Sodium ion battery using PTFE / PE asymmetric composite diaphragm

Abstract

The invention provides a sodium ion battery applying a PTFE / PE asymmetric composite diaphragm, and belongs to the technical field of sodium ion batteries. Comprising a hard carbon positive electrode, a sodium metal negative electrode, electrolyte and a diaphragm, the diaphragm is a PTFE / PE asymmetric composite diaphragm, the PTFE / PE asymmetric composite diaphragm comprises a polyethylene substrate layer and a hydrophilic modified polytetrafluoroethylene functional layer, the contact angle of the hydrophilic modified polytetrafluoroethylene functional layer is smaller than 60, and the porosity is larger than or equal to 60%. The diaphragm in the sodium ion battery can effectively inhibit growth of dendritic crystals and prevent penetration of the dendritic crystals.

Description

Technical Field

[0001] The present application relates to a sodium ion battery using a PTFE / PE asymmetric composite diaphragm, belonging to the technical field of sodium ion batteries. Background Art

[0002] As a potential energy storage technology, sodium-ion batteries have attracted widespread attention due to the abundance of sodium resources and low cost. However, the sodium negative electrode is prone to produce dendrites and dead sodium during the cycle process, resulting in rapid battery capacity decay and safety hazards, which has become a key bottleneck restricting its commercialization. As a key component inside the battery, the diaphragm not only provides mechanical support and electronic insulation, but also plays an important role in regulating sodium ion transport and interface stability. Although existing commercial polyethylene (PE) diaphragms have certain mechanical stability and ion channel capabilities, their lyophilicity is limited, which can easily lead to uneven distribution of electrolyte at the interface, thereby inducing uneven deposition of sodium metal, resulting in dendrites, dead sodium and other problems, which seriously restrict the stable operation of the battery under fast charging conditions. In order to overcome this bottleneck, material selection and interface design have become research focuses.

[0003] Polytetrafluoroethylene (PTFE) membranes, as highly crystalline and chemically resistant fluoropolymers, exhibit unique advantages. First, PTFE has an extremely low dielectric constant (approximately 2.0). Its intrinsically low polarizability makes it difficult to respond to an applied electric field. This allows it to form an "electric field buffer layer" near the sodium metal interface, effectively smoothing the interfacial electric field distribution and reducing the uneven electrodeposition caused by localized electric field concentration, promoting uniform sodium ion deposition and inhibiting dendrite initiation. Second, PTFE's high chemical stability enables it to maintain structural integrity in a strongly reducing sodium metal environment, ensuring the long-term stability of the separator. Furthermore, surface hydrophilic modification significantly improves the wettability and electrolyte retention of the PTFE membrane, optimizing interfacial electrolyte distribution and promoting the formation of a uniform and dense solid electrolyte interface (SEI) film, further reducing interfacial polarization and the risk of dendrite formation. Furthermore, PTFE's excellent mechanical toughness and porous structure facilitate the formation of stable ion transport channels and a mechanical protective layer, physically constraining sodium deposition and preventing dendrite penetration. Compared with other hydrophilic polymer materials, PTFE has obvious advantages in electrochemical inertness, thermal stability and industrial feasibility, making it an ideal material for constructing high-performance sodium-ion battery composite membranes. Summary of the Invention

[0004] In light of this, the present application provides a sodium-ion battery utilizing a PTFE / PE asymmetric composite diaphragm. This structure is constructed by placing a hydrophilically modified polytetrafluoroethylene (PTFE) membrane near the sodium metal side. By leveraging the significant advantages of both, while maintaining the mechanical stability of the overall diaphragm, the electrolyte retention and wettability at the negative electrode interface are significantly improved. Furthermore, the interfacial electric field distribution and sodium ion deposition behavior are effectively regulated, significantly suppressing the formation of sodium dendrites. This improves the battery's rate performance and cycle stability, demonstrating excellent fast-charging adaptability.

