Metal oxide-nanocellulose fiber, porous film comprising same, and separator for secondary battery

Abstract

The present invention relates to a metal oxide-nanocellulose fiber, a porous film comprising same, and a separator for a secondary battery manufactured therefrom and, more specifically, to a metal oxide-nanocellulose fiber, a porous film comprising same, and a separator for a secondary battery, wherein the metal oxide-nanocellulose fiber has excellent high-temperature heat resistance and achieves a thin film with a relatively small thickness by chemically binding a metal oxide to the surface of nanocellulose without an organic binder.

Description

Metal oxide-nanocellulose fibers, porous films containing the same, and separators for secondary batteries

[0001] The present invention relates to metal oxide-nanocellulose fibers, a porous film comprising the same, and a separator for a secondary battery manufactured using the same, and more particularly, to metal oxide-nanocellulose fibers which not only have excellent high-temperature heat resistance but also enable a relatively thin film thickness by chemically bonding a metal oxide to the surface of nanocellulose without an organic binder, a porous film comprising the same, and a separator for a secondary battery.

[0002] As the demand for high-performance electronic devices has increased recently, the demand for secondary batteries has also been increasing. Accordingly, high-performance secondary batteries, such as high energy density and high power density, are in demand.

[0003] A separator acts as an insulator, preventing short circuits by allowing ion movement between the anode and cathode within a secondary battery while blocking electronic conduction. Because separators for secondary batteries provide physical insulation, they play a crucial role in battery stability.

[0004] Conventional secondary battery separators have mostly been structured by coating inorganic particles on a polyolefin porous film substrate. These secondary battery separators typically enhance heat resistance by coating alumina particles sized 0.5-1 um on a polyolefin porous film such as PE or PP. However, when exposed to high temperatures, the adhesive strength of the organic binder with low thermal stability decreases, causing the alumina particles to fall off, resulting in reduced durability.

[0005] These alumina particle-coated separators, which coat the film substrate with inorganic particles, have the problem of increasing the thickness of the separator due to the coating layer. As the separator thickness increases in the battery, the amount of positive and negative electrode materials decreases, leading to lower energy density. (Related prior literature: KR Patent Publication No. 2013-0037386, published on April 16, 2013)

[0006] To address these issues, a method has been developed to manufacture porous films by forming silica nanoparticles on the surface of nanocellulose via the sol-gel method. While this approach may be beneficial in that it prevents silica nanoparticle detachment and inhibits hydrogen bonding, resulting in a uniform porous film, the silica formed via the sol-gel method has limitations: it easily traps lithium ions, hindering their movement during charging and discharging of secondary batteries. This can have a detrimental effect on the operation of secondary batteries. (Related prior literature: KR Patent Publication No. 2020-0042220, published on April 23, 2020)

[0007] In order to solve the above problems, the present invention aims to provide a metal oxide-nanocellulose fiber, a porous film including the same, and a separator for a secondary battery, which not only has excellent high-temperature heat resistance but also allows for a relatively thin film thickness by chemically bonding a metal oxide to the surface of nanocellulose without an organic binder.

[0008] In order to achieve the above-described purpose, the present invention relates to a nanocellulose fiber having a metal oxide represented by the following chemical formula 1 chemically bonded to the surface thereof and a porous film including the same.

[0009] [Chemical Formula 1]

[0010] M2O3 (where M is a metal belonging to group 13 of the periodic table)

[0011] The metal oxide may be alumina (Al2O3), and the metal oxide may be chemically bonded to the surface of the nanocellulose fibers in the form of particles, or may be present in the form of a coating by chemically bonding to the surface of the nanocellulose fibers.

