A highly wettable, dendrite-resistant composite separator for sodium-ion batteries and its preparation method

Abstract

This invention belongs to the field of sodium-ion battery materials technology and discloses a highly wettable, dendrite-resistant composite separator for sodium-ion batteries and its preparation method. The invention uses an electron beam radiation-grafted hydrophilic polyolefin microporous membrane as a substrate, coats its surface with a modified nanofiber coating, and then forms a stable composite separator through drying and hot pressing. This invention solves the core problem of poor adhesion of the nanofiber coating on a hydrophobic substrate through hydrophilic modification, achieving a synergistic improvement in electrolyte superwetting, high ion conduction efficiency, and dendrite suppression. Its preparation process is green and efficient, suitable for the high safety requirements of sodium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery materials technology, and in particular to a highly wettable, dendrite-resistant sodium-ion battery composite separator and its preparation method. Background Technology

[0002] Polypropylene (PP) and polyethylene (PE) microporous membranes are widely used as separators for lithium / sodium-ion batteries due to their excellent electrochemical stability and mechanical strength. However, their inherent hydrophobicity results in poor wettability to polar sodium-ion battery electrolytes, leading to high interfacial impedance and severely impacting the battery's rate performance and cycle life. More seriously, uneven ion flow and uneven sodium deposition can easily lead to sodium dendrite growth, which may puncture the separator, causing internal short circuits and posing a serious safety hazard.

[0003] Existing improvement methods mostly employ surface coating with inorganic ceramics (such as Al2O3, SiO2) or organic polymer coatings. While these can improve wettability to some extent, they generally suffer from problems such as easy coating peeling, weak interfacial adhesion, complex processes, or high costs. Nanocellulose has advantages such as good hydrophilicity, renewability, and high mechanical strength, but its adhesion when directly coated onto hydrophobic PP / PE membranes is poor, and its chemical stability at high potentials is insufficient, making long-term stable operation difficult. Therefore, there is an urgent need to develop a novel composite membrane that can firmly bond, maintain long-term stability, and synergistically enhance wettability and dendrite inhibition functions. Summary of the Invention

[0004] The purpose of this invention is to provide a highly wettable, dendrite-resistant sodium-ion battery composite separator and its preparation method, thereby solving the problems existing in the prior art.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a method for preparing a highly wettable, dendrite-resistant sodium-ion battery composite separator, comprising the following steps:

[0007] The base film and grafting solution were subjected to hydrophilic modification by co-irradiation with electron beam to graft acrylic acid, resulting in a modified base film.

[0008] The nanocellulose fiber hydrogel was modified with a modifier to obtain a modified nanocellulose fiber dispersion gel.

[0009] The modified nanocellulose fiber dispersion gel, 40wt% ceramic slurry, polyvinylidene fluoride, and 10wt% polyvinyl alcohol aqueous solution were mixed and then uniformly dispersed by high-speed stirring and ultrasonic dispersion to obtain a coating liquid.

[0010] The coating liquid is applied to the surface of the modified base film on one or both sides, and then vacuum dried and hot-pressed to form a nanocellulose fiber coating, thus obtaining the highly wettable anti-dendritic sodium-ion battery composite separator.

[0011] Preferably, the base membrane is a polypropylene microporous membrane or a polyethylene microporous membrane; the thickness of the base membrane is 5~20μm; the electron beam irradiation dose is 10~80KGy; the mass fraction of the grafting solution is 3~20%; and the mass of the grafting solution is 1~4 times the mass of the base membrane.

[0012] Preferably, the modifier is one or more of toluene-2,4-diisocyanate, polydiphenylmethane diisocyanate, and isophorone diisocyanate; the modification method is as follows: the water in the nanocellulose fiber hydrogel is replaced with the organic solvent N-methylpyrrolidone by rotary evaporation to obtain a nanocellulose fiber / N-methylpyrrolidone dispersion gel; then, under nitrogen protection, the modifier and catalyst are added to the nanocellulose fiber / N-methylpyrrolidone dispersion gel, and the reaction is carried out under heating conditions to obtain a modified nanocellulose fiber dispersion gel.

