Multi-layer diaphragm for lithium-manganese battery and preparation method of multi-layer diaphragm

Abstract

The invention relates to the technical field of diaphragms, in particular to a multi-layer diaphragm for a lithium-manganese battery and a preparation method of the multi-layer diaphragm. According to the multilayer diaphragm, a polypropylene / polyethylene / polypropylene three-layer microporous membrane is used as a base material, and lyophilic interface layers are constructed on the surface of the diaphragm and in pore channels through the steps of pretreatment, photoinitiator retention, epoxy group grafting, polyethylene glycol chain segment introduction, sulfonylation, zwitterionic ionization and the like. According to the method, the electrolyte absorption rate, the electrolyte retention rate and the ionic conductivity of the diaphragm are remarkably improved, the surface resistance is reduced, meanwhile, the pore structure and the mechanical strength are maintained, and the discharge performance and the consistency of the battery at high rate and low temperature are effectively improved.

Description

Technical Field

[0001] This invention relates to the field of separator technology, and in particular to a multilayer separator for lithium manganese batteries and its preparation method. Background Technology

[0002] Lithium-manganese batteries, as a primary battery system, are widely used in electronic devices, medical instruments, and other fields due to their high energy density and stable discharge platform. The separator, as a key component within the battery, must not only possess excellent electronic insulation and ion conductivity but also exhibit superior wetting and retention capabilities against non-aqueous electrolytes to ensure the effective construction and long-term stability of ion channels. Currently, primary lithium-manganese cylindrical batteries commonly use polyolefin microporous separators, such as polyethylene / polypropylene multilayer composite membranes, which have become the mainstream choice due to their high chemical stability and mechanical strength. However, polyolefin materials themselves are non-polar with low surface energy, resulting in poor interfacial compatibility with polar electrolytes (such as carbonate solvent systems). During electrolyte injection, the electrolyte has difficulty quickly penetrating the separator pores, easily forming localized unwetted areas and reducing the effective cross-sectional area for ion transport.

[0003] This insufficient wettability manifests in practical battery applications as prolonged electrolyte immersion time and uneven electrolyte distribution. Especially under high-rate discharge or low-temperature conditions, limited ion migration paths lead to a significant increase in internal resistance, decreased capacity output efficiency, and increased batch-to-batch consistency fluctuations. Further, insufficient interfacial stability between the separator and electrolyte can induce interfacial polarization, causing localized drying of the electrolyte within the pores, further weakening ion conduction continuity and accelerating battery performance degradation. Existing improvements include surface coating with hydrophilic coatings or physical modifications, but these often come at the cost of sacrificing the separator's pore structure or mechanical integrity. For example, excessively thick coatings may lead to pore blockage or increased thermal shrinkage, which is detrimental to long-term battery reliability.

[0004] Modification of polyolefin separators needs to improve their hydrophilicity while maintaining the connectivity and mechanical support of their microporous structure. Simply introducing hydrophilic groups through surface treatment may result in dissolution or migration in the electrolyte due to weak bonding between the modified layer and the substrate, potentially introducing side reactions. Furthermore, if the modification process fails to achieve uniform coverage of the pore walls, the wetting effect is limited to the surface, making it difficult to address electrolyte retention in deeper pore regions, leading to insufficient interfacial stability under dynamic operating conditions. Therefore, developing a modification method that significantly enhances the hydrophilicity of the separator without compromising its core pore structure and dimensional stability has become a key challenge in improving the performance of lithium-manganese batteries. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a multilayer separator for lithium manganese batteries and its preparation method, so as to solve the problem that existing polyolefin separators have low surface energy and poor wettability, resulting in insufficient electrolyte wetting, insufficient effective cross-sectional area of ​​ion channels, high internal resistance, capacity fluctuation and interface polarization, which makes it difficult to meet the requirements of high rate and low temperature applications.

[0006] To achieve the above objectives, the present invention provides a method for preparing a multilayer separator for lithium manganese batteries, comprising the following steps:

[0007] (1) Pretreatment of polyolefin microporous membrane base film;

[0008] (2) The photoinitiator is fixed on the surface of the pretreated base film and / or the inner wall of the pores and activated by ultraviolet irradiation to form grafting reaction active sites on the surface of the base film and / or the inner wall of the pores;

[0009] (3) Under ultraviolet irradiation, the monomer containing epoxy groups is grafted onto the active site to obtain a grafted membrane containing epoxy groups.

[0010] (4) The grafted membrane containing epoxy groups is placed in a reaction solution containing methoxy polyethylene glycol with single-end mercapto-terminated end and organic strong base, and a vacuum-venting cycle is performed to promote the reaction solution to enter the micropores of the base membrane. Then, the methoxy polyethylene glycol with single-end mercapto-terminated end undergoes a ring-opening grafting reaction with the epoxy groups to obtain a polyethylene glycol grafted membrane.

[0011] (5) A sulfonated membrane was obtained by quaternizing some of the sulfide sites in the polyethylene glycol graft membrane with haloalkanes.

[0012] (6) The sulfonyl lactone was used to perform a ring-opening quaternization reaction on the sulfonium ether sites in the sulfonium membrane that were not quaternized by haloalkanes to introduce sulfonate groups and form zwitterionic structures, thus obtaining a sulfonium / zwitterionic composite membrane.

[0013] (7) The sulfonium / zwitterion composite membrane was heat-set and vacuum dried to obtain a multilayer membrane for lithium manganese batteries.

