Porous composite negative electrode foil, preparation method thereof and sodium ion battery

Abstract

The invention discloses a porous composite negative electrode foil, a preparation method thereof and a sodium ion battery, and belongs to the technical field of sodium ion batteries. The foil comprises an organic base film and metal conducting layers on the two sides of the organic base film, the metal conducting layers are aluminum or aluminum alloy layers of a three-dimensional communicated porous structure, and nano tin or antimony metal layers are arranged on the surfaces of the metal conducting layers in a magnetron sputtering mode. The preparation method comprises the following steps: performing plasma activation on the organic base film; cleaning the alloy foil; performing hot rolling compounding; selectively corroding and forming pores; cleaning and drying; and depositing a nano metal layer through magnetron sputtering. The invention also provides a sodium ion battery directly using the foil as a negative electrode. When the composite foil is used as a negative electrode, the porous structure of the composite foil can buffer volume expansion and promote ion transmission, the nano metal layer can guide uniform deposition of sodium to inhibit dendritic crystals, and the middle organic base film can improve thermal safety. The energy density, the safety, the cycle life and the rate capability of the prepared battery are synergistically improved.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a porous composite negative electrode foil and its preparation method, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries (SIBs) have promising applications in large-scale energy storage due to their abundant sodium resources, excellent low-temperature performance, and environmental friendliness. However, their energy density is generally lower than that of lithium-ion batteries, resulting in high overall operating costs and a significant decline in market acceptance, thus limiting their overall competitiveness. To improve the energy density and reduce the cost of sodium-ion batteries, the industry has proposed a "no negative electrode" or "few negative electrodes" design concept. This involves using little or no traditional negative electrode active material (such as hard carbon) on the negative electrode side, and using aluminum foil as the negative electrode current collector with sodium metal directly deposited on its surface.

[0003] However, this design faces serious challenges: First, because this type of "no negative electrode" or "few negative electrodes" sodium-ion battery eliminates the negative electrode active material that allows sodium ions to be inserted and removed, the deposition of sodium ions on the flat aluminum foil surface is uneven due to the limited nucleation sites and high overpotential, leading to the formation of sodium dendrites. These dendrites can pierce the separator, causing a short circuit and serious safety issues. They can also break down during cycling, forming "dead sodium" and causing rapid capacity decay. Second, the significant volume change during sodium metal deposition / removal generates enormous mechanical stress at the current collector interface, leading to electrode structure damage and contact failure, resulting in an extremely short battery cycle life. Summary of the Invention

[0004] The purpose of this invention is to overcome the technical defects of existing conventional liquid sodium-ion batteries, such as low energy density, significant sodium dendrite growth, volume expansion effect, poor safety, and short cycle life in sodium-ion batteries without a negative electrode. This invention provides a porous composite negative electrode foil that can guide uniform sodium deposition, buffer volume stress, and possess high safety characteristics. This invention also provides a method for preparing the above-mentioned porous composite negative electrode foil, which is process-controllable and suitable for continuous production. Furthermore, this invention provides a sodium-ion battery using the above-mentioned porous composite negative electrode foil as the negative electrode, which exhibits high energy density, excellent safety performance, good rate performance, and good cycle performance.

[0005] The objective of this invention is mainly achieved through the following technical solutions: A porous composite negative electrode foil includes an organic base film and a metal conductive layer disposed on both main surfaces of the organic base film; wherein: The organic base film is made of polyester, polypropylene, or polyimide; The conductive metal layer is a porous aluminum metal layer or a porous aluminum alloy layer, and its surface is provided with a nano-tin metal layer or a nano-antimony metal layer generated by magnetron sputtering.

[0006] This invention provides a porous composite negative electrode foil with a "sandwich" structure. The middle layer is an organic base film (polyester PET, polypropylene PP, or polyimide PI), and the two sides are specially treated porous metal conductive layers. The middle organic base film can melt during abnormal battery temperature rise, acting as a "fuse" to physically isolate thermal runaway, thereby improving safety. The two metal conductive layers are made of porous aluminum or aluminum alloy, with a layer of nano-tin or antimony metal modified on the surface by magnetron sputtering. The porous structure provides a large specific surface area and three-dimensionally interconnected channels, which is beneficial for electrolyte wetting and rapid sodium ion transport, improving rate performance. The surface nano-tin / antimony layer is a "sodium-loving" material that can undergo an alloying reaction with sodium, significantly reducing the sodium deposition overpotential and guiding sodium ions to uniformly nucleate and deposit within the porous framework, rather than forming dendrites on the surface, fundamentally improving safety and cycle stability. The tin / antimony layer is formed by magnetron sputtering, resulting in strong adhesion and precisely controllable thickness, ensuring the durability of the functional layer and preventing clogging of the porous structure.