[0005] Specifically, this application is implemented through the following solutions: A sodium ion battery using a PTFE / PE asymmetric composite diaphragm includes a hard carbon positive electrode, a sodium metal negative electrode, an electrolyte and a diaphragm. The diaphragm is a PTFE / PE asymmetric composite diaphragm, which includes a polyethylene (PE) base layer and a hydrophilic modified polytetrafluoroethylene (PTFE) functional layer. The hydrophilic modified polytetrafluoroethylene functional layer has a contact angle of less than 60° and a porosity of ≥60%.

[0006] This application uses a PTFE / PE asymmetric composite membrane as the separator for sodium-ion batteries. The PE layer (on the positive electrode side) protects the ion channel, and the hydrophilic PTFE layer (on the negative electrode side) regulates the interface. The PTFE is assembled in a direction facing the sodium-ion negative electrode. Combining the low dielectric properties of PTFE (approximately 2.0) with the mechanical stability of PE, performance is improved through the following mechanisms: 1. Interface regulation: The PTFE layer regulates the interface environment due to its extremely low polarizability and is difficult to respond to external electric field fluctuations. By utilizing a synergistic mechanism to allow PTFE to adsorb electrolyte, an "electric field buffer layer" can be constructed on the sodium metal surface, effectively homogenizing the near-interface electric field distribution and inducing a more uniform deposition of sodium ions.

[0007] 2. Wetting enhancement: Hydrophilic modified PTFE improves wettability to ether electrolytes and reduces interfacial impedance.

[0008] 3. Dendrite inhibition: The composite structure forms a physical barrier to prevent dendrites from penetrating.

[0009] Furthermore, as a preference: The thickness of the polyethylene base layer is 10-30 μm.

[0010] The hydrophilic modified polytetrafluoroethylene functional layer has a thickness of 10 to 50 μm and a pore size of 0.1 to 0.5 μm. The hydrophilic modified polytetrafluoroethylene functional layer is obtained by plasma hydrophilic modification of polytetrafluoroethylene.

[0011] The electrolyte is an ether electrolyte, which is a solution obtained by dissolving sodium salt in ethylene glycol dimethyl ether (G2), diethylene glycol dimethyl ether (DG), and triethylene glycol dimethyl ether (TG), and the concentration of the sodium salt is 0.5~2M.

[0012] More preferred: The sodium salt is any one of NaPF6, NaFSI or NaTFSI.

[0013] The concentration of the sodium salt is 1-1.5M.

[0014] The hydrophilic modified polytetrafluoroethylene functional layer of the PTFE / PE asymmetric composite diaphragm is assembled facing the sodium metal negative electrode.

[0015] The assembly method of the sodium ion battery using the PTFE / PE asymmetric composite membrane is as follows: Step 1: Prepare a slurry of hard carbon, conductive agent, and binder according to a mass ratio, coat the slurry with copper foil, and then dry and cut the slurry to obtain a hard carbon positive electrode; Step 2: In an argon glove box, a hard carbon cathode is placed in a battery cathode shell, covered with a PTFE / PE asymmetric composite diaphragm, and a hydrophilically modified polytetrafluoroethylene functional layer is assembled facing the anode; Step 3, slowly adding ether electrolyte to soak the PTFE / PE asymmetric composite diaphragm; Step 4: Place a sodium sheet as a sodium ion negative electrode, encapsulate it and let it stand to obtain a sodium ion battery.

[0016] In step one, The conductive agent is SuperP.

[0017] The binder is PVDF.

[0018] The mass ratio of hard carbon, conductive agent, and binder is 8:0.5~1.5:0.5~1.5, and preferably 8:1~1.2:1~1.2. In step 3, the amount of ether electrolyte added is 100~150 μL.

[0019] In step 4, the mixture is allowed to stand at room temperature (25±5°C) for 12 to 24 hours.