[0014] Preferably, the polyolefin microporous membrane base membrane is a polypropylene / polyethylene / polypropylene three-layer microporous membrane with a thickness of 20μm to 30μm and a porosity of 35% to 45%.

[0015] Preferably, the pretreatment in step (1) includes: sequentially cleaning the base film with anhydrous ethanol and acetone, and then drying it at 50°C to 70°C for 20 min to 60 min.

[0016] Preferably, in step (2), the photoinitiator is benzophenone and acetone is used as a solvent for fixation. The mass fraction of benzophenone relative to acetone is 0.5% to 1.5%, and the soaking time is 5 min to 20 min.

[0017] Preferably, the ultraviolet wavelength for ultraviolet irradiation activation in step (2) is 340nm to 380nm, and the light intensity is 20mW / cm². 2 ~60mW / cm 2 The irradiation distance is 3cm to 10cm, and the irradiation time is 1min to 4min.

[0018] Preferably, the epoxy group-containing monomer in step (3) is glycidyl methacrylate, and the mass fraction of glycidyl methacrylate relative to the spreading solvent acetone is 1% to 5%.

[0019] Preferably, the solvent of the reaction solution in step (4) is a mixture of anhydrous ethanol and acetonitrile, and the mass ratio of anhydrous ethanol to acetonitrile is 60:40 to 80:20.

[0020] Preferably, in step (4), the mass fraction of the single-end thiol-capped methoxy polyethylene glycol relative to the total mass of the mixed solvent is 2% to 3%, and its number average molecular weight is 1800 to 2200.

[0021] Preferably, the strong organic base in step (4) is 1,8-diazabicycloundec-7-ene, and its amount is 8% to 12% of the mass fraction of the single-terminated thiol-capped methoxy polyethylene glycol.

[0022] Preferably, in step (4), the vacuuming pressure in the vacuuming-venting cycle reaches -85kPa to -92kPa, the pressure is maintained for 4min to 6min each time, and the cycle is repeated 3 times.

[0023] Preferably, the ring-opening grafting reaction temperature in step (4) is 35℃~45℃, and the reaction time is 1.5h~2.5h.

[0024] Preferably, the haloalkane in step (5) is iodomethane, the mass fraction of iodomethane relative to acetonitrile is 0.3% to 0.8%, the reaction temperature is 15°C to 30°C and the mixture is kept in the dark, and the reaction time is 15 min to 25 min.

[0025] Preferably, the sulfonyl lactone in step (6) is 1,3-propanesulfonyl lactone or 1,4-butanesulfonyl lactone, with a mass fraction of 0.9% to 1.6% relative to acetonitrile, a reaction temperature of 45°C to 55°C, and a reaction time of 3 to 5 hours.

[0026] Preferably, in step (7), the heat setting temperature is 85℃~95℃ and the heat setting time is 8min~12min; the vacuum drying temperature is 55℃~65℃ and the drying time is 10h~14h.

[0027] Furthermore, the present invention also provides a multilayer separator for lithium manganese batteries, which is obtained by the above preparation method.

[0028] The beneficial effects of this invention are:

[0029] This invention utilizes photo-initiated fixation technology to construct a grafted framework on the surface of a polyolefin microporous membrane, and sequentially introduces epoxy groups, polyethylene glycol segments, sulfonium salts, and zwitterionic structures to form a composite hydrophilic interface layer. This approach significantly improves the wettability and retention capacity of the membrane for non-aqueous electrolytes. The electrolyte can rapidly spread on the membrane surface and within the pores, forming a stable solvation layer, effectively reducing the liquid-solid interface contact angle and preventing localized drying. The flexible ether-oxygen structure of the polyethylene glycol segments can coordinate with lithium ions in the electrolyte, enhancing the interfacial solvation effect and promoting continuous ion migration. Simultaneously, the hydrophilicity of the segments endows the membrane with a higher electrolyte adsorption capacity, making the ion transport path smoother.

[0030] The introduction of sulfonium salt sites enhances the polarity of the membrane surface, fixing positive charges and attracting anions from the electrolyte through electrostatic interactions, optimizing the local solvation environment and improving the interfacial wetting driving force. The zwitterionic structure, possessing both positive and negative charge centers, forms an inner salt shielding layer, effectively suppressing ion aggregation or charge imbalance tendencies at the interface, reducing polarization, and improving interfacial stability during long-term cycling. The synergistic effect of polyethylene glycol segments and the sulfonium / zwitterionic structure forms a continuous hydrophilic liner on the pore inner wall, not only increasing the initial wetting rate but also strengthening capillary retention capacity, resulting in a more uniform electrolyte distribution within the pores and reducing the risk of capacity fluctuations caused by gravitational dripping.

[0031] This design maintains the original porosity and permeability of the separator while preventing pore blockage through a controlled grafting reaction. Heat setting further stabilizes the microstructure, ensuring good dimensional stability during battery assembly and operation. The high interfacial compatibility of the composite hydrophilic layer mitigates electrolyte decomposition side reactions, extending battery storage life. Simultaneously, the mechanical properties of the polyolefin-based film remain unaffected, and its puncture resistance and thermal shrinkage rate still meet high safety requirements. Overall, this separator exhibits more stable ionic conductivity under high-rate discharge and low-temperature conditions, which is beneficial for improving battery capacity output efficiency and batch consistency, meeting the requirements of high-performance lithium-manganese batteries for rapid wetting, low internal resistance, and long-term reliability. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0033] This invention provides a method for preparing a multilayer separator for lithium manganese batteries. Using a polyolefin microporous separator as the base membrane, a series of processes are employed: photoinitiation fixation, epoxy-containing monomer grafting, mercapto-terminated polyether ring-opening grafting, surface sulfonium treatment, zwitterionization, and heat setting / vacuum drying. While preserving the original pore structure and mechanical support of the separator as much as possible, a hydrophilic interface layer characterized by polyethylene glycol segments and sulfonium / zwitterion sites is constructed on the separator surface and the inner walls of the pores. This improves the wetting, impregnation, and retention capabilities of non-aqueous electrolytes on the separator, reduces surface resistivity and interfacial polarization risks, and improves the effective cross-sectional area and consistency of ion transport under rate and low-temperature discharge conditions.

