A composite fiber separator for sodium-ion batteries and its preparation method

Abstract

This invention discloses a composite fiber separator for sodium-ion batteries and its preparation method, belonging to the field of sodium battery separator technology. Addressing the shortcomings of existing separators, this invention provides a polyacrylonitrile / polyamide / SiO2 composite nanofiber membrane, composed of polyacrylonitrile fibers and polyamide fibers arranged in an alternating stacked pattern. Polyacrylonitrile provides good electrolyte wettability and inhibits sodium dendrite penetration, while polyamide imparts excellent mechanical strength and thermal stability to the separator. The SiO2 inorganic filler significantly improves ionic conductivity and promotes uniform sodium ion deposition. The preparation process employs a dual-needle cross-electrospinning technique, adding SiO2 to one of the spinning solutions. The independent spinning of the dual needles and cross-deposition achieve uniform distribution of the filler at the fiber level, avoiding the filler agglomeration problem in traditional methods. This invention significantly improves the overall electrochemical performance and safety of the separator through material synergy and structural control.

Description

Technical Field

[0001] This invention belongs to the field of sodium battery separator technology, and specifically relates to a composite fiber separator with a cross-stacked fiber structure, its preparation method and application. Background Technology

[0002] Sodium-ion batteries, with their advantages of low cost, abundant natural sodium reserves, and environmental friendliness, have gradually become a research hotspot in the field of grid-scale energy storage. As a core component of the battery, the separator not only prevents physical contact between the positive and negative electrodes but also serves as an electrolyte reservoir, undertaking the function of ion transport between the two electrodes. Therefore, it must possess high ionic conductivity, good thermal stability, electrolyte wettability, and mechanical strength, which are crucial to the overall battery performance. Currently, commercially available separators are mainly made of polypropylene (PP), polyethylene (PE), PP / PE composite membranes, and ceramic-modified separators. However, the pore size of these mass-produced separators is much larger than the radius of solvated sodium ions, and polyolefin-based separators in sodium batteries generally suffer from poor electrolyte wettability and significant thermal shrinkage. Furthermore, they cannot effectively suppress sodium dendrite growth, severely limiting the safety and cycle life of sodium batteries.

[0003] To address these issues, researchers have attempted to prepare nanofiber separators using electrospinning technology and introduce inorganic fillers (such as SiO2) to enhance ionic conductivity and thermal stability. For example, patent CN116315435A discloses a fiber separator for lithium batteries and its preparation method. This separator is a PI / oPAN nanofiber membrane, composed of polyimide fibers and pre-oxidized polyacrylonitrile fibers arranged in an alternating stack. It is prepared by cross-spinning PAA and PAN followed by high-temperature thermal imidization treatment at 250-350℃. While this technology achieves the cross-stacked fiber structure, it relies on the high-temperature imidization reaction of the polyamic acid precursor, resulting in a high process temperature, and it does not involve the introduction of inorganic fillers. Another example is patent CN121769427A, which discloses a multifunctional composite separator for sodium-ion batteries. This separator comprises structural polymers (such as PVDF and PAN), synergistic polymers (vinyl lactams and vinyl esters), inorganic nanofillers, and a sodium ion conduction promoter, and is prepared by electrospinning to obtain a three-dimensional fiber network structure. Although this technology introduces inorganic fillers, the fillers are uniformly dispersed inside or on the surface of all fibers, and soluble sodium salts must be added to promote ion conduction. It does not involve the selective distribution design of polyamide materials and fillers.

[0004] In summary, existing technologies still have many shortcomings: 1) In blend spinning or single-nozzle spinning methods, inorganic fillers (such as SiO2) are prone to agglomeration, resulting in uneven filler distribution and affecting the consistency of membrane performance; 2) Existing cross-spinning technologies are mainly for PI / oPAN systems, requiring high-temperature imidization treatment, with a narrow process window, and do not introduce inorganic fillers to synergistically enhance ion conduction and thermal stability; 3) Polyacrylonitrile (PAN) has good polarity and electrolyte wettability, which can effectively inhibit dendrite growth; polyamide (PA) has excellent mechanical properties and heat resistance; SiO2, as an inorganic filler, can provide additional ion transport channels and enhance structural stability. There is no known technical solution that synergistically introduces polyacrylonitrile (PAN), polyamide (PA), and SiO2, and achieves selective distribution of SiO2 in specific fibers through dual-needle electrospinning.