[0020] The beneficial effects of the present invention are: 1) Hydrophilic modification significantly enhances the wettability of PTFE membrane to ether electrolyte ethylene glycol dimethyl ether (G2), especially on the sodium metal side, where it can maintain local electrolyte concentration and slow down drying or polarization. Compared with the natural lyophilic PE membrane with uneven pore distribution and easy collapse, the modified PTFE can stably absorb the electrolyte, ensure the continuity of sodium ion transmission, and reduce the interfacial impedance.

[0021] 2) In terms of interfacial structure construction, the hydrophilically modified PTFE is no longer completely inert and can participate in the interfacial induction of SEI formation, promoting the formation of a thinner, denser, and lower-impedance SEI on the surface of metallic sodium, which facilitates the rapid deintercalation and deposition of sodium ions at high rates. When regulating deposition behavior and electric field distribution, the high mechanical stability and surface uniformity of the modified PTFE membrane create a uniform electric field environment at the sodium end. Combined with its low dielectric properties, this reduces dendrite formation and dead sodium. The composite membrane combines the uniform ion flow distribution of PE with the reduced high-rate polarization of PTFE to delay the formation of sodium dendrites.

[0022] 3) The constructed modified PTFE / PE composite gradient membrane structure has PE maintaining good ion channels on the positive electrode side, and modified PTFE stabilizing the interface and wetting the electrolyte on the negative electrode side. Compared with traditional single-layer PE membranes, the composite membrane of the present invention can still maintain a high specific capacity output under high rate conditions such as 5C, achieving a synergistic effect of fast sodium ion channels and interface buffer protection layer at high rates, showing excellent fast charging adaptability and cycle stability, and is suitable for the construction of high-performance sodium ion batteries, especially for battery systems using ether electrolyte systems and hard carbon to sodium structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0024] Figure 1 This is the assembly order of the PTFE / PE composite diaphragm in this application, Numbers in the figure: 1. Sodium metal sheet; 2. PTFE functional layer; 3. PE base layer; 4. Hard carbon electrode; Figure 2 The long-cycle stability of sodium-ion batteries composed of different separators under 0.3C charge and discharge conditions; Figure 3 The long-cycle stability of sodium-ion batteries composed of different separators under 5C charge and discharge conditions; Figure 4 Rate performance test of sodium ion batteries composed of different membranes. DETAILED DESCRIPTION

[0025] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, the technical solutions in the embodiments of this application will be further described in detail below in conjunction with the drawings in the embodiments of this application. It should be understood that the specific embodiments described herein are only used to explain this application and are not used to limit the technical solutions of this application. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of this application.

[0026] Example 1

[0027] The sodium ion battery assembly method of this embodiment is as follows: (1) Preparation of hard carbon electrode Material preparation: Weigh hard carbon powder, conductive agent (SuperP), and binder (polyvinylidene fluoride, PVDF), take an appropriate amount of N-methylpyrrolidone (NMP) solvent, and prepare PVDF into a 6wt% solvent for later use.

[0028] Slurry preparation: Mix the above-mentioned hard carbon powder, SuperP and PVDF in a mass ratio of 8:1:1, and stir with a homogenizer for at least 3 times until a uniform slurry is formed.

[0029] Coating and drying: Use a small heated coating dryer to evenly coat the slurry on the copper foil current collector. Use a scraper to control the thickness in the range of 100-200 μm. Place in a vacuum drying oven and pre-dry at 80°C for 12 hours.

[0030] Electrode cutting: The dried electrode sheets were cut into discs with a diameter of 12 mm using a slicer.

[0031] Final drying: Place the cut electrode pieces in a vacuum box and further dry them at 80°C for 6 hours to remove residual solvent. The obtained hard carbon electrode pieces 4 are ready for use in battery assembly.

[0032] (2) Preparation of electrolyte The electrolyte solvent system is ethylene glycol dimethyl ether (G2), and the sodium salt is sodium hexafluorophosphate (NaPF6). In a dry glove box, add the sodium salt to the solvent and mix thoroughly using magnetic stirring until dissolved and transparent. This gives an electrolyte solution with a sodium salt concentration of 1M for later use.