[0034] (I) Base film selection and pretreatment

[0035] In the multilayer membrane preparation method provided by this invention, the base membrane is preferably a polypropylene / polyethylene / polypropylene three-layer microporous membrane; its thickness is preferably 20μm to 30μm, more preferably 23μm to 27μm; and its porosity is preferably 35% to 45%, more preferably 37% to 41%. The thickness and porosity of the base membrane together affect the pore connectivity and effective pore volume, thereby affecting the depth and uniformity of the subsequent modification liquid entering the pores; when the porosity is too low, it is not conducive to the penetration of the modification liquid, while too high a porosity may bring the risk of thermal shrinkage and insufficient mechanical support.

[0036] In the pretreatment process provided by this invention, the base membrane is preferably cleaned sequentially with anhydrous ethanol and acetone to remove surface oil, plasticizer residue, and particulate contamination. Taking a 100mm×100mm diaphragm sheet as an example, the preferred amount of cleaning solvent is 40g to 70g, more preferably 45g to 55g; the preferred cleaning method is immersion with slight agitation, and the agitation time is preferably 1min to 5min, more preferably 2min to 3min. The preferred drying temperature after cleaning is 50℃ to 70℃, more preferably 55℃ to 65℃; the preferred drying time is 20min to 60min, more preferably 25min to 40min.

[0037] (II) Photoinitiator immobilization and UV activation

[0038] In the photoinitiation and fixation step provided by this invention, benzophenone is preferably used as the photoinitiator, and acetone is used as the solvent for immersion and fixation. Benzophenone can form an excited state under ultraviolet light and generate free radicals by abstracting hydrogen from polyolefin segments, thereby forming active sites on the membrane surface / pore wall that can be used for graft polymerization; the density of these active sites will affect the grafting density of subsequent epoxy group-containing monomers, and thus affect the apparent content of epoxy groups and the subsequent amount of polyethylene glycol grafted.

[0039] In the benzophenone fixation process provided by this invention, the mass fraction of benzophenone relative to acetone is preferably 0.5% to 1.5%, more preferably 0.8% to 1.2%, specifically, 200 mg to 450 mg of benzophenone can be added to 30 g of acetone. The soaking and standing time is preferably 5 min to 20 min, more preferably 8 min to 12 min. The acetone evaporation time after removal is preferably 3 min to 10 min, more preferably 4 min to 6 min; subsequently, drying is carried out at 50°C to 70°C for 5 min to 20 min, more preferably at 55°C to 65°C for 8 min to 12 min, to reduce the interference of residual acetone on the ultraviolet fixation and grafting process.

[0040] In the ultraviolet irradiation activation step provided by the present invention, the ultraviolet wavelength is preferably 340nm to 380nm, more preferably 360nm to 370nm; the light intensity is preferably 20mW / cm². 2 ~60mW / cm 2 More preferably 35mW / cm 2 ~45mW / cm 2 The preferred irradiation distance is 3cm to 10cm, more preferably 4cm to 6cm; the preferred irradiation time is 1min to 4min, more preferably 1.5min to 2.5min.

[0041] Furthermore, without departing from the technical effect of the present invention, the photoinitiator may also be preferably an initiation system that can generate free radicals under ultraviolet light, such as phenylethyl ketone or anthraquinone.

[0042] (iii) Grafted layer containing epoxy groups

[0043] In the epoxy grafting step provided by this invention, glycidyl methacrylate is preferably used as the grafting monomer containing epoxy groups, and acetone is used as the spreading solvent to fully immerse the membrane and allow surface grafting polymerization to occur under ultraviolet irradiation, resulting in a grafted layer containing epoxy groups. The epoxy group density will directly affect the ring-opening grafting efficiency of subsequent mercapto-terminated polyethylene glycol; insufficient epoxy sites will limit the amount of polyethylene glycol introduced, while too many epoxy sites or an excessively thick grafted layer may cause a reduction in the effective diameter of the pores or even the risk of pore blockage. Therefore, it is preferable to control the monomer concentration and irradiation conditions.

[0044] In the glycidyl methacrylate grafting process provided by this invention, the mass fraction of glycidyl methacrylate relative to acetone is preferably 1% to 5%, more preferably 2% to 4%, specifically, 400 mg to 800 mg of glycidyl methacrylate can be added to 20 g of acetone. The ultraviolet irradiation conditions are preferably the same as the aforementioned activation conditions (wavelength 360 nm to 370 nm), and the light intensity is preferably 30 mW / cm². 2 ~60mW / cm 2More preferably 35mW / cm 2 ~45mW / cm 2 The irradiation time is preferably 3 min to 10 min, more preferably 5 min to 7 min; the irradiation distance is preferably 4 cm to 7 cm, more preferably about 5 cm. After the reaction, the diaphragm is preferably removed immediately and rinsed sequentially with acetone and anhydrous ethanol to remove unreacted monomers and free polymers, reducing non-specific adsorption caused by impurities in subsequent reactions; the drying temperature is preferably 50℃ to 70℃, more preferably 60℃; the drying time is preferably 20 min to 60 min, more preferably 25 min to 40 min.

