Porous polybenzimidazole diaphragm with asymmetric pore diameter as well as method and application of porous polybenzimidazole diaphragm

Abstract

The invention discloses a porous polybenzimidazole diaphragm with an asymmetric aperture as well as a preparation method and application of the porous polybenzimidazole diaphragm. The diaphragm is prepared by adopting a non-solvent induced phase separation method, and a porous structure with asymmetric apertures on the front and back surfaces is formed by regulating and controlling the non-solvent proportion, the phase separation condition and the standing environment in the film forming process. The anode side diaphragm is designed to have a smaller aperture, so that the permeability of electrolyte is improved, excessive diffusion or precipitation of ions is prevented, the ionic conductivity is optimized, the interaction of the anode and the cathode is effectively isolated, and the short circuit phenomenon is prevented. The negative electrode side diaphragm adopts a large aperture design, so that a smoother conduction channel can be provided for lithium ions, meanwhile, the growth of lithium dendrites is inhibited, and the internal resistance of the battery is reduced. According to the design, the ion transmission performance of the battery is obviously optimized, the power output and the energy density are improved, and meanwhile, the cycling stability and the safety are effectively improved. The preparation method disclosed by the invention is simple in process and suitable for large-scale production, and an innovative solution is provided for developing a high-performance lithium metal battery.

Description

Technical Field

[0001] The present invention relates to the field of lithium metal battery materials, and in particular to a porous polybenzimidazole diaphragm with asymmetric pore size and a method and application thereof. Background Art

[0002] With the rapid advancement of science and technology, the miniaturization and lightweight of electronic devices, as well as the rapid development of electric vehicles and large-scale energy storage systems, have put forward unprecedented demands for high-performance, long-life, safe and reliable battery technology. In this context, lithium metal batteries are regarded as key materials to promote the next generation of high-performance energy storage technology revolution due to their extremely high theoretical energy density (thanks to the high specific capacity of lithium metal, reaching 3860mAh / g, far exceeding the currently widely used graphite negative electrode) and the lowest electrochemical potential (-3.04V vs SHE (Standard Hydrogen Electrode)). The application of lithium metal negative electrodes can not only significantly improve the energy density of batteries, but also bring a qualitative leap in the range of electric vehicles and the standby time of portable electronic devices.

[0003] However, one of the biggest challenges faced in the practical application of lithium metal anodes is the formation of lithium dendrites during the charge and discharge cycle. Lithium dendrites are slender structures caused by uneven lithium deposition. They can penetrate the separator layer (i.e., diaphragm) inside the battery, causing internal short circuits in the battery, and then cause thermal runaway and fire, seriously threatening the safety and cycle life of the battery. Therefore, how to effectively inhibit the growth of lithium dendrites has become a key technical bottleneck for the commercial application of lithium metal batteries.

[0004] Traditional commercial separator materials, such as polyethylene (PE) and polypropylene (PP), are low-cost and have good processing performance, but their mechanical strength is limited, especially prone to deformation at high temperatures, and the electrolyte wettability is insufficient, making it difficult to effectively inhibit the penetration of lithium dendrites, and unable to meet the high requirements of lithium metal batteries for separator materials. Therefore, the development of new separator materials to provide higher thermal stability, stronger mechanical strength and optimized electrolyte wettability has become an urgent need to solve the safety problems of lithium metal batteries.