[0005] Therefore, it is necessary to develop a composite separator that synergistically combines polyacrylonitrile (PAN), polyamide (PA), and SiO2, while simultaneously achieving uniform dispersion and selective distribution of SiO2, thereby comprehensively improving the overall performance of sodium battery separators. Summary of the Invention

[0006] This invention addresses the technical problems of insufficient performance of existing separators and uneven distribution of SiO2 filler by proposing a composite fiber separator for sodium-ion batteries and its preparation method.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0008] In a first aspect, the present invention provides a composite fiber separator for sodium-ion batteries, characterized in that: the composite fiber separator for sodium-ion batteries is a polyacrylonitrile / polyamide / SiO2 filler composite nanofiber membrane, composed of polyacrylonitrile fibers and polyamide fibers arranged in an alternating stacked manner, wherein the SiO2 filler is selectively distributed in either the polyacrylonitrile fibers or the polyamide fibers. The polyacrylonitrile is used to provide electrolyte wettability and inhibit sodium dendrite growth, the polyamide is used to provide mechanical strength and thermal stability, and the SiO2 filler is used to improve ionic conductivity and promote uniform sodium ion transport.

[0009] The SiO2 filler is preferably fumed silica with a particle size of 20-200 nm.

[0010] The polyamide fiber is selected from nylon 6 or nylon 66, with an average diameter of 100-800 nm.

[0011] The average diameter of the polyacrylonitrile fiber is 80-400 nm.

[0012] The necessity of the co-design of the materials selected in this invention:

[0013] 1) Polyacrylonitrile (PAN): It has strong polarity and good affinity with electrolyte, which can effectively wet the diaphragm and promote the migration of sodium ions; at the same time, its fibrous structure can physically block sodium dendrite puncture.

[0014] 2) Polyamide (PA): High mechanical strength and excellent thermal stability, which can significantly improve the overall structural stability and high temperature shrinkage resistance of the diaphragm.

[0015] 3) SiO2 inorganic filler: The surface is rich in hydroxyl groups, which can form hydrogen bonds with the electrolyte, build additional ion conduction pathways, and improve ion conductivity; at the same time, it acts as a rigid particle to reinforce the fiber structure and inhibit dendrite growth.

[0016] Secondly, the present invention provides a method for preparing the composite fiber separator for sodium-ion batteries, characterized by comprising the following steps:

[0017] (1) Polyacrylonitrile was dissolved in DMF to obtain spinning solution A; polyamide was dissolved in formic acid to obtain spinning solution B;

[0018] (2) Disperse the SiO2 filler in spinning solution A or spinning solution B using ultrasonication;

[0019] (3) Using a double-needle or multi-needle electrospinning device, spinning solution A and spinning solution B are cross-spun, so that the two fibers are stacked and deposited on the receiving device to form a composite fiber membrane; wherein, the double-needle structure allows the two spinning solutions to be sprayed out independently and cross-mixed and deposited at the receiving end, thereby achieving uniform distribution of SiO2 filler in the fiber layer and avoiding agglomeration; the obtained fibers are dried to obtain a composite fiber membrane.

[0020] In step (1), the mass fraction of polyacrylonitrile in spinning solution A is 8%-15%; the mass fraction of polyamide in spinning solution B is 12%-25%, preferably 15%-20%.

[0021] In step (2), the amount of SiO2 filler added is 1%-10% of the mass of polyacrylonitrile or polyamide.

[0022] The parameters for cross-spinning in step (3) are as follows: voltage 15-30 kV, preferably 18-25 kV; receiving distance 10-20 cm, preferably 15 cm; ambient temperature 20-35℃, preferably 25℃; relative humidity 30%-55%, preferably 40%-50%; propulsion rate of spinning solution A 0.5-1.5 mL / h, preferably 0.8-1.5 mL / h; propulsion rate of spinning solution B 0.3-1.0 mL / h; the two spinning solutions are sprayed alternately or simultaneously from different needles; the drying is vacuum drying, the temperature is 60-100℃, preferably 80-100℃; the time is 6-12h, preferably 6-8h.

