Cellulose battery diaphragm material as well as preparation method and application thereof

Abstract

The invention relates to a cellulose battery diaphragm material as well as a preparation method and application thereof. The method comprises the following steps: dissolving cellulose in an ionic liquid to form a first cellulose solution; adding an auxiliary agent into the first cellulose solution, and uniformly stirring to form a second cellulose solution; the additive is selected from polar solvents with the dielectric constant not less than 20, the viscosity less than 3.0 cP at 25 DEG C and the boiling point not less than 120 DEG C; the second cellulose solution is arranged on the substrate, so that the second cellulose solution forms a wet coating film on the substrate; the thickness of the wet coating film is 1-2 mm; the substrate is placed in a first anti-solvent coagulating bath for 3-5 h, and the wet coating film is gelatinized; immersing the gelatinized membrane in a second anti-solvent coagulating bath for 8-12 hours; and drying to obtain the cellulose battery diaphragm material. The technical problem to be solved is how to provide the cellulose battery diaphragm material which has excellent heat resistance, high mechanical strength and high porosity, and pores are micropores and macropores with gradient round hole structures.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery separator material preparation, and in particular relates to a cellulose battery separator material and a preparation method and application thereof. Background Art

[0002] With the rapid development of renewable energy, battery technology has become the core of the energy storage sector. Lithium-ion batteries are widely used in various electronic devices due to their high energy density and long cycle life. As a key component of lithium-ion batteries, battery separators primarily separate the positive and negative electrodes to prevent short circuits while allowing lithium ions to migrate between the two electrodes to complete the charge and discharge process, directly impacting battery performance and safety. With the increasing popularity of high-energy-density batteries and fast-charging technologies, separators must possess thermal stability, high porosity, mechanical strength, and environmental friendliness. Cellulose membranes have become a research hotspot due to their excellent thermal stability, good electrolyte wettability, and reproducibility. Cellulose-based separators require high porosity to absorb and retain sufficient electrolyte, while also possessing an appropriate pore size to reduce self-discharge and ensure excellent ionic conductivity and electrochemical performance. However, cellulose itself suffers from problems such as poor mechanical strength and difficult-to-control pore structure.

[0003] Studies have reported that by dissolving cellulose in a binary solvent of superbase-derived ionic liquid and dimethyl sulfoxide, and then coagulating and regenerating it in an ethanol bath, a regenerated cellulose membrane with a nanocrack structure can be obtained, which is beneficial for ion transport; however, in this regenerated cellulose membrane with a nanocrack structure, although the overall size of the nanocracks is very small, the membrane is subjected to macroscopic stress (such as bending, stretching, compression, etc.) during preparation or use. These cracks may become stress concentration areas, making it possible for cracks to merge and expand, and there is a certain risk of structural embrittlement. At the same time, the crack morphology can usually extend disorderly, and the pore shape is not easy to accurately control, which makes it limited in certain high-precision separation or more precise barrier scenarios (such as some battery separators). Summary of the Invention

[0004] The primary objective of this invention is to provide a cellulose battery separator material, its preparation method, and its application. The technical problem to be solved is how to provide a cellulose battery separator material that combines excellent heat resistance with high mechanical strength, while also possessing a high porosity and a gradient circular pore structure with micropores and macropores. The micropores effectively block dendrite penetration, the macropores enhance ion flux, and the separator exhibits excellent electrolyte absorption and wettability. Furthermore, the preparation method of this invention utilizes a conventional coating equipment film-forming process, significantly reducing equipment investment and energy consumption. Combined with recyclable ionic liquids, this method enables green and efficient production.

[0005] The purpose of the present invention and the technical problem solved are also achieved by adopting the following technical solutions. According to the present invention, a method for preparing a cellulose battery separator material comprises the following steps:

[0006] S11 dissolving cellulose in an ionic liquid to form a first cellulose solution;

[0007] S12 adding an auxiliary agent to the first cellulose solution and stirring uniformly to form a second cellulose solution; the auxiliary agent is selected from a polar solvent having a dielectric constant ≥ 20, a viscosity at 25°C < 3.0 cP, and a boiling point ≥ 120°C;

[0008] S13: placing the second cellulose solution on the substrate to form a wet coating film on the substrate; the wet coating film has a thickness of 1 to 2 mm;

[0009] S14: placing the substrate with the wet coating film in a first anti-solvent coagulation bath for 3 to 5 hours to gel the wet coating film;

[0010] S15: immersing the gelled membrane in a second anti-solvent coagulation bath for 8 to 12 hours;

[0011] S16 is dried to obtain a cellulose battery separator material.