[0007] The porous structure in the porous aluminum metal layer or porous aluminum alloy layer of the present invention is formed through selective corrosion. The porous structure is distributed throughout the surface of the conductive metal layer and extends into its interior, forming a three-dimensionally interconnected porous framework. The present invention utilizes the difference in corrosion potential of different metal elements (such as aluminum and zinc / magnesium) in the electrolyte to selectively remove the more reactive components, leaving the aluminum framework to form pores. This logic results in uniform pore distribution, internal interconnection, and a simple and controllable process.

[0008] Furthermore, the organic base film has a thickness of 4-6 µm; the metal conductive layer has a thickness of 10-25 µm. The thickness of the nano-tin metal layer or nano-antimony metal layer is 5-20 nm.

[0009] This invention limits the thickness of the organic base film to 4-6 µm, balancing sufficient mechanical strength with a sensitive thermal failure response to ensure effective triggering of the safety mechanism. It avoids the following problems caused by excessively thick organic base films: high heat capacity, slow thermal response, potential failure to melt in time, and gaps left after shrinkage that may still trigger an electric arc; it also avoids the following problems caused by excessively thin organic base films: low mechanical strength, susceptibility to breakage under battery assembly and cyclic stress, leading to internal short circuits. This invention limits the thickness of the nano-tin or nano-antimony metal layer to 5-20 nm. A thickness ≥ 5 nm ensures the formation of a continuous, defect-free functional film; a thickness ≤ 20 nm prevents excessively thick coatings from clogging the porous structure, ensuring unobstructed ion transport channels and controlling the weight of inactive materials. This balances interface modification and structural preservation.

[0010] A method for preparing the above-mentioned porous composite negative electrode foil includes the following steps: Step S1: Perform plasma treatment on the organic base film to activate its surface and introduce active groups; Step S2: Provide aluminum alloy foil and clean and dry it; Step S3: Aluminum alloy foil is thermally laminated onto both sides of the organic base film treated in step S1 to form a composite foil. Step S4: Immerse the composite foil in the etching solution to selectively etch the aluminum alloy foil on both sides to form a porous composite foil with porous areas on the surface. Step S5: Clean and dry the porous composite foil; Step S6: Deposit nano-tin metal or nano-antimony metal on both sides of the porous composite foil using magnetron sputtering to obtain a porous composite negative electrode foil.

[0011] In step S2 of this invention, the aluminum alloy foil is cleaned and dried using the following method: the aluminum alloy foil is immersed in a sodium hydroxide solution to remove the uneven oxide layer and oil stains on its surface, followed by ultrasonic cleaning and drying. In step S3, the peel strength between the organic base film and the aluminum alloy foil after thermal lamination is the relative adhesion between the organic base film and the alloy foil. In step S3, the thermal lamination is performed using a hot roller pressing method, which is completed by a heated roller using electric heating, steam heating, or heat transfer oil heating. This invention thermally laminates the aluminum alloy foil with the organic base film, ensuring the alloy foil adheres tightly to both sides of the organic base film. In step S5, the porous composite foil is cleaned and dried using the following method: ultrasonic cleaning with deionized water removes residual corrosion liquid and corrosion products from the surface of the porous composite foil, followed by vacuum drying.

[0012] Magnetron sputtering is a physical vapor deposition (PVD) technique. Its core principle involves using a vacuum chamber to generate a high-density plasma (formed by argon ionization) through the combined action of an electric and magnetic field. High-energy argon ions, accelerated by the electric field, bombard a target material made of pure tin or pure antimony, sputtering its atoms. These atoms travel at high speed to a porous composite foil substrate and deposit on its surface and the inner walls of the pores, forming a dense, uniform thin film.

[0013] This invention involves depositing tin or antimony nanocoatings on a three-dimensional porous aluminum framework formed through selective etching using magnetron sputtering. Both tin and antimony can undergo reversible alloying reactions with sodium at relatively low potentials (forming Na+-like nanocoatings). 15(Sn4, Na3Sb, etc.). This essentially provides a large number of pre-existing, thermodynamically favorable nucleation sites for sodium ion deposition. The energy barrier (overpotential) required for sodium ion deposition at these alloy sites is much lower than on a smooth aluminum surface. This guides sodium to nucleate uniformly, rather than randomly and dendritively, which is key to suppressing sodium dendrites.