[0045] Without departing from the technical effect of the present invention, alternative monomers containing epoxy groups may also be preferred such as glycidyl acrylate, allyl glycidyl ether, etc.

[0046] (iv) Thiol-terminated methoxy polyethylene glycol ring-opening grafting and vacuum-degassing to promote impregnation

[0047] In the polyethylene glycol grafting step provided by this invention, it is preferable to use methoxy polyethylene glycol with a single-end mercapto-terminated cap to undergo a ring-opening addition reaction with an epoxy group, and preferably, a strong organic base is added as a catalyst to promote the nucleophilic ring-opening of the epoxy group by the mercapto group, thereby forming a thioether bond connection structure on the membrane surface and introducing polyethylene glycol segments. The introduction of polyethylene glycol segments can significantly improve the polarity and hydrophilicity of the membrane surface, enhance the spreading and retention of electrolyte on the membrane surface and in the pores; at the same time, the length and grafting density of polyethylene glycol segments will affect the interfacial solvation layer structure and adhesion / retention ability, and thus affect the ion migration impedance at low temperature and high magnification.

[0048] In the reaction solvent system provided by this invention, a mixed solvent of anhydrous ethanol and acetonitrile is preferably used, and the mass ratio of ethanol to acetonitrile is preferably 60:40 to 80:20, more preferably 65:35 to 75:25. This solvent ratio affects the solubility of polyethylene glycol, the viscosity of the reaction solution, and the wetting and penetration ability of the membrane pores: increasing the proportion of acetonitrile is beneficial to reducing viscosity and enhancing penetration to the polyolefin surface, but too high a proportion may reduce the solubility of polyethylene glycol by ethanol and its contribution to the uniformity of the reaction.

[0049] Regarding the amount of polyethylene glycol used in this invention, the mass fraction of methoxy polyethylene glycol with single-end mercapto-terminated ends relative to the total mass of the mixed solvent is preferably 1.5% to 4%, more preferably 2% to 3%. Its number-average molecular weight is preferably 1500 to 3000, more preferably 1800 to 2200. While a lower molecular weight facilitates entry into the pores, its hydrophilic retention capacity may be insufficient; conversely, an excessively high molecular weight may reduce the penetration depth into the pores due to increased solution viscosity and increase the risk of enrichment at the pore opening.

[0050] Regarding the catalyst provided by the present invention, 1,8-diazabicycloundec-7-ene is preferably used as a promoter, and its amount relative to the mass fraction of the single-terminated thiol-capped methoxy polyethylene glycol is preferably 5% to 20%, more preferably 8% to 12%.

[0051] In the key wetting process of this invention, it is preferable to place an epoxy-grafted diaphragm in the reaction solution and then place the reaction vessel in a vacuum dryer for a vacuum-venting cycle to remove gas from the diaphragm pores and promote the entry of the reaction solution into the micropores. The vacuum pressure is preferably -70 kPa to -95 kPa, more preferably -85 kPa to -92 kPa; the holding time for each cycle is preferably 3 min to 10 min, more preferably 4 min to 6 min; and the number of cycles is preferably 2 to 5 times, more preferably 3 times. This step, by reducing the partial pressure of the gas inside the pores and creating a pressure difference drive when restoring to normal pressure, makes it easier for the reaction solution to enter the pores and fully contact the epoxy sites within the pores. This facilitates the formation of a hydrophilic inner liner layer on the inner wall of the pores, thereby reducing the risk of a decrease in the effective cross-sectional area of ​​the ion channels due to insufficient local wetting and improving the injection wetting speed and retention capacity.

[0052] Regarding the ring-opening reaction conditions provided by this invention, the reaction temperature is preferably 30℃~50℃, more preferably 35℃~45℃, and specifically 40℃; the reaction time is preferably 1h~4h, more preferably 1.5h~2.5h, and specifically 2h. Temperature and time together determine the ring-opening reaction conversion rate and the stability of the grafted layer: too low a temperature will reduce the reaction rate and lead to insufficient grafting, while too high a temperature or too long a time may cause excessive surface reaction and thickening of pores, thereby increasing the risk of pore blockage.

[0053] (v) Construction of composite surface layer by sulfonation and zwitterionization

[0054] In the sulfonium sulfonation step provided by this invention, it is preferable to quaternize the thioether sites using a haloalkane after a thioether bond structure has been formed on the membrane surface, thereby constructing sulfonium salt sites with fixed positive charges. Further, it is preferable to use sulfonolactones to undergo ring-opening quaternization with the unquaternized thioether sites and introduce sulfonate groups, thus forming a zwitterionic structure containing both positive and negative charge centers. The fixed charge and zwitterionic layer can enhance the polarity and wetting driving force of the membrane / electrolyte interface, improve interface stability, and slow down electrolyte interface polarization. Simultaneously, the zwitterionic structure helps reduce the tendency for ion aggregation and charge imbalance at the interface, thereby improving long-term wettability and resistance to drying.