[0012] Another invention relates to a method for producing a porous film, comprising: a first step of preparing a reaction solution by adding a metal alkoxide represented by the following chemical formula 2 and nanocellulose fibers to water or an organic solvent; a second step of synthesizing metal oxide-nanocellulose in which a metal oxide is chemically bonded to the surface of the nanocellulose fibers by adding a reaction catalyst to the reaction solution to cause a sol-gel reaction; and a third step of washing the synthesized metal oxide-nanocellulose using a washing solvent; and a fourth step of producing a film of a predetermined thickness by dispersing the washed metal oxide-nanocellulose in water or an organic solvent at a predetermined concentration through a film forming process.

[0013] [Chemical Formula 2]

[0014]

[0015] (Here, M is a metal corresponding to group 13 of the periodic table,

[0016] R1 to R3 are the same or different and each independently represent hydrogen or C 1-20 alkylene)

[0017] Another invention relates to a separator for a secondary battery and an electrolyte for an all-solid-state battery comprising the porous film.

[0018] Nanocellulose fibers chemically bonded with metal oxides according to the present invention have excellent heat resistance even at high temperatures of 200°C or higher.

[0019] In addition, in the case of the porous film according to the present invention, since the nanocellulose fibers themselves to which metal oxides are chemically bonded are used as the porous film, the process is simple, and since there is no separate coating layer, the thickness is thin, which can contribute to improving energy density.

[0020] In addition, according to the present invention, since the alumina particles, which are mainly used in conventional battery separators, are chemically bonded to nanocellulose in the form of inorganic particles, there is an advantage in that it does not affect the movement of lithium ions within the battery.

[0021] Furthermore, alumina nanoparticles are advantageous in forming nanopores, and thus can be applied to films that combine alumina nanocellulose films with solid electrolytes. The all-solid-state battery according to the present invention not only has improved thermal safety and durability, but also enables a film lamination process during manufacturing, thereby facilitating production and reducing costs.

[0022] In addition, according to the present invention, since the size and bonding form of the metal oxide particles can be controlled by controlling the reaction conditions during synthesis, it can be applied in an optimized film form in a required technical field.

[0023] Figure 1 is a schematic diagram of the structure of a metal oxide-nanocellulose fiber according to the present invention.

[0024] Figure 2 is a flow chart of a method for manufacturing a porous film including metal oxide-nanocellulose fibers according to the present invention.

[0025] Figure 3 is an SEM photograph of alumina particle-bonded nanocellulose fibers of Example 1 according to the present invention.

[0026] Figure 4 is an SEM photograph of an alumina-coated nanocellulose fiber of Example 2 according to the present invention.

[0027] Figure 5 is an SEM photograph of a non-surface-treated nanocellulose fiber of a comparative example.

[0028] Figure 6 is an SEM photograph of an alumina particle-bonded nanocellulose porous film of Example 1 according to the present invention.

[0029] Figure 7 is an SEM photograph of an alumina-coated nanocellulose porous film of Example 2 according to the present invention.

[0030] Figure 8 is an SEM photograph of a non-surface-treated nanocellulose film of a comparative example.

[0031] Figure 9 shows the results of EDS elemental analysis of alumina particle-bonded nanocellulose fibers of Example 1 according to the present invention.

[0032] Figure 10 shows the EDS elemental analysis results of non-surface-treated nanocellulose fibers of a comparative example.

[0033] Figure 11 shows the XPS elemental analysis results of the alumina particle-bonded nanocellulose fiber of Example 1 according to the present invention.

[0034] Figure 12 is the XPS elemental analysis result for the Al narrow peak of the alumina particle-bonded nanocellulose fiber of Example 1 according to the present invention.

[0035] Figure 13 shows the XPS elemental analysis results of non-surface-treated nanocellulose fibers of a comparative example.

[0036] Figures 14(a) and 14(b) are images of the results of a thermal shrinkage experiment of an alumina particle-bonded nanocellulose porous film and a polyolefin-based porous film of Example 1 according to the present invention, respectively.

[0037] Figure 15 is a graph showing the results of a thermal shrinkage experiment of an alumina particle-bonded nanocellulose porous film of Example 1 according to the present invention.

