A ZnF2 gradient structure carbon nanofiber current collector, its preparation method and application

Abstract

This invention discloses a ZnF2 gradient-structured carbon nanofiber current collector, comprising porous carbon nanofiber material with an interconnected network structure and ZnF2 distributed within the porous carbon nanofiber material. Along the thickness direction of the current collector, the mass ratio of ZnF2 to the porous carbon nanofiber material continuously increases, ranging from 0.05 to 1:1. This invention simultaneously achieves ZnF2 gradient distribution, directional lithium-ion transport, and in-situ LiF-rich SEI generation, solving two key problems: low internal space utilization and SEI instability in three-dimensional carbon-based current collectors. This invention uses conventional carbon sources such as polyacrylonitrile and raw materials such as zinc acetate, combined with mature electrospinning, carbonization, and plasma fluorination processes. The preparation process is simple, parameters are controllable, and it is suitable for large-scale production, showing significant application prospects in the field of high-performance lithium metal batteries.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, specifically relating to a ZnF2 gradient structure carbon nanofiber current collector, its preparation method, and its application. Background Technology

[0002] Lithium metal anodes have an extremely high theoretical specific capacity (3860 mAh g). - ¹) and its extremely low electrochemical potential (-3.04 V vs. SHE) have led to its widespread recognition as a core candidate material for building next-generation high-energy-density energy storage systems. However, its practical application is severely limited by factors such as uncontrolled lithium dendrite growth, instability of the solid-state electrolyte interphase (SEI) film, and drastic volume changes during cycling. These problems result in low coulombic efficiency, limited cycle life, and potential safety hazards, seriously hindering the commercialization of lithium metal anodes.

[0003] To address the aforementioned challenges, three-dimensional carbon-based current collectors, with their high specific surface area, porous structure, and excellent electronic conductivity, have become ideal lithium metal host materials. The three-dimensional structure can effectively buffer volume expansion and reduce local current density, thereby suppressing dendrite growth to some extent. However, existing three-dimensional carbon-based current collectors still face the following two key problems: First, lithium ions tend to preferentially deposit in the surface region near the separator, making it difficult to fully utilize the internal space of the three-dimensional structure. The current collector gradually evolves into a "pseudo-planar" structure, unable to leverage the advantages of its three-dimensional framework, thus making it difficult to achieve long-term stable cycle performance.

[0004] Secondly, the SEI film formed on the surface of the three-dimensional current collector mainly relies on the spontaneous decomposition of the electrolyte. Its composition is mainly organic, with insufficient mechanical strength and low ionic conductivity, making it difficult to effectively suppress the continuous consumption of electrolyte and the repeated growth of dendrites.

[0005] To address the issue of uneven lithium-ion deposition, existing research has proposed gradient engineering strategies, such as constructing conductivity gradients or lithiophilic gradients (e.g., introducing elements like Ag, Au, and Sb), aiming to create a driving force from the surface inwards, guiding lithium ions to migrate into the current collector. However, these strategies have failed to fundamentally improve the stability of the SEI film; the SEI at the gradient modification sites still depends on the spontaneous decomposition of the electrolyte, making it difficult to guarantee interfacial stability.

[0006] To address the stability issue of SEI films, existing technologies attempt to introduce fluorine-containing components to strengthen the inorganic components in the SEI, thereby improving its mechanical strength and ion conductivity. However, current fluorine modifications mostly employ a uniform distribution approach, making it difficult to achieve synergistic regulation with gradient structures and thus failing to simultaneously achieve the dual objectives of "directional lithium deposition" and "stable SEI construction."

[0007] Therefore, how to construct a three-dimensional carbon nanofiber current collector that combines the gradient distribution characteristics of ZnF2, enables the directional transport of lithium ions, and induces in-situ generation of LiF-rich SEI has become a key technical challenge that urgently needs to be overcome in the current research on lithium metal anodes. Summary of the Invention

[0008] The purpose of this application is to overcome the above-mentioned shortcomings of the prior art and provide a ZnF2 gradient structure carbon nanofiber current collector, its preparation method and application, which simultaneously realizes ZnF2 gradient distribution, directional lithium ion transport and in-situ LiF rich SEI generation, and solves the two key problems of low internal space utilization and SEI instability of three-dimensional carbon-based current collectors pointed out in the background art.

[0009] To achieve the above-mentioned objectives, this application provides a ZnF2 gradient structure carbon nanofiber current collector, wherein the current collector comprises a porous carbon nanofiber material with an interconnected network structure and ZnF2 distributed in the porous carbon nanofiber material; along the thickness direction of the current collector, the mass ratio of ZnF2 to porous carbon nanofiber material continuously increases, and the mass ratio of ZnF2 to porous carbon nanofiber material ranges from 0.05 to 1:1.