[0014] This invention achieves the integration of multiple functions by depositing nano-tin or nano-antimony metal: Structural synergy: The porous structure provides macroscopic space to alleviate the huge volume expansion of sodium deposition; while the nano-alloy coating on its surface guides the uniform deposition of sodium at the microscale. The combination of the two suppresses dendrites at both the macro and micro levels, ensuring safety and long-term cycling.

[0015] Synergistic Functions: The porous framework ensures ion transport kinetics (high-rate performance), while the nano-alloy coating optimizes the thermodynamics of sodium deposition (uniform nucleation), together enhancing electrochemical performance.

[0016] Process synergy: Magnetron sputtering, as a dry vacuum process, is perfectly integrated with the aforementioned wet etching and cleaning steps, avoiding the introduction of new contamination or stress.

[0017] Furthermore, in step S1, the plasma treatment is performed under an argon atmosphere, introducing hydroxyl and / or carboxyl active groups onto the surface of the treated organic base film. This invention employs argon plasma treatment because it can produce a pure physical bombardment and activation effect.

[0018] Furthermore, the aluminum alloy foil used in step S2 is either an aluminum-zinc alloy foil or an aluminum-magnesium alloy foil, wherein the mass percentage of aluminum is 65%-90%, and the mass percentage of zinc or magnesium is 10%-35%. In step S4 of this invention, the composite foil is immersed in an etching solution to remove zinc or magnesium from the alloy on both sides of the composite foil. This invention limits the zinc / magnesium content in the aluminum alloy to 10%-35% to ensure sufficient "sacrificial phase" (magnesium or zinc) for etching. If the content is too low, the pores formed after etching will be sparse and discontinuous, failing to construct a three-dimensionally interconnected porous framework, significantly reducing the efficiency of electrolyte wetting and sodium ion transport. Excessive magnesium or zinc content will form a large amount of brittle intermetallic compounds (such as Al3Mg2), making the foil itself too brittle and prone to cracking during rolling and subsequent processing. Simultaneously, excessive etching may loosen or even collapse the porous layer framework, losing the mechanical support and conductive network required for current collector operation. Aluminum as the main component ensures the foil has excellent electronic conductivity, a fundamental requirement for current collector operation. Aluminum content is crucial for maintaining low resistance and ensuring battery rate performance. Magnesium and zinc, as key additives, do not completely disappear after selective etching. Some zinc and magnesium atoms remain on or near the surface of the aluminum framework. These residual, highly reactive atoms, in synergy with the subsequently sputtered nano-tin / antimony layers, together form a "sodium-loving" interface that induces uniform sodium deposition, which is the core of suppressing dendrite formation.

[0019] This invention utilizes the differences in electrochemical corrosion susceptibility among the components in aluminum-magnesium alloys or aluminum-zinc alloys to construct porous structures. Magnesium (standard electrode potential approximately -2.37 V) or zinc (approximately -0.76 V) has a more negative potential than aluminum (approximately -1.66 V), and preferentially undergoes oxidation and dissolution as the anode in the corrosive solution, while aluminum is relatively protected as the cathode.

[0020] Furthermore, in step S3, the heating temperature for thermal lamination is 80-160℃, and the peel strength between the organic base film and the aluminum alloy foil after thermal lamination is not less than 1300 g / inch. The heating temperature for thermal lamination in this invention is limited to 80-160℃, which ensures that the organic base film is appropriately softened to promote adhesion while avoiding melting and deformation. This invention requires the composite foil to have a peel strength of not less than 1300 g / inch, a critical performance threshold that has been rigorously designed and verified. Its purpose is to ensure that the composite foil can withstand the processing stresses of subsequent wet etching, cleaning, and vacuum sputtering, avoiding delamination and ensuring manufacturing yield. It provides a stable mechanical and electrical interface for sodium-ion batteries during long-term cycling. High bonding force effectively suppresses interface degradation caused by volume changes, ensuring efficient electron conduction in the current collector, which is a prerequisite for obtaining high-rate performance and long cycle life. This strength index is the basis for leveraging the active safety function (heat-induced melting) of the "organic base film-metal foil" sandwich structure. Only when the interface is absolutely reliable under normal conditions can controllable circuit breaker isolation be achieved under abnormal conditions.