[0012] The purpose of the present invention and the solution to its technical problems can be further achieved by adopting the following technical measures.

[0013] Preferably, in the aforementioned preparation method, the ionic liquid is selected from at least one of imidazolium salt ionic liquids, pyridinium salt ionic liquids and amino acid choline ionic liquids.

[0014] Preferably, in the aforementioned preparation method, the pulp is selected from dissolving pulp and / or refined cotton; and in the first cellulose solution, the amount of cellulose is 6-8% of the amount of the ionic liquid by mass.

[0015] Preferably, in the aforementioned preparation method, the auxiliary agent is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide and N-methylpyrrolidone; and the amount of the auxiliary agent is 30-60% by mass of the ionic liquid.

[0016] Preferably, in the aforementioned preparation method, the temperature for dissolving the cellulose is 75-85°C; and the temperature for adding the auxiliary agent is 50-65°C.

[0017] Preferably, the aforementioned preparation method further includes a step of removing bubbles from the second cellulose solution before step S13.

[0018] Preferably, in the aforementioned preparation method, the first anti-solvent is selected from at least one of ethanol, methanol, propanol and acetone; and the water content of the first anti-solvent is ≤50% by mass.

[0019] The purpose of the present invention and the technical problem solved are achieved by adopting the following technical solutions: According to the present invention, a cellulose battery separator material is proposed, which includes micropores and macropores with a gradient circular pore structure; the pore diameter of the micropores is 20-50nm; the pore diameter of the macropores is 100-250nm.

[0020] The purpose of the present invention and the solution to its technical problems can be further achieved by adopting the following technical measures.

[0021] Preferably, the aforementioned cellulose battery separator material is prepared according to the aforementioned preparation method.

[0022] The purpose of the present invention and the technical problem solved are achieved by adopting the following technical solutions: The present invention proposes an application of the aforementioned cellulose battery separator material in the field of batteries.

[0023] By means of the above technical solution, the cellulose battery separator material and its preparation method and application proposed in the present invention have at least the following advantages:

[0024] The cellulose battery diaphragm material proposed in the present invention and its preparation method and application are as follows: cellulose is fully dissolved in an ionic liquid, and then a polar solvent with a dielectric constant ≥ 20, a viscosity of < 3.0 cP at 25°C, and a boiling point ≥ 120°C is added to the first cellulose solution and stirred to form a second cellulose solution; finally, a staged coagulation process is performed to coagulate the cellulose in the first anti-solvent and the second anti-solvent in sequence, thereby constructing pores with a gradient circular pore structure on the diaphragm, so that the prepared diaphragm material has both excellent heat resistance and high mechanical strength, and at the same time has a high porosity and Good electrolyte absorption and wettability; the prepared battery separator material will not show any thermal shrinkage and deformation after being placed in a 160°C environment for 1 hour; its tensile strength is ≥90MPa, porosity is ≥60%, solution absorption rate is ≥100%, and water contact angle is ≤43°; in the preferred technical solution, its tensile strength is ≥100MPa, porosity is ≥65%, solution absorption rate is ≥110%, and water contact angle is ≤40°; in a further preferred technical solution, its tensile strength is ≥110MPa, porosity is ≥70%, solution absorption rate is ≥120%, and water contact angle is ≤38°.

[0025] The preparation of the cellulose battery separator material proposed in the present invention is based on the film-forming process of conventional coating equipment, which can greatly reduce equipment investment and reduce costs; at the same time, it can greatly reduce equipment energy consumption, and combined with the ionic liquid recyclable process, realize green and efficient production.

[0026] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a morphology-SEM image of the battery separator material of Example 1 of the present invention;

[0028] Figure 2 This is a photograph of the thermal shrinkage behavior of the battery separator of Example 1 of the present invention;

[0029] Figure 3 This is a tensile strength test curve of the battery separator of Example 1 of the present invention. DETAILED DESCRIPTION

[0030] To further illustrate the technical means and effectiveness of the present invention in achieving its intended objectives, the following, in conjunction with the accompanying drawings and preferred embodiments, provides a detailed description of a cellulose battery separator material, its preparation method, and its specific implementation, structure, features, and effectiveness. In the following description, different references to "one embodiment" or "embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics of one or more embodiments may be combined in any suitable manner.

