A porous thin film with vertical channel structure, its preparation method and application

Abstract

This invention belongs to the field of nanoporous membrane technology, specifically relating to a porous thin film with a vertical pore structure, its preparation method, and its application. The method includes the following steps: S1. Melt-blending a crystalline polymer and a polymer compatible with the crystalline polymer to obtain a blend; S2. Placing the blend on a substrate with an epiphytic crystallization effect, hot-pressing it, and inducing the crystalline polymer to crystallize, forming a thin film precursor with a vertically oriented crystal structure; S3. Etching the thin film precursor with a selective solvent to remove the polymer compatible with the crystalline polymer, obtaining a porous thin film with a vertical pore structure. By controlling the crystal growth direction of the crystalline polymer through the epiphytic crystallization effect, combined with selective etching, a porous thin film with pores perpendicular to the film surface and continuously interconnected is successfully prepared. The tortuosity of the pores is significantly reduced, resulting in a shorter transport path and lower resistance for matter in the vertical direction.

Description

Technical Field

[0001] This invention belongs to the field of nanoporous membrane technology, specifically relating to a porous thin film with a vertical pore structure, its preparation method, and its application. Background Technology

[0002] Porous thin films, as important functional materials, directly determine their performance in various applications due to the tortuosity of their pore structure. Existing methods for preparing porous thin films mainly include non-solvent-induced phase separation (NIPS), thermally induced phase separation (TIPS), electrospinning, and template methods. NIPS induces phase separation by immersing a polymer solution in a non-solvent bath, forming asymmetric or symmetric porous structures. TIPS utilizes temperature changes to induce phase separation in the polymer solution, followed by curing to obtain a porous film. Electrospinning stretches a polymer solution or melt using a high-voltage electric field to form a nanofiber felt-like porous structure. Template methods use sacrificial templates (such as nanoparticles or nanofibers) to create pores. However, the pore structures obtained by these methods are usually disordered and highly tortuous, resulting in a tortuous transport path, high resistance, and low permeability in the direction perpendicular to the film. This disordered or transverse pore structure significantly limits the performance of the film in scenarios requiring efficient vertical transport. Meanwhile, these methods struggle to effectively control the tortuosity of the pore structure through adjustments to preparation parameters (such as solvent type, temperature, spinning voltage, or blending ratio), resulting in an inability to precisely optimize the pore orientation. This becomes a key technological bottleneck restricting the improvement of the vertical transport performance of porous films. Therefore, it is necessary to develop a porous film capable of achieving a vertical pore structure and its preparation method to overcome the shortcomings of existing technologies. Summary of the Invention

[0003] To address the problems mentioned in the background art, this invention proposes a porous thin film with a vertical channel structure and its preparation method. By precisely controlling the crystal growth direction of crystalline polymers through the epigenetic crystallization effect and combining it with selective etching, a porous thin film with channels perpendicular to the film surface and continuously connected is successfully prepared. Compared with the disordered channel film prepared by traditional methods, the channel tortuosity is greatly reduced, the material transport path in the vertical direction is shorter and the resistance is lower, significantly improving the permeability and material transport efficiency of the film, effectively breaking through the application limitations of traditional porous thin films in high-efficiency vertical transport scenarios.

[0004] The technical solution adopted by this invention to solve its technical problem is: to provide a method for preparing a porous thin film with a vertical pore structure, comprising the following steps:

[0005] S1. A crystalline polymer and a polymer compatible with the crystalline polymer are melt-blended to obtain a blend;

[0006] S2. The blend is placed on a substrate with an epigenetic crystallization effect, hot-pressed and the crystalline polymer is induced to crystallize, forming a thin film precursor with a vertically oriented crystal structure;

[0007] S3. The thin film precursor is etched using a selective solvent to remove the polymer that is compatible with the crystalline polymer, thereby obtaining a porous thin film with a vertical channel structure.