[0021] In specific implementation of the present invention, the etching solution used in step S4 can be an acidic etching solution, an alkaline etching solution, or a neutral etching solution. Preferably, the etching solution used in step S4 is a hydrochloric acid solution with a concentration of 0.1-1 mol / L, a sodium hydroxide solution with a concentration of 0.1-1 mol / L, or a sodium chloride solution with a concentration of 3-6 wt%. The etching temperature is 20-55℃, and the etching time is 0.5-6 h, so as to form porous areas on both sides of the composite foil.

[0022] This invention allows for precise control of the rate and depth of this electrochemical reaction by selecting acidic, alkaline, or neutral etching solutions and controlling their concentration, temperature, and processing time. The goal is to selectively remove magnesium or zinc from both surfaces of the composite foil, leaving behind a three-dimensional, interconnected nano / micro porous framework composed of aluminum. This structure not only inherits the excellent conductivity of aluminum, but its large specific surface area and abundant pores further facilitate electrolyte wetting and sodium ion transport, and provide a stable supporting substrate for the subsequent sputtering of nano-tin / antimony coatings.

[0023] In specific implementation, the present invention regulates the corrosion effect by controlling the concentration and temperature (20-55℃) of the etching solution and the contact time (0.5-6h) between the composite foil and the etching solution. By utilizing the difference in corrosion rate of different components in the aluminum alloy, magnesium or zinc elements are removed from both sides of the composite foil to form a porous composite foil.

[0024] Furthermore, the aluminum alloy foil preparation method used in step S2 is as follows: aluminum alloy is melted at 700-800℃ under an argon atmosphere, cast, and cooled to obtain an ingot; the ingot is rolled into aluminum alloy foil; and the aluminum alloy foil is annealed at 300-400℃. The alloy foil preparation process in this invention is a systematic design serving the final porous composite negative electrode function. Melting at 700-800℃ under argon protection ensures precise and uniform distribution of magnesium / zinc active elements, which is the material basis for subsequent controllable and uniform selective corrosion. Annealing at 300-400℃ not only eliminates rolling stress but also obtains a soft and uniform microstructure through recrystallization. This microstructure greatly optimizes the thermal composite interface bonding with the organic base film and ensures the isotropic reaction between the corrosion solution and the alloy, thereby obtaining an ideal porous structure with uniform pore size distribution and a robust conductive framework.

[0025] In summary, this customized process chain ensures full controllability from alloy composition to microstructure and macro thickness, providing a reliable guarantee for the preparation of high-performance porous composite anode foil.

[0026] Furthermore, the aluminum alloy foil used in step S2 has a thickness of 10-25µm, and the porous region on each side of the porous composite foil in step S4 has a thickness of 5-15µm; wherein the thickness of the porous region on each side of the porous composite foil is not greater than the thickness of the aluminum alloy foil on that side.

[0027] In this invention, the thickness of the metal foil (10-25µm) and the thickness of the porous region formed by selective etching (5-15µm per side) are two key parameters designed in a synergistic manner. They jointly define the microstructure of the final "conductive metal layer," which includes, but is not limited to, the following two scenarios, both of which are within the scope of this invention: 1. The formation of a "porous aluminum metal layer": When an aluminum alloy foil with a thickness near the lower limit (e.g., 10-15µm) is selected, and relatively sufficient corrosion is applied (e.g., the corrosion depth on each side is close to or reaches the thickness of the aluminum alloy foil on that side), the corroded elements (such as zinc or magnesium) in the aluminum alloy foil are substantially or completely removed throughout the thickness direction. At this point, the original "aluminum alloy layer" is transformed into a three-dimensional porous framework composed of pure or near-pure aluminum, i.e., the "porous aluminum metal layer." This structure achieves maximum weight reduction and provides a fully interconnected porous network.

[0028] 2. Situations where a "porous aluminum alloy layer" is formed: When using aluminum alloy foil with a relatively large thickness (e.g., 20-25µm), or when controlling the corrosion depth to act only on the surface area, the corrosion process will form a porous aluminum surface layer with a thickness of 5-15µm on both sides of the alloy foil, while the central part of the foil remains an uncorroded, dense aluminum alloy core layer. In this case, the conductive metal layer exhibits a "sandwich structure": porous aluminum layers with high specific surface area on both sides, and a dense aluminum alloy core with high strength and good conductivity in the middle. This belongs to the aforementioned "porous aluminum alloy layer." This structure, while maintaining excellent surface activity (for sodium deposition), possesses higher mechanical strength and conductive integrity.

