A cellulose film and its preparation method and application

Abstract

The present invention provides a cellulose film and its preparation method and application, the preparation method comprising the following steps: (1) mixing and dispersing cellulose with a first pore-forming agent to obtain a dispersion; (2) shaping the dispersion obtained in step (1) to obtain a first membrane material; (3) mixing the first membrane material obtained in step (2) with a second organic solvent to obtain the cellulose film; wherein the first pore-forming agent comprises porous polystyrene microspheres; the mass ratio of the cellulose to the first pore-forming agent is 1:(0.5-1.6) based on solid mass; and the first pore-forming agent is soluble in the second organic solvent. In the present invention, the cellulose film obtained by a specific process has good heat resistance, air permeability and wettability, which is conducive to the transmission of lithium ions and the infiltration of electrolytes.

Description

Technical Field

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

[0002] The mainstream lithium battery separators in the current market are mainly polyolefin separators. Polyolefin separators are mainly made of petrochemical raw materials and are difficult to degrade in the natural environment. At the same time, the existing production process makes it difficult to take into account the mechanical properties and heat resistance of the separator. Therefore, it is necessary to find a separator with biodegradable properties and high performance as a replacement. Cellulose is widely distributed in nature and has biodegradable properties. Pure fiber-coated separators have the characteristics of lightweight and high heat resistance. However, since the cellulose surface contains rich hydroxyl functional groups, hydrogen bonding forces will be formed in water. During the drying process, due to capillary action, the nanofibers tend to form a dense network, which hinders the migration of lithium ions and results in poor air permeability and wetting properties of the separator.

[0003] Therefore, developing a method for preparing a cellulose membrane with excellent mechanical properties, heat resistance, air permeability and wettability is an urgent problem to be solved in this field. Summary of the Invention

[0004] In response to the shortcomings of the prior art, the present invention aims to provide a cellulose membrane, a preparation method, and applications thereof. The cellulose membrane obtained using the specific preparation method of the present invention retains the mechanical properties, heat resistance, and lightness of the cellulose membrane while also improving its air permeability and wettability.

[0005] To achieve this object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a method for preparing a cellulose film, the method comprising the following steps:

[0007] (1) cellulose and a first pore-forming agent are mixed and dispersed to obtain a dispersion; (2) the dispersion obtained in step (1) is shaped to obtain a first membrane material; (3) the first membrane material obtained in step (2) is mixed with a second organic solvent to obtain the cellulose membrane; wherein the first pore-forming agent comprises porous polystyrene microspheres (porous PS microspheres); the mass ratio of the cellulose to the first pore-forming agent is 1:(0.5-1.6) based on solid mass; and the first pore-forming agent is soluble in the second organic solvent.

[0008] In the present invention, the porous polystyrene microspheres have a large specific surface area and a high porosity, and can be more evenly embedded in the cellulose matrix. The porous polystyrene microspheres are then removed by a second organic solvent, thereby leaving a large number of evenly distributed microporous structures in the cellulose matrix, thereby improving the porosity of the cellulose membrane. While ensuring the mechanical properties and heat resistance of the cellulose membrane, the air permeability and wettability of the cellulose membrane can be improved; at the same time, controlling the mass ratio of cellulose to the first pore-forming agent within a specific range can further improve the air permeability, heat resistance and wettability of the cellulose membrane; the cellulose membrane obtained by the specific method of the present invention has good mechanical properties, heat resistance, air permeability and wettability, is more conducive to the infiltration of the electrolyte and the transmission of lithium ions, and is lightweight.

[0009] In the present invention, based on solid mass, the mass ratio of the cellulose to the first pore-forming agent is 1:(0.5-1.6), wherein the specific value of (0.5-1.6) can be, for example, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, 1.1, 1.2, 1.3, 1.4, 1.5, etc.

[0010] Preferably, the cellulose in step (1) comprises nanocellulose.

[0011] Preferably, the cellulose comprises at least one of cellulose nanocrystals (CNCs), nanofiber filaments (CNFs) or bacterial cellulose (BC).

[0012] In the present invention, the diameter of the nanocellulose is 5-20 nm and the length is 50-100 nm.

[0013] Preferably, the cellulose and / or the first membrane material is treated with a first organic solvent, the polarity of the first organic solvent is less than that of water, and the boiling point of the first organic solvent is less than 100°C, for example, it can be 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 98°C, etc.

[0014] In the present invention, by treating cellulose and / or the first membrane material with an organic solvent of low polarity and low boiling point, the capillary action of cellulose during the drying process can be reduced, the distance between cellulose chains can be relatively increased, and the pore structure of the cellulose membrane can be increased, thereby further improving the air permeability and wettability of the cellulose membrane.

[0015] In the present invention, the cellulose is in the form of a cellulose dispersion having a solid content of 3-5%, for example, 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, 4.2%, 4.4%, 4.6%, 4.8%, 5%, etc.

[0016] In the present invention, the solvent of the cellulose dispersion includes water.

[0017] Preferably, the raw materials for preparing the porous polystyrene microspheres include styrene monomers, a second pore-forming agent, a dispersant, an initiator and a solvent.

