A kind of cellulose nanofibril and its preparation method and application

Abstract

The present invention discloses a cellulose nanofibril and a preparation method and application thereof, belonging to the technical field of nanocellulose. The preparation method of the cellulose nanofibril comprises the following steps: (1) pretreatment: pretreating a fir strip with a peracetic acid solution at high temperature, and then washing with a sodium hydroxide solution to obtain fir fiber; (2) mechanical microfibrillation: adding the fir fiber to an ionic liquid-water mixture, placing it in a stirrer for mechanical stirring, and obtaining a suspension; (3) dialysis: placing the suspension in a dialysis bag for dialysis to obtain cellulose nanofibril. The obtained cellulose nanofibril has a narrow diameter distribution range, uniform size, ultra-high aspect ratio, good dispersibility, and is green, non-toxic, and degradable. The preparation method is simple in process, green and environmentally friendly, mild in reaction conditions, low in energy consumption, low in cost, and the reaction solvent is recyclable. The obtained cellulose nanofibril can be used as a lithium battery separator, and has a good application prospect.

Description

Technical Field

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

[0002] Cellulose is the most abundant renewable biopolymer, which can be obtained from resource-rich industrial and agricultural wastes. It has the characteristics of low cost and renewable. As a local biomass resource in Guangxi, Chinese fir has stable and uniform components, with a wood cellulose content of more than 90%, including 40-50% cellulose, 15-25% hemicellulose, and 30-40% lignin. However, most Chinese fir is used as biomass fuel or directly discarded, and a small part is used for furniture and pulp and paper making, with problems such as low utilization rate and low utilization value.

[0003] Nanocellulose is a high value-added product obtained by nano-fiberization of cellulose fibers through mechanical, biological, chemical and other methods. Its fiber diameter is 5 to 100 nm and its aspect ratio is greater than 10. At present, the method of preparing nanocellulose from natural wood cellulose has the problems of using toxic and harmful chemicals, high energy consumption and high water consumption. Nanocellulose prepared by mechanical method has the defects of wide diameter distribution range and uneven size. In addition, natural fibers with compact structure and solid texture are very easy to entangle together, causing blockage and wear of high-pressure homogenizer.

[0004] The preparation of lignocellulose nanofibers mainly involves the decomposition / defibrillation of plant fiber cells into individual cellulose nanofibrils. The first step is to process cellulose fibers by removing lignin and hemicellulose, and then the fibers, which are about 3 mm long and about 30 μm in diameter, are pretreated (chemically or enzymatically) to destroy their dense hierarchical structure and improve fiber accessibility to reduce energy dissipation during subsequent mechanical defibrillation. There are many pretreatment methods reported in the literature, and the 2,2,6,6-tetramethylpiperidinyl oxide (TEMPO)-mediated oxidation reaction is the most widely used. The fibrils obtained by this method have a uniform diameter of less than 4 nm and a length of about 800 nm. However, TEMPO catalysts are toxic, which prevents the use of TEMPO-CNF in some applications. There are also CNFs prepared by enzyme (Enz) pretreatment, such as pretreatment of pulp by endoglucanase, which can produce selective cellulose hydrolysis and is an environmentally friendly and low-cost process, but Enz-CNF shows large differences in diameter and length distribution, and contains incompletely fibrillated branched fibrils, and the low hemicellulose content and low surface charge in industrial lignocellulose fibers lead to agglomeration problems in the suspension. CNFs prepared by TEMPO or enzyme pretreatment are no longer in their natural state in terms of fibril length, crystallinity, and mechanical damage.

[0005] Therefore, it is necessary to seek a method for preparing cellulose nanofibrils with good product dispersibility, simple preparation process, green environmental protection, mild reaction conditions, low energy consumption and low cost. Summary of the invention

[0006] In view of the above problems, the present invention provides a method for preparing cellulose nanofibrils, wherein the fir wood strips are pretreated with a peracetic acid solution to remove most of the lignin, and then the fiber microfibrillation is promoted by an ionic liquid-water system, and then the cellulose nanofibrils are dissociated by a mechanical method. The preparation process is simple, green and environmentally friendly, the reaction conditions are mild, the energy consumption is low, and the cost is low. The obtained cellulose nanofibrils have a narrow diameter distribution range, uniform size, and good dispersibility.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for preparing cellulose nanofibrils comprises the following steps:

[0009] (1) Pretreatment: Pretreating the fir strips with a peracetic acid solution at high temperature, and then washing with a sodium hydroxide solution to obtain fir fibers;

[0010] (2) Mechanical microfibrillation: adding fir fiber to an ionic liquid-water mixture and placing it in a stirrer for mechanical stirring to obtain a suspension;

[0011] (3) Dialysis: The suspension is placed in a dialysis bag for dialysis to obtain evenly dispersed cellulose nanofibrils.

