Method for preparing sodium superionic conductor coated pp separator and application thereof in sodium ion battery

Abstract

The application discloses a preparation method of a sodium super-ionic conductor coated PP diaphragm and application of the diaphragm in a sodium ion battery, and belongs to the technical field of battery materials. Mg2+ / Y3+ The NZSP powder is modified by co-doping, a four-layer gradient composite diaphragm structure is designed, a green double-solvent slurry system, gradient coating, in-situ interface cross-linking and gradient hot-pressing densification processes are adopted, and thus a composite diaphragm with high ionic conductance, high interface combination, excellent thermal stability and high sodium dendrite inhibition capacity is prepared. ‑3 The prepared diaphragm has a room-temperature ionic conductance of 2.8*10 S / cm, a sodium ion migration number higher than 0.88, a 150 DEG C thermal shrinkage rate lower than 3%, and a coating peeling strength higher than 150 N / m. When applied to a sodium metal battery, the capacity retention rate reaches 94.5% after 1C cycles for 1000 times, and the stable cycle of a symmetrical battery is more than 1600 hours. The process is green and environmentally friendly, and can be used for large-scale production, significantly improves the safety and cycle life of the battery, and is suitable for large-scale energy storage, low-speed vehicles and power-type sodium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, specifically to a method for preparing a sodium superionic conductor coated PP separator and its application in sodium-ion batteries. Background Technology

[0002] Against the backdrop of the rapid development of large-scale energy storage and lightweight power applications, sodium-ion batteries are gradually becoming an important development direction in energy storage systems due to their advantages such as abundant resources, controllable costs, and process compatibility. To further improve energy density, sodium metal anodes have attracted much attention due to their high specific capacity and low potential. However, they are prone to forming sodium dendrites during charge and discharge, which can not only puncture the separator and cause short circuits but also continuously undergo side reactions with the electrolyte, resulting in short battery life and poor safety, severely limiting their practical applications.

[0003] Commercial polypropylene separators offer good chemical stability and low cost, making them the mainstream substrate for sodium-ion batteries. However, they also have significant drawbacks. Their strong hydrophobic surface makes electrolyte wetting difficult, resulting in low ion conductivity. Furthermore, the low sodium ion transference number leads to significant polarization during charge and discharge, further exacerbating dendrite growth. Additionally, severe high-temperature thermal shrinkage and limited mechanical strength make it difficult to effectively prevent dendrite penetration during long-term cycling, posing significant safety hazards.

[0004] Sodium superionic conductors (NZSPs) possess high ionic conductivity, high mechanical strength, and good electrochemical stability, making them suitable for functional modification when coated onto PP membrane surfaces. However, existing coating technologies generally suffer from severe particle agglomeration, poor coating uniformity, and weak interfacial adhesion with the substrate. Furthermore, they often employ toxic solvents, hindering large-scale production. Single ceramic coatings also struggle to simultaneously achieve the synergistic functions of efficient ion transport, uniform sodium deposition, and interfacial stabilization, resulting in overall performance that still falls short of the requirements for highly stable sodium metal batteries.

[0005] Therefore, it is necessary to propose a method for preparing sodium superionic conductor coated PP separator and its application in sodium-ion batteries. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for preparing a sodium superionic conductor coated PP separator and its application in sodium-ion batteries. Through structural innovation, material innovation and process innovation, this invention solves the problems of powder agglomeration, weak interfacial bonding, insufficient ion conduction efficiency, limited dendrite suppression effect and environmentally unfriendly process of traditional NZSP coated separators. It achieves a comprehensive improvement in the composite separator's performance, including high ion conductivity, high wettability, high thermal stability, strong interfacial bonding and efficient dendrite suppression.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: The sodium superion conductor coated PP membrane provided by the present invention is a four-layer gradient composite structure, which consists of the following layers from top to bottom: (1) surface dendrite-inducing inhibition layer; (2) middle NZSP functional conductive layer; (3) bottom transition bonding layer; (4) PP base film.

