Sodium metal composite negative electrode material and preparation method thereof

Abstract

The application belongs to the technical field of sodium metal batteries, and particularly relates to a sodium metal composite negative electrode material and a preparation method. First, a sodium halide / CMC aqueous solution is slowly added into a PAN / DMF solution under stirring, and the mixed solution is electrospun. The obtained spinning film is pre-oxidized and carbonized to obtain a composite film. The composite film is fully absorbed with molten sodium, and after natural cooling, a Na@sodium halide / N-CNF composite negative electrode material is obtained. The application has simple process, easily obtained raw materials, low cost and easy scaling. The electrospinning combined with the pre-oxidation-carbonization process is simple to operate and easy to control parameters. The obtained negative electrode material has excellent thermal safety.

Description

Technical Field

[0001] This invention belongs to the field of sodium metal battery technology, specifically relating to a sodium metal composite anode material and its preparation method. Background Technology

[0002] Sodium metal batteries are considered a strong candidate for next-generation energy storage systems due to their high theoretical energy density. However, sodium metal anodes face serious safety and cycle stability challenges in practical applications. Regarding safety, sodium metal has a low melting point (97.8 ℃) and high chemical reactivity. Under thermal abuse (such as overcharging or internal short circuits), the accumulated heat can easily melt the sodium, potentially causing violent combustion or even explosion, posing a significant safety hazard. In terms of cycle performance, uneven deposition of sodium ions on the anode surface easily forms sodium dendrites. Dendrite growth can puncture the separator, leading to short circuits and continuously consuming active materials and electrolyte, resulting in rapid capacity decay.

[0003] Existing technologies for improving the performance of sodium metal anodes mainly focus on constructing three-dimensional conductive frameworks (such as porous carbon or metal foam) to reduce local current density, optimizing electrolyte formulations, or constructing artificial solid electrolyte interphase (SEI) films to stabilize the electrode / electrolyte interface. However, the fabrication process of three-dimensional frameworks is complex and reduces the volumetric energy density of the battery; while electrolyte modification and artificial SEI usually cannot fundamentally prevent the violent reaction of molten sodium during thermal runaway, and their long-term stability is insufficient.

[0004] Therefore, existing technologies still lack a solution that can simultaneously and efficiently improve the thermal safety and electrochemical cycle stability of sodium metal anodes from the perspective of material properties, while also being simple and cost-effective. This invention aims to address two key technical problems of existing sodium metal anodes: poor safety due to high-temperature melting and combustion, and short cycle life due to sodium dendrite growth. Summary of the Invention

[0005] To address the aforementioned problems in the prior art, this invention provides a high-safety sodium metal composite anode material, its preparation method, and its application in sodium-ion batteries. The core of this invention lies in the method of casting molten sodium metal into a three-dimensional framework prepared by electrospinning, thereby thoroughly mixing highly thermally stable sodium chloride (NaCl) particles with metallic sodium to form a composite structure filled with sodium metal and using NaCl as a sodium-affinity site, which can be described as a Na@NaCl / N-CNF composite structure.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solutions.

[0007] A sodium metal composite anode material comprises a three-dimensional porous nitrogen-doped carbon nanofiber framework, sodium halide nanoparticles uniformly loaded on the framework, and metallic sodium filling the pores of the framework. The sodium halide particles serve as a reinforcing phase with high chemical and thermal stability, and their mass accounts for 10% to 40% of the total mass of the sodium metal composite anode material, preferably 20% to 35%. The average particle size of the sodium halide particles is 0.1 μm to 20 μm, preferably 0.5 μm to 10 μm, and more preferably 1 μm to 5 μm.

[0008] Preferably, the mass ratio of sodium to sodium halide is 5:(1-3); more preferably, the mass ratio of sodium to sodium halide is 5:1, 5:2 or 5:3; most preferably, the mass ratio of sodium to sodium halide is 5:2.