[0012] As a preferred technical solution, in step (2), the ionic liquid in the ionic liquid-water mixture is 1-ethyl-3-methylimidazolium acetate, and the concentration ratio of the ionic liquid to water is 0 to 50%.

[0013] As a preferred technical solution, in step (2), the mass ratio of the fir fiber to the ionic liquid-water mixture is 1:150-200.

[0014] As a preferred technical solution, in step (2), the mechanical stirring time is 0.5 to 2.5 h.

[0015] As a preferred technical solution, in step (1), the mass ratio of the fir wood strips to the peracetic acid solution is 1:5-6.

[0016] As a preferred technical solution, in step (1), the concentration of the peracetic acid is 4 to 10 wt %, and the concentration of the sodium hydroxide solution is 0.01 to 0.02 mol / L.

[0017] As a preferred technical solution, in step (1), the fir wood strips are treated with peracetic acid solution at a temperature of 80-85° C. for 3-4 times, and each treatment time is 35-45 minutes.

[0018] As a preferred technical solution, in step (3), the dialysis time is 6 to 7 days.

[0019] A cellulose nanofibril prepared by the cellulose nanofibril preparation method as described above.

[0020] The cellulose nanofibrils of the present invention have a diameter distribution of 3 to 12 nm, a length distribution of 3 to 40 μm, and an aspect ratio of 950 to 1000.

[0021] An application of the cellulose nanofibrils as described above in lithium battery separators.

[0022] The reaction principle of preparing cellulose nanofibrils in the present invention is as follows:

[0023] The present invention first pre-treats fir wood strips with a peracetic acid solution to remove most of the lignin while well retaining the hemicellulose, then uses a "non-dissolving / degradable" ionic liquid-water solvent guided by weakening the hydrogen bond network of hemicellulose molecules as a medium system, and uses 1-ethyl-3-methylimidazolium acetate as an ionic liquid. Under the action of the ionic liquid, the intermolecular hydrogen bonds of cellulose and hemicellulose are weakened, and the degree of fiber microfibrillation increases accordingly. Then, a mechanical method is combined to gently and efficiently dissociate cellulose nanofibrils with an ultra-long-diameter ratio from high-content hemicellulose and cellulose fibers.

[0024] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0025] 1. The present invention first pre-treats fir wood strips with a peracetic acid solution to obtain fir wood fibers, adds the obtained fir wood fibers to an ionic liquid-water mixture, and mechanically stirs them, and then obtains uniformly dispersed cellulose nanofibrils through simple dialysis. The cellulose nanofibrils obtained by the present invention have a diameter distribution of 3 to 12 nm, a length distribution of 3 to 40 μm, an aspect ratio of 950 to 1000, a narrow diameter distribution range, uniform size, an ultra-high aspect ratio, good dispersibility, and are green, non-toxic, and degradable.

[0026] 2. The ionic liquid-water system used in the present invention is composed of 1-ethyl-3-methylimidazolium acetate and water. The ionic liquid has the characteristics of being non-toxic, biodegradable and recyclable. The ionic liquid-water system regulates the hydrogen bond distribution of the hemicellulose macromolecular structure, effectively promotes the swelling of hemicellulose, and then promotes the swelling of cellulose, weakens the hydrogen bonding effect and van der Waals force, promotes the microfibrillation process of the fiber, and significantly reduces the mechanical energy consumption.

[0027] 3. The present invention promotes fiber microfibrillation through an ionic liquid-water system, and then combines mechanical methods to dissociate cellulose nanofibrils, thereby avoiding the high-energy consumption and high-pollution delignification and hemicellulose removal processes. The hemicellulose retained in the process of preparing cellulose nanofibrils can reduce the mechanical energy consumption, prevent fiber agglomeration, and improve the dispersibility of cellulose nanofibrils.

[0028] 4. The preparation method of the present invention is simple in process, environmentally friendly, mild in reaction conditions, low in energy consumption, low in cost, and the reaction solvent is recyclable and highly valued. At the same time, preparing cellulose nanofibrils from fir raw materials can improve the utilization rate and utilization value of fir.