[0008] The bottom transitional adhesive layer consists of amino-modified polydopamine, CMC, and nano-SiO2. Through covalent bonding, it significantly enhances the interfacial adhesion between the inorganic coating and the PP base film, preventing coating detachment during cycling. The middle layer is Mg. 2+ / Y 3+ The co-doped NZSP functional layer provides a continuous and efficient sodium ion transport channel, enhancing overall ionic conductivity and providing physical dendrite inhibition. The surface layer, composed of NVP nanosheets and a CNT network, possesses sodium-loving properties and a uniform electric field distribution, inducing uniform sodium ion nucleation, inhibiting lateral dendrite growth, and stabilizing the SEI film while reducing side reactions. The four-layer structure works synergistically to achieve an integrated function of "strong adhesion, high conductivity, uniform deposition, and efficient dendrite inhibition".

[0009] This invention uses Mg 2+ / Y 3+ Co-doped NZSP, with the chemical formula Na 3.2 Zr 1.7 Mg 0.2 Y 0.1 Si2PO 12 Dual-cation co-doping can regulate lattice volume, expand ion transport channels, and increase carrier concentration, resulting in a significant improvement in room-temperature ionic conductivity compared to pure NZSP. By employing a polymer-assisted sol-gel method combined with spray drying granulation, spherical, highly dispersed, and uniformly sized nano-NZSP particles were obtained, with a particle size D50 controlled within the range of 80-150 nm. This significantly improved coating uniformity and density, reduced agglomeration defects, and achieved innovation in NZSP powder materials.

[0010] The intermediate NZSP slurry uses a dimethyl carbonate (DMC) and water dual-solvent system to replace the traditional highly toxic NMP solvent, significantly reducing VOC emissions and improving production safety. The silane coupling agent KH-550 is introduced into the slurry to achieve in-situ chemical bonding between inorganic powders and organic binders. Simultaneously, a compound ion-conducting agent of NaSI and NaTFSI is added to further enhance interfacial ion transport efficiency and reduce interfacial impedance.

[0011] The gradient coating and hot-pressing process employs precise slot coating sequentially from the base layer to the middle layer and then to the top layer, combined with segmented gradient drying to gradually remove solvents and prevent coating cracking, pinholes, and bulging. Gradient hot pressing achieves coating densification and triggers in-situ interfacial crosslinking reactions, significantly improving the peel strength between the coating and the base film, ensuring that the composite diaphragm does not detach or crack during long-term cycling.

[0012] The method for preparing a sodium superionic conductor coated PP separator according to the present invention includes the following steps: (1) Mg 2+ / Y 3+ Preparation of co-doped NZSP nanoparticles: Sodium nitrate, zirconium oxynitrate, magnesium nitrate, yttrium nitrate, tetraethyl orthosilicate, and ammonium dihydrogen phosphate were weighed according to stoichiometric ratio and dissolved in a mixed solvent of deionized water and ethanol. Citric acid was added as a complexing agent and polyethylene glycol as a dispersant, and the mixture was stirred to form a transparent sol. The sol was gelled in a water bath at 60-80℃ for 8-16 h and then vacuum dried at 100-120℃ for 12-24 h to obtain a dry gel. The dry gel was then subjected to segmented calcination: pre-calcination at 350-450℃ for 2-4 h to remove organic matter, followed by high-temperature crystallization at 850-950℃ for 6-10 h to form a NASICON phase structure. The calcined product was then wet-ball-milled in ethanol medium for 8-12 h and spray-dried at an inlet temperature of 180-220℃ and an outlet temperature of 80-100℃ to obtain spherical, highly dispersed NZSP nanoparticles.