[0013] Preferably, the solid content of the nanocellulose fiber hydrogel is 1.9-2.1%; the parameters of the modified nanocellulose fiber dispersion gel are: fiber diameter of 3-5 nm and length of 300-1000 nm; the mass of the modifier is 0.1-0.5% of the mass of the nanocellulose fiber hydrogel; the reaction temperature is 60-80℃; the reaction time is 5-8 h; and the catalyst is dibutyltin dilaurate.

[0014] Preferably, the solid content of the coating liquid is 2-6%; the mass fractions of the modified nanocellulose fiber dispersion gel, 40wt% ceramic slurry, polyvinylidene fluoride, and 10wt% polyvinyl alcohol aqueous solution in the coating liquid are 10-60 parts, 25-40 parts, 20-40 parts, and 0-10 parts, respectively.

[0015] Preferably, the ceramic slurry is one or more of alumina slurry, silica slurry, and titanium dioxide slurry; the particle size of the ceramic particles in the ceramic slurry is 10~200nm.

[0016] Preferably, the vacuum drying temperature is 80~95℃ and the vacuum drying time is 60~90min.

[0017] Preferably, the hot pressing temperature is 80~105℃; the hot pressing pressure is 1~5MPa; and the hot pressing time is 5~10min.

[0018] Preferably, the areal density of the nanocellulose fiber coating is 0.5~6 g / m³.2 The thickness of the nanocellulose fiber coating is 1~5μm.

[0019] The present invention also provides a highly wettable, dendrite-resistant sodium-ion battery composite separator prepared by the above preparation method.

[0020] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) Strong interfacial bonding: First, the polyolefin base film is hydrophilically modified and polar functional groups are introduced on its surface, which greatly enhances the chemical bonding force and physical adhesion between the hydrophilic nanocellulose coating and the hydrophobic base film, effectively preventing the coating from falling off during use.

[0022] (2) The isocyanate-modified nanocellulose fibers can form a continuous, interwoven, and strong network in the coating. This network structure has extremely high strength and modulus, and can effectively resist the mechanical stress during the growth of sodium dendrites. It greatly enhances the interfacial bonding force between the coating and the base film, ensuring that the membrane remains structurally intact during long-term charge-discharge cycles and electrolyte immersion, and that the coating does not powder or fall off.

[0023] (3) Isocyanate-modified nanocellulose fibers, when used as a membrane coating, can form efficient ion transport channels and improve electrochemical performance. The nanocellulose network constructs uniformly sized, interconnected nanoscale pores in the coating. Isocyanate modification improves the dispersibility of nanocellulose in organic systems, avoids filler agglomeration, ensures channel uniformity, helps balance the current density on the electrode surface, reduces local ion aggregation from the source, and inhibits dendrite initiation.

[0024] (4) The nanocellulose fibers themselves have excellent thermal stability, and their network structure can effectively inhibit the shrinkage of polyolefin-based membranes at high temperatures. At the same time, the polar functional groups on the coating surface enhance the interaction with the polar electrolyte, giving the membrane excellent thermal stability and electrolyte affinity. Detailed Implementation

[0025] This invention provides a method for preparing a highly wettable, dendrite-resistant sodium-ion battery composite separator, comprising the following steps:

[0026] The base film and grafting solution were subjected to hydrophilic modification by co-irradiation with electron beam to graft acrylic acid, resulting in a modified base film.

[0027] The nanocellulose fiber hydrogel was modified with a modifier to obtain a modified nanocellulose fiber dispersion gel.

[0028] The modified nanocellulose fiber dispersion gel, 40wt% ceramic slurry, polyvinylidene fluoride, and 10wt% polyvinyl alcohol aqueous solution were mixed and then uniformly dispersed by high-speed stirring and ultrasonic dispersion to obtain a coating liquid.