[0023] This invention employs a dual-needle independent liquid supply system, adding SiO2 to the spinning solution of PAN or PA, with the two types of fibers cross-stacking and depositing at the receiving end. Because the two polymers (PAN / PA) are dissolved in different solvents (DMF / formic acid), mixing them all in the same system would exacerbate SiO2 bridging and agglomeration due to the combined effects of solvent and polymer. With separate liquid supply, SiO2 only contacts a single polymer and a single solvent, eliminating the simultaneous adsorption and cross-linking of multiple silica particles by both polymer chains, thus eliminating polymer-induced agglomeration at the interface. Furthermore, SiO2 exists only within or on the surface of one type of fiber. After the two fibers are cross-stacking, the inorganic functional fibers are uniformly interwoven within the pure polymer fiber network. This results in a macroscopically uniform distribution of the filler throughout the membrane, and microscopically, the filler is confined within a single fiber, preventing large-area aggregation and the appearance of localized silica-free regions. Compared to single-nozzle mixed spinning, the dual-needle structure ensures that SiO2 is distributed only within or on the surface of specific fibers, significantly improving the consistency of membrane performance.

[0024] Thirdly, the present invention provides the application of the composite fiber separator in sodium-ion batteries.

[0025] The present invention also provides a sodium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and the aforementioned composite fiber separator.

[0026] The beneficial effects of this invention are:

[0027] This invention addresses key issues in membrane design, such as electrolyte wettability, mechanical strength, and ionic conductivity, through the synergistic effect of polyacrylonitrile, polyamide, and silica. Polyacrylonitrile imparts excellent polarity and electrolyte wettability to the membrane, effectively inhibiting sodium dendrite growth. Polyamide provides superior mechanical properties and heat resistance. Silica, as an inorganic filler, constructs additional ion transport channels and enhances structural stability. Simultaneously, the use of a dual-needle cross-electrospinning technique ensures uniform dispersion of silica at the fiber level, effectively avoiding filler agglomeration problems common in traditional blending or single-nozzle spinning, significantly improving the consistency of electrochemical performance. The resulting membrane exhibits a thermal shrinkage rate of less than 5% at 200℃, a tensile strength exceeding 12 MPa, and significantly better ionic conductivity than traditional polyolefin membranes. Furthermore, the preparation process is simple, requiring only the addition of a dual-nozzle assembly to an existing electrospinning apparatus, without the need for additional complex equipment, and shows promising prospects for industrial application. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 The image shown is a scanning electron microscope image of the composite fiber membrane prepared in Example 1.

[0030] Figure 2 The rate curve of the composite fiber diaphragm prepared in Example 1.

[0031] Figure 3 Cyclic performance tests were conducted on the composite membranes prepared for Examples 1-3 and the sodium-ion batteries assembled with commercial PP and GF, as well as the coulombic efficiency of Example 3.

[0032] Figure 4 The graph shows the ionic conductivity test results for Example 3. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1

[0035] A method for preparing a composite fiber membrane, comprising the following specific steps:

[0036] (1) Dissolve 2g of polyacrylonitrile in 23mL of DMF to prepare 8 wt% spinning solution A, and add 0.2g of SiO2 nanoparticles (average particle size 80 nm) of 10% of polyacrylonitrile by mass to spinning solution A, and ultrasonically disperse for 1 h;

[0037] (2) Dissolve 4g of nylon 6 in 13mL of formic acid to prepare 20 wt% spinning solution B. Electrospinning is carried out using a dual-nozzle electrospinning method: voltage 23 kV, receiving distance 15 cm, spinning solution A propulsion rate 1.0 mL / h, spinning solution B propulsion rate 0.5 mL / h, relative humidity 40%, temperature 25℃. Simultaneous spinning is used to obtain a fiber-interlaced membrane. Vacuum drying at 80℃ for 8 h yields a composite fiber membrane.

[0038] Figure 1The scanning electron microscope image of the composite fiber membrane prepared in this embodiment shows that after the two spinning solutions are electrospun synchronously by dual nozzles, a three-dimensional network fiber structure is formed with random interlacing and overlapping, without obvious defects such as beading, broken fibers, or droplets, and the fiber formation is uniform and complete.