[0013] (2) Preparation of three-layer gradient composite coating slurry: Bottom layer transition bonding layer slurry: composed of 30-45wt% amino-modified polydopamine, 40-55wt% CMC, and 5-15wt% nano-SiO2, using a 7:3 mixed solvent of deionized water and isopropanol, with a solid content of 8-12wt%, and high-speed shear dispersion for 1-2h. Middle layer NZSP functional conductive layer slurry: 85-92wt% co-doped NZSP powder, 6-12wt% composite binder, and 1-3wt% ion conduction aid are dispersed in a 6:4 mixed solvent of DMC and water, and then formed into a uniform slurry by planetary ball milling for 4-6h and mechanical stirring for 6-12h. Surface layer dendrite-inducing inhibition layer slurry: 70-80wt% NVP nanosheets, 10-20wt% CNT, and 5-15wt% waterborne SBR / CMC binder are ultrasonically dispersed in deionized water for 1-2h to obtain a low-viscosity uniform coating liquid with a solid content of 5-10wt%.

[0014] (3) Gradient coating and gradient drying: Using commercially available 12-25μm biaxially oriented PP membrane as the base film, the bottom layer, middle layer, and top layer are coated sequentially by slot extrusion coating. The bottom layer thickness is 0.5-1.5μm, the middle layer thickness is 2-8μm, and the top layer thickness is 0.3-1.0μm. After each coating, segmented temperature-controlled drying is performed within a temperature range of 40-100℃ to gradually remove the solvent and avoid coating defects.

[0015] (4) Gradient hot pressing densification and in-situ interfacial crosslinking: The dried composite membrane is subjected to two stages of hot pressing: pre-pressing at 80-100℃ and 0.5-1.0MPa for 1-2 min to eliminate pores; and main pressing at 110-130℃ and 1.5-3.0MPa for 3-5 min to achieve coating densification and trigger the in-situ crosslinking reaction between the coupling agent and the polymer. After hot pressing, vacuum annealing is carried out at 60℃ for 4-8 h to eliminate internal stress and improve structural stability.

[0016] (5) Slitting and vacuum packaging: After the composite diaphragm is cooled to room temperature, it is slitting according to the required size and vacuum-sealed to avoid moisture absorption and contamination, thus obtaining the final product.

[0017] Application in sodium-ion batteries: The composite separator of this invention is applied to sodium metal batteries, using NVP or NVPF as the positive electrode and metallic sodium as the negative electrode. A 1.0M NaPF6-EC / DEC / DMC electrolyte is used, with the addition of 5% FEC film-forming additive. After assembling CR2032 coin cells or pouch cells, they are formed at a low temperature of 45℃, and their rate performance, cycle life, symmetric cell stability, and safety performance are tested.

[0018] In summary, this invention provides a method for preparing a sodium superionic conductor coated PP separator and its application in sodium-ion batteries. This invention has the following beneficial effects: significantly improved ion conduction performance; the room temperature ion conductivity of the composite separator can reach 2.8 × 10⁻⁶. -3 The sodium ion transference number (S / cm) is increased to 0.88-0.92, which is much higher than that of traditional PP separators and ordinary NZSP coated separators, effectively reducing battery polarization and improving rate performance. The interfacial bonding is greatly enhanced. Through the transition layer design and in-situ interface cross-linking, the coating peel strength exceeds 150 N / m. After more than 1,000 cycles, there is no peeling or cracking, and the structural stability is excellent.

[0019] The electrolyte wettability and thermal stability are significantly improved, with an electrolyte contact angle of less than 15° and a significantly increased wetting rate. The thermal shrinkage rate after 0.5 hours of heat treatment at 150°C is less than 3%, far superior to pure PP separators, resulting in a substantial improvement in high-temperature safety performance. Simultaneously, it effectively inhibits sodium dendrite growth, with the surface NVP / CNT layer inducing uniform sodium ion deposition, and the middle high-modulus NZSP layer providing physical barriers. Na||Na symmetric cells can achieve a current of 0.5 mA / cm². 2 Under stable cycling conditions, it can be cycled for more than 1600 hours without obvious dendrite growth or short circuits.