[0010] This invention constructs a carbon nanofiber current collector with a continuous ZnF2 concentration gradient distributed along its thickness direction. ZnF2 exhibits strong lithiophilicity, and its gradient distribution within the three-dimensional current collector establishes a thermodynamic driving force, guiding lithium ions to migrate directionally inward, achieving uniform and controllable lithium deposition from bottom to top, and effectively suppressing lithium dendrite growth. The reaction between ZnF2 and lithium generates a LiF-rich solid electrolyte interface layer in situ. LiF possesses high mechanical strength and good ionic conductivity, effectively inhibiting the continuous decomposition of the electrolyte. Simultaneously, the gradient spatial distribution of ZnF2 ensures that it always matches the actual lithium deposition front, resulting in uniform and appropriate LiF generation at the actual deposition sites. This avoids the problems of excessive local consumption of LiF at the top and the thick, disordered organic SEI layer inside, common in uniformly distributed systems.

[0011] Preferably, the thickness of the porous carbon nanofiber material is 120-170 micrometers, and the mass ratio of ZnF2 to the porous carbon nanofiber material along the thickness direction of the current collector ranges from 0.1 to 1:1. In some embodiments of the present invention, the mass ratio of ZnF2 to the porous carbon nanofiber material continuously increases along the thickness direction of the current collector. Specifically, the mass ratio along the thickness direction includes multiple ratios from 0.1:1, 0.15:1, 0.2:1, 0.25:1, 0.3:1, 0.35:1, 0.4:1, 0.45:1, 0.5:1, 0.55:1, 0.6:1, 0.65:1, 0.7:1, 0.75:1, 0.8:1, 0.85:1, 0.9:1, 0.95:1, and 1.0:1, and these multiple ratios continuously increase.

[0012] This invention also provides a method for preparing the ZnF2 gradient structure carbon nanofiber current collector, comprising the following steps: S1. Dissolve the carbon source and the zinc-containing compound in a solvent to prepare at least two precursor solutions with increasing zinc content; S2. The precursor solution is electrospun layer by layer in order of increasing zinc content to obtain nanofiber material with zinc concentration gradient along the thickness direction. S3. The nanofiber material is subjected to pre-oxidation treatment in an oxygen-containing atmosphere and carbonization treatment in an inert atmosphere in sequence to obtain zinc-containing gradient carbon nanofiber material; S4. The zinc-containing gradient carbon nanofiber material is subjected to plasma fluorination treatment to convert the zinc-containing substances in the fiber into ZnF2 in situ, forming a ZnF2 gradient structure carbon nanofiber current collector that is continuously distributed along the thickness direction.

[0013] The above preparation method uses carbon source and zinc-containing compounds (such as zinc acetate) as raw materials. After layer-by-layer electrospinning and pre-oxidation carbonization, plasma fluorination is used to convert the zinc-containing species in the fiber gradient into ZnF2 in situ, forming a ZnF2 gradient carbon nanofiber material with continuous distribution along the thickness direction. The process is simple and controllable, and the raw material cost is low, making it suitable for large-scale production and showing important application prospects in the field of high-performance lithium metal batteries.

[0014] Preferably, in S1, the carbon source includes one or more of polyacrylonitrile, polyimide, and polymethyl methacrylate; the zinc-containing compound is one or more of zinc acetate, zinc nitrate, or zinc chloride; and the solvent includes N,N-dimethylformamide.

[0015] More preferably, polyacrylonitrile is used as the main carbon source, and its mass fraction in the solvent is 10-30%; when polyimide is added, the mass ratio of polyimide to polyacrylonitrile is 0.01-0.05:1; when polymethyl methacrylate is added, the mass ratio of polymethyl methacrylate to polyacrylonitrile is 0.01-0.1:1; the mass ratio of the zinc-containing compound to polyacrylonitrile increases sequentially in each layer, and the mass ratio ranges from 0.05 to 1:1.

[0016] N,N-Dimethylformamide is an aprotic polar solvent that can dissolve not only high molecular weight polymers such as polyacrylonitrile and polyimide, but also metal salts such as zinc acetate and zinc nitrate. This co-solubility ensures that zinc ions can be uniformly dispersed at the molecular level in the carbon source matrix. During heat treatment, polyacrylonitrile undergoes cyclization to form a stable ladder structure, resulting in a high carbonization yield and the formation of a layered graphitic nitrogen structure, which is beneficial for electrical conductivity. Polyimide itself has extremely high heat resistance and rigidity, and its aromatic ring structure is easily transformed into carbon materials after carbonization. Simultaneously, polyimide contains nitrogen, enabling self-doping to form nitrogen-doped carbon. During carbonization, PMMA decomposes and escapes, leaving numerous pores, which can significantly increase the specific surface area of ​​the material, allowing the loaded zinc compounds to be more fully exposed for reaction. In summary, carbon nanofiber current collectors with good flexibility, high specific surface area, and uniform ZnF2 gradient distribution along the thickness direction can be prepared.