[0039] Example 2

[0040] A method for preparing a composite fiber membrane, comprising the following specific steps:

[0041] (1) Dissolve 5g of polyacrylonitrile in 28.3 mL of DMF to prepare 15 wt% spinning solution A;

[0042] (2) Dissolve 5 g of nylon 66 in 23.2 mL of formic acid, then add 0.05 g of SiO2 nanoparticles (average particle size 80 nm) of 1% of nylon 66 by mass, and ultrasonically disperse for 1 h to prepare a spinning solution B of 15 wt% nylon 66 + 1% SiO2 (based on the mass of nylon 66). Electrospinning is carried out using a dual-nozzle system: voltage 18 kV, receiving distance 15 cm, spinning solution A propulsion rate 0.8 mL / h, spinning solution B propulsion rate 0.3 mL / h, relative humidity 50%, temperature 25℃. Simultaneous spinning is carried out to obtain a fiber-interlaced membrane, which is then vacuum dried at 80℃ for 8 h to obtain a composite fiber membrane.

[0043] Example 3

[0044] A method for preparing a composite fiber membrane, comprising the following specific steps:

[0045] (1) Dissolve 5g of polyacrylonitrile in 45mL of DMF to prepare 10 wt% spinning solution A;

[0046] (2) Dissolve 3g of nylon 6 in 11.2mL of formic acid, then add 0.15g of SiO2 nanoparticles (average particle size 80 nm) of 5% of nylon 6 by mass, and ultrasonically disperse for 1 h. Prepare spinning solution B of 18 wt% nylon 6 + 5% SiO2. Use dual-nozzle electrospinning: voltage 25 kV, receiving distance 15 cm, spinning solution A propulsion rate 1.5 mL / h, spinning solution B propulsion rate 1 mL / h, relative humidity 40%, temperature 25℃, and spin simultaneously to obtain a fiber-interlaced membrane. Vacuum dry at 100℃ for 6 h to obtain a composite fiber membrane.

[0047] Example of implementation effect 1

[0048] Sodium-ion batteries were assembled using the composite fiber membranes prepared in Examples 1-3, commercial polypropylene (PP), and commercial glass fiber (GF), as detailed below:

[0049] Battery assembly: Button cell assembly was carried out in an argon-filled glove box (O2 / H2O levels below 0.1 ppm). The positive electrode was made of NaNi. 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (NFM), polyvinylidene fluoride (PVDF), and Super-P active material were mixed in a mass ratio of 8:1:1. The electrolyte used was 1 M NaPF6 dissolved in a solution of EC and DEC mixed in a 1:1 (v / v) ratio. 100 μL of electrolyte was added to each coin cell, and the counter electrode was sodium metal. The electrochemical performance of the assembled sodium-ion batteries was tested, as detailed below:

[0050] The rate performance and cycle performance of the assembled batteries were tested at room temperature using the LAND battery testing system. For rate performance testing, the charge / discharge current density varied between 0.1C and 3.0C, with a voltage range of 2.0–4.0 V. For cycle performance testing, the charge / discharge current density was set to 0.1C.

[0051] The thermal shrinkage rate of the diaphragm is calculated as (S0-S) / S0×100%, where S0 and S are the areas of the diaphragm before and after high-temperature treatment; the liquid absorption rate of the diaphragm is calculated as (W-W0) / W0×100%, where W0 and W are the weights of the diaphragm before and after immersion in the electrolyte.

[0052] Tensile strength: A tensile testing machine was used to test the diaphragm, which was cut into rectangles of 100 mm × 50 mm. The tensile rate was set to 10 mm / min. -1 .

[0053] The porosity of the membrane was tested using the n-butanol method. The specific procedure was as follows: a membrane of volume V was completely immersed in a n-butanol solution of density ρ. After 1 hour, it was removed, and any remaining liquid on the membrane surface was wiped off with filter paper. The mass of the membrane before and after immersion in butanol was accurately weighed using an electronic balance and recorded as W0 and W1, respectively. The porosity of the membrane was calculated using the following formula: Porosity = (W1 - W0) / ρV

[0054] Stainless steel was used as the electrode, and the test frequency was 0.1 Hz-1 MHz. The equation for calculating ionic conductivity is: σ= L / (RxA). σ is the ionic conductivity, L represents the membrane thickness, R is the volume resistance, and A is the effective contact area of ​​the stainless steel electrode plate.

[0055] Specific capacity is the value obtained by dividing the battery's discharge capacity by the active mass of the positive electrode.

[0056] Capacity retention rate after 200 cycles, with the cycle cutoff voltage range being 2-4V and the charge / discharge current both set at 0.1 C.

[0057] The electrochemical performance test results are shown in Table 1:

[0058] Table 1. Electrochemical performance of sodium-ion batteries assembled with composite membranes prepared in Examples 1-3 and commercial PP and GF.