[0020] The battery cycle and rate performance are significantly improved. In the Na|| NVP battery system, the capacity retention rate reaches 94.5% after 1000 cycles at 1C rate, and the capacity retention rate exceeds 91% at 5C high rate. The coulombic efficiency is stable at over 99.5%. Moreover, it is green and environmentally friendly and can be mass-produced. It adopts a low-toxicity water / DMC dual solvent system, which significantly reduces VOC emissions. The process route is compatible with existing commercial separator coating production lines, and the yield rate is higher than 98%, making it suitable for continuous industrial production. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the preparation process of the sodium superionic conductor coated PP composite separator of the present invention; Figure 2 This is a schematic diagram comparing the microstructure of the composite membrane coating of the present invention with that of the traditional NZSP coating.

[0022] Figure 3 This is a schematic diagram of the gradient multilayer structure of the sodium superionic conductor coated PP composite membrane of the present invention; Figure 4 This is a schematic diagram of the composite separator of the present invention used in a sodium metal battery. Figure 3 In the middle layer: 100—PP base film, 200—bottom transition adhesive layer, 300—middle layer Mg 2+ / Y 3+ Co-doped NZSP functional conductive layer, 400-surface dendrite-induced suppression layer; Figure 4 In the middle: 1—sodium metal negative electrode, 2—composite separator of the present invention, 3—electrolyte, 4—positive electrode sheet, 5—copper foil for negative electrode current collector, 6—aluminum foil for positive electrode current collector, 7—battery casing; Detailed Implementation

[0023] The invention will now be further described with reference to the accompanying drawings.

[0024] like Figures 1 to 4 As shown: This invention relates to a method for preparing a sodium superion conductor coated PP separator and its application in sodium-ion batteries. The sodium superion conductor coated PP separator is a four-layer gradient composite structure, which consists of the following layers from top to bottom: (1) a surface dendrite-inducing inhibition layer; (2) a middle NZSP functional conductive layer; (3) a bottom transition bonding layer; and (4) a PP base film.

[0025] Example 1 This embodiment provides a complete method for preparing a sodium superionic conductor coated PP membrane, and describes its structure and properties. First, according to the target chemical formula Na... 3.2 Zr 1.7 Mg 0.2 Y 0.1 Si2PO12 According to the stoichiometric ratio, sodium nitrate, zirconium oxynitrate, magnesium nitrate, yttrium nitrate, tetraethyl orthosilicate, and ammonium dihydrogen phosphate were accurately weighed and added to a mixed solvent of deionized water and anhydrous ethanol. The mixture was stirred continuously at room temperature until completely dissolved. Citric acid was then added as a complexing agent, and polyethylene glycol as a dispersant. Stirring continued until the system became transparent and homogeneous, forming a stable precursor sol. The sol was heated in a 70°C constant temperature water bath for 12 hours to allow it to fully transform into a gel. It was then transferred to a vacuum drying oven and dried at 110°C for 18 hours to obtain a loose, dry gel precursor. The dry gel was placed in a high-temperature box furnace for segmented calcination. First, it was pre-calcined at 400°C for 3 hours to fully remove organic matter and residual solvent, then the temperature was increased to 900°C and held for 8 hours to allow the material to completely crystallize and form a pure-phase NASICON structure. The calcined product was wet-ball-milled with ethanol for 10 hours, followed by spray drying with the inlet temperature controlled at 200℃ and the outlet temperature at 90℃. This resulted in Mg2+ with high sphericity, good dispersibility, and a particle size D50 of approximately 120nm. 2+ / Y 3+ Co-doped NZSP nanoparticles.