[0009] The preparation method of the sodium metal composite anode material includes the following steps: (1) Dissolve polyacrylonitrile (PAN) in N,N-dimethylformamide (DMF), heat to 50~70 ℃ and stir until homogeneous to prepare PAN / DMF solution; (2) Dissolve sodium halide and sodium carboxymethyl cellulose (CMC) in deionized water and disperse them evenly by ultrasonication to obtain sodium halide / CMC aqueous solution; (3) Add the sodium halide / CMC aqueous solution slowly to the PAN / DMF solution while stirring, and continue stirring for 1 to 3 hours until a uniform and stable spinning solution is formed; (4) The prepared spinning solution is continuously spun for 10 hours to obtain a composite fiber membrane; the fiber membrane is placed in a vacuum oven and dried at 80 °C for 8 hours to remove residual solvent; (5) Place the dried composite fiber membrane in a tube furnace and heat it to 200~300 ℃ in an air atmosphere for pre-oxidation. After pre-oxidation, let it cool down naturally and transfer the fiber membrane to another tube furnace. Heat it to 600~1000 ℃ under inert gas protection for carbonization. After cooling with the furnace, sodium halide / nitrogen-doped carbon nanofiber composite membrane is obtained. (6) Under the protection of inert gas, place the sodium halide / nitrogen-doped carbon nanofiber composite film described in step (5) on the surface of molten sodium and press lightly to allow the composite film to naturally absorb sodium liquid; after the sodium liquid completely fills the pores of the composite film, take out the composite film and cool it naturally to room temperature to obtain Na@sodium halide / N-CNF composite anode material.

[0010] Preferably, in step (1), the mass fraction of PAN in the PAN / DMF solution is 8-15%.

[0011] Preferably, in step (2), the mass ratio of sodium halide to sodium carboxymethyl cellulose and deionized water is 1:(0.2-1):(5~20).

[0012] Preferably, in step (3), the mass ratio of PAN to CMC in the spinning solution is 5:1 to 20:1.

[0013] More preferably, in step (3), the mass fraction of sodium halide in the spinning solution is controlled between 0.5% and 5.0%.

[0014] In a further preferred embodiment, step (4) involves injecting the spinning solution into a syringe and attaching a stainless steel needle with an inner diameter of 0.6 mm. The spinning parameters are set as follows: positive electrode voltage 15 kV, negative electrode voltage -5 kV, receiving distance 15 cm, propulsion speed 1.0 mL / h, ambient temperature 25 ℃, and relative humidity 30%. Electrospinning is performed using aluminum foil as the receiving substrate.

[0015] Preferably, in step (5), the pre-oxidation step, the temperature is increased at a rate of 1~5℃ / min and held for 2~4 hours.

[0016] Preferably, in step (5), the pre-oxidation temperature is raised to 240~260 ℃.

[0017] Preferably, in step (5), the carbonization step, the temperature is increased at a rate of 5~20℃ / min and held for 1.5~3 hours.

[0018] Preferably, in step (5), the carbonization temperature is raised to 750~850 ℃.

[0019] The preparation method of Na@sodium halide / N-CNF composite anode also includes post-processing of the material: excess metallic sodium is gently removed from the surface of the composite anode using a scraper or sandpaper to make the surface smooth. The actual sodium loading is calculated using a precision balance, ensuring that the deviation from the theoretical value does not exceed ±10%.

[0020] The sodium metal composite negative electrode sheet prepared by the above method, together with the positive electrode sheet, separator, and electrolyte, can be made into button cells, pouch cells, or cylindrical cells according to the conventional sodium-ion battery assembly process, and used in various energy storage devices.

[0021] Compared with existing pure sodium metal anodes, the present invention has the following significant advantages: Intrinsic safety is significantly improved: uniformly dispersed NaCl particles form a physical barrier network within the sodium metal matrix (see appendix). Figure 4 , 5When thermal runaway occurs in the battery and sodium metal melts, this network effectively hinders the flow, aggregation, and splashing of molten sodium, thereby greatly suppressing the intensity of the combustion reaction. Compared with other sodium halides, NaCl has a stronger effect in reducing combustion intensity. Experiments show that the composite negative electrode prepared in this invention has a maximum combustion temperature that is 23% to 28% lower than that of pure sodium metal in open flame tests, and its combustion reduction effect is the highest compared with negative electrodes made of other sodium halides (see Appendix). Figure 6 (and example data).