[0029] 5. The preparation method of the present invention has low energy consumption and low cost. Compared with the traditional methods TEMPO-CNF and Enz-CNF, the present invention can reduce 1.6074 yuan / g and 2.457 yuan / g respectively in the whole process of preparing cellulose nanofibrils, indicating that the preparation of cellulose nanofibrils of the present invention has great advantages in low energy consumption, low cost and high value utilization.

[0030] 6. The preparation method of the present invention can also produce nanopaper, and the prepared nanopaper has high strength, thermal stability and high transparency. The cellulose nanofibrils prepared by the present invention can be used as lithium battery separators, which can effectively solve the problems of poor electrolyte wettability and thermal stability of polyolefin separators and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The scanning electron microscope images and atomic force microscope images of the cellulose nanofibrils prepared in Comparative Example 1, Example 1 and Example 2 are shown.

[0032] Figure 2 This is the intermolecular force diagram of cellulose nanofibrils prepared in Comparative Example 1 and Example 1.

[0033] FIG. 3 is a graph showing rheometer data of cellulose nanopaper fibrils prepared in Comparative Example 1 and Example 1.

[0034] Figure 4 The Fourier transform infrared spectra of the nanopaper prepared in Comparative Example 2, Example 5 and Example 6 are shown.

[0035] Figure 5 XPS graphs of the nanopaper prepared in Comparative Example 2 and Examples 5-7.

[0036] Figure 6 Schematic diagram of transmittance and haze of nanopaper prepared in Comparative Example 2, Example 5 and Example 6.

[0037] Figure 7Schematic diagram of the thermal stability of nanopaper prepared in Comparative Example 2, Example 5 and Example 6.

[0038] Figure 8 This is a graph showing the mechanical strength of the nanopaper prepared in Comparative Example 2 and Examples 5-8.

[0039] Fig. 9 This is a scanning electron microscope image of the tensile fracture surface of the nanopaper prepared in Example 5. DETAILED DESCRIPTION

[0040] The present invention is further described in detail below by way of examples. These examples are only used to illustrate the present invention and do not limit the protection scope of the present invention.

[0041] Example 1

[0042] Preparation of cellulose nanofibrils:

[0043] (1) Pretreatment: 200 g of fir strips were repeatedly washed with deionized water and ethanol until the washing liquid was clear, and then completely dried in an oven at 60°C. The washed and dried fir strips were cut into thin strips with a width of 2 mm, and then treated with 1200 g of peracetic acid solution (4 wt% aqueous solution, pH = 4.8 before reaction) at 85°C for 4 times, each treatment for 45 min, and then washed with 0.01 mol / L sodium hydroxide solution. Finally, rinsed with deionized water to remove residual chemicals to obtain fir fibers;

[0044] (2) Mechanical microfibrillation: 2 g of fir fiber was added to 300 g of an ionic liquid-water mixture with an ionic liquid concentration of 24% (1-ethyl-3-methylimidazolium acetate), and the mixture was mechanically stirred in a kitchen blender for 2.5 h to obtain a suspension.

[0045] (3) Dialysis: The suspension is placed in a dialysis bag and dialyzed for 7 days to obtain evenly dispersed cellulose nanofibrils and recover the ionic liquid.

[0046] After detection and analysis, the diameter distribution of the prepared cellulose nanofibrils is 3-12 nm, the length distribution is 3-40 μm, and the aspect ratio is 950-1000.

[0047] Comparative Example 1

[0048] The difference between Comparative Example 1 and Example 1 is that in Comparative Example 1, no ionic liquid is used during mechanical microfibrillation, the fir fiber is always in an aqueous solution, and the other preparation conditions are the same as those of Example 1 to obtain cellulose nanofibrils.

[0049] Example 2

[0050] Preparation of cellulose nanofibrils:

[0051] (1) Pretreatment: 200 g of fir strips were repeatedly washed with deionized water and ethanol until the washing liquid was clear, and then completely dried in an oven at 60°C. The washed and dried fir strips were cut into thin strips with a width of 2 mm, and then treated with 1200 g of peracetic acid solution (5 wt% aqueous solution, pH = 4.8 before reaction) at 80°C for 4 times, each treatment for 40 min, and then washed with 0.01 mol / L sodium hydroxide solution. Finally, rinsed with deionized water to remove residual chemicals to obtain fir fibers;

[0052] (2) Mechanical microfibrillation: 2 g of fir fiber was added to 300 g of an ionic liquid-water mixture with an ionic liquid concentration of 50%, where the ionic liquid was 1-ethyl-3-methylimidazolium acetate, and the mixture was mechanically stirred in a kitchen blender for 0.5 h to obtain a suspension.