[0029] The above parameter range provides flexibility and adjustability for this invention. By matching different initial foil thicknesses and etching processes, the porosity, mechanical strength, weight, and active surface area of ​​the final conductive metal layer can be precisely controlled, thereby adapting to the specific requirements of different battery models for current collector performance. Whether forming an integral "porous aluminum metal layer" or a composite "porous aluminum alloy layer," the core purpose and effect are the same: (a) the porous surface structure provides an ideal interface for electrolyte wetting, rapid sodium ion transport, and accommodating sodium deposition volume expansion; (b) the nano-tin / antimony layer deposited on the porous surface by magnetron sputtering can effectively guide uniform sodium deposition. The dense aluminum alloy core in the middle (if present) serves as an additional reinforcing skeleton, further enhancing the overall mechanical and electrical stability of the foil.

[0030] A sodium-ion battery discloses a negative electrode made of the aforementioned porous composite negative electrode foil, or a porous composite negative electrode foil prepared using the aforementioned method. This invention directly uses the porous composite negative electrode foil as the battery's negative electrode without adding any other negative electrode active materials. The sodium-ion battery of this invention directly uses the aforementioned porous composite negative electrode foil as the negative electrode, which integrates multiple functions such as a current collector, active sodium deposition space, dendrite suppression layer, and safety protection layer. When used directly as the negative electrode, no additional negative electrode active material needs to be coated, thereby significantly reducing the weight and volume of the negative electrode and significantly improving the battery's energy density. Simultaneously, its built-in porous structure and sodium-loving coating ensure the uniformity and structural stability of sodium deposition, while the intermediate organic base film provides thermal safety redundancy, enabling the battery to achieve high energy density while also possessing high safety and long cycle life.

[0031] In summary, the present invention has the following advantages compared with the prior art: (1) The porous composite negative electrode foil of the present invention achieves the composite of organic base film and aluminum alloy foil through thermal bonding. Utilizing the difference in corrosion rate of different components in the aluminum alloy, a porous structure is formed, and finally, a layer of nano-tin or antimony metal is sputtered onto its surface. This porous composite negative electrode foil exhibits excellent performance and can be directly used as the negative electrode of sodium-ion batteries. The prepared sodium-ion batteries show significantly improved energy density, good safety performance, and good electrochemical performance in terms of rate capability and cycle life.

[0032] (2) The porous composite negative electrode foil provided by the present invention adopts a sandwich-like structure, with an organic base film in the middle layer. When the battery is subjected to mechanical impact or thermal failure, the organic base film will shrink or even melt, thus blocking the thermal runaway reaction of the battery in time and improving the battery safety.

[0033] (3) This invention solves the problem of difficult composite of aluminum alloy and organic base film by thermal bonding. At the same time, it utilizes the difference in corrosion rate of different components in aluminum alloy to complete the construction of porous structure on the surface of composite foil. The resulting porous structure has the characteristics of uniform and adjustable pores and internal interconnection. Moreover, the synthesis method is simple and conducive to continuous production.

[0034] (4) The porous negative electrode composite foil provided by the present invention has micropores distributed on both sides of the foil surface. On the one hand, it is conducive to the immersion of electrolyte, improves sodium ion transport efficiency, and enhances the rate performance of sodium ion battery. In addition, the deposited nano-tin or antimony metal is conducive to the uniform deposition of sodium ions and provides effective sites for the deposition of sodium ions on the negative electrode. Its porous structure effectively alleviates the volume expansion of sodium metal during cycling and improves the cycle stability of sodium ion battery. Attached Figure Description

[0035] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings: Figure 1 A flowchart of a specific embodiment of the present invention; Figure 2 This is a schematic diagram of thermal bonding in a specific embodiment of the present invention; Figure 3 This is a cross-sectional schematic diagram of a porous composite negative electrode foil in a specific embodiment of the present invention.

[0036] The names corresponding to the reference numerals in the attached figures are: 1. Organic base film, 2. Aluminum-magnesium alloy foil, 3. Porous region. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0038] The following examples and comparative examples use the same positive electrode, electrolyte, and separator. The positive electrode material is a mixture of layered ternary nickel-iron-manganese oxide (NiFeMn 111), conductive agent Super P, and binder polyvinylidene fluoride (PVDF) in a mass ratio of 92:4:4. The electrolyte is a commercially available Kunlun 2016 model sodium-ion battery electrolyte. The separator is a polypropylene (PP) separator (base film thickness 14 μm).

