Non-shrinking cellulose film, its preparation and use

Abstract

The application discloses a non-shrinking cellulose film and a preparation method and application thereof, and relates to the technical field of cellulose films. The method adopts a "two-dimensional plane limiting method", places a cellulose gel in a rigid plane porous limiting structure, forms an anchor point by using a protruding point in the limiting structure, and effectively inhibits the shrinkage of the gel in the drying process. Under the anchoring effect, the gel can be dried at high temperature, so that the film preparation time is greatly shortened. The method not only significantly improves the production efficiency, but also guarantees the size stability and mechanical properties of the film, has a good application prospect, and is especially suitable for the field of high-performance materials such as water-based battery separators.

Description

Technical Field

[0001] This invention relates to the field of cellulose film technology, and more specifically, to a non-shrink cellulose film, its preparation method, and its application. Background Technology

[0002] To address the challenges of intermittency and instability in renewable energy generation such as wind and solar power, electrochemical energy storage technology is becoming increasingly important. Among these technologies, lithium-ion batteries stand out due to their high energy density (up to 250 Wh·kg⁻¹). - ¹), and has been widely used in mobile electronic devices and electric vehicles. However, secondary batteries, represented by lithium-ion batteries, still have some inherent problems, such as the risk of combustion and explosion caused by organic electrolytes and the safety hazards introduced by alkaline metal materials, which restrict their promotion in broader scenarios such as large-scale energy storage.

[0003] Aqueous zinc-ion batteries are considered a potential alternative system for large-scale energy storage due to their abundant resources, low cost, and relatively high safety. In particular, aqueous zinc-iodine batteries have received widespread attention in recent years due to their inherent safety and environmental friendliness. Aqueous zinc-ion batteries generally consist of a positive electrode, a separator, a negative electrode, and an electrolyte. The separator not only needs to isolate the positive and negative electrodes to prevent short circuits but also needs to ensure efficient transport of zinc ions. An ideal separator material should possess good chemical stability, a suitable pore structure, excellent wettability, and sufficient mechanical strength. Currently commonly used separators include glass fiber, porous polymers, and cellulose-based separators. Cellulose-based materials have advantages such as being lightweight, transparent, thermally stable, and biodegradable, showing good application potential in green packaging, flexible electronics, and battery separators.

[0004] However, in the preparation of cellulose films, the film is often obtained by drying the cellulose gel. During this process, water evaporation easily leads to gel shrinkage, affecting the dimensional stability and performance of the film. To alleviate shrinkage, existing studies mostly employ room temperature natural drying, but this method is time-consuming. To address this, existing techniques have proposed applying mechanical fixation to both ends of the gel to suppress shrinkage in that direction. However, this method only achieves dimensional constraint in a single direction and cannot effectively suppress shrinkage in other directions, making it difficult to efficiently prepare dimensionally stable and uniformly performing cellulose films. Therefore, how to achieve multi-directional uniform constraint of the cellulose gel during drying, thereby significantly improving preparation efficiency while ensuring performance, remains a technical problem that needs to be solved.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a shrinkage-free cellulose film, its preparation method, and its applications. The cellulose film is prepared using a "two-dimensional planar confinement method," which significantly shortens the film preparation time. This method not only significantly improves production efficiency but also ensures the dimensional stability and mechanical properties of the film.

[0007] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing a shrinkage-free cellulose film, comprising the following steps: Cellulose powder is dissolved in a low-temperature alkaline solvent to form a homogeneous cellulose solution. An epoxy crosslinking agent is added to the cellulose solution to carry out a crosslinking reaction and form a cellulose hydrogel. The cellulose hydrogel is then subjected to solvent exchange and placed in a rigid planar porous confinement structure to form a sandwich-type constraint system. Pressure is applied in a direction perpendicular to the plane. Finally, the film is dried under heating conditions to obtain a self-supporting and non-shrinking cellulose film.

[0008] In an optional embodiment, the low-temperature alkaline solvent is a mixed solution of sodium hydroxide and urea, wherein the mass percentage concentration of sodium hydroxide is 4-8% and the mass percentage concentration of urea is 2-6%.