[0031] The present invention provides a method for preparing a cellulose battery separator material, which comprises the following steps:

[0032] The first step is to dissolve cellulose. Add cellulose to the ionic liquid and stir to dissolve it to form a first cellulose solution. In order to dissolve the cellulose better, it is preferred that the dissolution be carried out under heating conditions. The dissolution temperature should not be too high to avoid changes in the properties of the ionic liquid or degradation of the cellulose; at the same time, the dissolution temperature should not be too low so that the system can maintain a better viscosity and the cellulose can be dissolved faster; through a large number of experiments, it has been verified that controlling the dissolution temperature of cellulose in the ionic liquid can effectively optimize the rheology and uniformity of the solution; the temperature for dissolving cellulose is preferably 70-90°C, and it is further preferred that the temperature for dissolving cellulose is 75-85°C, and it is dissolved under stirring for 1-1.5 hours, which can make the cellulose processable better and the final prepared battery separator have better performance.

[0033] In order to ensure the performance of the battery separator material, the cellulose in the above steps is preferably dissolving pulp and / or refined cotton. The reason is that these two types of cellulose have undergone a special processing process to remove most of the non-cellulose components in the raw materials, and their cellulose content is relatively high. The prepared first cellulose solution has a higher yield and is purer; at the same time, the polymerization degree of these two types of cellulose is relatively moderate and stable, which can ensure the strength and batch stability of the battery separator.

[0034] In order to ensure the performance of the battery separator material, the concentration of the first cellulose solution should be high. However, when the concentration is too high, it becomes more difficult to dissolve and the solution stability is poor. Through a large number of experiments, it has been verified that the amount of cellulose in the first cellulose solution is preferably 6-8% of the amount of ionic liquid by mass.

[0035] In order to control the pore structure of the battery separator material and construct pores with a gradient circular pore structure, the present invention preferably uses an ionic liquid selected from at least one of imidazolium salt ionic liquids, pyridinium salt ionic liquids and amino acid choline ionic liquids; when this ionic liquid acts on cellulose, it mainly destroys hydrogen bonds and does not truly deprotonate the -OH group of cellulose, so that the cellulose can still maintain molecular neutrality. It mainly relies on the hydrogen bonds between the anions of the ionic liquid and the -OH group of cellulose to untie the hydrogen bond network of the cellulose chain; since the cellulose molecules remain neutral, they may undergo a "liquid-liquid phase separation" mechanism during the later phase separation, which can allow the polymer-rich phase and the poor phase to wrap around each other to form a prototype of spherical holes; combined with the subsequent staged solidification process, a process of slow phase separation → hole growth → rapid shaping occurs, thereby forming clear, rounded multi-level holes on the battery separator, rather than crack-type holes.

[0036] In order to ensure the purity of the first cellulose solution, the first cellulose solution is preferably filtered or centrifuged to remove impurities and undissolved matter therein.

[0037] The second step is to add an auxiliary agent to the first cellulose solution and stir it evenly to form a second cellulose solution; the auxiliary agent added in this step is selected from polar solvents with a dielectric constant ≥20, a viscosity at 25°C <3.0cP, and a boiling point ≥120°C.

[0038] In the technical solution of the present invention, the polar solvent serves as a key auxiliary agent in the system. On the one hand, it can improve the fluidity of the second cellulose solution, making it easy to coat it into a uniform film; on the other hand, it works together with the anti-solvent in the subsequent staged coagulation bath to construct pores with a gradient circular pore structure on the diaphragm material; the pores of the gradient circular pore structure include micropores and macropores. The micropores can effectively block dendrite penetration, and the macropores can enhance ion flux.