[0005] As a high-performance polymer material, polybenzimidazole (PBI) has become an ideal candidate material for developing high-performance battery separators due to its excellent thermal stability (ability to maintain structural stability at high temperatures), excellent mechanical strength and good chemical stability. PBI separators can not only effectively block the penetration of lithium dendrites, but also maintain the normal operation of batteries under high temperature conditions, thereby improving the overall safety and reliability of the battery. However, despite the many advantages of PBI separators, most of their existing pore size designs adopt symmetrical structures. For example, a patent application with publication number CN109904370A discloses a porous polybenzimidazole polymer electrolyte separator and a preparation method and application, which belongs to the field of advanced lithium ion battery technology. The present invention prepares a porous membrane with high porosity, high solvent absorption, rapid electrolyte wetting and high mechanical strength by a rapid non-solvent induced phase inversion method (NIPS). The separator material of the present invention adopts highly rigid and fully aromatic polybenzimidazole (PBI). The electron-rich nitrogen atoms on the imidazole ring in the PBI molecule can form a coordination effect with lithium ions, improve the conductivity and migration number of lithium ions, and effectively inhibit the growth of lithium dendrites. This design fails to fully consider the different requirements and characteristics of the positive and negative electrode materials in lithium metal batteries during the charging and discharging process. For example, the negative electrode side requires a smaller pore size to effectively suppress lithium dendrites, while the positive electrode side may require a larger pore size to promote rapid electrolyte transport and effective ion exchange.

[0006] Therefore, the existing PBI separator design still has limitations in balancing battery performance and safety, limiting its potential for widespread application in lithium metal batteries. Summary of the invention

[0007] In order to address the deficiencies in the prior art, the purpose of the present invention is to provide a porous PBI membrane based on asymmetric pore size design and a preparation method thereof, and to prepare a membrane with an asymmetric porous structure having small pores on the positive electrode side and large pores on the negative electrode side by a non-solvent induced phase separation method, thereby simultaneously improving the ionic conductivity and safety of lithium metal batteries.

[0008] The positive electrode side of the separator is designed with a small pore size to improve the uniform wettability of the electrolyte and prevent excessive ion diffusion; the negative electrode side is designed with a large pore size to reduce the lithium ion conduction resistance and inhibit the growth of lithium dendrites. The asymmetric structure of the separator can optimize the permeability of the electrolyte and improve the cycle stability and power output of the battery.

[0009] In order to achieve the above-mentioned purpose, the technical solution of the present invention is as follows:

[0010] A method for preparing a porous polybenzimidazole (PBI) membrane with asymmetric pore size by using non-solvent induced phase separation (NIPS). By adjusting the ratio of water and ethanol in the non-solvent system and the separation conditions, as well as the static environment, PBI forms an asymmetric porous structure with small pores on the positive electrode side and large pores on the negative electrode side during the phase separation process, thereby optimizing the ionic conductivity, thermal stability and mechanical strength of the membrane, and significantly improving the electrochemical performance and safety of lithium metal batteries.

[0011] Specifically, a method for preparing a porous polybenzimidazole membrane with asymmetric pore size by using a non-solvent induced phase separation method comprises the following steps:

[0012] (1) dissolving polybenzimidazole in an organic solvent to prepare a uniform polybenzimidazole film-forming solution;

[0013] (2) spreading the uniform polybenzimidazole film-forming solution obtained in step (1) into a polybenzimidazole solution film layer and allowing it to stand;

[0014] (3) immersing the polybenzimidazole solution membrane layer after standing in step (2) in a solvent that is miscible with the organic solvent in step (1) but not miscible with polybenzimidazole, and performing phase separation at room temperature to form a porous polybenzimidazole membrane with an asymmetric porous structure;

[0015] (4) washing and drying to obtain the porous polybenzimidazole membrane with asymmetric pore size.

[0016] The organic solvent is at least one of dimethylacetamide (DMAc), dimethylformamide (DMF), and N-methylpyrrolidone (NMP) to ensure sufficient dissolution of the PBI resin and stability of the solution;

[0017] The mass fraction of polybenzimidazole in the polybenzimidazole film-forming solution is 10%-15%.

[0018] Specifically, in step (1), the dissolution is carried out at a temperature of 60-80°C.

[0019] Specifically, in step (2), a uniform polybenzimidazole film-forming solution is coated on a substrate and spread into a polybenzimidazole solution film liquid layer;

[0020] The coating speed is 1500-2500 rpm, and the coating thickness is 30-50 μm.