[0017] More preferably, the polyacrylonitrile has a weight-average molecular weight of 80,000 to 300,000, the polyimide has a weight-average molecular weight of 10,000 to 100,000, and the polymethyl methacrylate has a weight-average molecular weight of 100,000 to 600,000. The selection of these molecular weights aims to find a balance between processability and final properties, achieving an optimal molecular weight that facilitates spinning and produces fibers with excellent final fiber properties.

[0018] Preferably, in step S2, the electrospinning process parameters are: spinning distance 10~20 cm, voltage 15~20 kV, flow rate 0.015~0.025 mL / min, and roller speed 200~400 r / min. The selected voltage, distance, and flow rate range perfectly match the spinning material of the present invention and can control the surface morphology of the nanofiber material.

[0019] Preferably, in step S3, the heating rate of the pre-oxidation treatment is 1℃ / min, the temperature is 250~300℃, and the holding time is 0.5~2 h; the heating rate of the carbonization treatment is 2℃ / min, the temperature is 700~900℃, and the holding time is 1~3 h; the inert atmosphere is argon. The gentle pre-oxidation and slow carbonization maximize the preservation of the continuity and flexibility of the fiber material prepared by electrospinning, avoiding fiber breakage or pulverization. The slow heat treatment process reduces zinc salt agglomeration caused by localized overheating, which is beneficial for forming ultrafine, uniformly distributed ZnO nanoparticles.

[0020] Preferably, in S4, the plasma fluorination treatment is carried out in a mixed atmosphere containing NF3, wherein the volume fraction of NF3 in the mixed atmosphere is 2% and the balance is He; the discharge power is 20~80 W and the fluorination time is 20~50 min.

[0021] After carbonization, zinc-containing compounds (zinc acetate, zinc nitrate, etc.) decompose at high temperatures and combine with oxygen or residual oxygen-containing functional groups in the carbon material, mainly forming zinc oxide. Through NF3 plasma treatment, a gas-solid phase fluorination replacement reaction is successfully achieved between highly reactive fluorine radicals and ZnO in the carbonization products, realizing a phase transformation from oxide to fluoride. ZnF2 active sites are constructed in the composite material while preserving the integrity of the carbon nanofiber framework.

[0022] The present invention also provides the application of the ZnF2 gradient structure carbon nanofiber current collector in an electrochemical energy storage device, wherein the electrochemical energy storage device includes a lithium metal battery, a lithium-sulfur battery, or a solid-state battery.

[0023] Compared with the prior art, the present invention has the following technical effects: 1. This invention constructs a continuously distributed ZnF2 concentration gradient along the thickness direction by combining layer-by-layer electrospinning with plasma fluorination. ZnF2 has strong lithiophilicity, and the gradient distribution inside the three-dimensional current collector establishes a thermodynamic driving force, guiding lithium ions to migrate inward in a directional manner, achieving uniform and controllable lithium deposition from bottom to top, and effectively suppressing lithium dendrite growth.

[0024] 2. In this invention, ZnF2 reacts with lithium to generate a LiF-rich solid electrolyte interface layer in situ. LiF possesses high mechanical strength and good ionic conductivity, which can effectively suppress the continuous decomposition of the electrolyte. Simultaneously, the gradient spatial distribution of ZnF2 ensures that it always matches the actual lithium deposition front, resulting in uniform and appropriate LiF generation at the actual deposition sites. This avoids the problems of excessive local consumption of LiF at the top and thick, disordered organic SEI layers inside, common in uniformly distributed systems.

[0025] 3. The raw materials used in this invention are inexpensive, and the process is simple and controllable. This invention uses conventional carbon sources such as polyacrylonitrile and inexpensive zinc-containing compounds such as zinc acetate as raw materials, combined with mature electrospinning, carbonization, and plasma fluorination processes. The preparation process is simple, the parameters are controllable, and it is suitable for large-scale production.

[0026] 4. In this invention, lithium metal is pre-deposited in the current collector before assembling a symmetrical battery. This significantly extends the cycle life of the symmetrical battery and demonstrates outstanding stability under high DOD conditions. After pre-depositing lithium metal in the current collector, it is combined with a high-loading NCM90 cathode (18.77 mg / cm³). -2 (N / P=2.66) Matched and assembled full cells, the capacity retention rate reached 82% after 250 cycles under normal conditions. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the preparation process of the ZnF2 gradient structure carbon nanofiber current collector of the present invention.