[0055] In the sulfonation treatment conditions provided by this invention, acetonitrile is preferably used as the solvent, and iodomethane is preferably used as the haloalkane. The mass fraction of iodomethane relative to acetonitrile is preferably 0.2% to 1.0%, more preferably 0.3% to 0.8%, specifically, 150 mg to 300 mg of iodomethane can be added to 40 g of acetonitrile. The reaction temperature is preferably 15°C to 30°C, and it is preferably allowed to stand in the dark. The reaction time is preferably 10 min to 30 min, more preferably 15 min to 25 min, specifically 20 min. By using room temperature, darkness, and short standing time, quaternization tends to occur in the easily accessible surface region of the solution, which is beneficial for controlling the distribution of charge sites and reducing the probability of pore blockage due to over-reaction.

[0056] In the zwitterionization step provided by this invention, acetonitrile is preferably used as the solvent, and 1,3-propanesulfonyl lactone is preferred. Its amount relative to the mass fraction of acetonitrile is preferably 0.6% to 2.0%, more preferably 0.9% to 1.6%, specifically, 350 mg to 650 mg of 1,3-propanesulfonyl lactone can be added to 40 g of acetonitrile. The reaction temperature is preferably 40°C to 60°C, more preferably 45°C to 55°C, specifically 50°C; the reaction time is preferably 2 h to 6 h, more preferably 3 h to 5 h, specifically 4 h. This temperature and time affect the degree of ring-opening quaternization of sulfonyl lactone and the density of zwitterion sites, thereby affecting the hydrophilicity of the membrane surface, interfacial stability, and electrolyte retention capacity. Too low a temperature will result in insufficient introduction of zwitterions, while too high a temperature may lead to excessive surface reaction, causing increased local resistance in the pores.

[0057] Without departing from the technical effect of the present invention, haloalkanes can also be replaced with reagents such as iodoethane, bromomethane, or bromoethane that have quaternizing ability for thioethers; sulcolones can also be replaced with 1,4-butanesulcolones, etc.

[0058] (vi) Heat setting and vacuum drying

[0059] In the post-processing steps provided by this invention, the modified composite separator is preferably clamped within a rigid frame for heat setting to suppress thermal shrinkage and warping during subsequent battery electrolyte injection and assembly. The heat setting temperature is preferably 80℃~100℃, more preferably 85℃~95℃, and specifically 90℃; the heat setting time is preferably 5min~20min, more preferably 8min~12min, and specifically 10min. The heat setting temperature and time affect the dimensional stability and pore structure recovery of the separator: insufficient temperature is detrimental to releasing internal stress, while excessively high temperature may cause the polyolefin pore structure to collapse.

[0060] Subsequently, vacuum drying is preferably performed to remove residual solvent and unreacted small molecules. The vacuum drying temperature is preferably 50℃~70℃, more preferably 55℃~65℃, and specifically 60℃; the drying time is preferably 8h~16h, more preferably 10h~14h, and specifically 12h. Sufficient vacuum drying can reduce the impact of residual solvent on electrolyte composition and interfacial side reactions, thereby improving battery consistency and storage stability.

[0061] The present invention will be described below with reference to embodiments, but should not be construed as limiting the scope of protection of the present invention.

[0062] Example 1:

[0063] The base membrane used in this embodiment is Celgard 2325 polypropylene / polyethylene / polypropylene three-layer microporous membrane manufactured by Celgard Corporation, with a thickness of 25μm and a porosity of 39%; the methoxy polyethylene glycol with single-end mercapto-capped methoxy groups and a number average molecular weight of 2000 is selected from Shanghai Aladdin Biochemical Technology Co., Ltd. product number T164392.

[0064] Step 1: Weigh 140mg of polypropylene / polyethylene / polypropylene three-layer microporous membrane, cut it into 100mm×100mm pieces, place it in a petri dish, add 50g of anhydrous ethanol to immerse it and shake for 2 minutes, then discard the cleaning solution; add 50g of acetone to immerse it and shake for 2 minutes, then discard the cleaning solution; then lay the membrane flat on a clean glass plate and dry it in a 60℃ forced-air oven for 30 minutes to obtain the pretreated membrane;

[0065] Step 2: Place the pretreated septum obtained in Step 1 in a petri dish, add 30g of acetone and 300mg of benzophenone, and let it soak at room temperature for 10 minutes; remove the septum and evaporate the acetone under a clean airflow at room temperature for 5 minutes, then dry it in a 60℃ oven for 10 minutes; subsequently, place the septum under a 365nm ultraviolet light source with a light intensity of 40mW / cm². 2 Irradiation distance 5cm, irradiation for 2min, to obtain photo-induced fixation diaphragm;

[0066] Step 3: Place the photoinitiated immobilization diaphragm obtained in Step 2 in a glass dish, add 20g of acetone and 600mg of glycidyl methacrylate, spread and submerge the diaphragm; cover the quartz dish, and then expose it to a 365nm ultraviolet light source with a light intensity of 40mW / cm². 2Irradiation distance was 5 cm, irradiation time was 6 min; after the reaction was completed, the membrane was immediately removed and rinsed once with acetone and once with anhydrous ethanol, and then dried at 60℃ for 30 min to obtain an epoxy-grafted membrane; placed in a glass reaction flask with a ground glass stopper, 28 g of anhydrous ethanol and 12 g of acetonitrile were added to prepare a mixed solvent, and then 1000 mg of methoxy polyethylene glycol with a single-end mercapto-terminated end and 100 mg of 1,8-diazabicycloundec-7-ene were added. After stirring and dissolving at room temperature, the reaction flask was placed in a vacuum desiccator and evacuated to -90 kPa for 5 min, and then released to atmospheric pressure. The vacuum-release cycle was repeated 3 times to remove the gas in the pores and promote the reaction solution to enter the micropores; then the reaction flask was placed in a 40℃ water bath and allowed to stand for 2 h; after the reaction, the membrane was removed, rinsed once with 50 g of acetonitrile and once with 50 g of anhydrous ethanol, and dried at 60℃ for 30 min to obtain a polyethylene glycol-grafted membrane.