[0021] Preferably, in step (2), the standing time is 5-20 min.

[0022] Preferably, in step (3), the solvent is a mixture of water and ethanol, and the volume ratio of water to ethanol is 30:70-60:40, and the porosity and pore size distribution of the membrane are controlled to obtain an asymmetric porous structure with small pores on the positive electrode side and large pores on the negative electrode side, thereby simultaneously optimizing the electrolyte wettability and lithium ion conductivity.

[0023] Preferably, in step (3), the phase separation time is 20-30 min.

[0024] In the method described, during the preparation process, the ambient temperature is controlled at 25° C. and the humidity is controlled at 30% to 50% to ensure the stability of the membrane formation process and the uniformity of the asymmetric pore structure.

[0025] The prepared asymmetric PBI membrane was washed with deionized water for 24 h to remove residual solvent and non-solvent, and then dried in a vacuum drying oven at 60° C. to 80° C. for 12 to 24 h to obtain the final asymmetric porous PBI membrane.

[0026] Preferably, the small pore size of the positive electrode side of the porous polybenzimidazole separator is 50-100 nm, the large pore size of the negative electrode side of the porous polybenzimidazole separator is 200-500 nm, the total porosity is 80% to 90%, and the membrane thickness is 20-50 μm.

[0027] The present invention also provides a porous polybenzimidazole diaphragm with asymmetric pore size prepared by the method. The prepared asymmetric pore size PBI diaphragm has an electrolyte absorption rate of 300% to 400%, an ion conductivity of 1.0 to 1.5 mS / cm, excellent dimensional stability at 200°C, and a tensile strength of 35 to 45 MPa, which is significantly better than the traditional PP diaphragm.

[0028] The present invention also provides a lithium metal battery, comprising a positive electrode, a negative electrode and a separator, wherein the separator is a porous polybenzimidazole separator with an asymmetric pore size. When the prepared asymmetric pore size PBI separator is used in a lithium metal battery, the current density is 0.5 mA / cm 2 Under the conditions of , the battery cycle life is extended to more than 500h and the polarization voltage is kept within ±15mV.

[0029] The design ideas of the present invention are as follows:

[0030] The present invention adopts the non-solvent induced phase separation method (NIPS), combined with asymmetric pore design, precise regulation of the ratio of water and ethanol in the non-solvent system and the static environment, to prepare an asymmetric porous polybenzimidazole (PBI) diaphragm with small pores on the positive electrode side and large pores on the negative electrode side, aiming to simultaneously improve the ion conduction efficiency and cycle stability of lithium metal batteries. By adjusting the ratio of water and ethanol during the phase separation process and utilizing the regulation of the static environment, the solvent on the upper surface is volatilized, a small pore size is formed on the positive electrode side to optimize the uniform distribution and permeability of the electrolyte, and a large pore size is formed on the negative electrode side to enhance the rapid conduction channel of lithium ions, while effectively suppressing the risk of penetration of lithium dendrites. The design also ensures that the diaphragm has excellent thermal stability and mechanical strength in a high temperature environment, meeting the multiple requirements of lithium metal batteries for diaphragm performance.

[0031] The advantages and beneficial effects of the present invention are:

[0032] The present invention significantly optimizes the ion conduction performance and cycle stability of lithium metal batteries through asymmetric pore design. The design of small pores on the positive electrode side improves the wettability and uniformity of the electrolyte, which helps to reduce the ion diffusion resistance; the design of large pores on the negative electrode side provides a fast conduction channel for lithium ions, significantly reduces the internal resistance of the battery and improves the rate performance. At the same time, the asymmetric porous PBI membrane exhibits excellent thermal stability and dimensional stability under high temperature environment, can effectively avoid thermal shrinkage and melting, and reduce the risk of thermal runaway of the battery; in addition, its excellent mechanical strength effectively resists the piercing of lithium dendrites, ensuring the safety and cycle life of the battery. The prepared asymmetric PBI membrane exhibits low voltage polarization and stable lithium ion deposition / dissolution behavior in long-term cycles, significantly prolongs the service life of lithium metal batteries, and provides reliable guarantee for the development of high-performance lithium metal batteries. The preparation process of the present invention is simple and easy to control. The precise construction of the asymmetric porous structure can be achieved by adjusting the non-solvent ratio and phase separation conditions, which is suitable for large-scale production and practical applications. DETAILED DESCRIPTION