[0053] (3) Dialysis: The suspension is placed in a dialysis bag and dialyzed for 7 days to obtain evenly dispersed cellulose nanofibrils and recover the ionic liquid.

[0054] After detection and analysis, the diameter distribution of the prepared cellulose nanofibrils is 3-12 nm, the length distribution is 3-40 μm, and the aspect ratio is 950-1000.

[0055] Example 3

[0056] Preparation of cellulose nanofibrils:

[0057] (1) Pretreatment: 200 g of fir strips were repeatedly washed with deionized water and ethanol until the washing liquid was clear, and then completely dried in an oven at 60°C. The washed and dried fir strips were cut into thin strips with a width of 2 mm, and then treated with 1200 g of peracetic acid solution (4 wt% aqueous solution, pH = 4.8 before reaction) at 85°C for 4 times, each treatment for 45 min, and then washed with 0.01 mol / L sodium hydroxide solution. Finally, rinsed with deionized water to remove residual chemicals to obtain fir fibers;

[0058] (2) Mechanical microfibrillation: 2 g of fir fiber was added to 300 g of an ionic liquid-water mixture with an ionic liquid concentration of 15%, where the ionic liquid was 1-ethyl-3-methylimidazolium acetate, and the mixture was mechanically stirred in a kitchen blender for 2.5 h to obtain a suspension.

[0059] (3) Dialysis: The suspension is placed in a dialysis bag and dialyzed for 7 days to obtain evenly dispersed cellulose nanofibrils and recover the ionic liquid.

[0060] After detection and analysis, the diameter distribution of the prepared cellulose nanofibrils is 3-12 nm, the length distribution is 3-40 μm, and the aspect ratio is 950-1000.

[0061] Example 4

[0062] Preparation of cellulose nanofibrils:

[0063] (1) Pretreatment: 200 g of fir strips were repeatedly washed with deionized water and ethanol until the washing liquid was clear, and then completely dried in an oven at 60°C. The washed and dried fir strips were cut into thin strips with a width of 2 mm, and then treated with 1200 g of peracetic acid solution (5 wt% aqueous solution, pH = 4.8 before reaction) at 80°C for 4 times, each treatment for 40 min, and then washed with 0.01 mol / L sodium hydroxide solution. Finally, rinsed with deionized water to remove residual chemicals to obtain fir fibers;

[0064] (2) Mechanical microfibrillation: 2 g of fir fiber was added to 300 g of an ionic liquid-water mixture with an ionic liquid concentration of 12% (1-ethyl-3-methylimidazolium acetate), and the mixture was mechanically stirred in a kitchen blender for 2.5 h to obtain a suspension.

[0065] (3) Dialysis: The suspension is placed in a dialysis bag and dialyzed for 7 days to obtain evenly dispersed cellulose nanofibrils and recover the ionic liquid.

[0066] After detection and analysis, the diameter distribution of the prepared cellulose nanofibrils is 3-12 nm, the length distribution is 3-40 μm, and the aspect ratio is 950-1000.

[0067] Example 5

[0068] Preparation of nanopaper:

[0069] The suspension prepared in Example 1 was placed on a hydrophilic polyvinylidene fluoride filter membrane for vacuum filtration for 1 min. After the filtration was completed, a second filter membrane was placed on the top, and the entire assembly was dried at 25°C, 50% relative humidity and 500 g weight for 5 h, during which the bottom and top filter membranes were continuously replaced, and finally placed in a 60°C oven for drying for 6 h to obtain nanopaper.

[0070] Comparative Example 2

[0071] The difference between Comparative Example 2 and Example 5 is that the suspension in Comparative Example 2 is the suspension prepared in Comparative Example 1, and the other preparation conditions are the same as those in Example 5 to prepare nanopaper.

[0072] Example 6

[0073] The difference between Example 6 and Example 5 is that the suspension in Example 6 is the suspension prepared in Example 2, and the other preparation conditions are the same as those in Example 5 to prepare nanopaper.