[0028] Figure 2 The images are obtained by scanning electron microscopy (SEM): (a) is an SEM image of the Gr-ZnF2-CNF gradient structure carbon nanofiber current collector prepared in Example 2 of the present invention; (b) is an SEM image of the uniformly distributed ZnF2-CNF carbon nanofiber prepared in Comparative Example 2 of the present invention; (c) is an SEM image of the carbon nanofiber current collector prepared in Example 2 of the present invention after lithium metal deposition; and (d) is an SEM image of the uniformly distributed ZnF2-CNF carbon nanofiber prepared in Comparative Example 2 of the present invention after lithium metal deposition.

[0029] Figure 3 (a) A cross-sectional SEM image of the ZnF2 gradient structure carbon nanofiber current collector prepared in Example 2 of the present invention and its corresponding (b) EDS orientation line scan image of Zn and F elements along the thickness direction.

[0030] Figure 4 The Gr-ZnF2-CNF prepared in Example 2, the F-CNF prepared in Comparative Example 1, the ZnF2-CNF prepared in Comparative Example 2, and the Gr-Zn-CNF prepared in Comparative Example 3 were tested at 2 mA cm⁻¹. -2 2 mAh cm -2 Comparative test results of coulomb efficiency under the given conditions.

[0031] Figure 5 The Gr-ZnF2-CNF prepared in Example 2, the F-CNF prepared in Comparative Example 1, the ZnF2-CNF prepared in Comparative Example 2, and the Gr-Zn-CNF prepared in Comparative Example 3, after Li deposition, showed a temperature of 2 mA cm⁻¹. -2 6 mAh cm -2 Comparison of symmetrical battery performance under certain conditions.

[0032] Figure 6 The graph shows a comparison of the cycling performance of Gr-ZnF2-CNF prepared in Example 2, F-CNF prepared in Comparative Example 1, ZnF2-CNF prepared in Comparative Example 2, and Gr-Zn-CNF prepared in Comparative Example 3 when matched with NCM90. Detailed Implementation

[0033] To enable those skilled in the art to better understand the present invention, the present invention will now be further described in conjunction with specific embodiments.

[0034] In the following description, the embodiments of this application are for illustrative purposes and not for limiting purposes, so as to provide a thorough understanding of the embodiments. However, those skilled in the art will understand that the embodiments of this application can also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known preparation methods have been omitted so as not to obscure the description of the embodiments of this application with unnecessary details. Unless otherwise specified, the raw materials used in the following embodiments are all commercially available.

[0035] It should also be understood that the terms “including,” “comprising,” “having,” and their variations all mean “including but not limited to,” unless otherwise specifically emphasized.

[0036] This section only introduces content related to the inventive points; other details can be obtained from relevant technologies and will not be described in detail here. The following embodiments only illustrate several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

[0037] The specific implementation method is as follows: Example 1: A ZnF2 gradient structure carbon nanofiber current collector is prepared by the following steps: (1) Polyacrylonitrile (weight average molecular weight 150,000) and polyimide (weight average molecular weight 30,000) were selected and dissolved in N,N-dimethylformamide; the mass fraction of polyacrylonitrile was 12% and the mass ratio of polyimide to polyacrylonitrile was 5%.

[0038] (2) Prepare four portions of the above solution under the same conditions, add different amounts of zinc acetate to each, and the mass ratio of zinc acetate to polyacrylonitrile is 10%, 20%, 40% and 60% respectively. Stir magnetically for 6 hours until the solution is clear and there is no solid residue.

[0039] (3) Electrospinning was carried out layer by layer in order of increasing zinc acetate concentration. The spinning parameters were: spinning distance 12cm, voltage 15 kV, flow rate 0.015 mL / min, roller speed 200 r / min, and total spinning solution volume of each layer 8 mL, to obtain zinc concentration gradient nanofiber felt.

[0040] (4) The obtained nanofiber felt was placed in a tube furnace for pre-oxidation treatment at a heating rate of 1℃ / min and heated to 280℃ and held for 0.5 h; then heated to 700℃ at 2℃ / min for carbonization for 1 h. Argon gas was introduced for protection during the carbonization process to obtain zinc-containing gradient carbon nanofiber felt.

[0041] (5) The carbon nanofiber felt was placed in the plasma reaction chamber for fluorination treatment. A He mixed gas containing 2% NF3 was introduced, the discharge power was 20 W, and the fluorination time was 20 min to obtain a ZnF2 gradient structure carbon nanofiber current collector.

[0042] Example 2: A ZnF2 gradient structure carbon nanofiber current collector is prepared by the following steps: (1) Polyacrylonitrile (weight average molecular weight 150,000) and polyimide (weight average molecular weight 30,000) were selected and dissolved in N,N-dimethylformamide; the mass fraction of polyacrylonitrile was 10% and the mass ratio of polyimide to polyacrylonitrile was 3%.

[0043] (2) Prepare four portions of the above solution under the same conditions, with the mass ratio of zinc acetate to polyacrylonitrile being 25%, 50%, 75% and 100% respectively. Stir magnetically for 6 hours until the solution is clear and free of solid residue.