[0009] In an optional embodiment, the temperature range for the low-temperature dissolution is -20°C to -10°C.

[0010] In an optional embodiment, the epoxy crosslinking agent is 3-chloro-1,2-epoxypropane or citric acid, and the amount added is 2-15% of the mass of cellulose.

[0011] In an optional embodiment, the crosslinking reaction is carried out at a temperature range of 50-70°C for 1-4 hours.

[0012] In an optional embodiment, the rigid planar porous limiting structure is a metal mesh, a porous metal plate, or a porous polyimide plate, and the applied pressure is 0.1-0.5 MPa.

[0013] In an optional embodiment, the drying temperature is 70-90°C and the drying time is 1-5 hours.

[0014] Secondly, the present invention provides a shrinkage-free cellulose film prepared by the above-described preparation method.

[0015] Thirdly, the present invention provides an application of a non-shrinkage cellulose film in an aqueous battery separator.

[0016] Fourthly, the present invention provides a zinc-iodine battery, wherein the battery is formed by sequentially stacking a zinc sheet, a cellulose membrane impregnated with electrolyte, and an iodine positive electrode to form a sandwich structure; The cellulose membrane is the aforementioned non-shrink cellulose film.

[0017] The present invention has the following beneficial effects: This invention employs a method of coating cellulose gel onto a rigid planar porous confining structure. The uniform array of anchor points formed by the microstructure provides multidirectional constraint on the gel during drying, effectively suppressing overall shrinkage and deformation caused by water loss. This method not only achieves high dimensional stability of the film but also allows for rapid drying at higher temperatures, significantly shortening the production cycle. The resulting film, when used as a separator in an aqueous zinc-iodine battery, exhibits high cycle stability and excellent electrochemical performance. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 The diagram shows a schematic of the cellulose film in this invention; a) is a non-shrink cellulose film prepared at 80°C using the "two-dimensional planar confinement method"; b) is a cellulose film obtained at 80°C as a control group, exhibiting a severe shrinkage effect; c) is a large-size non-shrink cellulose film (20 × 85 cm) prepared at 80°C using the "two-dimensional planar confinement method".

[0020] Figure 2 The images shown are scanning electron microscope (SEM) images and atomic force microscope (AFM) characterization images of the cellulose film prepared using the "two-dimensional plane confinement method"; a is a scanning electron microscope image of the surface of the cellulose film prepared using the "two-dimensional plane confinement method"; b is an AFM characterization image of the surface of the cellulose film prepared using the "two-dimensional plane confinement method"; c is the three-dimensional morphology of the surface of the cellulose film prepared using the "two-dimensional plane confinement method". Figure 3 The diagram shows the tensile curve, Young's modulus, and toughness value of the cellulose film in this invention; a) is the tensile curve of the cellulose film prepared by the "two-dimensional plane confinement method"; b) is the Young's modulus and toughness value of the cellulose film prepared by the "two-dimensional plane confinement method". Figure 4 This is the Fourier transform infrared spectrum of the cellulose film in this invention; Figure 5 This is a diagram showing the long-cycle performance of the zinc-iodine secondary battery in this invention; Figure 6 This is a voltage test diagram and demonstration schematic diagram of the soft-pack battery in this invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0022] Cellulose films can be obtained by drying cellulose gels, but during this process, the cellulose gel inevitably shrinks due to water evaporation. Therefore, to mitigate shrinkage, researchers typically allow the cellulose film to dry naturally at room temperature, which is an extremely time-consuming process. To address this issue, Mredha et al. proposed the concept of "Drying in Confined Conditions" in 2018. This process involves fixing both ends of the gel with a mold before drying. Therefore, shrinkage in the direction of the fixed ends is significantly suppressed, but this strategy does not solve the shrinkage problem in other directions. Based on this, the inventors proposed "Anchored Drying," a process that simplifies by placing the gel on a planar woven metal mesh. The protrusions of this restraining structure form numerous anchor points, effectively suppressing gel shrinkage during drying. More importantly, the anchoring effect of these numerous anchor points allows the gel to be dried at high temperatures, such as 80°C, which greatly shortens the preparation time of the cellulose film and has significant application implications.