[0022] Excellent electrochemical cycling performance: The introduction of NaCl particles helps to homogenize the sodium ion flow and guides uniform sodium deposition, thereby effectively suppressing the growth of sodium dendrites. A full cell with Na(NiFeMn)O2 (NFM) as the positive electrode retains over 80% of its capacity after 200 cycles at 1 C rate (see appendix). Figure 7 ).

[0023] The process is simple, low-cost, and easily scalable: It employs electrospinning combined with a pre-oxidation-carbonization process, which is easy to operate and allows for precise parameter control. The raw materials used (PAN, CMC, sodium halides, etc.) are readily available and inexpensive, and their purity requirements meet standard laboratory and industrial standards, eliminating the need for special high-end raw materials and reducing preparation costs. This method is highly compatible, enabling large-scale preparation and making it suitable for industrial production. Attached Figure Description

[0024] Figure 1 A symmetrical cell assembled using a composite negative electrode prepared by the roll forming method and pure sodium as the electrode sheet was constructed at 1 mA cm⁻¹. -2 Long-cycle voltage-time curves at current density.

[0025] Figure 2 This is a schematic diagram of the process for preparing the composite negative electrode using the melt casting method of this invention.

[0026] Figure 3 The image shows a scanning electron microscope image of the micron-sized NaCl powder used.

[0027] Figure 4 This is a SEM image of the surface of the Na@NaCl-5:2 / N-CNF composite negative electrode obtained in Example 1.

[0028] Figure 5 The image shows the SEM image of the cross-section of the Na@NaCl-5:2 / N-CNF composite negative electrode obtained in Example 1.

[0029] Figure 6 Comparison of infrared thermal images of pure sodium metal anode and composite anode in the embodiments of the present invention during combustion tests.

[0030] Figure 7The cycling curves are for a battery assembled with Na(NiFeMn)O2 (NFM) as the positive electrode and the material of Example 1 of this invention as the negative electrode, at a 1 C rate. Detailed Implementation

[0031] The following examples are used to further illustrate the present invention, but are not intended to limit the scope of the invention.

[0032] The materials used in the embodiments and comparative examples are as follows: Polyacrylonitrile (PAN, Mw = 150,000 g mol) -1 ); Sodium carboxymethyl cellulose (CMC, degree of substitution ≈ 0.9) (used to adjust the viscosity of the spinning solution, not as the main component of the final composite anode). Sodium chloride, sodium fluoride, sodium iodide (NaCl, NaF, NaI, analytical grade, 99.5%); N,N-Dimethylformamide (DMF, anhydrous grade); Deionized water (resistivity 18.2 MΩ·cm); Sodium metal block (Na, 99.9%), Aladdin; Under an inert atmosphere, weigh out a predetermined amount of metallic sodium (purity ≥99.7%) and micron-sized sodium chloride powder (purity ≥99.9%).

[0033] To investigate the effect of the Na to NaCl ratio on the electrochemical performance of the anode material, Na@NaCl composite anodes were first prepared by the rolling method (Na:NaCl = 5:1, Na:NaCl = 5:2, Na:NaCl = 5:3 mass ratio).

[0034] (1) Weigh 2.50 g of metallic sodium block and 0.50 g, 1.00 g, and 1.50 g of NaCl powder with an average particle size D50≈5 μm respectively.

[0035] (2) Place the mixture in the gap between the rollers of a manual double-roll mill (100 mm wide rollers) and adjust the roller gap to about 0.05 mm. Roll the mixture at room temperature (~25 °C). After each roll, fold the resulting sheet in half and feed it back into the gap. Repeat this process for a total of 50 rolls.

[0036] (3) The composite foil with a final thickness of about 40 μm is punched out into a circular piece with a diameter of 15 mm using a die, which serves as the negative electrode.

[0037] (4) Electrochemical performance testing: Assemble a Na@NaCl symmetric battery, preferably with a composite negative electrode Na@NaCl-5:2 at 1.0 mAcm -2Current density, 1.0 mAh cm⁻¹ -2 Electrochemical performance tests were conducted at the specified capacity, and the results are as follows: Figure 1 As shown, stable cycling exceeds 400 hours, and the voltage curve is smooth.