[0044] (3) Electrospinning was carried out layer by layer in order of increasing zinc acetate concentration. The spinning parameters were: spinning distance 15cm, voltage 18 kV, flow rate 0.020 mL / min, roller speed 400 r / min, and total amount of spinning solution for each layer 10 mL, to obtain zinc concentration gradient nanofiber felt.

[0045] (4) The obtained nanofiber felt was placed in a tube furnace for pre-oxidation treatment at a heating rate of 1℃ / min and heated to 280℃ and held for 1 h; then heated to 800℃ at 2℃ / min for carbonization for 2 h. Argon gas was introduced for protection during the carbonization process to obtain zinc-containing gradient carbon nanofiber felt.

[0046] (5) The carbon nanofiber felt was placed in the plasma reaction chamber for fluorination treatment. A He mixed gas containing 2% NF3 was introduced, the discharge power was 60 W, and the fluorination time was 30 min to obtain a ZnF2 gradient structure carbon nanofiber current collector.

[0047] Example 3: A ZnF2 gradient structure carbon nanofiber current collector is prepared by the following steps: (1) Polyacrylonitrile (weight average molecular weight 150,000) and polymethyl methacrylate (weight average molecular weight 350,000) were selected and dissolved in N,N-dimethylformamide; the mass fraction of polyacrylonitrile was 15% and the mass ratio of polymethyl methacrylate to polyacrylonitrile was 10%.

[0048] (2) Prepare four portions of the above solution under the same conditions, with the mass ratio of zinc acetate to polyacrylonitrile being 15%, 35%, 55% and 75% respectively. Stir magnetically for 6 hours until the solution is clear and free of solid residue.

[0049] (3) Electrospinning was carried out layer by layer in order of increasing zinc acetate concentration. The spinning parameters were: spinning distance 18cm, voltage 20 kV, flow rate 0.025 mL / min, roller speed 300 r / min, and total amount of spinning solution for each layer 20 mL, to obtain zinc concentration gradient nanofiber felt.

[0050] (4) The obtained nanofiber felt was placed in a tube furnace for pre-oxidation treatment at a heating rate of 1 ℃ / min and heated to 280℃ and held for 0.5 h; then heated to 900℃ at 2 ℃ / min for carbonization for 3 h. Argon gas was introduced for protection during the carbonization process to obtain zinc-containing gradient carbon nanofiber felt.

[0051] (5) The carbon nanofiber felt was placed in the plasma reaction chamber for fluorination treatment. A He mixed gas containing 2% NF3 was introduced, the discharge power was 50 W, and the fluorination time was 50 min to obtain a ZnF2 gradient structure carbon nanofiber current collector.

[0052] Example 4: A ZnF2 gradient structure carbon nanofiber current collector is prepared by the following steps: (1) Polyacrylonitrile (weight average molecular weight 150,000), polymethyl methacrylate (weight average molecular weight 350,000) and polyimide (weight average molecular weight 30,000) were selected and dissolved in N,N-dimethylformamide; the mass fraction of polyacrylonitrile was 15%, the mass ratio of polymethyl methacrylate to polyacrylonitrile was 5%, and the mass ratio of polyimide to polyacrylonitrile was 5%.

[0053] (2) Prepare four portions of the above solution under the same conditions, with the mass ratio of zinc acetate to polyacrylonitrile being 10%, 20%, 30% and 40% respectively, and stir magnetically for 6 hours until the solution is clear and free of solid residue.

[0054] (3) Electrospinning was carried out layer by layer in order of increasing zinc acetate concentration. The spinning parameters were: spinning distance 13cm, voltage 16 kV, flow rate 0.018 mL / min, roller speed 350 r / min, and total spinning solution volume of each layer 15 mL, to obtain zinc concentration gradient nanofiber felt.

[0055] (4) The obtained nanofiber felt was placed in a tube furnace for pre-oxidation treatment at a heating rate of 1℃ / min and heated to 280℃ and held for 1 h; then heated to 900℃ at 2℃ / min for carbonization for 1.5 h. Argon gas was introduced for protection during the carbonization process to obtain zinc-containing gradient carbon nanofiber felt.

[0056] (5) The carbon nanofiber felt was placed in the plasma reaction chamber for fluorination treatment. A He mixed gas containing 2% NF3 was introduced, the discharge power was 30 W, and the fluorination time was 25 min to obtain a ZnF2 gradient structure carbon nanofiber current collector.

[0057] Example 5: A ZnF2 gradient structure carbon nanofiber current collector is prepared by the following steps: (1) Polyacrylonitrile (weight average molecular weight 150,000), polymethyl methacrylate (weight average molecular weight 350,000) and polyimide (weight average molecular weight 30,000) were selected and dissolved in N,N-dimethylformamide; the mass fraction of polyacrylonitrile was 25%, the mass ratio of polymethyl methacrylate to polyacrylonitrile was 10%, and the mass ratio of polyimide to polyacrylonitrile was 3%.