[0029] The coating liquid is applied to the surface of the modified base film on one or both sides, and then vacuum dried and hot-pressed to form a nanocellulose fiber coating, thus obtaining the highly wettable anti-dendritic sodium-ion battery composite separator.

[0030] In this invention, the base membrane is preferably a polypropylene microporous membrane or a polyethylene microporous membrane, more preferably a polyethylene microporous membrane; the thickness of the base membrane is preferably 5~20μm, more preferably 5~10μm, and even more preferably 5μm.

[0031] In this invention, the electron beam irradiation dose is preferably 10-80 KGy, more preferably 20-60 KGy, and even more preferably 50 KGy.

[0032] In this invention, the grafting solution is preferably an acrylic acid solution; the mass fraction of the grafting solution is preferably 3-20%, more preferably 5-18%, and even more preferably 15%; the mass of the grafting solution is preferably 1-4 times the mass of the base film, more preferably 1-3 times, and even more preferably 2 times.

[0033] In this invention, the modifier is preferably one or more of toluene-2,4-diisocyanate (TDI), polymeric diphenylmethane diisocyanate (PMDI), and isophorone diisocyanate (IPDI), more preferably TDI or IPDI, and even more preferably TDI.

[0034] In this invention, the preferred modification method is as follows: The water in the nanocellulose fiber hydrogel is replaced with the organic solvent N-methylpyrrolidone by rotary evaporation to obtain a nanocellulose fiber / N-methylpyrrolidone dispersion gel; then, under nitrogen protection, a modifier and a catalyst are added to the nanocellulose fiber / N-methylpyrrolidone dispersion gel, and the reaction is carried out under heating conditions. After the reaction is completed, unreacted modifier and byproducts are thoroughly removed by centrifugation and washing to obtain the modified nanocellulose fiber dispersion gel. In this invention, the diameter, length, and solid content of the nanocellulose fibers remain unchanged before and after modification.

[0035] In this invention, the solid content of the nanocellulose fiber hydrogel is preferably 1.9-2.1%, and more preferably 2.0%.

[0036] In this invention, the parameters of the modified nanocellulose fiber dispersion gel are: the fiber diameter is preferably 3~5nm, and the length is preferably 300~1000nm.

[0037] In this invention, the mass of the modifier is preferably 0.1 to 0.5% of the mass of the nanocellulose fiber hydrogel, more preferably 0.2 to 0.4%, and even more preferably 0.3%.

[0038] In this invention, the reaction temperature is preferably 60~80℃, more preferably 62~70℃, and even more preferably 65℃; the reaction time is preferably 5~8h, more preferably 5~6h, and even more preferably 5h.

[0039] In this invention, the catalyst is preferably dibutyltin dilaurate.

[0040] In this invention, the solid content of the coating liquid is preferably 2-6%, more preferably 4-6%, and even more preferably 5%; the mass fractions of the modified nanocellulose fiber dispersion gel, 40wt% ceramic slurry, polyvinylidene fluoride (PVDF), and 10wt% polyvinyl alcohol aqueous solution in the coating liquid are preferably 10-60 parts, 25-40 parts, 20-40 parts, and 0-10 parts, more preferably 20-50 parts, 30-35 parts, 23-35 parts, and 2-9 parts, and even more preferably 35 parts, 32 parts, 25 parts, and 8 parts.

[0041] In this invention, the ceramic slurry is preferably one or more of alumina slurry, silica slurry, and titanium dioxide slurry, more preferably alumina slurry or silica slurry, and even more preferably alumina slurry; the particle size of the ceramic particles in the ceramic slurry is preferably 10~200nm, more preferably 50~150nm, and even more preferably 100nm.

[0042] In this invention, the vacuum drying temperature is preferably 80~95℃, more preferably 80~90℃, and even more preferably 80℃; the vacuum drying time is preferably 60~90min, more preferably 80~90min, and even more preferably 90min.