[0074] Example 7

[0075] The difference between Example 7 and Example 5 is that the suspension in Example 7 is the suspension prepared in Example 3, and the other preparation conditions are the same as those in Example 5 to prepare nanopaper.

[0076] Example 8

[0077] The difference between Example 8 and Example 5 is that the suspension in Example 8 is the suspension prepared in Example 4, and the other preparation conditions are the same as those in Example 5 to prepare nanopaper.

[0078] Characterization and analysis of cellulose nanofibril materials

[0079] 1. Microscopic analysis of cellulose nanofibrils

[0080] The cellulose nanofibrils prepared in Comparative Example 1, Example 1 and Example 2 were tested and analyzed under a microscope. The test results are as follows: Figure 1 As shown, Figure 1 Transmission electron microscope (TEM) and atomic force microscope (AFM) images of cellulose nanofibrils prepared in Comparative Example 1, Example 1 and Example 2. Figure 1 In the figure, (a) and (d), (b) and (e), (c) and (f) respectively represent the transmission electron microscopy and atomic force microscopy images of the cellulose nanofibrils prepared in Comparative Example 1, Example 1, and Example 2.

[0081] from Figure 1 As can be seen from (a) and (d), the fibers in comparative example 1 that were not treated with ionic liquid, i.e., the fibers were only in aqueous solution, and the fiber bundles were in a dispersed state. The mild kitchen blender defibrillation process caused little mechanical damage, and the length was shortened to a small extent. The diameter of the fibers dispersed in water reached hundreds of nanometers. Figure 1 As can be seen from (b) and (e), in Example 1, when the fibers were treated with an appropriate ionic liquid concentration (24%), the cellulose was separated into thinner bundles, and even individual nanofibers were observed. Figure 1 As can be seen from (c) and (f), in Example 2, when the fibers were treated with a high concentration of ionic liquid (50%), the fiber length and diameter dropped sharply, and the figure shows a single short and thick fiber. From the comparison between the above Examples 1 and 2 and Comparative Example 1, in the presence of a low concentration of ionic liquid (24%), the hemicellulose between the nanofibrils is swelling and "pushing" them apart, and the hemicellulose is conducive to the separation of the fiber bundles.

[0082] 2. Analysis of the effect of holocellulose on the adhesion of colloidal probes

[0083] The effect of holocellulose on the adhesion of the colloidal probe in Comparative Example 1 and Example 1 was analyzed. The analysis results are as follows: Figure 2 As shown, Figure 2 This is the intermolecular force diagram of cellulose nanofibrils prepared in Comparative Example 1 and Example 1.

[0084] from Figure 2 It can be seen that in Example 1, in the presence of ionic liquid, the unique core-shell structure, that is, hemicellulose wrapped around cellulose, can still retain a good aspect ratio (>1000) under mechanical action. The AFM probe and the surface are in contact under the action of external force, so that the van der Waals force dominates the interaction, and the force is continuously measured during the contact and separation process between the probe and the surface. The fibers treated with an appropriate ion concentration (24%) have high adhesion, and their maximum force and maximum distance are the highest, which means that the adhesion energy is also the highest. The low adhesion and adhesion energy reflected in the two sets of data are due to the core-shell structure synthesized by natural biology. The swollen hemicellulose maintains the separation of the original fibers through steric hindrance and charge repulsion, reflecting that its potential gap is very small. After treatment with ionic liquid, the degree of microfibrillation increases, the aspect ratio increases, and the intermolecular hydrogen bonding is enhanced, thereby increasing the adhesion and adhesion energy.

[0085] 3. Viscoelastic analysis of suspension

[0086] The viscoelasticity of the suspensions in Comparative Example 1 and Example 1 was analyzed. The results are as follows: Figure 3 As shown, FIG3 is a rheometer data diagram of the cellulose nanopaper fibrils prepared in Comparative Example 1 and Example 1.

[0087] from Figure 3 (a) It can be seen that the frequency dependence of the elastic (G') and viscous (G'') moduli of 0.18 wt% cellulose dispersed in ionic liquid in comparative example 1. All suspensions show gel behavior, with G' higher than G'', which is a typical feature of CNF suspensions. After dialysis, the CNFs and CNFs between H 2 The hydrogen bonding between O is very weak, so the suspension is mainly physically entangled. In Example 1, the suspension obtained after treatment with ionic liquid (24%) has fibers with high aspect ratio and flexibility. Even at low concentrations, the physical entanglement is strong, and the fibers have a viscoelasticity similar to that of a solid. The viscoelasticity is higher than that of fibers in pure water.