[0008] Further, in step S1, the mass ratio of the crystalline polymer to the amorphous polymer compatible with the crystalline polymer is 30:70 to 70:30.

[0009] Furthermore, in step S1, the melt blending temperature is 180~200℃, the rotation speed is 40~60 rpm, and the time is 3~8 min; the blending method is internal mixing or twin-screw extrusion.

[0010] Furthermore, the crystalline polymer includes one or more of polyvinylidene fluoride and its copolymers, high-density polyethylene, and nylon 6.

[0011] Furthermore, when the crystalline polymer is polyvinylidene fluoride and its copolymers, the polymer compatible with the crystalline polymer is selected from one or more of polybutylene succinate and polymethyl methacrylate.

[0012] When the crystalline polymer is high-density polyethylene, the polymer compatible with the crystalline polymer is liquid paraffin.

[0013] Furthermore, in step S2, the substrate with the epigenetic crystallization effect is a polytetrafluoroethylene (PTFE) film.

[0014] Furthermore, step S2 specifically includes:

[0015] S21. The blend is placed on a PTFE substrate and hot-pressed at a temperature of 180°C to 200°C and a pressure of 5MPa to 15MPa, with a melting time of 3 to 7 minutes, a hot-pressing time of 3 to 7 minutes, and a holding time of 2 to 4 minutes to form an initial film.

[0016] S22. The initial film is subjected to isothermal crystallization at a temperature of 140°C to 150°C to induce the crystals of the crystalline polymer to grow perpendicular to the film surface.

[0017] Furthermore, in step S3, the selective solvent includes dichloromethane, tetrahydrofuran, or chloroform, and the etching time is from 0.5 hours to 10 hours.

[0018] The present invention also provides a porous film with a vertical channel structure, wherein the porous film is prepared by epigenetic crystallization and selective etching of the amorphous phase from a blend system composed of a crystalline polymer and an amorphous polymer compatible with the crystalline polymer.

[0019] This invention also provides an application of a porous film with a vertical pore structure in lithium batteries, aiming to solve the problems of high lithium-ion transport resistance, poor rate performance, severe concentration polarization, high internal resistance, and limited cycle life caused by the highly tortuous pores in traditional lithium-ion battery separators. By using a PVDF porous film with highly ordered and nearly vertically interconnected pores as the battery separator, near-linear rapid transport of lithium ions in the thickness direction of the separator is achieved, thereby significantly improving the overall electrochemical performance of the battery.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] (1) This invention precisely controls the crystal growth direction of crystalline polymers through epigenetic crystallization effect and combines selective etching process to successfully prepare porous films with channels perpendicular to the film surface and continuous through-holes. Compared with disordered channel films prepared by traditional methods, the channel tortuosity is greatly reduced, the material transport path in the vertical direction is shorter and the resistance is smaller, which significantly improves the permeability and material transport efficiency of the film and effectively breaks through the application limitations of traditional porous films in high-efficiency vertical transport scenarios.

[0022] (2) This invention adopts a technical approach of synergistic effect between a crystalline polymer compatibility blend system and an epiphytic crystalline substrate. By adjusting key parameters such as polymer blending ratio, crystallization induction temperature, and etching conditions, the orderedness of the pore structure can be precisely controlled. At the same time, this method is compatible with a variety of crystalline polymers (such as PVDF and its copolymers, HDPE, PA6, etc.) and their corresponding compatible polymers. The preparation process does not require complex equipment and has high process stability. It provides a convenient and efficient technical solution for the customized preparation of vertical pore porous films in different application scenarios and has broad prospects for industrial application. Attached Figure Description

[0023] Figure 1 This is a cross-sectional scanning electron microscope image of Example 2;

[0024] Figure 2 The image shows a cross-sectional scanning electron microscope image for comparison.

[0025] Figure 3 The aperture distribution diagrams for Example 2 and the comparative example are shown below.

[0026] Figure 4 This is a comparison chart of porosity between Example 2 and the comparative example;

[0027] Figure 5Electrochemical impedance spectroscopy (a) and ionic conductivity (b) of Example 2 and the comparative example.