[0058] (2) Prepare four portions of the above solution under the same conditions, with the mass ratio of zinc acetate to polyacrylonitrile being 20%, 40%, 60% and 80% respectively. Stir magnetically for 6 hours until the solution is clear and free of solid residue.

[0059] (3) Electrospinning was carried out layer by layer in order of increasing zinc acetate concentration. The spinning parameters were: spinning distance 15cm, voltage 18kV, flow rate 0.022 mL / min, roller speed 400 r / min, and total spinning solution volume of each layer 12 mL, to obtain zinc concentration gradient nanofiber felt.

[0060] (4) The obtained nanofiber felt was placed in a tube furnace for pre-oxidation treatment at a heating rate of 1℃ / min and heated to 280℃ and held for 0.5 h; then heated to 800℃ at 2℃ / min for carbonization for 1 h. Argon gas was introduced for protection during the carbonization process to obtain zinc-containing gradient carbon nanofiber felt.

[0061] (5) The carbon nanofiber felt was placed in the plasma reaction chamber for fluorination treatment. A He mixed gas containing 2% NF3 was introduced, the discharge power was 80 W, and the fluorination time was 40 min to obtain a ZnF2 gradient structure carbon nanofiber current collector.

[0062] Comparative Example 1: A carbon nanofiber current collector, the preparation method includes the following steps: (1) Polyacrylonitrile (weight average molecular weight 150,000) and polyimide (weight average molecular weight 30,000) were selected and dissolved in N,N-dimethylformamide; the mass fraction of polyacrylonitrile was 10% and the mass ratio of polyimide to polyacrylonitrile was 3%, and no zinc-containing compounds were added.

[0063] (2) Prepare a single concentration solution for electrospinning (without zinc concentration gradient). The spinning parameters are: spinning distance 15cm, voltage 18 kV, flow rate 0.020 mL / min, roller speed 400 r / min, and total spinning solution volume 10 mL.

[0064] (3) The pre-oxidation and carbonization conditions were the same as in Example 2, and a uniform carbon nanofiber felt was obtained.

[0065] (4) The obtained carbon nanofiber felt was subjected to plasma fluorination treatment by introducing He gas containing 2% NF3, with a discharge power of 60 W and a fluorination time of 30 minutes. Zinc-free plasma fluorinated carbon nanofibers (F-CNF) were obtained.

[0066] Comparative Example 2: A carbon nanofiber current collector, the preparation method includes the following steps: (1) Polyacrylonitrile (weight average molecular weight 150,000) and polyimide (weight average molecular weight 30,000) were selected and dissolved in N,N-dimethylformamide; the mass fraction of polyacrylonitrile was 10% and the mass ratio of polyimide to polyacrylonitrile was 3%.

[0067] (2) Prepare a single concentration solution without adding any zinc-containing compounds. The spinning parameters are the same as in Example 2 to obtain a uniform carbon nanofiber felt precursor.

[0068] (3) The pre-oxidation and carbonization conditions were the same as in Example 2, and a uniform pure carbon nanofiber felt was obtained.

[0069] (4) The obtained carbon nanofiber felt was immersed in a saturated aqueous solution of ZnF2 and soaked at room temperature for 12 hours. After being taken out, it was vacuum dried at 60°C for 12 hours to obtain uniformly distributed ZnF2 carbon nanofibers (ZnF2-CNF).

[0070] Comparative Example 3: A carbon nanofiber current collector, the preparation method includes the following steps: (1) Polyacrylonitrile (weight average molecular weight 150,000) and polyimide (weight average molecular weight 30,000) were selected and dissolved in N,N-dimethylformamide; the mass fraction of polyacrylonitrile was 10% and the mass ratio of polyimide to polyacrylonitrile was 3%.

[0071] (2) Prepare four portions of the above solution under the same conditions, with the mass ratio of zinc acetate to polyacrylonitrile being 25%, 50%, 75% and 100% respectively, which is completely consistent with Example 2. Stir magnetically for 6 hours until the solution is clear and free of solid residue.

[0072] (3) Electrospinning was carried out layer by layer in order of increasing zinc acetate concentration, with the same spinning parameters as in Example 2, to obtain zinc concentration gradient nanofiber felt.

[0073] (4) The pre-oxidation and carbonization conditions were the same as in Example 2, and zinc-containing gradient carbon nanofiber felt was obtained.

[0074] (5) Gradient zinc-containing carbon nanofibers (Gr-Zn-CNF; Gr stands for Gradient, i.e., gradient) are obtained directly without plasma fluorination treatment.