[0018] Preferably, the mass ratio of the second pore-forming agent to the styrene monomer is (1-3):1, wherein the specific values ​​of 1-3 can be, for example, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, etc.; further preferably, it is (1-2):1.

[0019] In the present invention, the content of the second pore-forming agent affects the porosity, pore diameter and specific surface area of ​​the porous polystyrene microspheres; when the content of the second pore-forming agent is too low, the porosity of the porous polystyrene microspheres decreases, the pore diameter decreases, and the specific surface area decreases; when the content is too high, the porosity is not significantly improved, and the porous polystyrene microspheres are easily broken. The reasons are: the content of the second pore-forming agent is too high, the relative concentration of the monomer decreases, and the viscosity is too low during the polymerization process; and it is easy to cause pore formation failure, and the obtained polystyrene microspheres have low porosity and small pore diameter.

[0020] Preferably, the second pore forming agent comprises polyethylene glycol.

[0021] Preferably, the mass percentage of the dispersant in the preparation raw material is 0.5-0.8%, for example, it can be 0.5%, 0.52%, 0.54%, 0.56%, 0.58%, 0.6%, 0.62%, 0.64%, 0.66%, 0.68%, 0.7%, 0.72%, 0.74%, 0.76%, 0.78%, 0.8%, etc.

[0022] In the present invention, the dispersant affects the D50 particle size of the porous polystyrene microspheres; when the amount of the dispersant is too low, the D50 particle size of the porous polystyrene microspheres increases; when the amount of the dispersant is too high, the D50 particle size of the porous polystyrene microspheres decreases; only when the amount of the dispersant is within the above range can porous polystyrene microspheres with a suitable D50 particle size be obtained, and then a cellulose membrane with good wettability and air permeability be obtained.

[0023] In the present invention, the mass percentage of the dispersant is 0.5-0.8%, which refers to the mass percentage of the solid mass of the dispersant in the entire system, including the solvent.

[0024] Preferably, the dispersant includes any one of polyvinyl alcohol, sodium dodecylbenzenesulfonate, glycerol, and polymethyl acrylate, or a mixture of at least two thereof.

[0025] In the present invention, by making the polystyrene microspheres porous, that is, adding a specific content of a second pore-forming agent and a specific content of a dispersant, the particle size, porosity and pore size of the polystyrene microspheres can be controlled within an appropriate range, thereby making the polystyrene microspheres uniformly dispersed in the cellulose matrix, improving the porosity of the cellulose membrane, and obtaining a cellulose membrane with high air permeability, high wettability, heat resistance and good mechanical properties.

[0026] Preferably, the styrene monomer includes styrene and / or divinylbenzene.

[0027] Preferably, the styrene monomer includes styrene and divinylbenzene, and the mass ratio of styrene to divinylbenzene is (40-60):1, wherein the specific value of (40-60) can be, for example, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, etc.

[0028] Preferably, the amount of the initiator is 1-5% of the mass of styrene, for example, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc.

[0029] In the present invention, the initiator includes but is not limited to benzoyl peroxide; the solvent includes water.

[0030] Preferably, the method for preparing the porous polystyrene microspheres comprises:

[0031] The porous polystyrene microspheres are obtained by mixing styrene monomers, a second pore-forming agent, a dispersant, an initiator and a solvent, reacting the mixture and removing the second pore-forming agent.

[0032] In the present invention, the dispersant reduces the surface tension of the liquid, allowing for a higher degree of dispersion of the monomer liquid. It also increases the viscosity of the polymer medium, thereby hindering collision and adhesion between monomer droplets. It also forms a protective film on the surface of the monomer droplets, preventing droplet aggregation and acting as a steric hindrance. The second pore-forming agent participates in pore formation, increasing the porosity of the polystyrene microspheres. The polystyrene microspheres are prepared using suspension polymerization. Styrene monomers are stirred and dispersed to form small droplets suspended in the polymerization medium for polymerization. Because the droplets are surrounded by the polymerization medium, the heat of polymerization can be directly and effectively dissipated. Bulk polymerization initially occurs within the droplets. When the conversion rate reaches a certain level, the liquid surface becomes sticky and easily clumps together. Adding the dispersant stabilizes the droplets. As the conversion rate further increases, the droplets gradually harden, reducing the risk of adhesion. In the later stages of the reaction, as the reactant concentration decreases, the second pore-forming agent dispersed in water participates in pore formation, resulting in porous polystyrene microspheres.

[0033] Preferably, the mixing comprises: pre-mixing and dispersing the solvent and the second pore-forming agent, and then adding the styrene monomer, the dispersant and the initiator thereto and mixing them.

[0034] Preferably, the premixing and dispersion temperature is 80-85°C, for example, 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, etc.; the rotation speed is 200-300 rpm, for example, 200 rpm, 220 rpm, 240 rpm, 260 rpm, 280 rpm, 300 rpm, etc.; and the premixing and dispersion is performed until the second pore-forming agent is fully dissolved.