[0067] Step 4: Place the polyethylene glycol grafted membrane obtained in Step 3 into a glass bottle with a ground glass stopper, add 40g of acetonitrile and 200mg of iodomethane, and let it stand at room temperature in the dark for 20min. After the reaction, remove the membrane, wash it twice with acetonitrile, then wash it once with anhydrous ethanol, and dry it at 60℃ for 30min to obtain a sulfonium-modified membrane. Place it in a glass reaction flask, add 40g of acetonitrile and 500mg of 1,3-propanesulfonyl lactone, and let it stand at 50℃ for 4h. After the reaction, remove the membrane, wash it twice with acetonitrile and once with anhydrous ethanol, and finally dry it under vacuum at 60℃ for 2h to obtain a sulfonium / zwitterion composite membrane.

[0068] Step 5: The sulfonium / zwitterion composite multilayer separator obtained in Step 4 is clamped in a stainless steel frame, placed in a 90°C forced-air oven for 10 minutes for heat setting, then removed and cooled to room temperature, and then dried in a 60°C vacuum drying oven for 12 hours to obtain a multilayer separator for lithium manganese batteries.

[0069] Example 2:

[0070] In step 3, the number-average molecular weight of the methoxy polyethylene glycol with single-thiol end capping was adjusted from 2000 to 1500; the amount of methoxy polyethylene glycol with single-thiol end capping was adjusted to 900 mg. The remaining conditions were the same as in Example 1.

[0071] Example 3:

[0072] In step 3, the number of vacuum-venting cycles is adjusted from 3 to 2; the remaining conditions are the same as in Example 1.

[0073] Example 4:

[0074] In step 4, the dosage of iodomethane was adjusted from 200 mg to 150 mg; the remaining conditions were the same as in Example 1.

[0075] Example 5:

[0076] In step 4, 1,3-propanesulfonyl lactone was replaced with 1,4-butanesulfonyl lactone, and the dosage remained at 500 mg; the remaining conditions were the same as in Example 1.

[0077] Comparative Example 1:

[0078] In step 3, the operation of placing the reaction flask in a vacuum dryer, evacuating it to -90 kPa and holding it for 5 minutes, then releasing the gas to atmospheric pressure and repeating the evacuation-release cycle 3 times to remove the gas from the pores and promote the reaction liquid to enter the micropores is omitted; the remaining conditions are the same as in Example 1.

[0079] Comparative Example 2:

[0080] In step 4, 40g of acetonitrile was added but 200mg of iodomethane was not added. The reaction was allowed to stand at room temperature in the dark for 20min. After the reaction, the diaphragm was removed and washed twice with acetonitrile and once with anhydrous ethanol. It was then dried at 60℃ for 30min. The remaining conditions were the same as in Example 1.

[0081] Comparative Example 3:

[0082] In step 4, 40g of acetonitrile was added but not 500mg of 1,3-propanesulfonyl lactone. The mixture was then allowed to stand at 50°C for 4 hours. After the treatment, the diaphragm was removed and rinsed twice with acetonitrile and once with anhydrous ethanol. Finally, it was vacuum dried at 60°C for 2 hours. The remaining conditions were the same as in Example 1.

[0083] Comparative Example 4:

[0084] In step 3, 1000 mg of methoxy polyethylene glycol with a single-thiol end capping was replaced with 40 mg of 2-mercaptoethanol (equimolar amount); the remaining conditions were the same as in Example 1.

[0085] Performance testing:

[0086] Sample preparation: After completing the corresponding processes, the diaphragms prepared according to the examples and comparative examples were dried again in a vacuum drying oven at 60℃ for 12 hours, cooled to room temperature, and then sealed for storage. When electrolyte wetting tests are involved, the non-aqueous electrolyte is uniformly 1.0 mol / L lithium hexafluorophosphate dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate, with a volume ratio of ethylene carbonate to dimethyl carbonate of 1:1. Unless otherwise specified, for each test item, 5 parallel samples are prepared independently for each sample, and the arithmetic mean is taken as the test result of that sample for that item.

[0087] Air permeability: The samples were cut into 100mm × 100mm pieces and placed in the Gurley air permeability meter fixture. The test pressure difference was set to 1.21 kPa, the test volume to 100 mL, and the effective test area to 6.45 cm².2 Record the time required for 100 mL of air to pass through the sample, in seconds.

[0088] Liquid absorption rate: Cut the sample into 50mm×50mm pieces, dry them in a vacuum drying oven at 60℃ for 2 hours, cool them, and weigh them. Record the mass M0. Immerse the sample completely in the aforementioned non-aqueous electrolyte and let it stand at 25℃ for 12 hours. Remove the sample and wipe both sides lightly with quantitative filter paper 10 times each to remove the free electrolyte on the surface. Weigh the sample immediately and record the mass M1. Calculate the liquid absorption rate P = (M1-M0) / M0×100%.

[0089] Liquid retention rate: The sample with the liquid absorption rate test completed and the mass M1 is weighed is suspended in an environment of 25℃ and left to stand for 60 minutes, so that the electrolyte in the sample channel will naturally drip and redistribute under the action of gravity. Then, the sample is wiped 10 times on each side with quantitative filter paper to remove the residual liquid droplets on the surface and the mass M2 is immediately recorded. The liquid retention capacity is calculated as liquid retention rate R = (M2-M0) / (M1-M0)×100%.