[0028] Figure 6 For Example 2 and the comparative example, the Li / Li symmetric cell was tested at 0.25 mA h cm⁻¹. -2 Constant current cyclic lithium deposition / dissolution test results;

[0029] Figure 7 The lithium sheet surface morphology images are shown in the Li / Li symmetric cells of Example 2 and the comparative example after 1000h cycling.

[0030] Figure 8 The graph shows the half-cell cycle capacity and retention rate test results for Example 2 and the comparative example. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0032] Example 1:

[0033] This embodiment provides a method for preparing a porous thin film with a vertical pore structure, the specific steps of which are as follows:

[0034] S1: Melt blending; crystalline polymer polyvinylidene fluoride (PVDF) and amorphous polymer polybutylene succinate (PBSU) were selected as raw materials and mixed at a mass ratio of 50:50. The mixture was added to an internal mixer and melt-blended for 5 minutes at 190°C and 50 rpm to obtain a homogeneous blend.

[0035] S2: Forming and crystallization induction;

[0036] S21: Place the above blend on a polytetrafluoroethylene (PTFE) film substrate, put it into a hot press, and perform hot pressing treatment at 190°C and 10MPa, successively melting for 5 minutes, hot pressing for 5 minutes, and holding pressure for 3 minutes to form an initial film.

[0037] S22: The initial film is transferred to a constant temperature oven and isothermal melt crystallization is performed at 145℃ to induce PVDF crystals to grow in a direction perpendicular to the film surface, thereby obtaining a film precursor with a vertically oriented crystal structure.

[0038] S3: Selective etching; The film precursor was immersed in chloroform for 3 hours at room temperature to remove PBSU. After washing and drying, a porous film with a vertical channel structure was obtained, as shown in the scanning electron microscope image. Figure 1 As shown.

[0039] Example 2:

[0040] This embodiment provides a method for preparing a porous thin film with a vertical pore structure, the specific steps of which are as follows:

[0041] S1: Melt blending; PVDF and PBSU were selected as raw materials and mixed at a mass ratio of 30:70. A twin-screw extruder was used for melt blending, with the temperature set at 185℃, the speed at 45 rpm, and the blending time at 6 min, to obtain a homogeneous blend.

[0042] S2: Forming and crystallization induction;

[0043] S21: Place the blend on a PTFE substrate and hot-press it at 185°C and 8MPa. Melt for 4 minutes, hot-press for 6 minutes, and hold for 2 minutes to form an initial film.

[0044] S22: The initial thin film is subjected to melt isothermal crystallization at 142℃ to induce vertical growth of PVDF crystals and obtain the thin film precursor.

[0045] S3: Selective etching; using chloroform as a selective solvent, the film precursor is immersed for 5 hours, PBSU is etched away, and the target porous film is obtained after drying.

[0046] Example 3:

[0047] This embodiment provides a method for preparing a porous thin film with a vertical pore structure, the specific steps of which are as follows:

[0048] S1: Melt blending; PVDF and polymethyl methacrylate (PMMA) are mixed at a mass ratio of 70:30 and placed in an internal mixer. The mixture is melt blended for 3 minutes at 200°C and 60 rpm to obtain a homogeneous blend.

[0049] S2: Forming and crystallization induction;

[0050] S21: Place the blend on a PTFE substrate and hot-press it at 200°C and 12MPa. Melt for 3 minutes, hot-press for 7 minutes, and hold pressure for 4 minutes to form an initial film.

[0051] S22: The initial thin film is subjected to melt isothermal crystallization at 148℃ to induce vertical growth of PVDF crystals and obtain the thin film precursor.

[0052] S3: Selective etching; using chloroform as a solvent, PMMA is removed by immersion for 1 hour, and a porous film is obtained after drying.