[0033] An appropriate high-purity polybenzimidazole (PBI) resin is selected as a raw material, and an asymmetric porous PBI membrane is prepared by a non-solvent induced phase separation method (NIPS). The PBI resin is first dissolved in a suitable solvent and prepared into a uniform film-forming solution. The film-forming solution is applied to a substrate by spin coating technology, and the spin coating speed and film thickness are regulated to form a uniform film layer. The coated membrane is quickly immersed in a non-solvent mixture, and an asymmetric porous structure with small pores on the positive electrode side and large pores on the negative electrode side is achieved by controlling the proportion of the non-solvent system and the phase separation time. Finally, a high-performance asymmetric porous PBI membrane is obtained through washing and drying steps. The specific preparation steps are shown in the specific embodiments.

[0034] Example 1

[0035] 12 g of PBI resin was dissolved in 88 g of dimethylacetamide (DMAc), stirred at 80 ° C for 12 h, and a uniform solution with a mass fraction of 12% was prepared. The solution was evenly coated on a clean glass substrate at a speed of 2000 rpm to form a membrane liquid layer with a thickness of about 30 μm. After standing for 5 min, the coated membrane was immersed in a non-solvent mixture with a volume ratio of 30:70 (deionized water: anhydrous ethanol) and phase separated for 30 min at room temperature (25 ° C). Subsequently, the membrane was taken out and washed in deionized water for 24 h to remove residual solvent and non-solvent, and finally dried in a vacuum drying oven at 60 ° C for 12 h to obtain an asymmetric porous PBI membrane.

[0036] Example 2

[0037] Dissolve 10 grams of PBI resin in 90 grams of N,N-dimethylformamide (DMF), stir at 80°C for 12 hours, and prepare a uniform solution with a mass fraction of 10%. The solution was evenly coated on a glass substrate at a speed of 1500rpm to form a membrane liquid layer with a thickness of about 40μm. After standing for 10 minutes, the membrane was immersed in a non-solvent mixture with a volume ratio of 50:50 (deionized water: anhydrous ethanol), and the phase separation time was 25 minutes. The washing and drying steps were the same as in Example 1 to obtain an asymmetric porous PBI membrane.

[0038] Example 3

[0039] Dissolve 15 grams of PBI resin in 85 grams of N-methylpyrrolidone (NMP), stir at 80°C for 12 hours, and prepare a uniform solution with a mass fraction of 15%. The solution was evenly coated on a glass substrate at a speed of 2500rpm to form a membrane liquid layer with a thickness of about 50μm, and allowed to stand for 20 minutes. Subsequently, the membrane was immersed in a non-solvent mixture with a volume ratio of 20:80 (deionized water: anhydrous ethanol), and the phase separation time was 20 minutes. The washing and drying steps were the same as in Example 1 to obtain an asymmetric porous PBI membrane.

[0040] Example 4

[0041] Dissolve 12 grams of PBI resin in 88 grams of a mixed solvent of DMAc and NMP (the volume ratio of DMAc and NMP is 1:1), stir at 80°C for 12 hours, and prepare a uniform solution with a mass fraction of 12%. The solution was evenly coated on a glass substrate at a speed of 2000rpm to form a membrane liquid layer with a thickness of about 35μm, and allowed to stand for 20 minutes. The membrane was immersed in a non-solvent mixture with a volume ratio of 40:60 (deionized water: anhydrous ethanol), and the phase separation time was 30 minutes. The washing and drying steps were the same as in Example 1 to obtain an asymmetric porous PBI membrane.