[0035] Preferably, the reaction is carried out in the presence of a protective atmosphere; the protective atmosphere includes but is not limited to nitrogen; the reaction temperature is 80-90°C, for example, 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, 86°C, 88°C, 90°C, etc.; the reaction time is 8-10h, for example, 8h, 9h, 10h, etc.

[0036] Preferably, the method of removing the second pore forming agent comprises extraction.

[0037] In the present invention, the extraction method comprises placing the polystyrene microspheres obtained by the reaction into a solvent capable of dissolving a second pore-forming agent (including but not limited to dichloromethane), removing the second pore-forming agent, and then drying at 50-60° C. to obtain porous polystyrene microspheres.

[0038] Preferably, the D50 particle size of the porous polystyrene microspheres is 0.8-1.2 μm, for example, it can be 0.8 μm, 0.82 μm, 0.84 μm, 0.85 μm, 0.86 μm, 0.88 μm, 0.9 μm, 0.92 μm, 0.94 μm, 0.95 μm, 0.96 μm, 0.98 μm, 1 μm, 1.02 μm, 1.04 μm, 1.06 μm, 1.08 μm, 1.1 μm, 1.12 μm, 1.14 μm, 1.16 μm, 1.18 μm, 1.2 μm, etc.

[0039] In the present invention, by controlling the D50 particle size of the porous polystyrene microspheres within a specific range, the porosity of the cellulose membrane can be regulated, thereby obtaining a cellulose membrane with high wettability and air permeability; if the D50 particle size of the porous polystyrene microspheres is too small, the porosity of the cellulose membrane will be reduced, and a small particle size will lead to an excessively large specific surface area. As time goes by, the porous polystyrene microspheres are prone to agglomeration, and the viscosity of the dispersion will increase sharply, affecting the fluidity of the dispersion, and cannot be evenly dispersed in the cellulose dispersion, affecting the air permeability, wettability or heat resistance of the cellulose membrane; if the D50 particle size is too large, the improvement of the porosity is not obvious, and it cannot be completely removed in the later stage, resulting in the porous polystyrene microspheres remaining in the cellulose membrane, causing abnormal phenomena such as black spots on the surface of the cellulose membrane and uneven membrane surface, affecting membrane-related properties.

[0040] In the present invention, other conditions are the same, the thickness of the cellulose membrane is 5-10 μm, and when the D50 particle size of the porous polystyrene microspheres is in the range of 0.8-1.2 μm, the corresponding cellulose membrane has a porosity of 42-48%, a pore size of 42-45 nm, and an air permeability of 165-195 s / 100 cc.

[0041] Preferably, the porosity of the porous polystyrene microspheres is 45-75%, for example, it can be 45%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 68%, 70%, 72%, 74%, 75%, etc.

[0042] Preferably, the pore size of the porous polystyrene microspheres is 9-20 nm, for example, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm, etc.

[0043] Preferably, the specific surface area of ​​the porous polystyrene microspheres is 85-170 m 2 / g, for example, 85m 2 / g、90m 2 / g、95m 2 / g、100m 2 / g、105m 2 / g、110m 2 / g, 115m 2 / g, 120m 2 / g, 125m 2 / g, 130m 2 / g, 135m 2 / g, 140m 2 / g, 145m 2 / g, 150m 2 / g、155m2 / g, 160m 2 / g, 165m 2 / g, 170m 2 / g, etc.

[0044] Preferably, the mixing and dispersing speed in step (1) is 1500-2000 rpm, for example, it can be 1500 rpm, 1600 rpm, 1700 rpm, 1800 rpm, 1900 rpm, 2000 rpm, etc.; the time is 2-4 h, for example, it can be 2 h, 2.5 h, 3 h, 3.5 h, 4 h, etc.

[0045] Preferably, step (2) further includes a step of degassing the dispersion before molding.

[0046] In the present invention, the degassing method includes degassing by vacuuming.

[0047] Preferably, the molding method in step (2) includes casting molding.

[0048] Preferably, the molding temperature in step (2) is 45-55°C, for example, it can be 45°C, 46°C, 48°C, 50°C, 52°C, 54°C, 55°C, etc.

[0049] In the present invention, the casting molding comprises casting the dispersion obtained in step (1) into a glass mold with a fixed thickness for molding; the thickness of the glass mold is 5 to 10 μm.

[0050] Preferably, the first organic solvent includes an alcohol solvent.

[0051] Preferably, the alcohol solvent includes at least one of methanol, ethanol, 1-propanol, isopropanol, 2,2,2-trifluoroethanol, 2-butanol, and isobutanol.

[0052] Preferably, the mass concentration of the first organic solvent is ≥98%.

[0053] Preferably, the method of treating with the first organic solvent comprises: soaking the cellulose and / or the first membrane material in the first organic solvent, and drying to obtain the treated material.

[0054] Since water and the hydroxyl groups in cellulose form hydrogen bonds, when cellulose is dried, as the water molecules are gradually evaporated, the cellulose chains will be closer together, the pore structure of the cellulose will become smaller, and a dense network structure will be formed; and by selecting a first organic solvent with a low boiling point and low polarity for treatment, the capillary effect between the fibers can be reduced, the distance between the cellulose chains can be relatively increased, and the pore structure of the fibers can be relatively increased, thereby improving the air permeability and wettability of the cellulose membrane.