[0039] Example 1: This embodiment provides a porous composite negative electrode foil and its preparation method, as well as a sodium-ion battery. The preparation method is as follows: Figure 1 As shown, it includes: Step S1, Organic Base Film Pretreatment: A 6µm thick polyester PET film was selected as the organic base film. It was placed in the plasma treatment chamber, evacuated, and then purged with argon gas. The film was treated for 2 minutes at a pressure of 50 Pa and a radio frequency power of 100 W. After treatment, it was removed, allowing active groups such as hydroxyl and carboxyl groups to be introduced onto its surface.

[0040] Step S2, Alloy Foil Pretreatment: Provide an aluminum-magnesium alloy foil with a thickness of 10µm, wherein the mass ratio of magnesium to aluminum in the aluminum-magnesium alloy foil is 7:3 (Mg content 30 wt%). Immerse it in a 0.2 mol / L sodium hydroxide solution for 2 minutes to remove the surface oxide layer and oil stains, then ultrasonically clean it with deionized water for 5 minutes, and dry it at 80°C under vacuum for 2 hours.

[0041] Step S3, Hot Roller Lamination: Using a hot roller lamination device, two pieces of processed aluminum-magnesium alloy foil are respectively laminated onto the two main surfaces of the PET base film. The temperature of the heating roller is 80℃. After rolling, a composite foil is obtained, and its peel strength is tested to be greater than 1300 g / inch. A schematic diagram of the hot lamination process in this embodiment is shown below. Figure 2 As shown, it includes four sets of extrusion rollers arranged in sequence. Each set of extrusion rollers includes two pressure rollers, one upper and one lower. After the organic base film passes through the first set of extrusion rollers, the second set of extrusion rollers thermally laminates an aluminum-magnesium alloy foil onto its upper surface. Then, the third set of extrusion rollers thermally laminates an aluminum-magnesium alloy foil onto its lower surface. Finally, the fourth set of extrusion rollers further presses the two aluminum-magnesium alloy foils of the organic base film together. Figure 2 The corresponding names of the symbols in the attached figures are: 1. Organic base film, 2. Aluminum-magnesium alloy foil.

[0042] Step S4, Selective Etching for Pore Creation: Prepare a 1 mol / L hydrochloric acid solution as the etching solution. Immerse the composite foil in the etching solution at 25°C for 1 hour to remove magnesium from the surface areas on both sides of the composite foil, forming a porous composite foil. Scanning electron microscopy (SEM) was used to observe the sample cross-section, and the thickness of the formed porous aluminum alloy layer was measured to be approximately 8 µm.

[0043] Step S5, post-cleaning and drying: The etched porous composite foil is ultrasonically cleaned with deionized water for 10 minutes to remove residual etching solution and products, and then dried in a vacuum oven at 80℃.

[0044] Step S6, Magnetron Sputtering Functionalization: The cleaned and dried porous composite foil is placed in a magnetron sputtering apparatus. Using a pure tin target (purity ≥ 99.99%), sputtering deposition is performed under an argon atmosphere to form a uniform nano-tin layer with a thickness of approximately 10 nm on the surface of the porous layer, ultimately obtaining the porous composite negative electrode foil. A cross-sectional schematic diagram of the porous composite negative electrode foil obtained in this embodiment is shown below. Figure 3 As shown, where, Figure 3 The corresponding names of the reference numerals in the attached figures are: 1. Organic base film, 2. Aluminum-magnesium alloy foil, 3. Porous region.

[0045] Step S7, Battery Assembly: Using the porous composite negative electrode foil prepared above as the negative electrode, it is assembled together with the prepared positive electrode, separator and electrolyte to form a soft-pack sodium-ion battery with a theoretical capacity of 5 Ah, denoted as battery E1.

[0046] Example 2: The difference between this embodiment and Embodiment 1 is that: In step S1, a polyimide (PI) film with a thickness of 4µm is selected as the organic base film.

[0047] In step S2, the aluminum-magnesium alloy foil has a thickness of 15µm and a magnesium content of 20 wt%.

[0048] In step S3, the temperature of the heating roller for hot rolling is set to 150°C.

[0049] In step S4, the etching time is 2 hours to form a deeper porous layer.

[0050] The final assembled battery is designated as E2.

[0051] Example 3: The difference between this embodiment and Embodiment 1 is that: In step S2, aluminum-zinc alloy foil (Zn content 20 wt%) with a thickness of 10µm is selected.

[0052] In step S4, the etching solution is changed to a 0.5 mol / L sodium hydroxide solution, the etching temperature is 30℃, and the etching time is 1 hour.

[0053] In step S6, a pure antimony target (purity ≥99.99%) is used for magnetron sputtering to deposit a nano-antimony layer with a thickness of about 10 nm.

[0054] The final assembled battery is designated as E3.