[0090] Ionic conductivity and sheet resistance: The samples were cut into 19.0 mm diameter discs and immersed in the aforementioned non-aqueous electrolyte at 25°C for 12 h. After removal, they were quickly placed into the blocking electrode fixture, so that the diaphragm was clamped between two 19.0 mm diameter stainless steel electrodes and a constant clamping pressure was applied. The AC impedance was tested at 25°C using an electrochemical workstation. The AC disturbance voltage was set to 10 mV and the frequency range was set to 1 MHz to 1 Hz. The bulk resistance Rb was obtained by taking the high-frequency intercept. The sheet resistance was calculated as AR = Rb × A, where A is the effective area of ​​the electrode. The ionic conductivity was calculated as σ = t / (Rb × A), where t is the sample thickness.

[0091] Puncture resistance: The samples were cut into 100mm×100mm pieces and fixed on the clamp. The diameter of the puncture needle was set to 1.0mm and the puncture speed was set to 100mm / min. The maximum puncture force was recorded and expressed in Newtons.

[0092] Heating dimensional change rate: Cut the sample into 100mm×100mm pieces and mark the reference length L0=100.0mm in the longitudinal and transverse directions respectively. Lay the sample flat on a glass plate and place it in a 150℃ forced-air oven for 60min. After heating, remove it and let it cool naturally at room temperature for 30min. Measure the length L1 after heating in the longitudinal and transverse directions and calculate the heating dimensional change rate S=(L1-L0) / L0×100%.

[0093] The test results are shown in Table 1.

[0094] Table 1 Performance Test Results

[0095] sample air permeability / s / 100mL Liquid absorption rate / % Liquid retention rate / % <![CDATA[Sheet Resistance / Ω·cm 2 > Ionic conductivity / mS / cm Maximum puncture force / N Heating dimensional change rate longitudinally / % Heating dimensional change rate (lateral) / % Example 1 690 206 70 1.61 1.55 4.72 -15.8 -9.6 Example 2 675 192 68 1.70 1.48 4.68 -16.4 -10.2 Example 3 660 183 66 1.83 1.38 4.61 -17.5 -11.1 Example 4 670 188 67 1.78 1.42 4.66 -16.9 -10.7 Example 5 680 199 70 1.67 1.52 4.72 -15.2 -9.4 Comparative Example 1 650 169 56 2.05 1.25 4.88 -20.4 -13.1 Comparative Example 2 660 176 61 1.94 1.33 4.74 -18.3 -11.5 Comparative Example 3 710 201 52 1.71 1.51 4.55 -17.1 -10.8 Comparative Example 4 640 155 50 2.22 1.18 4.92 -19.3 -12.4

[0096] Data Analysis:

[0097] As can be seen from the data in Table 1, the multilayer separator for lithium manganese batteries prepared by this invention has significant advantages in electrolyte affinity: with the increase of the amount of methoxy polyethylene glycol segments with single-end mercapto-capped ends and the degree of pore wetting, the relative content of organic grafted layers on the separator surface and pore walls increases, the pore structure remains within the permeable range but becomes more uniform, and the electrolyte absorption and retention capacity are enhanced simultaneously, thereby reducing ion migration resistance and improving ionic conductivity. This may be because the grafted framework formed by glycidyl methacrylate under the photoinitiation of benzophenone provides anchoring points for subsequent segments and ionic groups; the sulfonium groups and zwitterionic structures jointly enhance surface polarity and form a stable solvation layer; and the flexible ether oxygen groups of the polyethylene glycol segments can coordinate with the electrolyte. These three elements generate a synergistic wetting and capillary retention effect at the pore wall micro-interface, enabling the separator to possess high liquid absorption, liquid retention, and ion conduction capabilities, and maintain good dimensional stability and puncture resistance after heat setting.

[0098] As can be seen from the data in Table 1 for Example 1 and Comparative Example 1, when the vacuum-venting cycle is omitted and the degree of reaction liquid entering the micropores is reduced, the permeability of the diaphragm is closer to the level of the base membrane, but the liquid absorption rate, liquid retention rate, and ionic conductivity decrease, while the surface resistivity increases. The main reason for this is that the grafted layer covering the inner wall of the pores is discontinuous, and the polar sites are insufficient to quickly form a continuous wetting channel, making it easy for the electrolyte to be partially inadequately wetted and to drip and be lost within the pores.

[0099] As can be seen from the data in Table 1 for Example 1 and Comparative Example 2, when iodomethane is not added, the air permeability of the membrane changes little, but the liquid absorption rate, liquid retention rate, and ionic conductivity decrease, while the sheet resistance increases. This may be because the methylation process of iodomethane on the thiol-related reaction sites on the pore walls to form sulfonium structures is missing, making the subsequent introduction of 1,3-propanesulfonyl lactone more restricted by diffusion and steric hindrance. This results in insufficient generation and uniform distribution of sulfonium groups and zwitterionic structures, making it difficult to form a stable polar solvation layer within the pores. Therefore, iodomethane pretreatment plays a crucial role in promoting the construction of ionic groups and exhibits a significant synergistic effect with the wetting effect of polyethylene glycol segments.