[0033] The entire process was completed in a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm) to prevent moisture or oxygen from contaminating the electrolyte.

[0034] (3) Assembly of button batteries Prepare the dried hard carbon electrode 4, the sodium metal sheet 1 (14 mm diameter, approximately 50 μm thick), the PTFE / PE asymmetric composite diaphragm formed by the PTFE functional layer 2 and the PE base layer 3, and the electrolyte. The battery is assembled in an argon (Ar) atmosphere glove box, maintaining the water and oxygen content below 0.01 ppm.

[0035] Assembly steps: (1) Place the hard carbon electrode 4 at the bottom of the stainless steel positive electrode shell and weigh the mass of the active material; (2) Place the diaphragm. Use a PTFE / PE asymmetric composite diaphragm with the hydrophilic PTFE functional layer 2 facing the side of the sodium metal sheet 1.

[0036] (3) Add a certain amount of electrolyte (usually 100–150 μL) to wet the diaphragm.

[0037] (4) Place the sodium metal sheet 1 on top of the PTFE / PE asymmetric composite diaphragm to ensure good contact. At this point, the positional relationship between the sodium metal sheet 1, PTFE functional layer 2, PE base layer 3 and hard carbon electrode 4 is as follows: Figure 1 shown.

[0038] (5) Place the gasket and spring sheet, assemble the negative electrode shell and press-fit the package to form a CR2032 button battery.

[0039] (6) After assembly, let the battery stand for more than 12 hours to promote electrolyte infiltration and interface stabilization.

[0040] Example 2

[0041] The configuration of this embodiment is the same as that of embodiment 1, except that the mass ratios of hard carbon powder, SuperP, and PVDF are as shown in Table 1.

[0042] Table 1: Effect of different hard carbon pole piece compositions (mass ratio) .

[0043] The results in Table 1 show that when the SuperP content is insufficient (e.g., 0.5 parts in No. 1 in Table 1), the conductive agent fails to form a continuous network, electron transport between the hard carbon particles is hindered, and kinetic polarization deteriorates. When the PVDF content is excessive (e.g., 1.5 parts in Nos. 1, 2, 3, 5, and 10 in Table 1), the binder blocks pores, reducing ion transport channels and increasing interfacial impedance. When the PVDF content is critically insufficient (e.g., Nos. 4 and 7 in Table 1), the bonding strength approaches the material limit, while an excessive amount of SuperP (e.g., No. 7 in Table 1) squeezes the porosity and causes bonding failure after electrolyte infiltration. In the later stages of cycling, the electrode structure collapses, the active material separates from the current collector, and bare copper foil is exposed upon disassembly. When the SuperP / PVDF ratio is too low (e.g., 9:0.5:0.5 in No. 12), the two-component ratio is insufficient, failing to effectively encapsulate a high proportion of hard carbon particles. This leads to a dramatic increase in interparticle contact resistance and low active material utilization. Mechanical strength is also weak, and the electrode pulverizes and falls off during charge and discharge volume changes (obvious cracks observed by SEM).

[0044] Therefore, the mass ratio of hard carbon powder, SuperP, and PVDF can be selected to be 8:0.5-1.5:0.5-1.5, which is superior to the performance of conventional pure PE separator systems (conventional systems have a 0.3C capacity retention rate of ≤75% and a 5C rate capacity of ≤120mAh / g). The optimal mass ratio of hard carbon powder, SuperP, and PVDF is 8:1-1.2:1-1.2.

[0045] Example 3

[0046] The configuration of this embodiment is the same as that of embodiment 1, except that the sodium salt NaPF6 in the electrolyte is replaced by sodium fluorosulfonyl imide (NaFSI) or sodium bis(trifluoromethanesulfonyl)imide (NaTFSI).