[0055] Comparative Example 1: This comparative example uses a conventional liquid sodium-ion battery, and the specific steps include: Step S1: Select hard carbon material as the negative electrode active material for sodium-ion batteries. Weigh hard carbon, conductive agent Super P, binder styrene-butadiene rubber SBR and dispersant carboxymethyl cellulose CMC, and mix them at high speed in a mass ratio of 92.5:2.5:3.5:1.5. Then coat the mixture onto conventional aluminum foil and prepare hard carbon negative electrode sheets through drying, rolling, slitting and cutting processes. Step S2: Using the hard carbon negative electrode sheet prepared in S1, and combined with the prepared positive electrode, negative electrode and electrolyte, assemble a soft-pack battery with a theoretical capacity of 5Ah, denoted as battery C1.

[0056] Comparative Example 2: This comparative example is a sodium-ion battery using ordinary aluminum foil as the negative electrode. The specific steps include: Step S1: Select ordinary commercial aluminum foil as the negative electrode of sodium-ion battery, with a thickness of 12µm; Step S2: Using the ordinary commercial aluminum foil prepared in step S1 as the negative electrode, and combining it with the prepared positive electrode, negative electrode, and electrolyte, assemble a soft-pack battery with a theoretical capacity of 5Ah, denoted as battery C2.

[0057] Performance testing and data analysis The performance of the batteries E1, E2, E3, C1, and C2 prepared above was tested, and the main results are summarized in Table 1 below: Table 1. Performance test results of batteries prepared in Examples 1, 2, 3, Comparative Example 1, and Comparative Example 2. Data comparison and analysis: 1. Energy Density: The energy density of the batteries (E1-E3) of this invention (164-173 Wh / kg) is significantly higher than that of the traditional hard carbon anode battery C1 (128 Wh / kg). This directly verifies that the "anode-free" design achieved by using the porous composite anode foil of this invention can effectively reduce battery weight by removing the traditional anode active material layer, thereby achieving the core objective of increasing energy density.

[0058] 2. Cycle performance: The batteries of this invention (E1-E3) maintained a capacity retention of over 90% after 500 cycles, significantly higher than battery C2 (33.2%) which uses ordinary aluminum foil directly. This fully demonstrates the effective buffering effect of the "three-dimensional porous aluminum skeleton" on the volume expansion of sodium deposition / removal, and the key contribution of the "nano-tin / antimony sodium-affinity layer" to inducing uniform sodium deposition and suppressing dendrites, thus achieving an ultra-long cycle life.

[0059] 3. Rate Performance: Under 3C high-rate discharge, the capacity retention rate of the battery of this invention (E1-E3) exceeds 88%, while that of the comparative battery C2 is only 41.8%. This strongly demonstrates that the three-dimensional interconnected porous structure constructed in this invention greatly optimizes the electrolyte wetting and sodium ion transport path, enabling the battery to have excellent fast charge and discharge capabilities.

[0060] 4. Overall Advantages: Although the cycle and rate performance of traditional C1 batteries are slightly better, their energy density is a significant weakness. This invention successfully improves energy density (compared to C1) while completely solving the fatal flaw of the original negative electrode-less scheme (C2) in terms of extremely poor cycle and rate performance. It achieves synergistic optimization and balance of multiple key performance aspects, resulting in unexpected technical effects.

[0061] In summary, the porous composite negative electrode foil and its battery provided by this invention, through the innovative combination of materials, structure and process, offer a promising new solution for developing sodium-ion batteries with high energy density, high safety, long life and high power.

[0062] The porous composite negative electrode foil provided by this invention, through an integrated design of "materials-structure-process", synergistically solves the core contradiction in anode-free sodium-ion batteries where energy density, safety, cycle life, and rate performance are difficult to balance simultaneously. In terms of energy density, by eliminating the traditional negative electrode active material layer and using an ultra-thin composite current collector, direct "weight reduction and efficiency improvement" is achieved; its three-dimensional porous structure provides high loading space for sodium metal, laying the foundation for high capacity.

[0063] In terms of safety, a dual protection mechanism was constructed, consisting of "active defense" (thermal melting of organic base film) and "inherent defense" (porous structure synergistic sodium-loving coating to suppress dendrites).

[0064] In terms of cyclic performance, the combination of the "spatial buffering" effect of the porous skeleton and the "interface stabilization" effect of the nano-alloy coating effectively resists cyclic stress and ensures the long-term integrity of the structure.