[0026] Subsequently, a three-layer gradient coating slurry was prepared. The bottom transitional bonding layer slurry consisted mainly of amino-modified polydopamine, CMC, and nano-silica, mixed in a mass ratio of 40:50:10. A mixed solvent of deionized water and isopropanol in a volume ratio of 7:3 was used to adjust the solid content to 10%, and the mixture was dispersed by high-speed shearing for 1.5 hours to form a uniform and stable low-viscosity coating liquid. The middle NZSP functional conductive layer slurry consisted mainly of the above-mentioned co-doped NZSP powder, combined with a composite binder and an ion conduction aid. The mass fractions of each component were 90% NZSP powder, 9% composite binder, and 1% ion conduction aid. The composite binder was composed of PVDF-HFP, polyacrylic acid, and silane coupling agent KH-550 in a ratio of 55:40:5. The ion conduction aid was composed of NaSI and NaTFSI in a 1:1 ratio. The solvent was a mixture of dimethyl carbonate and deionized water in a volume ratio of 6:4. The slurry was formed by planetary ball milling for 5 hours and mechanical stirring for 8 hours. The surface dendrite-inducing inhibition layer slurry is made by mixing NVP nanosheets, carbon nanotubes and water-based SBR / CMC binder in a mass ratio of 75:15:10, using deionized water as solvent, controlling the solid content at 8%, and obtaining a uniform coating liquid after ultrasonic dispersion for 1 hour.

[0027] Using a 25μm thick commercial biaxially oriented PP membrane as the base film, a three-layer gradient coating was applied sequentially using a slot extrusion coating machine. First, a bottom transition bonding layer was coated on the base film surface, with the wet film thickness controlled to approximately 1μm after drying. Solvent was gradually removed through segmented gradient drying. Next, a middle NZSP functional conductive layer was coated on top of the bottom layer, with a drying thickness of approximately 5μm. Finally, a surface dendrite-inducing inhibition layer was coated, with a drying thickness of approximately 0.5μm. Each layer was dried using a gentle temperature rise process to avoid defects such as cracking and pinholes caused by rapid solvent evaporation. The dried composite membrane was then subjected to gradient hot pressing. Pre-pressing was performed at 90℃ and 0.8MPa for 1.5 min to eliminate internal pores in the coating, followed by main pressing at 120℃ and 2.5MPa for 4 min to densify the coating and trigger silane coupling agent-mediated in-situ interfacial crosslinking. After hot pressing, the membrane was annealed in a vacuum environment at 60℃ for 6 h to eliminate internal stress and improve structural stability. Finally, the composite diaphragm is cut to the required size and vacuum-sealed for moisture protection to obtain the finished diaphragm.

[0028] Example 2 The difference between this embodiment and Embodiment 1 lies in adjusting the doping ratio of NZSP, using Na... 3.0 Zr 1.8 Mg 0.1 Y 0.1 Si2PO 12 The composition of the coating remained consistent, with all other raw materials, powder preparation process, slurry preparation method, coating parameters, hot pressing regime, and post-treatment process remaining the same. The effects of the doping ratio on the ionic conductivity and density of the coating were investigated by appropriately reducing the magnesium and yttrium doping amounts. The overall structure of the resulting composite membrane was similar to that of Example 1, still a four-layer gradient structure, with the thickness of each layer controlled within the same range, used to compare the differences in electrochemical performance under different doping amounts.

[0029] Example 3 The difference between this embodiment and Embodiment 1 is that the thickness of the middle NZSP functional conductive layer is reduced, and its thickness is controlled to 3 μm after drying. The thicknesses of the bottom and top layers remain unchanged, and the coating sequence, solvent system, hot pressing parameters, and annealing process are also unchanged. By reducing the thickness of the functional layer, the effect of coating thickness on the overall porosity of the diaphragm, ion transport resistance, and sodium dendrite suppression ability is investigated, providing a process reference for balancing performance and cost in actual production.