[0023] In a first aspect, the present invention provides a method for preparing a shrinkage-free cellulose film, comprising the following steps: S1. Dissolve cellulose powder in a low-temperature alkaline solvent to form a homogeneous cellulose solution; Specifically, the low-temperature alkaline solvent is a mixed solution of sodium hydroxide and urea, wherein the mass percentage concentration of sodium hydroxide is 4-8%, preferably 6%; and the mass percentage concentration of urea is 2-6%, preferably 4%.

[0024] In an optional embodiment, the temperature range for the low-temperature dissolution is -20°C to -10°C.

[0025] S2. Add an epoxy crosslinking agent to the cellulose solution to carry out a crosslinking reaction and form a cellulose hydrogel; In an optional embodiment, the temperature range of the crosslinking reaction is 50-70°C, and the reaction time is 1-4 hours; wherein the epoxy crosslinking agent is 3-chloro-1,2-epoxypropane or citric acid, and the amount added is 2-15% of the mass of cellulose.

[0026] S3. Then, solvent exchange is performed on the cellulose hydrogel, and the treated cellulose hydrogel is placed in a rigid planar porous confinement structure to form a sandwich confinement system, and pressure is applied in a direction perpendicular to the plane. Specifically, the solvent exchange includes the following steps: placing the cellulose hydrogel in a large amount of deionized water, weakly acidic deionized water, or an organic solvent (such as anhydrous ethanol, acetone, or tert-butanol), and allowing it to stand or be gently stirred. The solvent is replaced with fresh solvent every few hours, and this process is repeated 2-4 times until the water in the gel is completely replaced. This step effectively reduces the capillary forces caused by the surface tension of water during subsequent drying, thereby minimizing drying shrinkage and network collapse, and laying the foundation for maintaining the microporous structure and macroscopic flatness of the film.

[0027] Furthermore, it is preferable to use weakly acidic deionized water, which can effectively remove sodium hydroxide and urea.

[0028] In an optional embodiment, the rigid planar porous limiting structure includes, but is not limited to, a metal mesh, a porous metal plate, or a porous polyimide plate, and the applied pressure is 0.1-0.5 MPa.

[0029] It should be noted that the rigid planar porous restraint structure used in this application includes, but is not limited to, high-temperature resistant structures such as metal woven mesh, porous metal plate with ceramic protrusions, and porous polyimide plate with regular array protrusions. Any commercially available rigid planar plate that can apply multi-directional constraints to the gel is acceptable.

[0030] S4. Finally, the film is dried under heating conditions to obtain a self-supporting and non-shrink cellulose film.

[0031] In an optional embodiment, the drying temperature is 70-90°C and the drying time is 1-5 hours.

[0032] Secondly, the present invention provides a shrinkage-free cellulose film prepared by the above-described preparation method.

[0033] Thirdly, the present invention provides an application of a non-shrinkage cellulose film in an aqueous battery separator.

[0034] Fourthly, the present invention provides a zinc-iodine battery, wherein the battery is formed by sequentially stacking a zinc sheet, a cellulose membrane impregnated with electrolyte, and an iodine positive electrode to form a sandwich structure; The cellulose membrane is the aforementioned non-shrink cellulose film.

[0035] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0036] Example 1 This embodiment provides a method for preparing a shrinkage-free cellulose film, including the following steps: S1. Preparation of cellulose hydrogel: 40 g of cellulose powder was uniformly dispersed in 960 g of sodium hydroxide / urea solution (6% / 4%), and then dissolved in an environment of -20℃. After complete dissolution, 2-15% of 3-chloro-1,2-epoxypropane was added, and cross-linking was initiated at 60℃ to form a cellulose hydrogel. Finally, the obtained cellulose hydrogel was subjected to solvent exchange to wash away free sodium hydroxide and urea molecules.