[0075] Performance Characterization and Comparison: The ZnF2 gradient structure carbon nanofiber current collector (Gr-ZnF2-CNF) prepared in Example 2, the ZnF2 uniformly distributed carbon nanofiber (ZnF2-CNF) prepared in Comparative Example 1 (F-CNF), the ZnF2 uniformly distributed carbon nanofiber (ZnF2-CNF) prepared in Comparative Example 2, and the gradient zinc-containing carbon nanofiber (Gr-Zn-CNF) prepared in Comparative Example 3 were systematically characterized and their performance was tested. The results are as follows: Figures 2-6 As shown.

[0076] Morphological characteristics ( Figure 2 The Gr-ZnF2-CNF obtained in Example 2, after plasma fluorination, exhibited granular protrusions on the fiber surface. The ZnF2-CNF obtained in Comparative Example 2, prepared by an immersion method, showed a smoother fiber surface with ZnF2 uniformly adhered to it. (Deposition of 10 mAh cm⁻¹) -2 After lithium metal deposition, the surface morphology of Example 2 is dense and uniform, with lithium deposition mainly concentrated in the interior of the ZnF2-rich side, effectively utilizing the three-dimensional structure. In contrast, the fiber surface of Comparative Example 2 shows obvious dendrites and a loose deposition layer, with blockage of the top pores, demonstrating that the ZnF2 gradient structure can effectively suppress lithium dendrite growth and guide lithium to deposit inward in a directional manner, while a uniform ZnF2 distribution cannot achieve this effect.

[0077] Gradient structure verification ( Figure 3Cross-sectional EDS line scanning was performed on Example 2. The signal intensities of Zn and F elements decreased in a gradient along the thickness direction, confirming the successful construction of the ZnF2 gradient structure. The fiber thickness was 150 micrometers.

[0078] Coulomb efficiency comparison ( Figure 4 ): at 2 mA cm -2 2 mAh cm -2 Under the given conditions, the coulombic efficiency of the four groups of samples was compared as follows: Example 2 (Gr-ZnF2-CNF) maintained an average coulombic efficiency of 99.2% for over 550 cycles; Comparative Example 3 (Gr-Zn-CNF) showed fluctuations in coulombic efficiency after approximately 320 cycles, with an average of about 98.8%, indicating insufficient long-term stability of the simple gradient structure without fluorine-containing SEI protection; Comparative Example 2 (ZnF2-CNF) had an average coulombic efficiency of approximately 97.4%, and showed significant decay within 100 cycles, proving that the uniform ZnF2 distribution caused three-dimensional structural failure due to blockage of the top pores. Comparative Example 1 (F-CNF) had an average coulombic efficiency of 91.7%, and began to decay rapidly after 50 cycles, proving that without ZnF2 modification and without a gradient structure, the current collector could not effectively control lithium deposition behavior, leading to continuous interface failure. The above results directly prove that only by introducing ZnF2 into the three-dimensional CNF current collector in a gradient form can the directional lithium deposition regulation and stable LiF enrichment SEI construction be realized simultaneously, and both functions are indispensable.

[0079] Comparison of high DOD performance of symmetrical batteries ( Figure 5 ): at 2 mA cm -2 6 mAh cm -2 Under high areal capacity deep discharge conditions (DOD=60%), the symmetric cell of Example 2 (Gr-ZnF2-CNF) could cycle stably for nearly 2000 hours with low and stable overpotential; the symmetric cell of Comparative Example 3 (Gr-Zn-CNF) experienced a continuous increase in overpotential and eventually failed after about 800 hours; the symmetric cell of Comparative Example 2 (ZnF2-CNF) experienced a short circuit within 200 hours; the symmetric cell of Comparative Example 1 (F-CNF) experienced a rapid increase in polarization after about 80 hours and failed after about 100 hours. This indicates that relying solely on fluorination without introducing ZnF2 cannot form effective lithiophilic sites, resulting in uncontrolled lithium deposition behavior and continuous interface deterioration. The significant difference in lifetime among the four groups of samples fully demonstrates the ability of the ZnF2 gradient structure to continuously regulate lithium deposition behavior under high load conditions, and the resulting advantage in interface stability, which far exceeds the effects achievable by gradient modification or fluorination modification alone.

[0080] Comparison of full battery cycle performance ( Figure 6 Four groups of samples were pre-deposited with lithium metal and then combined with high-loading LiNi 0.9 Co 0.05Mn 0.05 O2 (NCM90) positive electrode (18.77 mg cm⁻¹) -2 A full cell with N / P=2.66 was assembled and cycled under normal conditions. Example 2 (Gr-ZnF2-CNF) achieved 82% capacity retention after 250 cycles, demonstrating excellent long-term cycle stability. Comparative Example 2 (ZnF2-CNF) failed suddenly after 100 cycles. Comparative Example 3 (Gr-Zn-CNF) began to show significant degradation at 140 cycles. Comparative Example 1 (F-CNF) showed less than 80% capacity retention and rapid failure after 80 cycles, indicating that simple fluorination without constructing a ZnF2 gradient structure has extremely limited improvement on the long-term cycle performance of the full cell. Example 2 significantly outperformed the three comparative examples in both cycle life and capacity retention, further validating the comprehensive advantages of ZnF2 gradient design in practical battery systems.