[0039] In the technical solution of the present invention, the timing of adding the polar solvent is very critical. During the cellulose dissolution stage, the polar solvent is not added first, but the cellulose dissolution is completed only by the ionic liquid. This technical setting can avoid the polar solvent from diluting the ionic liquid during the cellulose dissolution stage, allowing the ionic liquid to exert its maximum dissolving capacity, ensuring that the cellulose chains can fully unfold and disperse, maintaining the initial solubility and dispersibility of the cellulose, and obtaining a higher concentration and more homogeneous first cellulose solution. At the same time, not adding the polar solvent first during the cellulose dissolution stage can prevent the polar solvent from interfering with the cellulose dissolution process early, thereby avoiding its interference with the dispersion of the cellulose molecular chains or causing localized uneven pre-phase separation, so that the cellulose molecules are fully dispersed in the solution, laying the foundation for the subsequent formation of a pore-controlled structure. The purpose of adding the polar solvent to the first cellulose solution is, on the one hand, to quickly reduce the viscosity of the cellulose solution and improve the fluidity of the film-forming solution to facilitate uniform coating and film formation; on the other hand, during the subsequent staged solidification process, the polar solvent acts as a dynamic phase separation regulator to control the phase separation rate. Specifically, due to the large difference in polarity between the polar solvent and the ionic liquid, in the subsequent coagulation bath, the polar solvent will preferentially dissolve and diffuse with the external antisolvent, forming a "solvent diffusion front". This front allows the antisolvent to penetrate into the gel layer by layer, and the precipitation of cellulose proceeds gradually from the surface to the inside, which is a controlled stage-by-stage phase separation, thereby regulating the phase separation dynamics; moreover, the polar solvent is added relatively late and its reaction time with cellulose is relatively short, so the phase separation at this stage is more gentle and gradual, which can avoid the rapid and disordered phase separation that leads to irregular nanocracks on the diaphragm, so that the pore formation of the cellulose network is dominated by the nucleation-growth mechanism, thereby forming circular holes rather than random crack-like holes.

[0040] The technical solution of the present invention dynamically adjusts the phase separation kinetics and cooperates with the staged coagulation bath to achieve the regulation of the gradient circular pore structure, thereby accurately adjusting the pore structure and mechanical properties of the diaphragm and realizing controllable optimization of the diaphragm strength.

[0041] In order to make the auxiliary agent cooperate with the ionic liquid and the antisolvent to control the pores of the battery separator to be pores with a gradient circular pore structure under their joint action, the auxiliary agent of the present invention is preferably selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide and N-methylpyrrolidone.

[0042] By controlling the amount of the additive, the rheology and uniformity of the solution can be effectively optimized. According to a large number of experiments, the preferred amount of the additive in the present invention is 30-60% of the amount of the ionic liquid by mass.

[0043] When cellulose is dissolved under heating conditions, at 75-85°C, cellulose is fully dissolved in the ionic liquid, and the hydrogen bonding between cellulose and the ionic liquid is stable. It is preferred to cool the first cellulose solution before adding the additive; the additive is added to the first cellulose solution after the temperature has dropped to 50-65°C. At this point, the addition of the additive only serves to "reduce viscosity" and "post-production regulation" without significantly changing the established solubility equilibrium between cellulose chains. Furthermore, lowering the temperature to 50-65°C allows the additive to be stably incorporated into the solution, maintaining a moderate viscosity and preventing evaporation loss of the additive at high temperatures.

[0044] The third step is to place the second cellulose solution on the substrate, so that the second cellulose solution forms a wet coating film on the substrate.

[0045] The wet coating can be formed using any conventional coating method in the art, and the present invention does not impose any specific limitations thereto. In some specific embodiments of the present invention, the coating is applied by scraping with an applicator. The present invention also does not specifically limit the material of the substrate on which the coating is applied. The substrate serves only as a carrier for the wet coating, as long as it is compatible with the second cellulose solution and suitable for use as a medium in the subsequent staged coagulation bath. In some specific embodiments of the present invention, a glass substrate or a plastic substrate can be used.

[0046] In order to ensure the performance of the battery separator material, the present invention preferably removes bubbles in the second cellulose solution before coating to avoid separator defects or affecting its mechanical properties; for example, the second cellulose solution is degassed under vacuum conditions.

[0047] To ensure the performance of the battery separator material, the present invention preferably controls the wet coating thickness to 1-2 mm. If the coating thickness is too thin, the mechanical strength of the battery separator will be poor; if the coating thickness is too thick, it will shrink significantly during the drying stage, affecting the appearance and performance of the battery separator.