[0030] Comparative Example 1: This comparative example uses existing conventional NZSP coating technology, with undoped pure NZSP powder as raw material. After ordinary ball milling, the particle size distribution is relatively wide and agglomeration is obvious. The slurry is prepared with 85wt% NZSP powder, 12wt% PVDF binder, and 3wt% SuperP conductive agent, using traditional NMP as solvent. It is directly coated on the surface of the PP separator in a single layer. The coating thickness after drying is approximately 6μm. After drying at 80℃ and hot pressing at 100℃, the comparative sample is obtained. No transition layer or surface-induced structure is set, and no doping modification or green solvent system is used.

[0031] Comparative Example 2: This comparative example directly uses a commercially available Celgard 2400 PP membrane with a thickness of 25 μm without any coating or modification treatment, as a blank control group to compare the improvement of the composite membrane of the present invention in wettability, thermal stability, ionic conductivity and battery cycle performance.

[0032] Performance Testing and Result Analysis: Ionic conductivity, sodium ion transport number, thermal shrinkage rate, coating peel strength, symmetric cell cycle, and full cell cycle tests were conducted on Examples 1-3 and Comparative Examples 1-2, respectively. The results showed that the composite separator of Example 1 achieved a room temperature ionic conductivity of 2.8 × 10⁻⁶. -3 The coating exhibits a strength of S / cm, a sodium ion transference number of 0.90, a heat shrinkage rate of only 2.3% after 0.5 hours of heat treatment at 150℃, and a peel strength of 168 N / m at 0.5 mA / cm. 2 At the specified current density, the Na||Na symmetric cell can cycle stably for over 1600 hours, and in the Na||NVP full cell, the capacity retention after 1000 cycles at 1C rate is 94.5%. In Example 2, due to the reduced doping amount, the ionic conductivity is slightly lower, but still significantly better than Comparative Example 1. In Example 3, due to the thinner functional layer, the ionic impedance is lower, but the dendrite resistance is slightly lower than in Example 1. The ionic conductivity of Comparative Example 1 is only 1.0 × 10⁻⁶. -3 The thermal shrinkage rate was 8.5%, and a micro-short circuit appeared after approximately 980 hours of cycling. Comparative Example 2 showed the worst performance, with an ionic conductivity of only 8.5 × 10⁻⁶. -5 With a thermal shrinkage rate as high as 32.6% (S / cm), symmetrical batteries fail in less than 200 hours of cycling. The test results clearly demonstrate that this invention, through structural design, material doping, and process optimization, achieves a comprehensive improvement in the overall performance of the separator, exhibiting significant technical advantages and application value.

[0033] The embodiments described in this invention are for illustrative purposes only and do not constitute a limitation on the scope of the claims. Other substantially equivalent substitutions that can be conceived by those skilled in the art are all within the scope of protection of this invention.

Claims

1. A method for preparing a sodium superionic conductor coated PP membrane, characterized in that, Includes the following steps: Step 1: Preparation of Mg 2+ / Y 3+ Co-doped Na 3.2 Zr 1.7 Mg 0.2 Y 0.1 Si2PO 12 Nanoparticles were obtained by using a polymer-assisted sol-gel method combined with segmented calcination and spray drying granulation to obtain spherical, highly dispersed NZSP powder with a particle size D50 of 80-150 nm. Step 2: Prepare three-layer gradient composite coating slurry, including bottom transition bonding layer slurry, middle NZSP functional conductive layer slurry, and surface dendrite induction inhibition layer slurry; Step 3: Gradient coating is performed on the commercial PP separator base film. First, the bottom transition bonding layer is coated, then the middle NZSP functional conductive layer is coated, and finally the surface dendrite-inducing inhibition layer is coated. Then, the film is dried in stages. Step 4: Perform gradient hot pressing densification and in-situ interface crosslinking treatment on the coated composite membrane to enhance the bonding strength between the coating and the base film, as well as between the coating layers. Step 5: After vacuum annealing to eliminate internal stress, the product is slit, vacuum packaged, and then coated with sodium superionic conductor PP composite membrane.