[0038] Hereinafter, an embodiment of the present invention will be described in detail with reference to the attached drawings.

[0039] While the present invention is susceptible to numerous modifications and variations, specific embodiments thereof are illustrated in the drawings and will be described in detail below. However, the invention is not intended to be limited to the particular forms disclosed; rather, the invention includes all modifications, equivalents, and alternatives consistent with the spirit of the invention as defined by the claims.

[0040]

[0041] The present invention relates to nanocellulose fibers having a metal oxide represented by the following chemical formula 1 chemically bonded to the surface thereof and a porous film comprising the same.

[0042] [Chemical Formula 1]

[0043] M2O3 (where M is a metal belonging to group 13 of the periodic table)

[0044] Preferably, the metal oxide may be alumina (Al2O3), and the metal oxide may be chemically bonded to the surface of the nanocellulose fibers in the form of particles, or may be present in the form of a coating by chemically bonding to the surface of the nanocellulose fibers.

[0045] These metal oxide-nanocellulose fibers can be manufactured through a sol-gel reaction by adding a metal alkoxide represented by the following chemical formula 2 and a reaction catalyst to water or an organic solvent.

[0046] [Chemical Formula 2]

[0047]

[0048] (Here, M is a metal corresponding to group 13 of the periodic table, and R1 to R3 are the same or different, and each independently represents hydrogen or C 1-20 alkylene)

[0049] The metal M above is a metal belonging to Group 13 of the periodic table of elements, which has the advantage of not easily trapping lithium ions when the porous film according to the present invention is applied to a separator for a secondary battery. If the metal M is a metal belonging to Group 14, such as silica, it hinders the movement of ions during charging and discharging of the secondary battery, making it difficult to apply it to a separator for a secondary battery.

[0050] A feature of the present invention is that, depending on the reaction conditions such as the composition and amount of metal alkoxide or reaction catalyst, the metal oxide can be controlled to be in the form of particles on the surface of nanocellulose fibers as in Fig. 1(a), or to coat part or the entire surface of nanocellulose fibers as in Fig. 1(b).

[0051] When the metal oxide is chemically bonded in the form of particles, the particle diameter of the metal oxide may be 50 to 500 nm, and when the metal oxide is chemically bonded in the form of a coating, the coating thickness may be 2 to 50 nm.

[0052] The above nanocellulose fibers may have a width of 5 to 500 nm and a length of 100 nm to 10 μm.

[0053] The porous film according to the present invention may further include an additive such as a dispersion stabilizer or a binder, and more specifically, the additive may be at least one selected from the group consisting of acrylic, polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyvanillin oxalate (PVO), styrene butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyurethane (PU), and polyvinylidene fluoride (PVDF).

[0054] In conventional nanocellulose fibers, the pores are blocked by hydrogen bonding due to -OH groups on the surface, but in the metal oxide-nanocellulose according to the present invention, the pores of the film can be secured by forming nano pores on the surface of the metal oxide particles and inhibiting hydrogen bonding.

[0055]

[0056] Hereinafter, a method for manufacturing a porous film including metal oxide-nanocellulose fibers according to the present invention will be described with reference to FIG. 2.

[0057]

[0058] (1) Step 1: Preparation of reaction solution (metal alkoxide + nanocellulose fiber + solvent)

[0059] In this step, a reaction solution is prepared by adding a metal alkoxide represented by the following chemical formula 2 and nanocellulose fibers to water or an organic solvent.

[0060] [Chemical Formula 2]

[0061]

[0062] (Here, M is a metal corresponding to group 13 of the periodic table,

[0063] R1 to R3 are the same or different and each independently represent hydrogen or C 1-20 alkylene)

[0064] It is preferable that the above metal alkoxide is aluminum alkoxide represented by the following chemical formula 3.