[0043] In this invention, the hot pressing temperature is preferably 80~105℃, more preferably 90~105℃, and even more preferably 105℃; the hot pressing pressure is preferably 1~5MPa, more preferably 2~5MPa, and even more preferably 5MPa; the hot pressing time is preferably 5~10min, more preferably 5~8min, and even more preferably 5min.

[0044] In this invention, the areal density of the nanocellulose fiber coating is preferably 0.5~6 g / m³. 2 Further preferred is 2~5g / m 2 More preferably 3~4.5g / m 2The thickness of the nanocellulose fiber coating is preferably 1~5μm, more preferably 3~5μm, and even more preferably 5μm.

[0045] The present invention also provides a highly wettable, dendrite-resistant sodium-ion battery composite separator prepared by the above preparation method.

[0046] 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.

[0047] Example 1

[0048] This embodiment provides a method for preparing a highly wettable, dendrite-resistant sodium-ion battery composite separator, comprising the following steps:

[0049] (1) Preparation of modified base film

[0050] 1) Cut a 5μm thick polyethylene microporous membrane base into 20cm×30cm pieces, clean the surface with ethanol, dry to constant weight, and place in a polyethylene vacuum bag for later use; 2) Prepare an aqueous solution containing 15% acrylic acid and 0.05mol / L copper sulfate (polymerization inhibitor) as the grafting solution, seal it in a flat-bottomed test tube, and deoxygenate it for 30 minutes under a vacuum of 0.098MPa for later use; 3) Inject the deoxygenated grafting solution into the vacuum bag containing the pretreated base membrane from step 1), ensuring that the mass of the injected grafting solution is twice the mass of the base membrane. The base film is completely immersed to allow it to fully adsorb the monomer solution. Then, all the air inside the bag is expelled, and the vacuum bag is sealed with a vacuum sealer. 4) The sealed vacuum bag is placed flat on the sample tray and transferred to the area under the electron beam irradiation beam to receive irradiation. The irradiation dose is set to 50 KGy. After irradiation, the vacuum bag is placed at room temperature for 2 hours. 5) After the reaction is completed, the base film is taken out and washed repeatedly with deionized water at 60°C 3 times to remove ungrafted acrylic monomers and homopolymers. Then, it is dried in a vacuum oven at 60°C for 4 hours to obtain an acrylic graft modified hydrophilic base film, which is referred to as the modified base film.

[0051] (2) Preparation of modified nanocellulose fiber dispersion gel

[0052] 100g of cellulose nanofiber hydrogel (NCF) with a fiber diameter of 3-5nm, a length of 300-1000nm, and a solid content of 2.0wt% and 98g of N-methylpyrrolidone (NMP) were weighed and placed in a rotary evaporator. The NMP dispersion gel of NCF was obtained by solvent displacement. The above dispersion gel was transferred to a three-necked flask equipped with a mechanical stirrer, a thermometer, and a nitrogen inlet and outlet. Under nitrogen protection, toluene-2,4-diisocyanate (TDI) equivalent to 0.3wt% of the NCF weight was added to the gel, and 0.01g of dibutyltin dilaurate was added as a catalyst. The temperature of the reaction system was raised to 65℃ and the reaction was carried out for 5h with continuous stirring. After the reaction was completed, the reaction product was transferred to a centrifuge tube and washed three times with fresh NMP (8000 rpm, 10 min each time) to completely remove unreacted TDI and byproducts, resulting in a modified nanocellulose fiber dispersion gel with isocyanate active groups on the surface. The solid content of the dispersion gel was adjusted to 2.0 wt%, and the gel was sealed and stored for later use.

[0053] (3) Preparation of coating solution

[0054] Weigh the following components by weight: 35 parts modified nanocellulose fiber dispersion gel, 32 parts 40wt% alumina ceramic slurry, 25 parts polyvinylidene fluoride, and 8 parts 10wt% polyvinyl alcohol aqueous solution. Add all the above components to a stirred tank containing NMP solvent, controlling the solid content of the coating solution to 5%. Stir at high speed at 2000 rpm for 2 hours, followed by ultrasonic treatment at 500W power for 1 hour to finally obtain a uniformly dispersed coating solution.