[0055] Preferably, the immersion time in the first organic solvent is 12-20 hours, for example, it can be 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, etc.

[0056] Preferably, the drying temperature after the first organic solvent treatment is 60-75°C, for example, 60°C, 62°C, 64°C, 66°C, 68°C, 70°C, 72°C, 74°C, 75°C, etc.; the drying time is 1-3h, for example, 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0057] Preferably, the second organic solvent in step (3) comprises at least one of toluene, tetrahydrofuran, acetone, butanone, ethyl acetate, N,N-dimethylformamide, chloroform, benzene or xylene.

[0058] In the present invention, the second organic solvent can dissolve the porous polystyrene microspheres, thereby forming a large number of pore structures in the cellulose membrane, increasing the porosity of the cellulose membrane, and further improving the air permeability and wettability of the cellulose membrane.

[0059] Preferably, the mixing method in step (3) includes: immersing the first film material in the second organic solvent.

[0060] In the present invention, if the first film material is treated with a first organic solvent, the first film material is mixed with the second organic solvent after being treated with the first organic solvent.

[0061] Preferably, the immersion time in the second organic solvent is 5-7 hours, for example, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, etc.; and the temperature is room temperature.

[0062] Preferably, step (3) further includes a drying step after the mixing.

[0063] In the present invention, the drying temperature is 100-110° C. and the drying time is 1-2 hours.

[0064] As a preferred technical solution of the present invention, the preparation method comprises the following steps:

[0065] (1) mixing and dispersing a cellulose material and a first pore-forming agent at 1500-2000 rpm for 2-4 hours to obtain a dispersion; based on solid mass, the mass ratio of the cellulose to the first pore-forming agent is 1:(0.5-1.6);

[0066] (2) casting the dispersion obtained in step (1) at 45-55° C. to obtain a first membrane material; soaking the first membrane material in a first organic solvent for 12-20 hours, and drying at 60-75° C. for 1-3 hours to obtain a treated first membrane material;

[0067] (3) Soaking the treated first membrane material obtained in step (2) in a second organic solvent at room temperature for 5-7 hours and drying to obtain the cellulose membrane.

[0068] In the present invention, the solid-liquid ratio when the first membrane material is immersed in the first organic solvent in step (2) and the solid-liquid ratio when the treated first membrane material obtained in step (2) is immersed in the second organic solvent in step (3) are each independently 1:(5-8), wherein the specific value of (5-8) can be, for example, 5, 5.5, 6, 6.5, 7, 7.5, 8, etc.

[0069] In the present invention, the first membrane material is preferably treated with a first organic solvent, which can not only reduce the capillary effect between cellulose fibers and increase the pore structure, but also accelerate the evaporation of water carried by the low boiling point, dehydrate the polystyrene-cellulose dispersion into a gel state, further reduce the capillary effect, increase the pore size, and shorten the process flow and reduce costs.

[0070] In a second aspect, the present invention provides a cellulose film, which is prepared according to the preparation method described in the first aspect.

[0071] Preferably, the porosity of the cellulose membrane is 40-50%, for example, it can be 40%, 40.5%, 41%, 41.5%, 42%, 42.5%, 43%, 43.5%, 44%, 44.5%, 45%, 45.5%, 46%, 46.5%, 47%, 47.5%, 48%, 49%, 50%, etc.

[0072] In the present invention, the thickness of the cellulose film is 5-10 μm, for example, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

[0073] In a third aspect, the present invention provides a separator, comprising the cellulose membrane prepared according to the preparation method of the first aspect or the cellulose membrane according to the second aspect.

[0074] In a fourth aspect, the present invention provides a lithium-ion battery, comprising the cellulose membrane prepared according to the preparation method of the first aspect or the separator according to the third aspect.

[0075] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0076] Compared with the prior art, the present invention has the following beneficial effects:

[0077] The preparation method provided by the present invention selects porous polystyrene microspheres as a pore-forming agent and controls the mass ratio of the first pore-forming agent to cellulose within a specific range. While ensuring the mechanical properties and heat resistance of the cellulose membrane, it can improve the air permeability and wettability of the cellulose membrane; it is more conducive to the infiltration of the electrolyte and the transmission of lithium ions, and is lightweight. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 This is a transmission electron micrograph of the porous polystyrene microspheres provided in Preparation Example 1 of the present invention;

[0079] Figure 2 This is the infrared spectrum of the porous polystyrene microspheres provided in Preparation Example 1 of the present invention;

[0080] Figure 3 This is a scanning electron microscope image of the cellulose film obtained in Example 1 of the present invention;

[0081] Figure 4 This is a graph showing the weight change of the first film material as the immersion time in toluene increases in the preparation methods provided in Example 1, Example 14, and Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0082] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0083] Preparation Example 1

[0084] This preparation example provides a porous polystyrene microsphere, and the preparation method of the porous polystyrene microsphere comprises the following steps:

[0085] Deionized water and polyethylene glycol (PEG) were mixed and stirred at a temperature of 85°C and a rotation speed of 200 r / min until the PEG was fully dissolved; nitrogen was introduced, styrene (ST) and divinylbenzene (DVB) were added thereto, the dispersion speed was adjusted to 400 r / min, benzoyl peroxide (BPO) and polyvinyl alcohol (PVA) were added thereto, and the mixture was reacted at 85°C for 9 hours, followed by cooling and rotary evaporation to obtain white PS microspheres; the PS microspheres were placed in a dichloromethane solvent, the residual PEG was removed by extraction, and the mixture was dried in an oven at 60°C to obtain the porous PS microspheres; wherein, the mass of PEG (M PEG ) and the total mass of ST and DVB (M ST+DVB ) ratio is 2:1; the mass ratio of ST to DVB is 50:1; the mass of BPO is 1.5% of the mass of ST; the mass of PVA (M PVA ) accounts for 0.65% of the total mass of the system.

[0086] The morphology of the porous PS microspheres obtained in Preparation Example 1 was characterized by transmission electron microscopy. Figure 1 shown by Figure 1 It can be seen that the porous PS microspheres have a pore structure and present a certain core-shell structure.

[0087] The structure of the porous PS microspheres obtained in Preparation Example 1 was characterized by Fourier transform infrared spectroscopy. Figure 2 shown by Figure 2 It can be seen that 2922cm -1 、1492cm -1 and 1452cm -1 The absorption peak at 668 cm is the characteristic absorption peak of the benzene ring skeleton vibration. -1 The peak at represents the CH plane vibration characteristic of the benzene ring, and there is no vinyl C=C characteristic absorption peak, which means that the polymerization reaction is successful and a copolymer of styrene and divinylbenzene is obtained.

[0088] Preparation Examples 2 to 10, Comparative Preparation Example 1

[0089] Preparation Examples 2 to 9 respectively provide a porous PS microsphere, which differs from Preparation Example 1 only in the different contents of PEG and PVA (see Table 1 for details). Other raw materials, amounts and preparation methods are the same as those in Preparation Example 1.

[0090] Preparation Example 10 provides a porous PS microsphere, which differs from Preparation Example 1 only in that the dispersant is polymethyl acrylate, and the other raw materials, amounts and preparation methods are the same as those in Preparation Example 1.

[0091] Comparative Preparation Example 1 provides a PS microsphere, in which PEG and PVA are not added to the preparation raw materials, and other raw materials, amounts and preparation methods are the same as those in Preparation Example 1.

[0092] The formulations of Preparation Examples 1 to 10 and Comparative Preparation Example 1, as well as the D50 particle size, porosity, pore size, and specific surface area of ​​the obtained PS microspheres are shown in Table 1; wherein, “ / ” in Table 1 indicates that no test is required or cannot be obtained.

[0093] D50 particle size of PS microspheres (test method): The dispersion was tested using a TOPSIZER laser particle size analyzer. The corresponding refractive index was selected to obtain the cumulative particle size distribution of the dispersion. The particle size at which 50% of the cumulative size was measured from the particle side was recorded as D50.

[0094] Porosity test method: Use the water saturation method to test. Place the PS microsphere sample to be tested in a vacuum container, then apply a certain vacuum pressure to evacuate the gas in the container. Then connect the vacuum container to a water tank, and inject water in the water tank into the container to apply a certain water pressure to the sample. Then measure the water level change to calculate the porosity.

[0095] Pore ​​size test method: The mercury intrusion method is used to test, by applying external pressure to allow mercury to enter the pores of the material, and the pore size of the pore structure of the PS microsphere sample is directly obtained by measuring the amount of mercury entering.

[0096] Specific surface area test method: refer to GB / T 19587-2004.

[0097] Table 1

[0098]

[0099] Example 1

[0100] This embodiment provides a method for preparing a cellulose film, which specifically includes the following steps:

[0101] (1) 50 g of porous PS microspheres (Preparation Example 1, M PEG :M ST+DVB =2:1) ​​was mixed with 833 mL of cellulose nanocrystal dispersion (CNC dispersion, solvent: water, solid content: 4%, CNC diameter: 8-10 nm, length: 50-100 nm), and dispersed at a speed of 1500 rpm for 3 h to obtain a uniform dispersion (labeled as CNC&PS);

[0102] (2) The CNC&PS dispersion obtained in step (1) was vacuum-degassed and uniformly cast into a glass mold of fixed thickness (7 μm) at a casting temperature of 50° C. to obtain a first film material; the first film material was immersed in an ethanol solution (mass concentration of 99 wt%) for 12 h (dehydration) according to a solid-liquid ratio of 1:6 to obtain a CNC&PS gel; the obtained gel was then baked in an oven at 65° C. for 2 h to obtain a CNC&PS film;

[0103] (3) The baked CNC&PS membrane was placed in a toluene solution (mass concentration 50 wt%) according to a solid-liquid ratio of 1:8, and after being placed for 6 hours, it was baked in an oven at 105° C. for 1.5 hours to obtain the cellulose membrane.