[0065] [Chemical Formula 3]

[0066]

[0067] (Here, R1 to R3 are as defined in Chemical Formula 1)

[0068] More preferably, it may be selected from the group consisting of aluminum isopropoxide, aluminum ethoxide, aluminum tri-sec-butoxide, aluminum tert-butoxide, and aluminum trimethoxide.

[0069] The metal alkoxide may be included in an amount of 0.01 to 5 wt% based on the total mass of the reaction solution, and more preferably, 0.01 to 1 wt%.

[0070] The organic solvent of this step may be selected from the group consisting of ethanol, isopropyl alcohol, butanol, NMP (N-Methyl-2-Pyrrolidone), and acetonitrile, and it is preferable to disperse the nanocellulose fibers in the organic solvent at a concentration of 0.05 to 2 wt%.

[0071] The above nanocellulose fibers may have a width of 5 to 500 nm and a length of 100 nm to 10 μm.

[0072]

[0073] (2) Step 2: Manufacturing of metal oxide-nanocellulose fibers

[0074] In this step, a reaction catalyst is added and a sol-gel reaction is performed to synthesize metal oxide-nanocellulose fibers in which metal oxide is chemically bonded to the surface of nanocellulose fibers in the reaction solution.

[0075] By introducing a reaction catalyst in this step, not only is the sol-gel reaction of the metal alkoxide promoted, but the surface of the nanocellulose is also activated, thereby inducing better chemical bonding of the metal oxide to the surface of the nanocellulose.

[0076] The above reaction catalyst may be an acid catalyst selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, and acetic acid, or may be a basic catalyst selected from the group consisting of sodium hydroxide, lithium hydroxide, barium hydroxide, and ammonium hydroxide.

[0077] The above reaction catalyst may be included in an amount of 1 to 30 wt% based on the total mass of the reaction solution.

[0078] As mentioned above, the characteristic of the present invention is that the structure of the metal oxide chemically bonded to the surface of the nanocellulose fiber may be different depending on the reaction conditions such as the composition and amount of the metal alkoxide or reaction catalyst in the reaction solution, and specifically, it may be chemically bonded in the form of particles or in the form of a coating, and if necessary, the particle form and the coating form may coexist and exist in the form.

[0079] In addition, the size of the chemically bonded particles or the coating thickness can be appropriately controlled according to the above reaction conditions.

[0080] When the metal oxide is chemically bonded in the form of particles, the diameter of the particles may be 50 to 500 nm, and when it is chemically bonded in the form of a coating, the particles may be coated to a thickness of 2 to 50 nm.

[0081]

[0082] (3) Step 3: Cleaning of metal oxide-nanocellulose fibers

[0083] In this step, the metal oxide-nanocellulose fibers synthesized in the previous step are washed using a washing solvent.

[0084] The above cleaning method may be performed through centrifugation, vacuum filtration, or osmotic filtration, but is not necessarily limited thereto.

[0085]

[0086] (4) Step 4: Preparation of metal oxide-nanocellulose fiber dispersion and film formation

[0087] In this step, the metal oxide-nanocellulose fibers that have gone through the above-mentioned washing step are dispersed in water or an organic solvent at a certain concentration to prepare a dispersion, and then a film of a certain thickness is prepared through a certain film forming process.

[0088] That is, the metal oxide-nanocellulose fiber dispersion is applied, the applied nanocellulose fiber dispersion is dried to form a metal oxide-nanocellulose thin film, and then a porous film is manufactured through a drying process such as electron beam irradiation.

[0089] The metal oxide-nanocellulose fibers may be included in an amount of 0.01 to 5 wt% based on the total mass of the dispersion, and preferably, may be included in an amount of 0.01 to 1 wt% based on the total mass of the dispersion.

[0090] The organic solvent of this step may be selected from the group consisting of acetone, methanol, ethanol, isopropyl alcohol, butanol, NMP (N-Methyl-2-Pyrrolidone), and acetonitrile, but is not necessarily limited thereto.