[0088] There is a significant difference between the ionic liquid-water suspension of Example 1 and the pure water suspension of Comparative Example 1: the modulus of the 0.18wt%-24% ionic liquid-water suspension in Example 1 is about 1 order of magnitude higher than the response value of Comparative Example 1 without ionic liquid. The frequency-independent regions of G' and G'' of the 0.18wt%-24% ionic liquid-water suspension are wider than those of the 0.18wt%-water suspension, which also indicates stronger gel behavior. When containing ionic liquid, there is not only physical entanglement between the fibers, but also interaction between hydrogen bonds, which increases the modulus by about 1 order of magnitude.

[0089] from Figure 3 (b) It can be seen that the complex viscosity of the 0.18wt%-24wt% ionic liquid-water suspension in Example 1 is also much higher than the complex viscosity of the pure water suspension in Comparative Example 1.

[0090] from Figure 3 (c) It can be seen that the viscosity of the suspension decreases with increasing shear rate, a typical shear-thinning behavior of nanocellulose suspension, which indicates that the suspension is a pseudoplastic fluid and its dispersion in water is unhindered. The thinner the fiber bundle, the easier it is to form a highly entangled elastic network, which increases both the viscoelastic modulus and the viscosity.

[0091] Characterization and analysis of nanopaper materials

[0092] 1. Analysis of chemical bond changes in nanopaper

[0093] The chemical bond changes of the nanopaper prepared in Comparative Example 2 and Examples 5-7 were analyzed. The analysis results are as follows: Figure 4 and Figure 5 As shown, Figure 4 The Fourier transform infrared spectra of the nanopaper prepared in Comparative Example 2, Example 5 and Example 6 are shown. Figure 5 XPS graphs of the nanopaper prepared in Comparative Example 2 and Examples 5-7.

[0094] from Figure 4 and Figure 5 It can be seen that in Example 5, after the fiber was treated with an appropriate concentration of ionic liquid (24%), the C2 peak area increased, the exposed -0H peak increased, and the C=O bond stretching vibration was almost unchanged, that is, the degree of microfibrillation increased, corresponding to the fiber diameter becoming smaller as described above, and the fiber was not degraded. With the increase of ion concentration in Example 6 (50%), the C1 peak area decreased sharply, the exposed -0H peak increased sharply, and the C=O bond stretching vibration became smaller, corresponding to the fiber diameter becoming smaller and the length being very short as described above, the fiber was degraded, and the CC bond was broken.

[0095] (II) Transmittance and haze analysis of nanopaper

[0096] The transmittance and haze of the nanopaper prepared in Comparative Example 2, Example 5 and Example 6 were analyzed. The analysis results are as follows: Figure 6 As shown, Figure 6 Schematic diagram of transmittance and haze of nanopaper prepared in Comparative Example 2, Example 5 and Example 6.

[0097] from Figure 6 It can be seen that the optical transmittance at a wavelength of 550 nm is higher than 85%. As the wavelength increases, the optical haze varies from 65% to 40%. A rougher surface will lead to an increase in haze, which is also confirmed by the roughness values ​​Ra and Rq.

[0098] (III) Thermal stability analysis of nanopaper

[0099] The thermal stability of the nanopapers prepared in Comparative Example 2, Example 5 and Example 6 was analyzed. The results are as follows: Figure 7 As shown, Figure 7 Schematic diagram of the thermal stability of nanopaper prepared in Comparative Example 2, Example 5 and Example 6.

[0100] from Figure 7 It can be seen that the thermal stability of the nanopaper obtained after ionic liquid treatment is improved. TGA 5% As high as nearly 300°C. This also shows that the lithium battery separator made of cellulose nanofibrils should be able to maintain dimensional stability within a certain temperature range, and the separator made of cellulose nanofibrils can ensure the normal operation of lithium batteries at higher temperatures.

[0101] (III) Analysis of tensile properties of nanopaper

[0102] The tensile properties of the nanopapers prepared in Comparative Example 2 and Examples 5-8 were analyzed. The results are as follows: Figure 8 As shown, Figure 8 This is a graph showing the mechanical strength of the nanopaper prepared in Comparative Example 2 and Examples 5-8.

[0103] from Figure 8 It can be seen that the tensile strength of the nanopaper obtained after ionic liquid treatment can reach 400 MPa, and the elongation is also increased.