[0065] In terms of rate performance, the porous interconnected structure optimizes ion transport, the complete aluminum skeleton ensures electron conduction, and the sodium-loving coating reduces the reaction barrier. Together, these three factors achieve excellent "fast charge and fast discharge" capabilities.

[0066] In particular, the comparative data of Examples 1-3 and Comparative Examples 1-2 (Table 1) fully demonstrate the aforementioned synergistic effect: compared with conventional hard carbon anode batteries (Comparative Example 1), the battery of the present invention achieves a significant improvement in energy density (e.g., 173 vs. 128 Wh / kg); compared with ordinary aluminum foil anode batteries (Comparative Example 2), the present invention achieves a qualitative leap in cycle retention (94.8% vs. 33.2%) and rate performance (91.1% vs. 41.8%). This proves that the technical effect of the present invention is far from a simple summation of known features, but rather produces unexpected synergistic progress.

[0067] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A porous composite negative electrode foil, characterized in that, It includes an organic base film and a metal conductive layer disposed on both sides of the main surface of the organic base film; wherein: The organic base film is made of polyester, polypropylene, or polyimide; The conductive metal layer is a porous aluminum metal layer or a porous aluminum alloy layer, and its surface is provided with a nano-tin metal layer or a nano-antimony metal layer generated by magnetron sputtering.

2. The porous composite negative electrode foil according to claim 1, characterized in that, The organic base film has a thickness of 4-6 µm; the metal conductive layer has a thickness of 10-25 µm. The thickness of the nano-tin metal layer or nano-antimony metal layer is 5-20 nm.

3. A method for preparing a porous composite negative electrode foil as described in claim 1 or 2, characterized in that, Includes the following steps: Step S1: Perform plasma treatment on the organic base film to activate its surface and introduce active groups; Step S2: Provide aluminum alloy foil and clean and dry it; Step S3: Aluminum alloy foil is thermally laminated onto both sides of the organic base film treated in step S1 to form a composite foil. Step S4: Immerse the composite foil in the etching solution to selectively etch the aluminum alloy foil on both sides to form a porous composite foil with porous areas on the surface. Step S5: Clean and dry the porous composite foil; Step S6: Deposit nano-tin metal or nano-antimony metal on both sides of the porous composite foil using magnetron sputtering to obtain a porous composite negative electrode foil.

4. The method for preparing a porous composite negative electrode foil according to claim 3, characterized in that, In step S1, the plasma treatment is carried out under an argon atmosphere, and hydroxyl and / or carboxyl active groups are introduced onto the surface of the treated organic base film.

5. The method for preparing a porous composite negative electrode foil according to claim 3, characterized in that, The aluminum alloy foil used in step S2 is aluminum-zinc alloy foil or aluminum-magnesium alloy foil, wherein the mass percentage of aluminum is 65%-90% and the mass percentage of zinc or magnesium is 10%-35%.

6. The method for preparing a porous composite negative electrode foil according to claim 3, characterized in that, The method for preparing aluminum alloy foil in step S2 is as follows: aluminum alloy is melted at 700-800℃ under an argon atmosphere, cast and cooled to obtain an ingot; the ingot is rolled into aluminum alloy foil; and the aluminum alloy foil is annealed at 300-400℃.

7. The method for preparing a porous composite negative electrode foil according to claim 3, characterized in that, The heating temperature for thermal lamination in step S3 is 80-160℃, and the peel strength between the organic base film and the aluminum alloy foil after thermal lamination is not less than 1300 g / inch.

8. The method for preparing a porous composite negative electrode foil according to claim 3, characterized in that, The etching solution used in step S4 is a hydrochloric acid solution with a concentration of 0.1-1 mol / L, a sodium hydroxide solution with a concentration of 0.1-1 mol / L, or a sodium chloride solution with a concentration of 3-6 wt%; the etching temperature is 20-55℃, and the etching time is 0.5-6 h, so as to form porous areas on both sides of the composite foil.

9. A method for preparing a porous composite negative electrode foil according to any one of claims 3 to 8, characterized in that, The aluminum alloy foil used in step S2 has a thickness of 10-25µm, and the porous region on each side of the porous composite foil in step S4 has a thickness of 5-15µm; wherein the thickness of the porous region on each side of the porous composite foil is not greater than the thickness of the aluminum alloy foil on that side.

10. A sodium-ion battery, characterized in that, The negative electrode is made of the porous composite negative electrode foil as described in claim 1 or 2, or a porous composite negative electrode foil prepared by any one of the preparation methods of porous composite negative electrode foil as described in any one of claims 3 to 9.

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