[0091] In addition, the dispersion may further include one or more additives selected from the group consisting of acrylic, polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyvanillin oxalate (PVO), styrene butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyurethane (PU), and polyvinylidene fluoride (PVDF) to improve physical properties.

[0092] The film forming process forms a film with a certain thickness by drying the solvent through vacuum filtration, coating, or casting processes.

[0093]

[0094] Another invention relates to a secondary battery separator or an all-solid-state battery electrolyte comprising the porous film.

[0095] As described above, the metal oxide according to the present invention is a metal oxide corresponding to Group 13 of the periodic table of elements, and since the metal ions do not easily trap lithium ions, they can be used as a separator for a secondary battery or an electrolyte for an all-solid-state battery.

[0096] Metal oxide particles, such as alumina, used in conventional secondary battery separators have the advantage of having little effect on lithium ion movement during battery charging and discharging because they are bound to nanocellulose in the form of inorganic particles. Furthermore, the present invention allows for control of particle size and shape through reaction conditions, offering the advantage of providing an optimized separator tailored to specific usage conditions.

[0097] In addition, the separator manufactured using the porous film according to the present invention has excellent high-temperature durability, and since the metal oxide chemically bonds with the surface of the nanocellulose fiber, the phenomenon of particle detachment can be fundamentally prevented, and since thin film formation is possible without a separate coating process, the battery energy density can be improved.

[0098]

[0099] [Example]

[0100] [Example 1] Preparation of a nanocellulose porous film chemically bonded with alumina particles

[0101] 1) Manufacturing of alumina particle-bonded nanocellulose fibers

[0102] First, nanocellulose fibers dispersed in DI (water) were replaced with an alcohol-based organic solvent (isopropyl alcohol). Solvent replacement was performed by repeating centrifugation three times.

[0103] Solvent-exchanged nanocellulose fibers were dispersed in an alcohol-based organic solvent (isopropyl alcohol) with an acid concentration of 0.5 wt%. Then, aluminum alkoxide reagent was added to the solvent in which the nanocellulose fibers were dispersed to 0.5 wt% based on the total solution and dispersed for several hours. Afterwards, a basic catalyst was added to the prepared dispersion solution (7 v / v% based on the total solution) to initiate a sol-gel reaction. Afterwards, a uniform reaction was achieved by stirring, and after a certain period of time, centrifugation was performed three times using isopropyl alcohol as a washing solution, thereby removing the reaction reagent.

[0104] It was confirmed that the manufactured alumina nanocellulose fibers had alumina particles chemically bonded to the surface of the nanocellulose fibers, as shown in Fig. 3.

[0105]

[0106] 2) Manufacturing of porous films

[0107] Next, the alumina nanocellulose fibers manufactured above were added to isopropyl alcohol at a concentration of 0.1 wt% based on the solution, and then uniformly dispersed using a homogenizer at 6000 rpm for more than 10 minutes.

[0108] Thereafter, the manufactured dispersion was filtered using a vacuum filtration method, and the amount of filtering solution was controlled to form a film with a thickness of 10 μm or less. Thereafter, the film was separated from the filter and dried with hot air in an oven at 100°C or higher, thereby producing a nanocellulose porous film chemically bonded with alumina particles, as shown in Fig. 6.

[0109]

[0110] [Example 2] Preparation of alumina-coated nanocellulose porous film

[0111] Except for adding 0.07 wt% of the alkoxide reagent, the reaction was carried out in the same manner as in Example 1 to manufacture nanocellulose fibers, thereby manufacturing alumina-coated nanocellulose fibers in which alumina is present in a form coated on the surface, as shown in Fig. 4.

[0112] Thereafter, the same process as for manufacturing a porous film as in Example 1 was carried out to manufacture a porous film in which alumina was coated on a portion of the surface of nanocellulose, as shown in Fig. 7.

[0113]

[0114] [Comparative Example] Manufacturing of Nanocellulose Film without Surface Treatment

[0115] Except for not adding the amount of alkoxide reagent, the reaction was carried out in the same manner as in Example 1 to manufacture nanocellulose fibers, thereby manufacturing nanocellulose fibers without surface treatment as shown in Fig. 5.