[0104] The morphology of the cellulose membrane prepared by the preparation method provided in Example 1 was characterized by scanning electron microscopy. Figure 3 As shown; it can be clearly seen from the figure that the surface of the cellulose membrane has a microporous structure, indicating that the porosity of the cellulose membrane is significantly improved.

[0105] Example 2

[0106] This embodiment provides a method for preparing a cellulose film, which specifically includes the following steps:

[0107] (1) 50 g of porous PS microspheres (Preparation Example 1) were mixed with 833 mL of cellulose nanocrystal dispersion (CNC dispersion, solvent: water, solid content: 4%, CNC diameter: 8-10 nm, length: 50-100 nm) and dispersed at 2000 rpm for 4 h to obtain a uniform dispersion (labeled as CNC&PS);

[0108] (2) The CNC&PS dispersion obtained in step (1) was vacuum-degassed and uniformly cast into a glass mold of fixed thickness (7 μm) at a casting temperature of 55° C. to obtain a first film material; the first film material was immersed in an ethanol solution (mass concentration of 99 wt%) for 18 h (dehydration) at a solid-liquid ratio of 1:6 to obtain a CNC&PS gel; the obtained gel was then baked in an oven at 75° C. for 3 h to obtain a CNC&PS film;

[0109] (3) The baked CNC&PS membrane was placed in an acetone solution (mass concentration 99.5 wt%) according to a solid-liquid ratio of 1:8, and after being placed for 7 hours, it was baked in an oven at 105° C. for 2 hours to obtain the cellulose membrane.

[0110] Examples 3 to 11

[0111] Examples 3 to 11 respectively provide a method for preparing a cellulose membrane, which differs from Example 1 only in that the porous PS microspheres are provided by Preparation Examples 2 to 10, respectively, and the other components, amounts, and process parameters are the same as those in Example 1.

[0112] Example 12

[0113] This embodiment provides a method for preparing a cellulose membrane, which differs from Example 1 only in that the content of the porous PS microspheres is adjusted so that the mass ratio of CNC to porous PS microspheres is 1:0.5 (based on solid mass). Other components, amounts, and process parameters are the same as those in Example 1.

[0114] Example 13

[0115] This embodiment provides a method for preparing a cellulose membrane, which differs from Example 1 only in that the content of the porous PS microspheres is adjusted so that the mass ratio of CNC to porous PS microspheres is 1:1 (calculated by solid mass). Other components, amounts, and process parameters are the same as those in Example 1.

[0116] Example 14

[0117] This embodiment provides a method for preparing a cellulose film, which differs from Example 1 only in that, in step (2), no ethanol solution is used for soaking treatment, and the first film material is directly baked in an oven at 65°C for 2 hours. The other steps are the same as in Example 1.

[0118] Example 15

[0119] This embodiment provides a method for preparing a cellulose membrane, which differs from Example 1 only in that the ethanol solution is replaced with propylene glycol methyl ether of equal mass, and the other steps are the same as those in Example 1.

[0120] Comparative Example 1

[0121] This comparative example provides a method for preparing a cellulose membrane, which differs from Example 1 only in that, in the preparation method, porous PS microspheres are not added and ethanol solution is not used for soaking treatment. Other steps are the same as those in Example 1.

[0122] Comparative Example 2

[0123] This comparative example provides a method for preparing a cellulose membrane, which differs from Example 1 only in that the porous PS microspheres are replaced with PS microspheres of equal mass provided in Comparative Preparation Example 1, and the other steps are the same as those in Example 1.

[0124] Comparative Example 3

[0125] This comparative example provides a method for preparing a cellulose membrane, which differs from Example 1 only in that the content of the porous PS microspheres is adjusted so that the mass ratio of CNC to porous PS microspheres is 1:5 (based on solid mass). Other components, amounts and process parameters are the same as those in Example 1.

[0126] Comparative Example 4

[0127] This comparative example provides a method for preparing a cellulose membrane, which differs from Example 1 only in that the content of the porous PS microspheres is adjusted so that the mass ratio of CNC to porous PS microspheres is 1:0.3 (based on solid mass). Other components, amounts and process parameters are the same as those in Example 1.

[0128] Performance Testing

[0129] (1) Observe the weight change of the first film material in toluene

[0130] Taking Example 1, Example 14 and Comparative Example 1 as examples, the weight of the membrane material was weighed at different times during the toluene immersion process. The results are as follows: Figure 4 shown by Figure 4It can be seen that the weight of the pure cellulose membrane (Comparative Example 1) in toluene does not change, which indirectly proves the good stability of cellulose in toluene solution. In both Examples 1 and 14, the weight of the first membrane material drops sharply at the initial stage, then reaches equilibrium and stabilizes until the PS spheres are completely dissolved. However, the time to reach equilibrium is 22 minutes (Example 1) and 30 minutes (Example 14), respectively. The first membrane material obtained by the preparation method provided by the present invention reaches equilibrium in a shorter time, indicating that the permeability and porosity of the two membranes are also different. In descending order of porosity and permeability, the following order is: Example 1 > Example 14 > Comparative Example 1.

[0131] (2) Breathability

[0132] The air permeability index reflects the smoothness of lithium ion transmission in the channel of the lithium battery. If the air permeability value is too large, it will cause difficulty in lithium ion transmission and easily lead to battery short circuit.