[0053] Example 4:

[0054] This embodiment provides a method for preparing a porous thin film with a vertical pore structure, the specific steps of which are as follows:

[0055] S1: Melt blending; crystalline polymer high-density polyethylene (HDPE) and amorphous polymer liquid paraffin are mixed at a mass ratio of 50:50 and melt blended using a twin-screw extruder. The temperature is set at 180℃, the speed at 40 rpm, and the blending time is 8 min to obtain a homogeneous blend.

[0056] S2: Forming and crystallization induction;

[0057] S21: Place the blend on a PTFE substrate and hot-press it at 180°C and 5MPa. Melt for 7 minutes, hot-press for 3 minutes, and hold pressure for 3 minutes to form an initial film.

[0058] S22: The initial film is subjected to melt isothermal crystallization at 140℃ to induce HDPE crystals to grow perpendicular to the film surface, thereby obtaining the film precursor.

[0059] S3: Selective etching; The film precursor is immersed in chloroform for 10 hours to remove liquid paraffin by etching, and the target porous film is obtained after drying.

[0060] Example 5:

[0061] This embodiment provides a method for preparing a porous thin film with a vertical pore structure, the specific steps of which are as follows:

[0062] S1: Melt blending; PVDF and PMMA were mixed at a mass ratio of 60:40 and melt blended using a twin-screw extruder. The temperature was set at 188℃, the speed at 52 rpm, and the blending time was 7 min to obtain a homogeneous blend.

[0063] S2: Forming and crystallization induction;

[0064] S21: Place the blend on a PTFE substrate and hot-press it at 188°C and 9MPa. Melt for 5 minutes, hot-press for 5 minutes, and hold for 3 minutes to form an initial film.

[0065] S22: The initial thin film is subjected to melt isothermal crystallization at 143℃ to induce vertical growth of PVDF crystals and obtain the thin film precursor.

[0066] S3: Selective etching; using chloroform as a solvent, PMMA is removed by immersion for 2 hours, and the target porous film is obtained after drying.

[0067] Example 6:

[0068] This embodiment provides a method for preparing a porous thin film with a vertical pore structure, the specific steps of which are as follows:

[0069] S1: Melt blending; Select PVDF copolymer (PVDF-TrFE) and PBSU at a mass ratio of 55:45, put them into an internal mixer, and melt blend for 5 min at 192℃ and 48 rpm to obtain a uniform blend.

[0070] S2: Forming and crystallization induction;

[0071] S21: Place the blend on a PTFE substrate and hot-press it at 192°C and 11MPa. Melt for 4 minutes, hot-press for 6 minutes, and hold pressure for 4 minutes to form an initial film.

[0072] S22: The initial thin film is subjected to melt isothermal crystallization at 146℃ to induce vertical growth of PVDF-TrFE crystals, thus obtaining the thin film precursor.

[0073] S3: Selective etching; using chloroform as solvent, PBSU is removed by immersion for 4 hours, and a porous film is obtained after drying.

[0074] Example 7:

[0075] This embodiment provides a method for preparing a porous thin film with a vertical pore structure, the specific steps of which are as follows:

[0076] S1: Melt blending; HDPE and liquid paraffin are mixed at a mass ratio of 60:40 and melt blended using a twin-screw extruder. The temperature is set at 183℃, the speed at 58 rpm, and the blending time is 6 min to obtain a uniform blend.

[0077] S2: Forming and crystallization induction;

[0078] S21: Place the blend on a PTFE substrate and hot-press it at 183°C and 7MPa. Melt for 6 minutes, hot-press for 4 minutes, and hold for 3 minutes to form an initial film.

[0079] S22: The initial film is subjected to melt isothermal crystallization at 141℃ to induce vertical growth of HDPE crystals and obtain the film precursor.

[0080] S3: Selective etching; using chloroform as a solvent, the liquid paraffin is removed by immersion for 7 hours, and the target porous film is obtained after drying.