[0059]

[0060] Examples 1-3 systematically demonstrated the adjustability and feasibility of polyacrylonitrile / nylon / SiO2 composite fiber membranes in terms of material ratio (PAN concentration 8-15%, PA concentration 12-25%), SiO2 addition location and content (1%-10%, which can be added separately to the PAN side or PA side), and dual-needle electrospinning parameters (voltage 18-30 kV, receiving distance 13-15 cm, feed rate 0.3-1.5 mL / h). The examples showed the relationship between structure and performance: Example 1 (SiO2 added only to the PAN side) achieved the highest liquid absorption rate (308%) and relatively high porosity (71%); Example 2 (SiO2 added only to the PA66 side) exhibited excellent tensile strength (9.6 MPa) and thermal stability (shrinkage rate 2.0% at 200℃); Example 3 (SiO2 added only to the PA6 side) achieved the highest ionic conductivity (1.90 mS / cm). Figure 4 By comparing commercial PP membranes and commercial GF membranes, it is demonstrated that the present invention has significant comprehensive advantages in thermal stability, mechanical strength, ionic conductivity and cycling performance. The capacity retention rate after 200 cycles reaches 88.2%-92.5%, which is significantly better than PP (74.8%) and GF (82.0%). Figure 2 This further corroborates the excellent rate performance of the diaphragm in Example 1 at rates of 0.1C-3C. Figure 3 The results show that the half-cell in this embodiment exhibits significantly weaker capacity decay during cycling compared to the half-cell containing a PP separator. After 200 cycles, its capacity retention rate is higher than that of the half-cell containing a CF separator, and its coulombic efficiency is also higher. This demonstrates that the use of the separator in this patent has a positive promoting effect on the cycle stability of the battery.

[0061] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite fiber separator for sodium-ion batteries, characterized in that: The composite fiber separator for sodium-ion batteries is a polyacrylonitrile / polyamide / SiO2 filler composite nanofiber membrane, which is composed of polyacrylonitrile fibers and polyamide fibers stacked alternately, wherein the SiO2 filler is uniformly distributed in either polyacrylonitrile fiber or polyamide fiber.

2. The composite fiber separator for sodium-ion batteries according to claim 1, characterized by: The SiO2 filler is preferably fumed silica with a particle size of 20-200 nm.

3. The composite fiber separator for sodium-ion batteries according to claim 2, characterized by: The polyamide fiber is selected from nylon 6 or nylon 66, with an average diameter of 100-800 nm.

4. The composite fiber separator for sodium-ion batteries according to claim 3, characterized by: The average diameter of the polyacrylonitrile fiber is 80-400 nm.

5. The method for producing a composite fiber separator for a sodium-ion battery according to any one of claims 1 to 4, characterized by, Includes the following steps: (1) Polyacrylonitrile was dissolved in DMF to obtain spinning solution A; polyamide was dissolved in formic acid to obtain spinning solution B; (2) Disperse the SiO2 filler in spinning solution A or spinning solution B using ultrasonication; (3) Using a double-needle or multi-needle electrospinning device, the spinning solution A and the spinning solution B are cross-spun, and the resulting fibers are dried to obtain a composite fiber diaphragm.

6. The method of claim 5, wherein: In step (1), the mass fraction of polyacrylonitrile in spinning solution A is 8%-15%; and the mass fraction of polyamide in spinning solution B is 12%-25%.

7. The method of claim 6, wherein: In step (2), the amount of SiO2 filler added is 1%-10% of the mass of polyacrylonitrile or polyamide.

8. The method of claim 7, wherein: The parameters for cross-spinning in step (3) are: voltage 15-30 kV, receiving distance 10-20 cm, ambient temperature 20-35℃, relative humidity 30%-55%, propulsion rate of spinning solution A 0.5-1.5 mL / h, and propulsion rate of spinning solution B 0.3-1.0 mL / h; the two spinning solutions are sprayed alternately or simultaneously from different needles; the drying is vacuum drying at a temperature of 60-100℃ for 6-12 hours.

9. The application of the composite fiber separator for sodium-ion batteries according to any one of claims 1-4 in sodium-ion batteries.

10. A sodium-ion battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, characterized in that: The separator is the composite fiber separator for sodium-ion batteries as described in any one of claims 1-4.

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