[0042] Example 5

[0043] Dissolve 10 grams of PBI resin in 90 grams of a mixed solvent of DMF and NMP (the volume ratio of DMF and NMP is 1:1), stir at 80°C for 12 hours, and prepare a uniform solution with a mass fraction of 10%. The solution was evenly coated on a glass substrate at a speed of 1500rpm to form a membrane liquid layer with a thickness of about 45μm, and allowed to stand for 10 minutes. The membrane was immersed in a non-solvent mixture with a volume ratio of 60:40 (deionized water: anhydrous ethanol), and the phase separation time was 25 minutes. The washing and drying steps were the same as in Example 1 to obtain an asymmetric porous PBI membrane.

[0044] Comparative Example 1

[0045] Dissolve 10 grams of PBI resin in 90 grams of a mixed solvent of DMF and NMP (the volume ratio of DMF and NMP is 1:1), stir at 80°C for 10 hours, and prepare a uniform solution with a mass fraction of 10%. The solution is evenly coated on a glass substrate at a speed of 1000 rpm to form a membrane liquid layer with a thickness of about 45 μm. Quickly immerse the membrane in a non-solvent mixture with a volume ratio of 60:40 (deionized water: anhydrous ethanol), and the phase separation time is 10 minutes. The washing and drying steps are the same as in Example 1 to obtain a symmetrical porous PBI membrane.

[0046] Test Example 1

[0047] When preparing lithium metal batteries, lithium iron phosphate (LiFePO4) is selected as the positive electrode material, high-purity lithium metal is used as the negative electrode, and the electrolyte is 1M LiPF6 dissolved in EC / DMC / EMC (1:1:1, volume ratio), and 2wt.% of fluoroethylene carbonate (FEC) is added as an additive. The diaphragm uses polypropylene (PP) material (such as Celgard 2400). First, lithium iron phosphate, Super P conductive agent and PVDF binder are mixed in a mass ratio of 80:10:10, and an appropriate amount of NMP is added to prepare a slurry. The slurry is evenly coated on aluminum foil, vacuum dried at 80°C for 12 hours, and cut into discs with a diameter of 14mm as the positive electrode. The negative electrode uses lithium metal foil with a thickness of 50μm, which is cut into discs with a diameter of 16mm. The battery is assembled in a glove box with an Ar atmosphere, and the order is negative electrode (lithium metal) → diaphragm → positive electrode → injection of 40μL electrolyte → CR2032 battery shell packaging. After assembly, the battery was left at room temperature for 12 hours to ensure that the electrolyte completely soaked the electrodes and separator.

[0048] The ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) using a Solartron electrochemical workstation. A lithium symmetric cell (Li / electrolyte / Li) was used, the frequency range was set to 100kHz to 0.1Hz, the AC amplitude was 10mV, and the test temperature was 25°C. The ionic conductivity of the electrolyte was calculated by the impedance value of the high frequency band in the Nyquist diagram.

[0049] Table 1 Process parameters and membrane performance of different embodiments

[0050]

[0051]

[0052] From the results in Table 1, it can be seen that the present invention successfully optimizes the pore structure and performance of the asymmetric porous PBI membrane by precisely controlling key process parameters such as PBI concentration, solvent type, spin coating speed, film thickness, non-solvent ratio, and standing time. Specifically, PBI concentration and solvent type directly affect the viscosity of the solution and the film quality; spin coating speed and film thickness determine the thickness uniformity and pore distribution of the membrane; the ratio of water to ethanol in the non-solvent system and the standing time significantly affect the kinetic behavior of the phase separation process, and further control the pore size and porosity distribution on the positive and negative electrode sides. Through the optimization of these parameters, the prepared asymmetric porous PBI membrane exhibits the following excellent comprehensive properties: the small pore size on the positive electrode side is 60 to 90 nanometers, the large pore size on the negative electrode side is 210 to 475 nanometers, the porosity reaches 82% to 91%, the electrolyte absorption rate is between 310% and 400%, the ionic conductivity is 1.1 to 1.5 mS / cm, the tensile strength is 38 to 45 MPa, and zero thermal shrinkage is maintained at 200°C.