[0081] Comparative example:

[0082] This comparative example provides a method for preparing porous thin films, the specific steps of which are as follows:

[0083] Melt blending: Crystalline polymer polyvinylidene fluoride (PVDF) and amorphous polymer polybutylene succinate (PBSU) are mixed at a mass ratio of 30:70. The mixture is added to an internal mixer and melt-blended for 5 minutes at 190°C and 50 rpm to obtain a homogeneous blend.

[0084] Molding and crystallization induction: The above blend was placed on a polyimide (PI) film substrate without epigenetic crystallization effect and placed in a hot press. The hot pressing parameters were exactly the same as in Example 1: 190°C and 10MPa, and the mixture was sequentially melted for 5 minutes, hot pressed for 5 minutes, and held under pressure for 3 minutes to form the initial film.

[0085] The initial film was transferred to a constant temperature oven and subjected to melt isothermal crystallization at 145°C to obtain the film precursor.

[0086] Selective etching: The film precursor was immersed in dichloromethane and soaked at room temperature for 3 hours to etch away PBSU. After removal, the film was washed and dried to obtain a porous film. The scanning electron microscope image is shown below. Figure 2 As shown.

[0087] Table 1: Performance Test Data of Examples and Comparative Examples

[0088]

[0089] like Figure 3 The image shows the pore size distribution diagrams for Example 1 and the comparative example; as shown... Figure 4 The figure shown is a comparison of porosity between the embodiment and the comparative example.

[0090] In summary, all examples used a PTFE substrate, which can induce the growth of crystalline polymer crystals perpendicular to the film surface, forming vertically penetrating channels after etching. The comparative example used a PI substrate, which did not induce epigenetic crystal growth; the crystal growth direction was random, and the channels were randomly distributed after etching. Furthermore, the pore size distribution and porosity of each example were similar to the comparative example, with only a significant difference in channel tortuosity. This precisely verifies the effect of the present invention on controlling the channel direction and improving vertical transport performance.

[0091] Example 8:

[0092] 1. Raw materials and equipment related to battery manufacturing

[0093] Positive electrode preparation: Lithium iron phosphate was selected as the active material and mixed with conductive agent SuperP and binder polyvinylidene fluoride (PVDF) at a mass ratio of 8:1:1. N-methylpyrrolidone (NMP) was added to prepare a uniform slurry. The slurry was coated onto the surface of an aluminum foil current collector and dried in a vacuum drying oven at 60℃ for 12 hours. After drying, it was cut into 14mm circular electrode sheets, and the compaction density was controlled to be 1.5g / cm³.3 .

[0094] Negative electrode preparation: A 100μm thick lithium metal foil was selected and cut into 15mm circular electrode sheets to serve as the negative electrode of the battery.

[0095] Membrane selection: The experimental group used the vertical pore PVDF porous film prepared in Example 2, which was cut into 16mm circular pieces with a pore tortuosity of 1.3, an average pore diameter of 150nm, a porosity of 63%, and a film thickness of 60μm. The comparative example used a disordered pore PVDF porous film prepared without epigenetic crystallization effect. Except for the pore structure, the other parameters such as pore diameter, porosity, and film thickness were basically the same as those of the experimental group.

[0096] Electrolyte preparation: Prepare 1.0 M LiPF6 in 1 / 1 / 1 vol% EC / DMC / EMC

[0097] Battery assembly: In an argon-atmosphere glove box (water and oxygen content <0.1ppm), assemble the CR2032 button cell in the following order: "positive electrode shell → sulfur positive electrode sheet → electrolyte → porous membrane separator → electrolyte → lithium foil negative electrode → spring sheet → negative electrode cap". The electrolyte volume for each electrode is 20μL, ensuring that the separator is completely wetted by the electrolyte.

[0098] 2. Battery performance testing methods

[0099] Electrochemical impedance spectroscopy (EIS) and ionic conductivity testing: The assembled battery was tested using an electrochemical workstation with a frequency range of 0.01 Hz to 100 kHz. The ionic conductivity of the battery was calculated simultaneously, and the results are shown in the attached table. Figure 5 .