[0038] Example 1 The preparation of Na@sodium halide composite negative electrodes using a melt casting method with optimized electrochemical proportions (Na:sodium halide (NaF, NaI, NaCl) = 5:2, mass ratio) is as follows: Figure 2 As shown, the morphology of the NaCl powder used in the experiment is as follows. Figure 3 As shown.

[0039] (1) Weigh 1.2 g PAN and dissolve it in 8.8 g DMF. Stir at 60 °C for 6 hours to obtain a PAN solution. Separately, dissolve 0.12 g sodium halides (NaF, NaI, NaCl) and 0.1 g CMC in 1.0 g deionized water and sonicate for 10 minutes to disperse them evenly. Add the sodium halide aqueous solution dropwise to the PAN / DMF solution and continue stirring for 2 hours to obtain a spinning solution.

[0040] (2) A needle with an inner diameter of 0.6 mm was used. The spinning parameters were: positive electrode voltage 15 kV, negative electrode voltage 5 kV, receiving distance 15 cm, feed speed 1.0 mL / h, temperature 25 ℃, and relative humidity 30%. The spinning was carried out continuously for 8 hours, and the fiber membrane was collected and vacuum dried at 80 ℃ for 8 hours.

[0041] (3) The dried fiber membrane was pre-oxidized by heating it to 250 °C at 1 °C / min in air atmosphere and holding it for 2.5 hours; then carbonized it by heating it to 800 °C at 10 °C / min in argon atmosphere and holding it for 2 hours to obtain sodium halide (NaF, NaI, NaCl) / N-CNF composite membrane.

[0042] (4) Thermogravimetric analysis was performed on the carbonized film, and the total mass of the carbonized film was found to be 0.76 mg / cm³. 2 The actual sodium halide content was measured to be 0.48 mg / cm³. 2 Based on a sodium halide:Na ratio of 2:5, the required sodium loading per square centimeter of carbon film is 1.20 mg (i.e., the sodium halide mass fraction is 24.5% of the composite anode). A sodium block of the corresponding mass was heated to 180 °C in a glove box to melt. The carbon film was placed on the surface of the molten sodium, and after complete absorption, it was cooled to obtain the Na@sodium halide (NaF, NaI, NaCl) / N-CNF composite anode material. The composite anode was cut into 16 mm discs for use in button cell fabrication. The SEM image of the obtained Na@NaCl-5:2 / N-CNF composite anode surface is shown below. Figure 4As shown, the uniform dispersion of NaCl particles (bright color) in the sodium matrix is ​​illustrated. The SEM image of the resulting Na@NaCl-5:2 / N-CNF composite anode cross-section is shown below. Figure 5 As shown in the figure, the uniform distribution of NaCl particles in three-dimensional space is confirmed from a cross-sectional perspective.

[0043] (5) Effect verification: A combustion test was conducted on the composite negative electrode material, such as... Figure 6 As shown, using an infrared thermal imager, the peak temperatures of combustion tests containing NaF, NaI, and NaCl all decreased. Among them, the peak temperature of combustion tests of Na@NaCl-5:2 / N-CNF decreased by about 28%, demonstrating the best thermal safety.

[0044] Electrochemical cycling performance of the anode material of the present invention: Anode sheet was prepared using the anode material in Example 1, and a full cell was assembled using Na(NiFeMn)O2 (NFM) as the positive electrode. After cycling 200 times at a 1 C rate, the capacity retention rate was still more than 80%.

[0045] Example 2 The difference from Example 1 is that in step (1), the amount of PAN used is changed to 1.0 g dissolved in 9.0 g DMF; all other aspects are the same as in Example 1.

[0046] Example 3 The difference from Example 1 is that in step (1), the amount of PAN used is changed to 1.5 g dissolved in 8.5 g DMF; all other aspects are the same as in Example 1.

[0047] Example 4 The difference from Example 1 is that in step (3), the temperature is increased to 850 ℃ at 10 ℃ / min under an argon atmosphere and held for 2 hours for carbonization; all other aspects are the same as in Example 1.

[0048] Example 5 The difference from Example 1 is that in step (3), the temperature is increased to 900 ℃ at 10 ℃ / min under an argon atmosphere and held for 2 hours for carbonization; all other aspects are the same as in Example 1.