[0048] The fourth step is to perform the first stage of coagulation, specifically placing the substrate with the wet coating film in a first anti-solvent coagulation bath for 3 to 5 hours to gel the wet coating film. The fifth step is to perform the second stage of coagulation, specifically immersing the gelled film in a second anti-solvent coagulation bath for 8 to 12 hours.

[0049] In the above technical solution, regulating the cellulose regeneration rate through a staged coagulation bath is one of the key points of the technical solution of the present invention. The staged coagulation bath dynamically regulates the solvent diffusion rate and phase separation path through step-by-step connection to construct a gradient pore, which can ensure the structural stability of the diaphragm during the cycle. This process does not require additional pore-forming agents.

[0050] According to the cellulose ionic liquid solution system and polar solvent system in the present invention, the use of different anti-solvents will have different effects on the number and morphology of pores formed in the battery separator. In order to control the number and morphology of pores on the battery separator prepared by the present invention, the present invention sets a solvent gradient according to the cellulose ionic liquid solution and polar solvent system. Preferably, the first anti-solvent is selected from at least one of ethanol, methanol, propanol and acetone, and can also be an aqueous solution containing the above solvents; if it is an aqueous solution, the water content in the first anti-solvent is ≤50% by mass; the present invention preferably uses deionized water as the second anti-solvent. Specifically, after coating, it is first placed in the first anti-solvent coagulation bath and stays there for several hours, preferably for 3 to 5 hours, so that the outer layer of cellulose is first gelled to form a preliminary structure, and then it is transferred to the second coagulation bath for sufficient immersion, preferably for 8 to 12 hours, so that the residual ionic liquid and polar solvent inside are gradually replaced, thereby completing the complete regeneration of cellulose.

[0051] The above-mentioned staged coagulation strategy can make the pore formation of the cellulose membrane undergo a slow transition: in the first stage, the first anti-solvent and the polar solvent quickly mix and diffuse, and cellulose begins to precipitate from the surface to form a pore skeleton of a certain size; in the second stage, the introduction of the second anti-solvent further expands and penetrates these pores, and at the same time removes the residual ionic liquid; due to the gradient effect of the two successive coagulation media, the resulting membrane forms a gradient circular pore structure with a step-by-step transition from large pores to small pores from the surface to the inside, and its pore size distribution ranges from hundreds of nanometers to tens of nanometers. The morphology of these circular pores is uniform in all directions and clearly layered. The pore size of the micropores is 20-50 nm, which can effectively block the penetration of dendrites. The pore size of the large pores is 100-250 nm, which can improve the ion flux, thus having good performance.

[0052] Finally, the gelled film is dried to obtain the cellulose battery separator material.

[0053] Drying can be carried out by a conventional drying method for cellulose membranes, which is not specifically limited in the present invention. In some specific embodiments, drying is carried out by placing the cellulose gel in a vacuum oven for heating and drying.

[0054] The present invention also provides a cellulose battery separator material, which comprises micropores and macropores with a gradient circular pore structure; the pore diameter of the micropores is 20 to 50 nm; the pore diameter of the macropores is 100 to 250 nm.

[0055] The above-mentioned cellulose battery separator material is prepared according to the aforementioned preparation method.

[0056] The present invention also provides an application of the aforementioned cellulose battery separator material in the battery field.

[0057] The present invention will be further described below with reference to specific embodiments, but this should not be construed as limiting the scope of protection of the present invention. Non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above-mentioned contents of the present invention still fall within the scope of protection of the present invention.

[0058] Unless otherwise specified, the materials and reagents mentioned below are commercially available products familiar to those skilled in the art. Unless otherwise specified, the methods described are all well-known methods in the art. Unless otherwise defined, technical or scientific terms used shall have the same meanings as those commonly understood by those skilled in the art.

[0059] Example 1

[0060] Hardwood dissolving pulp containing 8 wt% of the ionic liquid was added to 1-allyl-3-methylimidazolium chloride ([Amim][Cl]) and stirred at 85°C for 60 minutes until completely dissolved. After the solution cooled to 60°C, dimethyl sulfoxide (DMSO) containing 50 wt% of the ionic liquid was added and thoroughly stirred.