[0100] Lithium dendrite suppression performance test: Assembled Li / Li symmetric cells, at 0.25 mA·h / cm -2 Constant current cycling lithium deposition / dissolution tests were conducted at a current density of [value missing], and the voltage polarization changes of the battery were recorded. The cycling time was up to 1000 hours. The results are attached [details missing]. Figure 6 After cycling, the battery was disassembled and the surface morphology of the lithium foil was observed using a scanning electron microscope (SEM). The results corresponded to the attached... Figure 7 .

[0101] Cycle capacity and capacity retention testing: The assembled lithium-sulfur half-cells were subjected to charge-discharge cycle tests at room temperature. The charge-discharge rate was 1C, the charging cut-off voltage was 3.8V, and the discharging cut-off voltage was 2.5V. The cycle capacity and capacity retention of the batteries were recorded, and the results are shown in the attached table. Figure 8 .

[0102] 3. Test Results and Analysis

[0103] (1) Electrochemical impedance and ionic conductivity

[0104] Appendix Figure 5 The figures show the electrochemical impedance spectroscopy (a) and ionic conductivity (b) of the experimental group (vertically porous membrane) and the comparative group (disordered porous membrane). It is clear from the figures that the impedance of the experimental group is significantly lower than that of the comparative group, with an ionic conductivity reaching 0.85 mS / cm, while the comparative group's ionic conductivity is only 0.6 mS / cm. This is because the vertically porous membrane has a significantly reduced channel tortuosity, allowing lithium ions to transport almost linearly along the membrane thickness direction, resulting in significantly reduced transport resistance and a substantial increase in ion migration efficiency. In contrast, the high tortuosity of the disordered porous membrane leads to a tortuous lithium ion transport path, increased impedance, and decreased ionic conductivity.

[0105] (2) Cyclic performance of Li / Li symmetric cells

[0106] Appendix Figure 6 The experimental and comparative Li / Li symmetric cells were tested at 0.25 mA·h / cm. -2 The constant current cycling test results for lithium deposition / dissolution are shown below. During the 1000-hour long cycling process, the voltage polarization of the experimental group batteries remained below 0.15V, and the voltage curve was stable without significant fluctuations, showing no short circuit. In contrast, the voltage polarization of the comparative battery increased sharply after approximately 500 hours of cycling, and the curve showed obvious oscillations, indicating that lithium dendrite growth had led to a decrease in the stability of the battery's internal interface. These results demonstrate that the vertical pore structure allows for uniform lithium ion transport within the separator, resulting in a more uniform current distribution within the battery and effectively suppressing the nucleation and disordered growth of lithium dendrites. Disordered pore separators, due to uneven lithium ion transport and excessively high local current density, are highly susceptible to inducing lithium dendrite growth, leading to increased battery polarization.

[0107] (3) Surface morphology of lithium sheet after cycling

[0108] Appendix Figure 7 The images show the SEM morphology of the lithium-ion electrode surfaces in the experimental and comparative Li / Li symmetric batteries after 1000 hours of cycling. The experimental lithium-ion electrode surfaces remained smooth and dense, with no obvious lithium dendrite protrusions or needle-like dendrites, only slight traces of lithium deposition. In contrast, the comparative lithium-ion electrode surfaces were rough and uneven, with a large number of needle-like lithium dendrites. This morphological difference directly confirms the excellent suppression effect of vertically channeled porous films on lithium dendrites. The uniform ion transport environment avoids localized enrichment of lithium ions on the negative electrode surface, thus preventing the growth of lithium dendrites. Conversely, the uneven ion transport in disordered porous membranes leads to the formation of a large number of lithium dendrites.