[0100] As can be seen from the data in Table 1 for Example 1 and Comparative Example 3, when only iodomethane treatment is performed without the introduction of 1,3-propanesulfonyl lactone, the diaphragm still achieves a high liquid absorption rate and low sheet resistance, but the liquid retention rate decreases significantly and the heating dimensional change rate tends to increase. The main reason for this is that the single sulfonium structure, with the added anion as the counter ion, easily leads to ion pair aggregation in the electrolyte environment, weakening the binding effect on the solvent, making it easier for the electrolyte to leak out during settling. Simultaneously, the lack of an internal salt shielding layer formed by the zwitterionic structure results in insufficient thermal constraint on the polymer chain segments of the pore walls. This indicates that the combination of the sulfonium group and the zwitterionic structure does not simply increase polarity, but rather achieves a synergistic effect of integrated absorption-retention-conduction enhancement.

[0101] As can be seen from the data in Example 1 and Comparative Example 4 in Table 1, when the methoxy polyethylene glycol with single-end mercapto-capped groups is replaced with an equimolar amount of 2-mercaptoethanol, the membrane's permeability and maximum puncture force increase, but the liquid absorption rate, liquid retention rate, and ionic conductivity decrease significantly. The main reason for this is that the 2-mercaptoethanol chain segments are short and have few ether oxygen atoms, making it difficult to form continuous, flexible solvated segments on the pore walls. The polar sites lack sufficient adsorption and capillary retention capacity for the electrolyte. Simultaneously, the short-chain modified layer has limited flexible bridging effect on the pore walls and cannot jointly construct a stable wetting-conducting network with the sulfonium / zwitterionic structure. Therefore, the significant synergy between the polyethylene glycol chain segment length and the ionic group composite is key to achieving both high liquid absorption and high liquid retention.

[0102] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a multilayer separator for lithium manganese batteries, characterized in that, Includes the following steps: (1) Pretreatment of polyolefin microporous membrane base film; (2) The photoinitiator is fixed on the surface of the pretreated base film and / or the inner wall of the pores and activated by ultraviolet irradiation to form grafting reaction active sites on the surface of the base film and / or the inner wall of the pores; (3) Under ultraviolet irradiation, the monomer containing epoxy groups is grafted onto the active site to obtain a grafted membrane containing epoxy groups. (4) The grafted membrane containing epoxy groups is placed in a reaction solution containing methoxy polyethylene glycol with single-end mercapto-terminated end and organic strong base, and a vacuum-venting cycle is performed to promote the reaction solution to enter the micropores of the base membrane. Then, the methoxy polyethylene glycol with single-end mercapto-terminated end undergoes a ring-opening grafting reaction with the epoxy groups to obtain a polyethylene glycol grafted membrane. (5) A sulfonated membrane was obtained by quaternizing some of the sulfide sites in the polyethylene glycol graft membrane with haloalkanes. (6) The sulfonyl lactone was used to perform a ring-opening quaternization reaction on the sulfonium ether sites in the sulfonium membrane that were not quaternized by haloalkanes to introduce sulfonate groups and form zwitterionic structures, thus obtaining a sulfonium / zwitterionic composite membrane. (7) The sulfonium / zwitterion composite membrane was heat-set and vacuum dried to obtain a multilayer membrane for lithium manganese batteries.

2. The method for preparing a multilayer separator for lithium manganese batteries according to claim 1, characterized in that, The polyolefin microporous membrane base is a polypropylene / polyethylene / polypropylene three-layer microporous membrane with a thickness of 20μm to 30μm and a porosity of 35% to 45%.

3. The method for preparing a multilayer separator for lithium manganese batteries according to claim 1, characterized in that, In step (2), the photoinitiator is benzophenone and acetone is used as a solvent for fixation. The mass fraction of benzophenone relative to acetone is 0.5% to 1.5%, and the soaking time is 5 min to 20 min.

4. The method for preparing a multilayer separator for lithium manganese batteries according to claim 1, characterized in that, The epoxy group-containing monomer in step (3) is glycidyl methacrylate, and the mass fraction of glycidyl methacrylate relative to the spreading solvent acetone is 1% to 5%.

5. The method for preparing a multilayer separator for lithium manganese batteries according to claim 1, characterized in that, In step (4), the single-end thiol-terminated methoxy polyethylene glycol has a mass fraction of 2% to 3% relative to the total mass of the mixed solvent, and its number average molecular weight is 1800 to 2200.

6. The method for preparing a multilayer separator for lithium manganese batteries according to claim 1, characterized in that, In step (4), the vacuuming-venting cycle reaches a vacuum pressure of -85kPa to -92kPa, and the pressure is maintained for 4 to 6 minutes each time, with a cycle count of 3 times.

7. The method for preparing a multilayer separator for lithium manganese batteries according to claim 1, characterized in that, The haloalkane in step (5) is iodomethane, with a mass fraction of 0.3% to 0.8% relative to acetonitrile. The reaction temperature is 15°C to 30°C and the mixture is kept in the dark for 15 min to 25 min.

8. The method for preparing a multilayer separator for lithium manganese batteries according to claim 1, characterized in that, The sulfonyl lactone mentioned in step (6) is 1,3-propanesulfonyl lactone or 1,4-butanesulfonyl lactone, with a mass fraction of 0.9% to 1.6% relative to acetonitrile, a reaction temperature of 45℃ to 55℃, and a reaction time of 3h to 5h.

9. The method for preparing a multilayer separator for lithium manganese batteries according to claim 1, characterized in that, In step (7), the heat setting temperature is 85℃~95℃ and the heat setting time is 8min~12min; the vacuum drying temperature is 55℃~65℃ and the drying time is 10h~14h.

10. A multilayer separator for lithium manganese batteries, characterized in that, It is obtained by the preparation method of the multilayer separator for lithium manganese batteries according to any one of claims 1-9.