[0037] S2. Preparation of non-shrink cellulose films using the "two-dimensional planar confinement method": A cellulose hydrogel, after complete solvent exchange, is placed between two planar woven metal meshes to form a sandwich structure. Appropriate pressure is then applied in the direction perpendicular to the normal to the metal mesh planes, ensuring close contact between the cellulose hydrogel surface and the metal mesh planes. Finally, the apparatus is dried at 80°C. After drying, a self-supporting, non-shrink cellulose film is obtained. Figure 1 As shown.

[0038] according to Figure 1 The results show that the cellulose film obtained at 80℃ without the two-dimensional plane confinement method exhibits a severe shrinkage effect. In contrast, the cellulose film prepared by the two-dimensional plane confinement method does not produce an obvious shrinkage effect and has a smooth and flat surface.

[0039] Experimental Example 1 The performance of the self-supporting, non-shrinkage cellulose film prepared in Example 1 was tested.

[0040] (1) Characterization results: The surface microstructure of the thin film was observed using scanning electron microscopy (SEM); simultaneously, the surface of the thin film was scanned using atomic force microscopy (AFM) in tapping mode to obtain quantitative data of its surface roughness (expressed as arithmetic mean roughness Ra) and three-dimensional morphology images. The results are as follows: Figure 2 As shown.

[0041] like Figure 2 As shown in the SEM images, the film surface exhibits a smooth and uniform morphology, without significant defects or shrinkage wrinkles. AFM quantitative analysis determined the average surface roughness Ra of the film to be 9.59 nm, further confirming its smoothness. The three-dimensional morphology image visually demonstrates the low-undulation characteristics of the film surface, consistent with the SEM and roughness results. These results collectively indicate that the cellulose film prepared by the "two-dimensional planar confinement method" possesses excellent surface smoothness and low roughness, effectively overcoming the shrinkage and surface degradation problems caused by traditional high-temperature drying.

[0042] (2) Mechanical property testing: Quasi-static uniaxial tensile tests were performed on the self-supporting cellulose films prepared by the "two-dimensional plane confinement method". The testing process followed the standard material mechanical property characterization method. The film samples were cut into dumbbell-shaped specimens of specified dimensions and stretched at a constant rate on a universal testing machine until fracture. The stress-strain curves were recorded simultaneously. The results are as follows: Figure 3 As shown.

[0043] The results are as follows Figure 3 As shown in the tensile curve, the cellulose film exhibits high tensile strength, with a maximum value of approximately 60 MPa, and an elongation at break of approximately 5%. Further analysis indicates that the film has a Young's modulus of approximately 0.93 GPa, demonstrating good rigidity; its toughness is as high as approximately 2.31 MJ·m. -3 This indicates that the material can effectively dissipate energy before fracture, exhibiting excellent damage resistance. The cellulose film prepared by this method maintains high strength while also possessing good rigidity and toughness, demonstrating balanced mechanical properties and meeting the basic requirements for high-performance film materials.

[0044] (3) Infrared spectroscopy detection: To analyze the chemical structure and surface functional groups of the prepared cellulose film, Fourier transform infrared spectroscopy (FT-IR) was used to characterize it. After drying, the film sample was placed in the sample chamber of the spectrometer and the infrared spectroscopy was performed at 4000-400 cm⁻¹. -1 Transmission mode scanning was performed within the wavenumber range to obtain infrared absorption spectra. To further analyze characteristic peaks, peak fitting was performed on the original spectra.

[0045] The results are as follows Figure 4 As shown. After peak fitting, the peak value is at 3317 cm⁻¹. -1 The characteristic absorption peak at 1650 cm⁻¹ corresponds to the stretching vibration of the OH bond in the cellulose molecule, indicating that the film surface is rich in hydroxyl groups (–OH). -1 The characteristic peaks appearing at this point are attributed to the stretching vibration of C=O, indicating the presence of carbonyl (–C=O) functional groups in the film. Both of these functional groups exhibit zinc affinity, which helps to lower the desolvation energy barrier of zinc ions in aqueous electrolytes and guides uniform deposition of zinc ions during battery cycling, thereby contributing to improved electrochemical performance and cycle stability of the battery.