[0109] (4) Half-cell cycle capacity and retention rate

[0110] Appendix Figure 8The graphs show the cycle capacity and capacity retention of the experimental and comparative lithium-sulfur half-cells. After 400 cycles at 1C, the experimental group battery maintained a high discharge specific capacity with a capacity retention of 85.6%. In contrast, the comparative battery exhibited a rapid capacity decay, with a capacity retention of less than 75% after 500 cycles. This improvement is attributed to two factors: firstly, the high ionic conductivity of the vertical-pore separator enhances the battery's reaction kinetics, reducing capacity loss during charge and discharge; secondly, effective suppression of lithium dendrites prevents internal short circuits caused by dendrites piercing the separator, while also reducing side reactions between lithium dendrites and the electrolyte, thus lowering the consumption of active materials and significantly improving the battery's cycle stability and capacity retention.

[0111] Porous thin films with vertically oriented channels were used as separators in lithium metal batteries (lithium-sulfur systems). Their low tortuosity and vertically interconnected channel structure enabled efficient and uniform lithium-ion transport, increasing the battery's ionic conductivity from 0.6 mS / cm to 0.85 mS / cm. Furthermore, they effectively suppressed the disordered growth of lithium dendrites, enabling Li / Li symmetric batteries to achieve a conductivity of 0.25 mA·h / cm. -2 The lithium sheet surface remained flat after 1000 hours of stable cycling. Furthermore, when applied to lithium-sulfur half-cells, this separator significantly improved the battery's cycle stability, maintaining 85.6% capacity retention after 400 cycles at 1C rate. This embodiment demonstrates that porous films with vertical pore structures, used as lithium battery separators, can fundamentally solve problems such as high ion transport resistance, lithium dendrite growth, and short cycle life caused by traditional disordered pore separators. This significantly improves the overall electrochemical performance of lithium batteries, and the application does not require changes to existing lithium battery assembly processes, possessing extremely high industrial feasibility and application value.

[0112] This invention innovatively introduces the epigenetic crystallization effect by constructing a compatible blend system of crystalline polymers. Guided by substrates such as polytetrafluoroethylene (PTFE) with epigenetic crystallization capabilities, and combining melt blending, directional crystallization induction, and selective etching, it successfully overcomes the core bottleneck of difficult-to-control pore direction and high tortuosity in traditional porous film preparation technology. Comparison results from various embodiments and comparative examples show that, while ensuring the uniformity of pore size distribution and porosity stability of the porous film, this invention precisely controls the crystal growth direction of the crystalline polymer, enabling vertically connected pores after etching. This significantly reduces pore tortuosity and thus greatly improves the material transport efficiency and permeability of the film in the vertical direction.

[0113] Applying porous films with vertical pore structures to lithium-ion batteries can achieve breakthroughs in multiple core performance aspects, demonstrating significant application value and technological advantages. This porous film, through epiphytic crystallization and selective etching processes, constructs a vertically interconnected pore structure that completely overcomes the drawbacks of highly tortuous and disordered pore distribution in traditional lithium-ion battery separators. The tortuosity of the pores is reduced to below 2.0, providing a near-linear transport path for lithium ions along the separator thickness, significantly reducing ion transport resistance—compared to the 0.6 mS / cm ionic conductivity of traditional separators, its ionic conductivity is increased to 0.85 mS / cm, significantly improving ion migration efficiency and concentration polarization. In terms of cycle performance, thanks to the uniform current distribution resulting from the ordered pore structure, the capacity retention rate after 800 cycles at 1C rate increases from 55.8% for traditional separators to 79.3%, effectively extending battery life. Simultaneously, the regular vertical pores suppress the disordered growth of lithium dendrites, keeping the lithium sheet surface smooth after cycling, significantly reducing the safety risks of battery short circuits and thermal runaway. More notably, this application does not require changes to the core fundamental parameters of the separator, such as its material chemical composition, porosity, and pore size distribution. It can achieve a leapfrog performance improvement simply by optimizing the pore orientation. It is not only compatible with various lithium-based battery systems such as liquid, gel, and quasi-solid state, but also has the characteristics of stable process and controllable cost for industrialization. It provides an efficient and feasible solution for upgrading lithium batteries in terms of energy density, power density, and safety, and powerfully promotes the technological progress and industrialization of high-performance lithium-based batteries.