[0053] The present invention provides a simple, controllable, and easily industrialized method for preparing asymmetric porous PBI membranes for large-scale production. The prepared membranes show broad application prospects in lithium metal batteries. They not only effectively improve the conduction efficiency of lithium ions, but also inhibit the growth of lithium dendrites, significantly improving the safety and cycle stability of the battery. During the preparation process, sufficient washing and drying steps can completely remove residual solvents and maintain the structural integrity of the membrane. The method of the present invention provides key material technology support for the development of high-performance lithium metal batteries and lays a foundation for the industrial application of next-generation high-energy density batteries.

Claims

1. A method for preparing a porous polybenzimidazole membrane with asymmetric pore size by non-solvent induced phase separation method, characterized in that: The following steps are involved: (1) dissolving polybenzimidazole in an organic solvent to prepare a uniform polybenzimidazole film-forming solution; (2) spreading the uniform polybenzimidazole film-forming solution obtained in step (1) into a polybenzimidazole solution film layer and allowing it to stand; (3) immersing the polybenzimidazole solution membrane layer after standing in step (2) in a solvent that is miscible with the organic solvent in step (1) but not miscible with polybenzimidazole, and performing phase separation at room temperature to form a porous polybenzimidazole membrane with an asymmetric porous structure; (4) washing and drying to obtain the porous polybenzimidazole membrane with asymmetric pore size.

2. The method for preparing a porous polybenzimidazole membrane with asymmetric pore size by non-solvent induced phase separation according to claim 1, characterized in that: In step (1), the organic solvent is at least one of dimethylacetamide, dimethylformamide, and N-methylpyrrolidone; The mass fraction of polybenzimidazole in the polybenzimidazole film-forming solution is 10%-15%.

3. The method for preparing a porous polybenzimidazole membrane with asymmetric pore size by non-solvent induced phase separation method according to claim 1, characterized in that: In step (1), dissolving is carried out at a temperature of 60-80°C.

4. The method for preparing a porous polybenzimidazole membrane with asymmetric pore size by non-solvent induced phase separation method according to claim 1, characterized in that: In step (2), a uniform polybenzimidazole film-forming solution is coated on a substrate and spread into a polybenzimidazole solution film layer; The coating speed is 1500-2500 rpm, and the coating thickness is 30-50 μm.

5. The method for preparing a porous polybenzimidazole membrane with asymmetric pore size by non-solvent induced phase separation method according to claim 1, characterized in that: In step (2), the standing time is 5-20 min.

6. The method for preparing a porous polybenzimidazole membrane with asymmetric pore size by non-solvent induced phase separation method according to claim 1, characterized in that: In step (3), the solvent is a mixture of water and ethanol, and the volume ratio of water to ethanol is 30:70-60:

40.

7. The method for preparing a porous polybenzimidazole membrane with asymmetric pore size by non-solvent induced phase separation method according to claim 1, characterized in that: In step (3), the phase separation time is 20-30 min.

8. The method for preparing a porous polybenzimidazole membrane with asymmetric pore size by non-solvent induced phase separation method according to claim 1, characterized in that: The small pore size of the porous polybenzimidazole membrane on the positive electrode side is 50-100 nm, and the large pore size of the porous polybenzimidazole membrane on the negative electrode side is 200-500 nm.

9. A porous polybenzimidazole membrane with asymmetric pore size prepared by the method according to any one of claims 1 to 8.

10. A lithium metal battery comprising a positive electrode, a negative electrode and a separator, characterized in that: The diaphragm is the porous polybenzimidazole diaphragm with asymmetric pore size as claimed in claim 9.

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