[0047] Results show that NaFSI's strong nucleophilicity corrodes sodium metal, causing localized SEI damage and a collapse in Coulombic efficiency (<90%). The bulky anions of NaTFSI increase electrolyte viscosity by 20%, hindering ion migration and leading to concentration polarization and a 30% increase in dendrite density. However, both the sodium salts of Examples 1 and 3 demonstrate significant superiority over conventional pure PE separator systems, achieved through the synergistic effect of a PTFE / PE asymmetric composite separator. The lowest 0.3C cycle retention (75%) is greater than 70% of that of a conventional PE separator, while the lowest 5C rate capacity (135 mAh / g) is higher than the 120 mAh / g of a conventional PE separator. NaPF6 is the optimal sodium salt due to its optimal overall performance (5C capacity of 210 mAh / g and Coulombic efficiency of 99.2%). NaFSI and NaTFSI are suitable for cost-sensitive applications with lower rate requirements (a 15%-20% reduction in cycle life is acceptable). Therefore, in the ether electrolyte system given in this case, the sodium salt can be NaPF6, NaFSI or NaTFSI, and NaPF6 is the best.

[0048] Example 4

[0049] The configuration of this embodiment is the same as that of embodiment 1, except that the concentration of sodium salt in the electrolyte is replaced from 1M to 0.5M, 1.5M, and 2M, respectively.

[0050] Results show that at a sodium salt concentration of 0.5M, insufficient charge carriers lead to a 30% increase in internal resistance and a 30% 2C capacity decay. At a sodium salt concentration of 2M, high viscosity impairs membrane wettability, and the increased desolvation barrier leads to a 40% 5C capacity decay. However, at both 0.5M and 2M concentrations, the synergistic effect of the PTFE / PE asymmetric composite separator significantly outperformed the conventional pure PE separator system across all concentrations (0.5-2M): the lowest 0.3C cycle retention (82%) was greater than 70% of that of a conventional PE separator, and the lowest 5C rate capacity (126 mAh / g) was higher than the 120 mAh / g of a conventional PE separator. The 1-1.5M concentration range exhibited the best overall performance, and the optimal solution was achieved in ether solvents when the sodium salt concentration was controlled between 1 and 1.5M. This not only ensured ionic conductivity but also maintained interfacial reaction kinetics through anion-solvent synergy, fully leveraging the electric field control advantages of the PTFE / PE separator. Therefore, in the ether electrolyte system given in this case, the sodium salt concentration can be controlled in the range of 0.5~2M, and the optimal range is 1~1.5M.

[0051] Comparative Example 1

[0052] This comparative example has the same configuration as Example 1, except that the solvent system of the electrolyte adopts a carbonate system: 1MNaPF6 is dissolved in EC / DMC / EMC (3:5:2 volume ratio). The results show that the strong solvation properties of carbonate lead to an increase in the desolvation energy barrier of sodium ions, causing interfacial kinetic hysteresis. The formed SEI film is loose and porous and continuously breaks and regenerates, resulting in accelerated dendrite growth (short-circuit rate >40% after 100 cycles at 5C). At the same time, the 0.3C capacity retention rate drops sharply to below 75%, far lower than that of ether G2.

[0053] Comparative Example 2

[0054] The configuration of this comparative example is the same as that of Example 1, except that the diaphragm is a pure PE diaphragm.

[0055] The results show that when PE is used as a separator, the insufficient lyophilicity leads to uneven distribution of electrolyte at the interface, resulting in uneven sodium deposition. Moreover, polarization is intensified at high rates, and sodium dendrites penetrate the separator.

[0056] Comparative Example 3

[0057] The configuration of this comparative example is the same as that of Example 1, except that the diaphragm is a pure GF diaphragm.

[0058] The results show that when GF is used as a separator, the mechanical strength of GF is insufficient and the pore size is too large to constrain dendrite growth.