[0133] In the present invention, the standard method for testing the air permeability value of cellulose membrane is: GB / T36363-2018.

[0134] (3) Standard method for testing the porosity of cellulose membranes: GB / T36363-2018.

[0135] (4) Thermal stability

[0136] Thermal stability is an important indicator for evaluating the performance of diaphragms. Taking polyolefin diaphragms as an example, when the temperature rises to the melting temperature of polyolefin diaphragms, diaphragms with high thermal stability can maintain dimensional integrity at high temperatures and block direct contact between the positive and negative electrodes.

[0137] In the present invention, the standard method for testing the thermal stability of cellulose membranes is GB / T12027-2004.

[0138] The cellulose film was heat-treated at 180°C for 60 min, and the dimensions of the cellulose film in the transverse direction (TD) and longitudinal direction (MD) before and after treatment were measured, and the thermal shrinkage was calculated; wherein, thermal shrinkage = (dimension before treatment - dimension after treatment) / dimension before treatment × 100%.

[0139] (5) Wetting performance

[0140] The electrolyte wettability of a separator is a key performance indicator for lithium-ion separators, directly reflecting lithium-ion transport performance and electrolyte penetration. Electrolyte absorption rate demonstrates differences in wettability through contact angle characteristics. Specific testing methods are described in GB / T30447-2018.

[0141] The polyolefin separator was used as the control group.

[0142] Control group: polyethylene membrane (SW807C, Shenzhen Xingyuan Material Technology Co., Ltd.), with a thickness of 7 μm.

[0143] The specific test results are shown in Table 2:

[0144] Table 2

[0145]

[0146]

[0147] As can be seen from Table 2, the cellulose membrane obtained by the specific preparation method of the present invention has polar groups on its surface, large pore size, and high porosity; therefore, the membrane has a small air permeability value and good air permeability, which is more conducive to the transmission of lithium ions; it has good heat resistance and low thermal shrinkage when treated at 180°C; and it has a low contact angle and good wettability to the electrolyte.

[0148] It can be seen from Comparative Example 1 that the air permeability of the cellulose membrane obtained in the preparation method does not exceed the standard when porous PS microspheres are not added and ethanol solution is not used for treatment. The reason is that during the CNC drying process, the strong hydrogen bonding effect and capillary action around the cellulose surface cause the distance between the cellulose fibers to be shortened, resulting in the almost closed porosity of the diaphragm, which to a certain extent hinders the transmission of lithium ions; and the contact angle of the cellulose membrane increases, and the wettability is slightly poor.

[0149] It can be seen from Comparative Example 2 that when the first pore-forming agent is not porous polystyrene microspheres but polystyrene microspheres are used instead, the air permeability, thermal stability and wettability of the cellulose membrane are all deteriorated.

[0150] It can be seen from Comparative Examples 3 and 4 that the mass ratio of the first pore-forming agent to cellulose is not within the range specified in the present invention, the thermal stability of the cellulose membrane deteriorates, and the air permeability is slightly reduced.

[0151] As can be seen from Example 14, when the preparation method does not use ethanol solution for treatment, the air permeability and contact angle of the obtained cellulose membrane are slightly higher, and the air permeability and wettability of the cellulose membrane are slightly worse.

[0152] It can be seen from the control group that compared with the polyolefin separator, the cellulose membrane obtained in the present invention has better air permeability, heat resistance and electrolyte wettability.

[0153] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a cellulose film, characterized in that: The preparation method comprises the following steps: (1) mixing and dispersing cellulose and a first pore-forming agent to obtain a dispersion; (2) forming the dispersion obtained in step (1) to obtain a first film material; (3) mixing the first membrane material obtained in step (2) with a second organic solvent to obtain the cellulose membrane; Wherein, the first pore-forming agent comprises porous polystyrene microspheres; The D50 particle size of the porous polystyrene microspheres is 0.8-1.2 μm; The porosity of the porous polystyrene microspheres is 45-75%; The pore size of the porous polystyrene microspheres is 9-20 nm; Calculated on a solid mass basis, the mass ratio of the cellulose to the first pore-forming agent is 1:(0.5-1.6); The first pore former is soluble in the second organic solvent.

2. The preparation method according to claim 1, characterized in that The cellulose in step (1) includes nanocellulose.

3. The preparation method according to claim 1, characterized in that The cellulose comprises at least one of cellulose nanocrystals, nanofiber filaments or bacterial cellulose.

4. The preparation method according to claim 1, characterized in that The first film material is further treated with the first organic solvent before being mixed with the second organic solvent. The polarity of the first organic solvent is lower than that of water, and the boiling point of the first organic solvent is lower than 100°C.

5. The preparation method according to claim 1, characterized in that The raw materials for preparing the porous polystyrene microspheres include styrene monomers, a second pore-forming agent, a dispersant, an initiator and a solvent.

6. The preparation method according to claim 5, characterized in that The mass ratio of the second pore-forming agent to the styrene monomer is (1-3):

1.

7. The preparation method according to claim 6, characterized in that The mass ratio of the second pore-forming agent to the styrene monomer is (1-2):

1.

