could easily lead to localized exposure of the copper foil surface, layer stacking, or imbalance of the pore structure, resulting in uneven distribution of interfacial current or obstruction of ion channels, manifested as decreased capacity retention and increased DCIR; removing a functional layer or changing the arrangement of the three layers would disrupt the gradient conductivity and pore structure design from the inside out, making it impossible to simultaneously achieve good interfacial stability and transport continuity; when the concentration of MXene or graphene dispersion was too low, the coating coverage was insufficient and the conductive network was discontinuous, while when it was too high, severe agglomeration and pore blockage occurred; when the solid content of the graphite slurry was too low, the coating became loose and the surface resistivity increased, while when it was too high, the coating became too dense and the lithium-ion diffusion path became longer, both of which exacerbated polarization and accelerated capacity decay. As can be seen from the results in Table 1, the limitations on the thickness of each layer, the concentration of the dispersion, and the range of solid content / viscosity of the graphite slurry in this invention are reasonable and necessary. They can significantly reduce polarization, stabilize the electrode / current collector interface, and inhibit the shedding of active materials, thereby achieving excellent cycle stability and interface reliability.

[0098] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A composite negative electrode current collector, characterized in that, It includes a copper foil, and an MXene layer, a graphene layer, and a graphite layer sequentially located on the surface of the copper foil; The thickness of the MXene layer is 0.2~0.35 μm, the thickness of the graphene layer is 0.3~0.6 μm, and the thickness of the graphite layer is 0.9~1.2 μm.

2. The composite negative electrode current collector according to claim 1, characterized in that, The surface roughness Ra of the copper foil is 0.2~0.6 μm, and the thickness of the copper foil is 5~20 μm; The surface resistivity of the composite negative electrode current collector is ≤15 Ω·sq. -1 The pore volume is 0.05~0.20 cm³. 3 ·g -1 The average pore size is 50~500 nm.

3. The composite negative electrode current collector according to claim 1, characterized in that, The MXene layer and graphene layer do not contain a binder; the graphite layer also includes a conductive agent and a carboxymethyl cellulose / styrene-butadiene rubber composite binder.

4. The composite negative electrode current collector according to claim 3, characterized in that, The graphite layer comprises, by weight percentage: 94-97.5% graphite; 0.5-2% conductive agent; 1-2% carboxymethyl cellulose; and 1-3% styrene-butadiene rubber.

5. The method for preparing the composite negative electrode current collector according to any one of claims 1 to 4, characterized in that, Includes the following steps: The MXene aqueous dispersion is first sprayed onto the surface of the copper foil to obtain an MXene layer on the surface of the copper foil; A second aqueous dispersion of reduced graphene oxide is sprayed onto the surface of the MXene layer to obtain a graphene layer on the surface of the MXene layer. A graphite-based ink slurry is coated onto the surface of the graphene layer and then rolled to form a graphite layer on the surface of the graphene layer. A composite negative electrode current collector was obtained by heat-treating copper foil containing MXene, graphene and graphite layers.

6. The preparation method according to claim 5, characterized in that, The mass concentration of the MXene aqueous dispersion is 1~10 mg·mL. -1 ; The temperature of the substrate during the first spraying is 40~60℃, the nozzle diameter is 0.2~0.5 mm, the atomization pressure is 0.1~0.3 MPa, the nozzle-substrate distance is 50~150 mm, and the travel speed is 0.1~0.5 m·s. -1 .

7. The preparation method according to claim 5, characterized in that, The mass concentration of the reduced graphene oxide aqueous dispersion is 0.2~3 mg·mL. -1 ; The temperature of the substrate during the second spraying is 40~60℃, the nozzle diameter during the second spraying is 0.2~0.5mm, the atomization pressure is 0.1~0.3 MPa, the spray trajectory spacing is 2~5 mm, and the round-trip interval is 5~15 mm.

8. The preparation method according to claim 5, characterized in that, The solid content of the graphite-based slurry is 48~55wt%, and the viscosity at 25℃ is 3000~5000 mPa·s; The graphite-based ink slurry is coated using gravure printing, with a line count of 150-300 lpi and a cell volume of 2-8 cm³. 3 ·m -2 The scraper angle is 35~55°, the scraper pressure is 0.2~0.6 MPa, and the conveyor speed is 0.5~3 m·min. -1 ; The rolling temperature is 25~100℃, and the linear pressure is 50~200 N·cm. -1 ; The heat treatment is performed at a temperature of 80~120℃ for a time of 0.5~2 h.

9. The application of the composite negative electrode current collector according to any one of claims 1 to 4 or the composite negative electrode current collector prepared by the preparation method according to any one of claims 5 to 8 in a negative electrode-free lithium metal battery.

10. A negative electrode-free lithium metal battery, characterized in that it comprises a positive electrode, a negative electrode current collector, a separator, and an electrolyte; The negative electrode current collector is the composite negative electrode current collector according to any one of claims 1 to 4 or the composite negative electrode current collector prepared by the preparation method according to any one of claims 5 to 8.

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