[0116] Afterwards, the same process as for manufacturing a porous film in Example 1 was carried out to manufacture a nanocellulose film without surface treatment as shown in Fig. 8.

[0117]

[0118] [Analysis Results]

[0119] 1. Analysis of nanocellulose fibers

[0120] The nanocellulose fibers manufactured by Examples 1 and 2 and Comparative Examples were analyzed as follows.

[0121] (1) SEM image analysis

[0122] As shown in Fig. 3, the nanocellulose fiber surface of Example 1 was coated with large alumina particles (average diameter: 160 nm) and fine particles (approximately 10 nm, <20 nm or less). Furthermore, it was confirmed that the fine particles coated the fiber surface thickened the nanocellulose fiber. (Fiber diameter: 30.3 nm after initial non-surface treatment → 47.6 nm after alumina particle bonding)

[0123] By controlling the amount of aluminum alkoxide reagent added, fine alumina particles were coated on the surface of the cellulose fibers without forming large alumina particles, as shown in Fig. 4. At this time, it was confirmed that the fiber diameter had thickened to 44.1 nm. (Fiber diameter: 30.3 nm without surface treatment → 44.1 nm after alumina particle coating)

[0124] As shown in Fig. 5, the non-surface-treated nanocellulose of the comparative example has no particles attached to the surface of the nanocellulose fibers, so the fiber surface is smooth in the SEM image, and the fiber diameter at this time was measured to be 30.3 nm on average.

[0125]

[0126] (2) EDS elemental analysis

[0127] EDS analysis was performed to analyze the elements of nanocellulose fibers manufactured by Example 1 and Comparative Example.

[0128] The alumina particle-bonded nanocellulose fibers of Example 1 were measured for particle fraction, and approximately 28.7% of the Al element was detected. This confirmed that the formed nanoparticles were Al2O3 nanoparticles. (See Fig. 9)

[0129] In contrast, in the comparative example, the Al element was not analyzed due to the non-surface-treated nanocellulose. (See Fig. 10)

[0130]

[0131] (3) XPS elemental analysis

[0132] The formation of alumina nanoparticles in Example 1 and Comparative Example was confirmed through XPS analysis.

[0133] The alumina particle-bonded nanocellulose fibers of Example 1 detected 22.2 atonic % Al. (See Fig. 11) In addition, the narrow peak analysis result of Al showed that the peak was due to Al-O bonds, confirming the formation of Al2O3 particles. (See Fig. 12)

[0134] On the other hand, no Al element was detected in the non-surface-treated nanocellulose fiber of Comparative Example 1. (See Fig. 13)

[0135]

[0136] 2. Porous film analysis

[0137] The porous films manufactured by Examples 1 and 2 and Comparative Examples were analyzed as follows.

[0138]

[0139] (1) SEM image analysis

[0140] SEM analysis was performed to confirm the microstructure of the nanocellulose porous film.

[0141] As shown in FIGS. 6 and 7, it was confirmed that the alumina of Examples 1 and 2 was chemically bonded to form pores on the surface of the surface-treated cellulose. This is because the alumina nanoparticles on the surface of the nanocellulose fiber formed nanopores and inhibited hydrogen bonding, enabling the film to have pores.

[0142] On the other hand, as shown in Fig. 8, it was confirmed that the non-surface-treated nanocellulose porous film of the comparative example had almost no pores formed. This was understood to be because most of the pores were blocked by hydrogen bonds due to -OH groups on the surface of the nanocellulose fibers.

[0143]

[0144] (2) Film heat shrinkage rate experiment

[0145] The following heat shrinkage rate experiment was conducted on the porous film of Example 1 and the conventional polyolefin-based porous film (Toray PE separator).