8. The preparation method according to claim 5, characterized in that The second pore forming agent includes polyethylene glycol.

9. The preparation method according to claim 5, characterized in that The mass percentage of the dispersant in the preparation raw materials is 0.5-0.8%.

10. The preparation method according to claim 5, characterized in that The dispersant includes any one of polyvinyl alcohol, sodium dodecylbenzene sulfonate, glycerol, and polymethyl acrylate, or a mixture of at least two of them.

11. The preparation method according to claim 5, characterized in that The styrene monomers include styrene and / or divinylbenzene.

12. The preparation method according to claim 11, characterized in that The styrene monomers include styrene and divinylbenzene, and the mass ratio of styrene to divinylbenzene is (40-60):

1.

13. The preparation method according to claim 5, characterized in that The amount of the initiator used is 1-5% of the mass of styrene.

14. The preparation method according to claim 5, characterized in that The preparation method of the porous polystyrene microspheres comprises: The porous polystyrene microspheres are obtained by mixing styrene monomers, a second pore-forming agent, a dispersant, an initiator and a solvent, reacting the mixture and removing the second pore-forming agent.

15. The preparation method according to claim 14, characterized in that The mixing comprises: pre-mixing and dispersing the solvent and the second pore-forming agent, and then adding the styrene monomer, the dispersant and the initiator thereto and mixing them.

16. The preparation method according to claim 15, characterized in that The temperature of the pre-mixing and dispersing is 80-85° C., and the rotation speed is 200-300 rpm.

17. The preparation method according to claim 14, characterized in that The reaction is carried out in the presence of a protective atmosphere, at a temperature of 80-90° C., and for 8-10 hours.

18. The preparation method according to claim 14, characterized in that The method of removing the second pore former includes extraction.

19. The preparation method according to claim 1, characterized in that The specific surface area of ​​the porous polystyrene microspheres is 85-170 m 2 / g.

20. The preparation method according to claim 1, characterized in that The mixing and dispersing process in step (1) is carried out at a speed of 1500-2000 rpm and for a time of 2-4 hours.

21. The preparation method according to claim 1, characterized in that Step (2) also includes a step of degassing the dispersion before forming.

22. The preparation method according to claim 1, characterized in that The molding method in step (2) includes casting molding.

23. The preparation method according to claim 1, characterized in that The molding temperature in step (2) is 45-55°C.

24. The preparation method according to claim 4, characterized in that The first organic solvent includes an alcohol solvent.

25. The preparation method according to claim 24, characterized in that The alcohol solvent includes at least one of methanol, ethanol, 1-propanol, isopropanol, 2,2,2-trifluoroethanol, 2-butanol, and isobutanol.

26. The preparation method according to claim 4, characterized in that The mass concentration of the first organic solvent is ≥98%.

27. The preparation method according to claim 4, characterized in that The method of treating with the first organic solvent includes: soaking the first film material in the first organic solvent, and drying to obtain a treated material.

28. The preparation method according to claim 27, characterized in that The time of soaking in the first organic solvent is 12-20 hours.

29. The preparation method according to claim 27, characterized in that The drying temperature after the first organic solvent treatment is 60-75° C. and the drying time is 1-3 hours.

30. The preparation method according to claim 1, characterized in that In step (3), the second organic solvent comprises at least one of toluene, tetrahydrofuran, acetone, butanone, ethyl acetate, N,N-dimethylformamide, chloroform, benzene or xylene.

31. The preparation method according to claim 1, characterized in that The mixing method in step (3) includes: immersing the first film material in the second organic solvent.

32. The preparation method according to claim 31, characterized in that The time of immersion in the second organic solvent is 5-7 hours, and the temperature is room temperature.

33. The preparation method according to claim 1, characterized in that The step (3) further includes a drying step after the mixing.

34. The preparation method according to claim 1, characterized in that The preparation method comprises the following steps: (1) mixing and dispersing cellulose and a first pore-forming agent at 1500-2000 rpm for 2-4 hours to obtain a dispersion; based on solid mass, the mass ratio of the cellulose to the first pore-forming agent is 1:(0.5-1.6); (2) casting the dispersion obtained in step (1) at 45-55° C. to obtain a first membrane material; soaking the first membrane material in a first organic solvent for 12-20 hours, and drying at 60-75° C. for 1-3 hours to obtain a treated first membrane material; (3) Soaking the treated first membrane material obtained in step (2) in a second organic solvent at room temperature for 5-7 hours and drying to obtain the cellulose membrane.

35. A cellulose film, characterized in that The cellulose film is prepared according to the preparation method according to any one of claims 1 to 34.

36. The cellulose film according to claim 35, characterized in that The porosity of the cellulose membrane is 40-50%.

37. A diaphragm, characterized in that The separator includes a cellulose membrane prepared by the preparation method according to any one of claims 1 to 34 or a cellulose membrane according to claim 35 or 36.

38. A lithium ion battery, characterized in that The lithium-ion battery comprises the cellulose film prepared by the preparation method according to any one of claims 1 to 34 or the separator according to claim 37.

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