Claims

1. A sodium metal composite anode material, characterized in that, The composite anode material comprises a three-dimensional porous nitrogen-doped carbon nanofiber framework, sodium halide nanoparticles uniformly loaded on the framework, and metallic sodium filling the pores of the framework.

2. The sodium metal composite anode material according to claim 1, characterized in that, The sodium halide particles account for 10% to 40% of the total mass of the sodium metal composite anode material, preferably 20% to 35%.

3. The sodium metal composite anode material according to claim 1, characterized in that, The mass ratio of sodium to sodium halide is 5:(1-3); preferably, the mass ratio of sodium to sodium halide is 5:1, 5:2 or 5:3; more preferably, the mass ratio of sodium to sodium halide is 5:

2.

4. The method for preparing the sodium metal composite anode material according to any one of claims 1-3, characterized in that, Includes the following steps: (1) Dissolve polyacrylonitrile in N,N-dimethylformamide, heat to 50~70 ℃ and stir until homogeneous to prepare PAN / DMF solution; (2) Dissolve sodium halide and sodium carboxymethyl cellulose in deionized water and disperse them evenly by ultrasonication to obtain sodium halide / CMC aqueous solution; (3) Add the sodium halide / CMC aqueous solution slowly to the PAN / DMF solution while stirring, and continue stirring for 1 to 3 hours until a uniform and stable spinning solution is formed; (4) The prepared spinning solution is continuously electrospun for 10 hours to obtain a composite fiber membrane; the fiber membrane is dried in a vacuum oven at 80 °C for 8 hours to remove residual solvent; (5) Place the dried composite fiber membrane in a tube furnace and heat it to 200~300 ℃ in an air atmosphere for pre-oxidation. After pre-oxidation, let it cool down naturally and transfer the fiber membrane to another tube furnace. Heat it to 600~1000 ℃ under inert gas protection for carbonization. After cooling with the furnace, sodium halide / nitrogen-doped carbon nanofiber composite membrane is obtained. (6) Under the protection of inert gas, place the sodium halide / nitrogen-doped carbon nanofiber composite film described in step (5) on the surface of molten sodium and press lightly to allow the composite film to naturally absorb sodium liquid; after the sodium liquid completely fills the pores of the composite film, take out the composite film and cool it naturally to room temperature to obtain Na@sodium halide / N-CNF composite anode material.

5. The method for preparing the sodium metal composite anode material according to claim 4, characterized in that, In step (1), the mass fraction of PAN in the PAN / DMF solution is 8-15%.

6. The method for preparing the sodium metal composite anode material according to claim 4, characterized in that, In step (2), the mass ratio of sodium halide to sodium carboxymethyl cellulose and deionized water is 1:(0.2-1):(5~20).

7. The method for preparing the sodium metal composite anode material according to claim 4, characterized in that, In step (3), the mass ratio of PAN to CMC in the spinning solution is 5:1 to 20:1; Preferably, in step (3), the mass fraction of sodium halide in the spinning solution is controlled between 0.5% and 5.0%; preferably, in step (4), the spinning solution is injected into a syringe and fitted with a stainless steel needle with an inner diameter of 0.6 mm; the spinning parameters are set as follows: positive electrode voltage 15 kV, negative electrode voltage -5 kV, receiving distance 15 cm, advancing speed 1.0 mL / h, ambient temperature 25 ℃, relative humidity 30%; electrospinning is performed using aluminum foil as the receiving substrate.

8. The method for preparing the sodium metal composite anode material according to claim 4, characterized in that, In step (5), the pre-oxidation step involves heating at a rate of 1~5℃ / min and holding at that temperature for 2~4 hours; preferably, in step (5), the pre-oxidation temperature is raised to 240~260℃.

9. The method for preparing the sodium metal composite anode material according to claim 4, characterized in that, Step (5), the carbonization step, involves heating at a rate of 5~20℃ / min and holding at that temperature for 1.5~3 hours; Preferably, in step (5), the carbonization temperature is raised to 750~850 ℃.

10. A sodium metal battery anode made from the sodium metal composite anode material according to any one of claims 1-3.

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