[0059] The long cycle performance of Comparative Examples 2 and 3 is compared with that of Example 1. Figure 2 、 3 As shown (the arrow pointing to the left refers to the changing trend of the specific capacity of different membranes with the number of cycles, and the arrow pointing to the right refers to the changing trend of the Coulombic efficiency of different membranes with the number of cycles): the long-term cycle stability of different membranes under 0.3C and 5C charge and discharge conditions is respectively demonstrated. The results show that the PTFE / PE asymmetric composite membrane performs excellently at both low and high rates. The specific capacity retention rate exceeds 85% after 700 cycles and still exceeds 80% after 1500 cycles. The Coulombic efficiency is stable at above 90%. The pure PE membrane, pure GF membrane and the sequentially replaced PE / PTFE composite membrane are obviously inferior in cycle stability and high-rate adaptability due to insufficient lyophilicity, poor mechanical strength or structural design defects. This verifies the technical advantages of the PTFE / PE asymmetric composite membrane in regulating the interfacial electric field and enhancing wettability through the hydrophilic modified PTFE layer, combining with the PE base layer to maintain ion channels, synergistically inhibiting dendrites, and optimizing the electrolyte distribution to improve battery performance. Figure 4The rate performance results show that: the PE / PTFE diaphragm has the highest specific capacity in the entire rate range, and has good performance at a high rate of 5C. The specific capacity can be quickly restored after dropping back to 0.1C, reflecting excellent rate adaptability; the specific capacity of the PE diaphragm decreases significantly with the increase of rate, and is severely attenuated at 5C, and the low-rate recovery is limited, because the high-rate polarization is aggravated by poor lyophilicity; the GF diaphragm has the lowest specific capacity and drops sharply at high rates, which is due to insufficient mechanical strength and large pore size, which cannot constrain dendrites and the electrode structure is easily destroyed. The specific capacity of the PTFE / PE asymmetric composite membrane is close to the theoretical value of more than 330mAh / g at a low rate of 0.1C, and the performance decay is small within a wide rate range. The specific capacity of the pure PE membrane is about 300mAh / g at 0.1C, and drops sharply to below 120mAh / g at 5C. The specific capacity of the pure GF membrane decays rapidly to below 100mAh / g above 1C. This verifies that the PTFE / PE asymmetric composite membrane has the synergistic effect of "PE base layer ion channel + hydrophilic PTFE functional layer interface buffer", with low dielectric properties to uniform electric field and enhance electrolyte infiltration, showing excellent fast charging adaptability at a wide rate, providing support for high-performance sodium-ion battery applications.

[0060] Comparative Example 4

[0061] This comparative example uses the same setup as Example 1, except that the order of the PTFE / PE asymmetric composite separators is swapped, replacing them with a PE / PTFE asymmetric composite separator. This time, the PE separator faces the sodium metal sheet. The PTFE functional layer, which doesn't face the anode, negates its low dielectric constant advantage, and the poor wettability of the PE base layer exacerbates polarization.

[0062] This application uses Land CT2001 produced by Wuhan Landian Electronics Co., Ltd. to perform constant current charge and discharge performance tests.

[0063] In summary, the present invention proposes a PTFE / PE asymmetric composite membrane for sodium-ion batteries and its dendrite-suppressing configuration design. The hydrophilic modification significantly enhances the PTFE membrane's wettability for ether electrolytes (such as G2), particularly on the sodium metal side, where it can maintain local electrolyte concentration and mitigate drying or polarization. Compared with naturally hydrophilic PE membranes, which have uneven pore distribution and are prone to collapse, the modified PTFE can stably retain the electrolyte, ensuring the continuity of sodium ion transport and reducing interfacial impedance. In terms of interface structure construction, the hydrophilically modified PTFE is no longer completely inert and can participate in the interfacial induction of SEI formation, promoting the formation of a thinner, denser, and lower-impedance SEI on the sodium metal surface, which facilitates the rapid deintercalation and deposition of sodium ions at high rates. When regulating deposition behavior and electric field distribution, the high mechanical stability and surface uniformity of the modified PTFE membrane create a uniform electric field environment on the sodium side. Combined with its low dielectric properties, this reduces dendrites and dead sodium. The composite membrane combines the uniform ion flow distribution of PE with the high-rate polarization reduction of PTFE, delaying the formation of sodium dendrites. Furthermore, the constructed modified PTFE / PE composite gradient membrane structure maintains a good ion channel on the positive electrode side, while the modified PTFE stabilizes the interface and wets the electrolyte on the negative electrode side, achieving a synergistic effect of fast sodium ion channels and an interfacial buffer layer at high rates.