[0061] Step 2: Degas the solution under vacuum. Then, pour the homogenized solution onto a plastic plate on a coater and apply a knife coat to a film thickness of 2 mm. The plate is then placed in an ethanol bath for regeneration. The plate is allowed to sit in the bath for 4 hours to ensure complete regeneration and a stable gel structure. The gel is then immersed in a deionized water bath for 8 hours. Finally, the cellulose gel is regenerated in a vacuum at 80°C to produce a regenerated cellulose battery separator.

[0062] The regenerated cellulose battery separator prepared by the embodiment of the present invention was scanned by electron microscope, as shown in the attached Figure 1 As shown, it includes micropores and macropores with a gradient circular pore structure; the pore diameter of the micropores is 20 to 50 nm; the pore diameter of the macropores is 100 to 250 nm.

[0063] The regenerated cellulose battery separator prepared in the embodiment of the present invention was placed in an environment at 25°C, 100°C, 120°C, 140°C, and 160°C for 1 hour. The separator showed no thermal shrinkage or deformation, and had excellent thermal stability. Figure 2 shown.

[0064] Attachment Figure 3 This is the tensile strength test curve of the regenerated cellulose battery separator prepared in the embodiment of the present invention. The test results show that the tensile strength of the separator reaches 116 MPa, which is much higher than that of traditional commercial polyolefin separators. The excellent mechanical properties ensure that the separator can maintain structural integrity during battery assembly and long-term cycling, avoiding battery performance degradation due to mechanical damage or deformation, and providing strong protection for battery safety and stability.

[0065] The membrane thickness, solution absorption rate, water contact angle and porosity were measured using conventional test methods in the art. The results are shown in Table 1.

[0066] Example 2

[0067] Same as Example 1. The difference is that the mass concentration of the dissolving pulp is 6%. The test results are shown in Table 1.

[0068] Example 3

[0069] Same as Example 1. The differences are that the cellulose is refined cotton, the auxiliary agent is N,N-dimethylformamide (DMF), and the first anti-solvent is acetone. The test results are detailed in Table 1.

[0070] Example 4

[0071] Same as Example 1. The differences are that the stirring time during dissolving the slurry is 90 minutes, the amount of additive added is 30%, the film thickness is 1 mm, and the vacuum drying temperature is 90° C. The test results are detailed in Table 1.

[0072] Example 5

[0073] Same as Example 1. The difference is that the ionic liquid is 1-butylpyridinium chloride. The test results are shown in Table 1.

[0074] Example 6

[0075] Same as Example 1. The difference is that the ionic liquid is choline glycinate. The test results are shown in Table 1.

[0076] Example 7

[0077] Same as Example 1, except that the cellulose dissolution temperature is 75° C. The test results are shown in Table 1.

[0078] Example 8

[0079] Same as Example 1, except that the first cellulose solution was cooled to 50° C. The test results are shown in Table 1.

[0080] Example 9

[0081] Same as Example 1, except that the first cellulose solution was cooled to 65° C. The test results are shown in Table 1.

[0082] Example 10

[0083] Same as Example 1. The difference is that the auxiliary agent is N-methylpyrrolidone. The test results are shown in Table 1.

[0084] Example 11

[0085] Same as Example 1. The difference is that the amount of adjuvant is 60%. The test results are shown in Table 1.

[0086] Comparative Example 1

[0087] Same as Example 1, except that the ethanol bath was replaced with a deionized water bath. The test results are shown in Table 1.

[0088] Comparative Example 2

[0089] Same as Example 1, except that the deionized water bath was replaced with an ethanol bath. The test results are detailed in Table 1.

[0090] Comparative Example 3

[0091] Same as Example 1. The difference is that the additive is mixed with the ionic liquid before the cellulose is added for dissolution. The test results are shown in Table 1.

[0092] Comparative Example 4

[0093] Same as Example 1. The difference is that the wet film thickness is 0.4 mm. The test results are shown in Table 1.

[0094] The performance of the battery separators obtained in the above examples and comparative examples was tested according to conventional testing methods in the art. The test results are shown in Table 1:

[0095] Table 1

[0096]

[0097] Note: In the above table, the test solution medium for solution absorption rate is 1M ZnSO4 solution, and the percentage is mass percentage.