[0114] The technical solution of this invention possesses significant flexibility and versatility. It is compatible with various crystalline polymers such as polyvinylidene fluoride (PVDF) and its copolymers, high-density polyethylene (HDPE), and nylon 6, as well as corresponding compatible amorphous polymer combinations. Furthermore, by adjusting the blending ratio, melt blending parameters, hot pressing conditions, crystallization temperature, and etching parameters, customized preparation of vertically channeled porous films for different application scenarios can be achieved. The entire preparation process requires no complex equipment, exhibits high process stability and strong operability, and not only solves the key technical defects of existing technologies but also provides an efficient and feasible technical path for the industrial application of vertically channeled porous films in multiple fields such as membrane separation, catalyst supports, energy storage, and biomedicine. It possesses significant technological innovation value and broad market prospects.

[0115] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a porous thin film with a vertical pore structure, characterized in that, Includes the following steps: S1. A crystalline polymer and a polymer compatible with the crystalline polymer are melt-blended to obtain a blend. S2. The blend is placed on a substrate with an epigenetic crystallization effect, hot-pressed and the crystalline polymer is induced to crystallize to form a thin film precursor with a vertically oriented crystal structure. S3. The thin film precursor is etched using a selective solvent to remove the polymer that is compatible with the crystalline polymer, thereby obtaining a porous thin film with a vertical channel structure.

2. The method for preparing a porous thin film with a vertical pore structure according to claim 1, characterized in that, In step S1, the mass ratio of the crystalline polymer to the polymer compatible with the crystalline polymer is 30:70 to 70:

30.

3. The method for preparing a porous thin film with a vertical pore structure according to claim 1, characterized in that, In step S1, the melt blending temperature is 180~200℃, the rotation speed is 40~60 rpm, and the time is 3~8 min; the blending method is internal mixing or twin-screw extrusion.

4. The porous thin film with a vertical channel structure according to claim 1, characterized in that, The crystalline polymer includes one of polyvinylidene fluoride and its copolymers, high-density polyethylene, and nylon 6.

5. The method for preparing a porous thin film with a vertical pore structure according to claim 3, characterized in that, When the crystalline polymer is polyvinylidene fluoride and its copolymers, the polymer compatible with the crystalline polymer is selected from polybutylene succinate and polymethyl methacrylate. When the crystalline polymer is high-density polyethylene, the polymer compatible with the crystalline polymer is liquid paraffin.

6. The method for preparing a porous thin film with a vertical pore structure according to claim 1, characterized in that, In step S2, the substrate with epigenetic crystallization effect is a polytetrafluoroethylene (PTFE) film.

7. The method for preparing a porous thin film with a vertical pore structure according to claim 1, characterized in that, Step S2 specifically includes: S21. The blend is placed on a PTFE substrate and hot-pressed at a temperature of 180°C to 200°C and a pressure of 5MPa to 15MPa, with a melting time of 3 to 7 minutes, a hot-pressing time of 3 to 7 minutes, and a holding time of 2 to 4 minutes to form an initial film. S22. The initial film is subjected to melt isothermal crystallization at a temperature of 140°C to 150°C to induce the crystals of the crystalline polymer to grow perpendicular to the film surface.

8. The method for preparing a porous thin film with a vertical pore structure according to claim 1, characterized in that, In step S3, the selective solvent includes dichloromethane, tetrahydrofuran, or chloroform, and the etching time is from 0.5 hours to 10 hours.

9. A porous thin film with a vertical pore structure, characterized in that, The porous film is prepared by epigenetic crystallization and selective etching of the amorphous phase from a blend system composed of a crystalline polymer and a polymer compatible with the crystalline polymer.

10. The application of the porous thin film with a vertical pore structure as described in claim 9 in a lithium battery.

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