[0104] (IV) Analysis of tensile properties of nanopaper

[0105] The tensile properties of the nanopaper prepared in Example 5 were analyzed. The results are as follows: Fig. 9 As shown, Fig. 9 This is a scanning electron microscope image of the tensile fracture surface of the nanopaper prepared in Example 5.

[0106] from Fig. 9It can be seen that when the nanopaper prepared in Example 5 is broken, the fibers are obviously pulled out, which also verifies that hydrogen bonds play a good role in hindering the tensile slip.

[0107] Comparison of the costs required to produce cellulose nanofibrils

[0108] The cost required for preparing cellulose nanofibrils in Example 1 of the present invention is compared with the traditional methods TEMPO-CNF and Enz-CNF, and the comparison results are shown in Table 1 below:

[0109] Table 1 Cost table for preparing cellulose nanofibrils, TEMPO-CNF and Enz-CNF in Example 1

[0110]

[0111] As shown in Table 1 above, compared with the traditional methods TEMPO-CNF and Enz-CNF, the present invention can reduce 0.8186 yuan / g and 0.581 yuan / g respectively in the delignification process, 0.1912 yuan / g and 0.3928 yuan / g respectively in the defibrillation process, and 0.5976 yuan / g and 1.4832 yuan / g respectively in the centrifugation process. In summary, in the whole process of preparing cellulose nanofibrils, the present invention can reduce 1.6074 yuan / g and 2.457 yuan / g respectively.

[0112] The above cost comparison shows that, compared with the traditional methods of TEMPO-CNF and Enz-CNF, the preparation of cellulose nanofibrils by the present invention has great advantages in low energy consumption, low cost and high value utilization.

[0113] Application Example 1

[0114] Preparation of lithium battery separator:

[0115] The cellulose nanofibrils prepared in Example 1 were filtered under reduced pressure using a sand core funnel and a hydrophilic polytetrafluoroethylene filter membrane. The filtration was stopped when the water in the funnel was drained. The filter membrane was removed together with the wet cellulose membrane and moved into anhydrous ethanol for solvent replacement for 2 h. The replaced wet membrane was clamped with a layer of filter membrane, placed in a glass plate to keep it flat, and dried at 60°C in a constant temperature blast drying oven for 24 h to obtain a dry nanocellulose membrane. The thickness of the final fiber membrane was controlled by controlling the volume of the suspension used for filtration, and lithium battery separators with thicknesses of 15 μm, 25 μm, and 35 μm were prepared. The prepared lithium battery separator can effectively solve the problem of poor electrolyte wettability and thermal stability of polyolefin separators.

[0116] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing cellulose nanofibrils, characterized in that: The following steps are involved: (1) Pretreatment: pretreating the fir strips with a peracetic acid solution at a high temperature, and then washing with a sodium hydroxide solution to obtain fir fibers; the fir strips are treated with the peracetic acid solution at a temperature of 80 to 85° C. for 3 to 4 times, and each treatment time is 35 to 45 minutes; (2) Mechanical microfibrillation: adding fir fiber to an ionic liquid-water mixture and placing the mixture in a stirrer for mechanical stirring to obtain a suspension; the ionic liquid in the ionic liquid-water mixture is 1-ethyl-3-methylimidazolium acetate, and the concentration ratio of the ionic liquid to water is 12-50%; the mass ratio of the fir fiber to the ionic liquid-water mixture is 1:150-200; (3) Dialysis: The suspension is placed in a dialysis bag for dialysis to obtain evenly dispersed cellulose nanofibrils.

2. The method for preparing cellulose nanofibrils according to claim 1, characterized in that: In step (2), the mechanical stirring time is 0.5 to 2.5 h.

3. The method for preparing cellulose nanofibrils according to claim 1, characterized in that: In step (1), the mass ratio of the fir wood strips to the peracetic acid solution is 1:5-6.

4. The method for preparing cellulose nanofibrils according to claim 1, characterized in that: In step (3), the dialysis time is 6 to 7 days.

5. Cellulose nanofibrils prepared by the method for preparing cellulose nanofibrils according to any one of claims 1 to 4.

6. The cellulose nanofibrils according to claim 5, characterized in that The cellulose nanofibrils have a diameter distribution of 3 to 12 nm, a length distribution of 3 to 40 μm, and an aspect ratio of 950 to 1000.

7. Use of the cellulose nanofibrils as claimed in claim 5 in lithium battery separators.

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