[0098] From the test data shown in Table 1, it can be seen that the battery separator prepared by the embodiment of the present invention has the following characteristics: Figure 1The gradient circular pore structure shown includes micropores and macropores of the gradient circular pore structure; the pore diameter of the micropores is 20-50 nm; the pore diameter of the macropores is 100-250 nm; at the same time, the prepared diaphragm has good mechanical properties, high porosity, high solution absorption rate, good water wettability, and excellent heat resistance. Its tensile strength is ≥90 MPa, porosity is ≥60%, solution absorption rate is ≥100%, water contact angle is ≤43°, and no thermal shrinkage or deformation occurs after being placed in a 160°C environment for 1 hour, as shown in Examples 1 to 11. Further, through process optimization and adjustment, its tensile strength is ≥100 MPa, porosity is ≥65%, solution absorption rate is ≥110%, and water contact angle is ≤40°, as shown in Examples 1, 3, and 8 to 11. Furthermore, through process optimization and adjustment, the tensile strength is ≥110 MPa, the porosity is ≥70%, the solution absorption rate is ≥120%, and the water contact angle is ≤38°, as shown in Examples 1 and 9. Comparative Examples 1 and 2 differ from Example 1 only in that a staged coagulation bath process is not adopted, resulting in their performance failing to achieve the technical effect of the diaphragm of the present invention. The diaphragm prepared in Comparative Example 1 has a very low porosity and a low solution absorption rate, while the diaphragm prepared in Comparative Example 2 has a low solution absorption rate and an excessively large water contact angle. In Comparative Example 3, the addition of the additive is advanced to the cellulose dissolution stage, resulting in a low solution absorption rate of the prepared diaphragm. In Comparative Example 4, the wet coating is thin, resulting in the prepared diaphragm being too thin and having poor mechanical properties.

[0099] The technical features in the claims and / or the specification of the present invention may be combined, and the manner of combination is not limited to the combination obtained by reference in the claims. The technical solutions obtained by combining the technical features in the claims and / or the specification are also within the scope of protection of the present invention.

[0100] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiment based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a cellulose battery separator material, characterized in that: It includes the following steps: S11 dissolving cellulose in an ionic liquid to form a first cellulose solution; S12 adding an auxiliary agent to the first cellulose solution and stirring uniformly to form a second cellulose solution; the auxiliary agent is selected from a polar solvent having a dielectric constant ≥ 20, a viscosity at 25°C < 3.0 cP, and a boiling point ≥ 120°C; S13: placing the second cellulose solution on the substrate to form a wet coating film of the second cellulose solution on the substrate; The wet coating thickness is 1 to 2 mm; S14: placing the substrate with the wet coating film in a first anti-solvent coagulation bath for 3 to 5 hours to gel the wet coating film; S15: immersing the gelled membrane in a second anti-solvent coagulation bath for 8 to 12 hours; S16 is dried to obtain a cellulose battery separator material.

2. The preparation method according to claim 1, characterized in that The ionic liquid is selected from at least one of imidazolium salt ionic liquids, pyridinium salt ionic liquids and amino acid choline ionic liquids.

3. The preparation method according to claim 1, characterized in that The cellulose is selected from dissolving pulp and / or refined cotton; in the first cellulose solution, the amount of cellulose is 6-8% of the amount of the ionic liquid by mass.

4. The preparation method according to claim 1, characterized in that The auxiliary agent is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide and N-methylpyrrolidone; and the amount of the auxiliary agent is 30-60% of the amount of the ionic liquid by mass.

5. The preparation method according to claim 1, characterized in that The temperature for dissolving the cellulose is 75-85°C; the temperature for adding the auxiliary agent is 50-65°C.

6. The preparation method according to claim 1, characterized in that Before step S13, a step of removing bubbles from the second cellulose solution is also included.

7. The preparation method according to claim 1, characterized in that The first anti-solvent is selected from at least one of ethanol, methanol, propanol and acetone; and the water content of the first anti-solvent is ≤50% by mass.

8. A cellulose battery separator material, characterized in that: The invention comprises micropores and macropores of a gradient circular pore structure; the pore diameter of the micropores is 20-50 nm; the pore diameter of the macropores is 100-250 nm.

9. The cellulose battery separator material according to claim 8, characterized in that It is prepared according to the preparation method according to any one of claims 1 to 7.

10. Use of the cellulose battery separator material according to claim 8 or 9 in the field of batteries.

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