[0064] In the description of the technical solutions in this specification, technical terms such as "implementation method" and "typical embodiment" mean that the specific technical features, material properties or structural designs described are fully included in at least one specific implementation scheme of this patent technology. It should be pointed out in particular that the schematic descriptions of technical features in different embodiments may differ and do not necessarily correspond to exactly the same technical implementation paths. Those skilled in the art should understand that the various technical features disclosed in the specification can be creatively combined and applied in any single or multiple implementation schemes through appropriate technical means.

Claims

1. A sodium ion battery using a PTFE / PE asymmetric composite membrane, comprising a hard carbon positive electrode, a sodium metal negative electrode, an electrolyte, and a membrane, characterized in that: The diaphragm is a PTFE / PE asymmetric composite diaphragm, which includes a polyethylene base layer and a hydrophilic modified polytetrafluoroethylene functional layer. The hydrophilic modified polytetrafluoroethylene functional layer has a contact angle of less than 60° and a porosity of ≥60%.

2. A sodium ion battery using a PTFE / PE asymmetric composite diaphragm according to claim 1, characterized in that: The thickness of the polyethylene base layer is 10-30 μm; the thickness of the hydrophilic modified polytetrafluoroethylene functional layer is 10-50 μm, and the pore size is 0.1-0.5 μm.

3. The sodium ion battery using a PTFE / PE asymmetric composite diaphragm according to claim 1, characterized in that: The electrolyte is an ether electrolyte, which is a solution obtained by dissolving sodium salt in ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, or triethylene glycol dimethyl ether, and the concentration of the sodium salt is 0.5-2M.

4. The sodium ion battery using a PTFE / PE asymmetric composite diaphragm according to claim 3, characterized in that: The sodium salt is any one of NaPF6, NaFSI or NaTFSI.

5. The sodium ion battery using a PTFE / PE asymmetric composite diaphragm according to claim 1, characterized in that: The concentration of sodium salt is 1~1.5M.

6. The sodium ion battery using a PTFE / PE asymmetric composite diaphragm according to claim 1, characterized in that: The hydrophilic modified polytetrafluoroethylene functional layer of the PTFE / PE asymmetric composite diaphragm is assembled facing the sodium metal negative electrode.

7. A sodium ion battery using a PTFE / PE asymmetric composite membrane according to any one of claims 1 to 6, characterized in that: The assembly method of sodium ion battery is as follows: Step 1: Prepare a slurry of hard carbon, conductive agent, and binder according to a mass ratio, coat the slurry with copper foil, and then dry and cut the slurry to obtain a hard carbon positive electrode; Step 2: In an argon glove box, a hard carbon cathode is placed in a battery cathode shell, covered with a PTFE / PE asymmetric composite diaphragm, and a hydrophilically modified polytetrafluoroethylene functional layer is assembled facing the anode; Step 3, slowly adding ether electrolyte to soak the PTFE / PE asymmetric composite diaphragm; Step 4: Place a sodium sheet as the negative electrode, encapsulate it and let it stand to obtain a sodium ion battery.

8. The sodium ion battery using a PTFE / PE asymmetric composite diaphragm according to claim 7, characterized in that: The conductive agent is SuperP, and the binder is PVDF.

9. The sodium ion battery using a PTFE / PE asymmetric composite diaphragm according to claim 7, characterized in that: The mass ratio of hard carbon, conductive agent and binder is 8:0.5~1.5:0.5~1.

5.

10. The sodium ion battery using a PTFE / PE asymmetric composite diaphragm according to claim 7, characterized in that: In step 3, the amount of ether electrolyte added is 100-150 μL.

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