[0055] (4) Preparation of nanocellulose fiber coating

[0056] Using a coating machine, the above coating solution was uniformly coated onto one side of the modified base film via a wire rod, controlling the wet film thickness to be 80 μm. The wet film was then placed in a vacuum drying oven at 80°C for 90 min to completely remove the solvent. Finally, the dried film was placed in a flatbed hot press and hot-pressed at 105°C and 5 MPa for 5 min to ensure a tight bond and densification between the functional coating and the hydrophilic base film, forming a nanofiber cellulose coating with an areal density of 3.92 g / m³. 2 The thickness is 5μm, thus obtaining the highly wettable, dendrite-resistant sodium-ion battery composite separator.

[0057] Example 2

[0058] (1) Preparation of modified base film

[0059] For details, please refer to Example 1, except that: the base film is a 7μm polyethylene microporous membrane, the mass fraction of acrylic monomer in the grafting solution is 10%, the mass of the grafting solution is 2.5 times the mass of the base film, and the irradiation dose is 40KGy.

[0060] (2) Preparation of modified nanocellulose fiber dispersion gel

[0061] For details, please refer to Example 1, except that: the solid content of NCF is 1.9 wt%, the modifier is polydiphenylmethane diisocyanate (PMDI), the weight of the modifier is 0.1 wt% of the weight of NCF, the reaction temperature is 75°C and the time is 7 h, and the solid content of the modified nanocellulose fiber dispersion gel is 1.9 wt%.

[0062] (3) Preparation of coating solution

[0063] For details, please refer to Example 1, except that: 42 parts of modified nanocellulose fiber dispersion gel, 30 parts of 40wt% alumina ceramic slurry, 20 parts of polyvinylidene fluoride, and 8 parts of 10wt% polyvinyl alcohol aqueous solution.

[0064] (4) Preparation of nanocellulose fiber coating

[0065] See Example 1 for details, except that: the coating solution is coated on both sides of the modified base film, and the thickness of the wet film on one side is controlled to be 40 μm, resulting in a final areal density of 4.02 g / m² for the nanocellulose fiber coating. 2 The thickness is 5μm.

[0066] Example 3

[0067] (1) Preparation of modified base film

[0068] For details, please refer to Example 1, except that: the base film is a 9μm polyethylene microporous membrane, the mass fraction of acrylic monomer in the grafting solution is 5%, the mass of the grafting solution is 3 times the mass of the base film, and the irradiation dose is 20KGy.

[0069] (2) Preparation of modified nanocellulose fiber dispersion gel

[0070] For details, please refer to Example 1, except that: the solid content of NCF is 2.1wt%, the modifier is isophorone diisocyanate (IPDI), the weight of the modifier is 0.5wt% of the weight of NCF, the reaction temperature is 80℃ and the time is 6h, and the solid content of the modified nanocellulose fiber dispersion gel is 2.1wt%.

[0071] (3) Preparation of coating solution

[0072] For details, please refer to Example 1, except that: 35 parts of modified nanocellulose fiber dispersion gel, 30 parts of 40wt% alumina ceramic slurry, 25 parts of polyvinylidene fluoride, and 10 parts of 10wt% polyvinyl alcohol aqueous solution.

[0073] (4) Preparation of nanocellulose fiber coating

[0074] See Example 1 for details, except that: the coating solution is coated on both sides of the modified base film, and the thickness of the wet film on one side is controlled to be 50 μm, resulting in a final areal density of 3.96 g / m² for the nanocellulose fiber coating. 2 The thickness is 5μm.