[0081] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A ZnF2 gradient structure carbon nanofiber current collector, characterized in that, The current collector includes a porous carbon nanofiber material with an interconnected network structure and ZnF2 distributed in the porous carbon nanofiber material; along the thickness direction of the current collector, the mass ratio of ZnF2 to the porous carbon nanofiber material increases continuously, and the mass ratio of ZnF2 to the porous carbon nanofiber material ranges from 0.05 to 1:

1.

2. The ZnF2 gradient structure carbon nanofiber current collector as described in claim 1, characterized in that, The thickness of the porous carbon nanofiber material is 120~170 micrometers, and the mass ratio of ZnF2 to the porous carbon nanofiber material along the thickness direction of the current collector is in the range of 0.1~1:

1.

3. The method for preparing the ZnF2 gradient structure carbon nanofiber current collector according to claim 1 or 2, characterized in that, Includes the following steps: S1. Dissolve the carbon source and the zinc-containing compound in a solvent to prepare at least two precursor solutions with increasing zinc content; S2. The precursor solution is electrospun layer by layer in order of increasing zinc content to obtain nanofiber material with zinc concentration gradient along the thickness direction. S3. The nanofiber material is subjected to pre-oxidation treatment in an oxygen-containing atmosphere and carbonization treatment in an inert atmosphere in sequence to obtain zinc-containing gradient carbon nanofiber material; S4. The zinc-containing gradient carbon nanofiber material is subjected to plasma fluorination treatment to convert the zinc-containing substances in the fiber into ZnF2 in situ, forming a ZnF2 gradient structure carbon nanofiber current collector that is continuously distributed along the thickness direction.

4. The method for preparing the ZnF2 gradient structure carbon nanofiber current collector as described in claim 3, characterized in that, In S1, the carbon source includes one or more of polyacrylonitrile, polyimide, and polymethyl methacrylate; the zinc-containing compound is one or more of zinc acetate, zinc nitrate, or zinc chloride; and the solvent includes N,N-dimethylformamide.

5. The method for preparing the ZnF2 gradient structure carbon nanofiber current collector as described in claim 4, characterized in that, Polyacrylonitrile is used as the main carbon source, and its mass fraction in the solvent is 10-30%. When polyimide is added, the mass ratio of polyimide to polyacrylonitrile is 0.01-0.05:

1. When polymethyl methacrylate is added, the mass ratio of polymethyl methacrylate to polyacrylonitrile is 0.01-0.1:

1. The mass ratio of the zinc-containing compound to polyacrylonitrile increases sequentially in each layer, and the mass ratio ranges from 0.05 to 1:

1.

6. The method for preparing the ZnF2 gradient structure carbon nanofiber current collector as described in claim 5, characterized in that, The polyacrylonitrile has a weight-average molecular weight of 80,000 to 300,000, the polyimide has a weight-average molecular weight of 10,000 to 100,000, and the polymethyl methacrylate has a weight-average molecular weight of 100,000 to 600,000.

7. The method for preparing the ZnF2 gradient structure carbon nanofiber current collector as described in claim 3, characterized in that, In S2, the electrospinning process parameters are: spinning distance 10~20 cm, voltage 15~20 kV, flow rate 0.015~0.025 mL / min, and roller speed 200~400 r / min.

8. The method for preparing the ZnF2 gradient structure carbon nanofiber current collector as described in claim 3, characterized in that, In S3, the heating rate of the pre-oxidation treatment is 1℃ / min, the temperature is 250~300℃, and the holding time is 0.5~2 h; the heating rate of the carbonization treatment is 2℃ / min, the temperature is 700~900℃, and the holding time is 1~3 h; the inert atmosphere is argon atmosphere.

9. The method for preparing the ZnF2 gradient structure carbon nanofiber current collector as described in claim 3, characterized in that, In S4, the plasma fluorination treatment is carried out in a mixed atmosphere containing NF3, wherein the volume fraction of NF3 in the mixed atmosphere is 2% and the balance is He; the discharge power is 20~80 W and the fluorination time is 20~50 min.

10. The application of the ZnF2 gradient structure carbon nanofiber current collector according to claim 1 in an electrochemical energy storage device, wherein the electrochemical energy storage device includes a lithium metal battery, a lithium-sulfur battery, or a solid-state battery.

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