[0046] Experimental Example 2 The non-shrink cellulose film prepared in Example 1 was used to prepare a cellulose separator, which was then immersed in an electrolyte. A zinc sheet, the electrolyte-immersed cellulose separator, and an iodine cathode were sequentially stacked to form a sandwich-structured zinc-iodine battery. Its long-cycle performance was then tested. Figure 5 As shown.

[0047] according to Figure 5As shown, the assembled zinc-iodine battery operates at 20 mA cm⁻¹. -2 Achieving ~1.8 mAh cm⁻¹ at high current density -2 It has high capacity and a capacity retention of ~70% after 4000 cycles, with an average coulombic efficiency of 99.5%.

[0048] Experimental Example 3 The non-shrinkage cellulose film prepared in Example 1 was used to prepare a cellulose separator, which was then immersed in an electrolyte. A pouch battery was assembled using a carbon felt loaded with iodine active material as the positive electrode, zinc foil as the negative electrode, and the cellulose film as the separator in a customized electrolyte. Subsequently, the open-circuit voltage of the battery pack was tested using an electrochemical workstation, and its actual power supply capability was verified by driving a standard LED bulb through an external circuit.

[0049] The results are as follows Figure 6 As shown, the open-circuit voltage of this zinc-iodine pouch battery pack reaches 3.55 V, indicating that the battery has a high output voltage platform. Further application demonstrations show that the battery pack can successfully light a commercial LED bulb, proving that it not only has a high voltage output but also provides stable current to drive the load, demonstrating high application potential.

[0050] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a shrinkage-free cellulose film, characterized in that, Includes the following steps: Cellulose powder is dissolved in a low-temperature alkaline solvent to form a homogeneous cellulose solution. An epoxy crosslinking agent is added to the cellulose solution to carry out a crosslinking reaction and form a cellulose hydrogel. The cellulose hydrogel is then subjected to solvent exchange, and the treated cellulose hydrogel is placed in a rigid planar porous confinement structure to form a sandwich-type constraint system. Pressure is applied in a direction perpendicular to the plane. Finally, it is dried under heating conditions to obtain a self-supporting and non-shrinking cellulose film.

2. The method for preparing a non-shrink cellulose film according to claim 1, characterized in that, The low-temperature alkaline solvent is a mixed solution of sodium hydroxide and urea, wherein the mass percentage concentration of sodium hydroxide is 4-8% and the mass percentage concentration of urea is 2-6%.

3. The method for preparing a non-shrink cellulose film according to claim 1, characterized in that, The temperature range for the low-temperature melting is -20℃ to -10℃.

4. The method for preparing a non-shrink cellulose film according to claim 1, characterized in that, The epoxy crosslinking agent is 3-chloro-1,2-epoxypropane or citric acid, and its addition amount is 2-15% of the cellulose mass.

5. The method for preparing a non-shrink cellulose film according to claim 1, characterized in that, The crosslinking reaction is carried out at a temperature range of 50-70℃ for 1-4 hours.

6. The method for preparing a shrinkage-free cellulose film according to claim 1, characterized in that, The rigid planar porous limiting structure is a metal mesh, a porous metal plate, or a porous polyimide plate, and the applied pressure is 0.1-0.5 MPa.

7. The method for preparing a non-shrink cellulose film according to claim 1, characterized in that, The drying temperature is 70-90℃, and the drying time is 1-5 hours.

8. A shrinkage-free cellulose film prepared by any one of claims 1-7.

9. The application of the non-shrinkage cellulose film as described in claim 8 in an aqueous battery separator.

10. A zinc-iodine battery, characterized in that, The battery is formed by stacking zinc sheets, a cellulose membrane soaked in electrolyte, and an iodine positive electrode in sequence to form a sandwich structure. The cellulose membrane is the non-shrink cellulose film as described in claim 8.

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