[0075] Example 4

[0076] (1) Preparation of modified base film

[0077] For details, please refer to Example 1, except that: the base membrane is a 20μm polypropylene microporous membrane, the mass fraction of acrylic acid monomer in the grafting solution is 20%, the mass of the grafting solution is twice the mass of the base membrane, and the irradiation dose is 50KGy.

[0078] (2) Preparation of modified nanocellulose fiber dispersion gel

[0079] See Example 1 for details, except that the modifier is TDI and IPDI in a mass ratio of 1:1, and the reaction temperature is 70°C and the time is 5.5h.

[0080] (3) Preparation of coating solution

[0081] See Example 1 for details.

[0082] (4) Preparation of nanocellulose fiber coating

[0083] See Example 1 for details, except that: the coating solution is coated on both sides of the modified base film, and the thickness of the wet film on one side is controlled to be 50 μm, resulting in a final areal density of 4.07 g / m² for the nanocellulose fiber coating. 2 The thickness is 5μm.

[0084] Example 5

[0085] (1) Preparation of modified base film

[0086] See Example 1 for details.

[0087] (2) Preparation of modified nanocellulose fiber dispersion gel

[0088] See Example 1 for details, except that the modifier is 0.5 wt% of NCF, the reaction temperature is 70°C, and the reaction time is 6 h.

[0089] (3) Preparation of coating solution

[0090] For details, please refer to Example 1, except that: 50 parts of modified nanocellulose fiber dispersion gel, 25 parts of 40wt% alumina ceramic slurry, 20 parts of polyvinylidene fluoride, 5 parts of 10wt% polyvinyl alcohol aqueous solution, and the solid content of the coating liquid is 4%.

[0091] (4) Preparation of nanocellulose fiber coating

[0092] See Example 1 for details, except that: the coating solution is coated on both sides of the modified base film, and the wet film thickness on one side is controlled to be 50 μm, resulting in a final nanofiber coating with an areal density of 4.11 g / m². 2 The thickness is 5μm.

[0093] Example 6

[0094] (1) Preparation of modified base film

[0095] See Example 1 for details.

[0096] (2) Preparation of modified nanocellulose fiber dispersion gel

[0097] See Example 2 for details, except that the modifier is 0.3 wt% of the NCF weight, the reaction temperature is 75°C, and the reaction time is 5 h.

[0098] (3) Preparation of coating solution

[0099] For details, please refer to Example 1, except that: 40 parts of modified nanocellulose fiber dispersion gel, 30 parts of 40wt% alumina ceramic slurry, 20 parts of polyvinylidene fluoride, 10 parts of 10wt% polyvinyl alcohol aqueous solution, and the solid content of the coating liquid is 4%.

[0100] (4) Preparation of nanocellulose fiber coating

[0101] See Example 1 for details, except that: the coating solution is coated on both sides of the modified base film, and the thickness of the wet film on one side is controlled to be 50 μm, resulting in a final areal density of 4.04 g / m² for the nanocellulose fiber coating. 2 The thickness is 5μm.

[0102] Example 7

[0103] (1) Preparation of modified base film

[0104] See Example 1 for details.

[0105] (2) Preparation of modified nanocellulose fiber dispersion gel

[0106] See Example 3 for details, except that the modifier weighs 0.3 wt% of NCF, the reaction temperature is 75°C, and the reaction time is 5 h.

[0107] (3) Preparation of coating solution

[0108] For details, please refer to Example 1, except that: 40 parts of modified nanocellulose fiber dispersion gel, 30 parts of 40wt% alumina ceramic slurry, 20 parts of polyvinylidene fluoride, 10 parts of 10wt% polyvinyl alcohol aqueous solution, and the solid content of the coating liquid is 4%.

[0109] (4) Preparation of nanocellulose fiber coating

[0110] See Example 1 for details, except that: the coating solution is coated on both sides of the modified base film, and the wet film thickness on one side is controlled to be 50 μm, resulting in a final nanofiber coating with an areal density of 3.98 g / m². 2 The thickness is 5μm.