2. The method for preparing a sodium superionic conductor coated PP separator according to claim 1, characterized in that, The segmented calcination process described in step (1) is as follows: pre-calcination at 350-450℃ for 2-4 hours, and high-temperature crystallization at 850-950℃ for 6-10 hours; the spray drying process parameters are inlet temperature 180-220℃ and outlet temperature 80-100℃.

3. The method for preparing a sodium superionic conductor coated PP separator according to claim 1, characterized in that, The bottom transition bonding layer slurry mentioned in step (2) is composed of amino-modified polydopamine, carboxymethyl cellulose (CMC) and nano-silica, with deionized water and isopropanol as the solvent and a solid content of 8-12 wt%.

4. The method for preparing a sodium superionic conductor coated PP separator according to claim 1, characterized in that, The mass ratio of the intermediate NZSP functional conductive layer slurry in step (2) is: Mg 2+ / Y 3+ The slurry contains 85-92 wt% co-doped NZSP powder, 6-12 wt% composite binder, and 1-3 wt% ion-conducting agent. The composite binder is composed of PVDF-HFP, PAA polyacrylic acid, and KH-550 silane coupling agent in a mass ratio of 50-60:30-40:5-10. The ion-conducting agent is a compound system of sodium succinimide (NaSI) and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) in a mass ratio of 1:

1. The slurry uses dimethyl carbonate (DMC) and deionized water as a mixed solvent.

5. The method for preparing a sodium superionic conductor coated PP separator according to claim 1, characterized in that, The surface dendrite-inducing inhibition layer slurry mentioned in step (2) is composed of Na3V2(PO4)3 nanosheets, carbon nanotubes (CNTs), and water-based SBR / CMC binder, with deionized water as the solvent and a solid content of 5-10 wt%.

6. The method for preparing a sodium superionic conductor coated PP membrane according to claim 1, characterized in that, The gradient hot pressing process described in step (4) includes: pre-pressing stage: temperature 80-100℃, pressure 0.5-1.0MPa, time 1-2min; main pressing stage: temperature 110-130℃, pressure 1.5-3.0MPa, time 3-5min; after hot pressing, vacuum annealing at 60℃ for 4-8h.

7. The method for preparing a sodium superionic conductor coated PP separator according to claim 1, characterized in that, The prepared composite membrane structure comprises, in sequence: a PP base membrane, a bottom transition bonding layer, a middle NZSP functional conductive layer, and a surface dendrite-inducing inhibition layer; wherein the thickness of the PP base membrane is 12-25 μm, the thickness of the bottom layer is 0.5-1.5 μm, the thickness of the middle layer is 2-8 μm, and the thickness of the surface layer is 0.3-1.0 μm.

8. A sodium superionic conductor coated PP composite membrane, characterized in that, Prepared by the method described in any one of claims 1-7, it has a four-layer gradient composite structure and possesses high sodium ion conductivity, high electrolyte wettability, high thermal stability and efficient sodium dendrite suppression capability.

9. The application of sodium superionic conductor-coated PP composite separator in sodium-ion batteries, characterized in that, According to claim 8, the sodium superionic conductor coated PP composite separator is applied to a sodium metal battery system, wherein the positive electrode is Na3V2(PO4)3 or Na3V2(PO4)2F3, the negative electrode is a sodium metal sheet, and the electrolyte is a sodium salt organic electrolyte containing fluorinated ethylene carbonate (FEC).

10. The application of the sodium superionic conductor coated PP composite separator according to claim 9 in a sodium-ion battery, characterized in that, The assembled sodium metal battery retains ≥94% capacity after 1000 cycles at 1C rate, and the Na||Na symmetric cell maintains capacity at 0.5 mA / cm². 2 The stable cycling time at current density exceeds 1600 hours.

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