[0111] Example 8

[0112] (1) Preparation of modified base film

[0113] See Example 2 for details.

[0114] (2) Preparation of modified nanocellulose fiber dispersion gel

[0115] See Example 1 for details.

[0116] (3) Preparation of coating solution

[0117] For details, please refer to Example 1, except that: 30 parts of modified nanocellulose fiber dispersion gel, 30 parts of 40wt% alumina ceramic slurry, 30 parts of polyvinylidene fluoride, 10 parts of 10wt% polyvinyl alcohol aqueous solution, and the solid content of the coating liquid is 4%.

[0118] (4) Preparation of nanocellulose fiber coating

[0119] See Example 1 for details, except that: the coating solution is coated on both sides of the modified base film, and the thickness of the wet film on one side is controlled to be 50 μm, resulting in a final areal density of 3.94 g / m² for the nanocellulose fiber coating. 2 The thickness is 5μm.

[0120] Example 9

[0121] (1) Preparation of modified base film

[0122] See Example 4 for details.

[0123] (2) Preparation of modified nanocellulose fiber dispersion gel

[0124] See Example 1 for details.

[0125] (3) Preparation of coating solution

[0126] For details, please refer to Example 1, except that: 20 parts of modified nanocellulose fiber dispersion gel, 40 parts of 40wt% alumina ceramic slurry, 30 parts of polyvinylidene fluoride, 10 parts of 10wt% polyvinyl alcohol aqueous solution, and the solid content of the coating liquid is 4%.

[0127] (4) Preparation of nanocellulose fiber coating

[0128] See Example 1 for details, except that: the coating solution is coated on both sides of the modified base film, and the thickness of the wet film on one side is controlled to be 50 μm, resulting in a final areal density of 3.91 g / m² for the nanocellulose fiber coating. 2 The thickness is 5μm.

[0129] Comparative Example 1

[0130] For details of this comparative example, please refer to Example 1, except that the base film was not grafted with acrylic acid. The final nanofiber coating has an areal density of 4.01 g / m³. 2 The thickness is 5μm.

[0131] Comparative Example 2

[0132] This comparative example is detailed in Example 1, except that the coating solution does not contain the modified nanocellulose fiber dispersion gel component. The coating solution was prepared by weighing the following components in parts by weight: 45 parts of 40wt% alumina ceramic slurry, 45 parts of polyvinylidene fluoride, and 10 parts of 10wt% polyvinyl alcohol aqueous solution. The final coating had an areal density of 4.06 g / m³. 2 The thickness is 5μm.

[0133] Comparative Example 3

[0134] This comparative example is detailed in Example 1, except that the nanocellulose fiber hydrogel is not modified with isocyanate. The final nanocellulose fiber coating has an areal density of 3.91 g / m³. 2 The thickness is 5μm.

[0135] The performance of the composite membranes prepared in the above embodiments and comparative examples was tested, and the results are shown in Table 1.

[0136] Table 1 Performance test results of composite membranes

[0137]

[0138] Note: The puncture strength increase ratio in the test results is calculated as the ratio of "test value of each sample" to "test value of the original base film (blank control) of the same model that has not been modified or coated".

[0139] As shown in Table 1, the composite membranes prepared in Examples 1-9 of this invention benefit from the ultra-high hydrophilicity imparted to the base membrane by electron beam co-irradiation grafted acrylic acid, the excellent dispersibility of isocyanate-modified nanocellulose fibers in organic solvents, and the interfacial bonding with the polymer matrix. Combined with the synergistic effect of ceramic slurry, polyvinylidene fluoride, and polyvinyl alcohol aqueous solution, a composite coating with high electrolyte wettability, high ionic conductivity, excellent mechanical strength, and superior resistance to sodium dendrite penetration is formed. Comparative Examples 1-3, lacking any key technical feature, are significantly inferior to the examples in all performance aspects, especially in puncture resistance, fully demonstrating the necessity and synergistic effect of each step and component of the technical solution of this invention.

[0140] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a highly wettable, dendrite-resistant composite separator for sodium-ion batteries, characterized in that, Includes the following steps: A modified base membrane is obtained by hydrophilic modification of a base membrane and a grafting solution by co-irradiation with an electron beam to graft acrylic acid. The base membrane is a polypropylene microporous membrane or a polyethylene microporous membrane. The thickness of the base membrane is 5-20 μm. The electron beam co-irradiation dose is 10-80 KGy. The mass fraction of the grafting solution is 3-20%. The mass of the grafting solution is 1-4 times the mass of the base membrane. The nanocellulose fiber hydrogel was modified with a modifier to obtain a modified nanocellulose fiber dispersion gel. The modifier was one or more of toluene-2,4-diisocyanate, polydiphenylmethane diisocyanate, and isophorone diisocyanate. The modification method was as follows: the water in the nanocellulose fiber hydrogel was replaced with the organic solvent N-methylpyrrolidone by rotary evaporation to obtain a nanocellulose fiber / N-methylpyrrolidone dispersion gel; then, under nitrogen protection, the modifier and catalyst were added to the nanocellulose fiber / N-methylpyrrolidone dispersion gel, and the reaction was carried out under heating conditions to obtain the modified nanocellulose fiber dispersion gel. The modified nanocellulose fiber dispersion gel, 40wt% ceramic slurry, polyvinylidene fluoride, and 10wt% polyvinyl alcohol aqueous solution were mixed and then uniformly dispersed by high-speed stirring and ultrasonic dispersion to obtain a coating liquid. The coating liquid is applied to the surface of the modified base film on one or both sides, and then vacuum dried and hot-pressed to form a nanocellulose fiber coating, thus obtaining the highly wettable anti-dendritic sodium-ion battery composite separator.

2. The method for preparing a highly wettable, dendrite-resistant sodium-ion battery composite separator according to claim 1, characterized in that, The solid content of the nanocellulose fiber hydrogel is 1.9~2.1%; the parameters of the modified nanocellulose fiber dispersion gel are: fiber diameter of 3~5nm and length of 300~1000nm; the mass of the modifier is 0.1~0.5% of the mass of the nanocellulose fiber hydrogel; the reaction temperature is 60~80℃; the reaction time is 5~8h; and the catalyst is dibutyltin dilaurate.

3. The method for preparing a highly wettable, dendrite-resistant sodium-ion battery composite separator according to claim 1, characterized in that, The solid content of the coating solution is 2-6%; the mass fractions of the modified nanocellulose fiber dispersion gel, 40wt% ceramic slurry, polyvinylidene fluoride, and 10wt% polyvinyl alcohol aqueous solution in the coating solution are 10-60 parts, 25-40 parts, 20-40 parts, and 0-10 parts, respectively.

4. The method for preparing a highly wettable, dendrite-resistant sodium-ion battery composite separator according to claim 3, characterized in that, The ceramic slurry is one or more of alumina slurry, silica slurry, and titanium dioxide slurry; the particle size of the ceramic particles in the ceramic slurry is 10~200nm.

5. The method for preparing a highly wettable, dendrite-resistant sodium-ion battery composite separator according to claim 1, characterized in that, The vacuum drying temperature is 80~95℃; the vacuum drying time is 60~90min.

6. The method for preparing a highly wettable, dendrite-resistant sodium-ion battery composite separator according to claim 1, characterized in that, The hot pressing temperature is 80~105℃; the hot pressing pressure is 1~5MPa; and the hot pressing time is 5~10min.

7. The method for preparing a highly wettable, dendrite-resistant sodium-ion battery composite separator according to claim 1, characterized in that, The areal density of the nanocellulose fiber coating is 0.5~6 g / m³. 2 The thickness of the nanocellulose fiber coating is 1~5μm.

8. A highly wettable, dendrite-resistant sodium-ion battery composite separator prepared by the preparation method according to any one of claims 1 to 7.

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