[0146] First, the porous film to be evaluated was cut to a size of 5x5 cm, and a sample was prepared by marking dots on the top, bottom, left, and right at 4 cm intervals. After placing the sample in a high-temperature (150 to 200°C) oven for 30 minutes, the distance between dots was measured to evaluate the thermal shrinkage rate. (See Fig. 14)

[0147]

[0148] As can be seen in Fig. 14(a) and Fig. 15, the porous film of Example 1 exhibited excellent high-temperature heat resistance, with almost no heat shrinkage up to 200°C. The results showed that high-temperature durability was maintained with a similar tendency regardless of the sheet thickness (5 to 50 μm). For reference, a similar tendency was also confirmed in the porous film manufactured using the alumina-coated nanocellulose fibers of Example 2.

[0149] On the other hand, the conventional polyolefin porous film exhibited a heat shrinkage rate of more than 60% at 150°C, as shown in Fig. 14(b).

[0150]

[0151] The embodiments disclosed herein are merely specific examples intended to aid understanding of the invention and are not intended to limit the scope of the invention. It will be apparent to those skilled in the art that other modifications based on the technical concepts of the present invention are possible beyond the embodiments disclosed herein.

Claims

1. Nanocellulose fibers having a metal oxide chemically bonded to the surface, represented by the following chemical formula 1. [Chemical Formula 1] M2O3 (where M is a metal belonging to group 13 of the periodic table) 2. In paragraph 1, Nanocellulose fiber, characterized in that the metal oxide is alumina (Al2O3).

3. In paragraph 1, Nanocellulose fibers, characterized in that the metal oxide is compounded in the form of particles on the surface of the nanocellulose fibers.

4. In paragraph 3, Nanocellulose fiber, characterized in that the particle diameter of the metal oxide is 50 to 500 nm.

5. In paragraph 1, Nanocellulose fibers, characterized in that the metal oxide is chemically bonded to the surface of the nanocellulose fibers and coated.

6. In paragraph 1, Nanocellulose fibers characterized in that the metal oxide is coated to a thickness of 2 to 50 nm.

7. In paragraph 1, Nanocellulose fibers, characterized in that the width of the nanocellulose fibers is 5 to 500 nm.

8. In paragraph 1, Nanocellulose fibers, characterized in that the length of the nanocellulose fibers is 100 nm to 10 ㎛.

9. A porous film comprising nanocellulose fibers according to paragraph 1.

10. In paragraph 9, A porous film characterized in that the above porous film further comprises at least one additive selected from the group consisting of acrylic, polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyvanillin oxalate (PVO), styrene butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyurethane (PU), and polyvinylidene fluoride (PVDF).

11. In the method for manufacturing a porous film according to Article 9, A first step of preparing a reaction solution by adding a metal alkoxide represented by the following chemical formula 2 and nanocellulose fibers to water or an organic solvent; A second step of synthesizing a metal oxide-nanocellulose fiber in which a metal oxide is chemically bonded to the surface of the nanocellulose fiber by injecting a reaction catalyst into the above reaction solution to cause a sol-gel reaction; A third step of washing the synthesized metal oxide-nanocellulose fiber using a washing solvent; A fourth step of manufacturing a film of a certain thickness through a film forming process using a dispersion of the above-mentioned washed metal oxide-nanocellulose fibers dispersed in water or an organic solvent at a certain concentration; A method for producing a porous film comprising: [Chemical Formula 2] (Here, M is a metal corresponding to group 13 of the periodic table, R1 to R3 are the same or different and each independently represent hydrogen or C 1-20 alkylene) 12. In paragraph 11, A method for producing a porous film, characterized in that the metal alkoxide of the first step is aluminum alkoxide represented by the following chemical formula 3. [Chemical Formula 3] (Here, R1 to R3 are as defined in Chemical Formula 1) 13. In paragraph 12, A method for producing a porous film, characterized in that the aluminum alkoxide is selected from the group consisting of aluminum isopropoxide, aluminum ethoxide, aluminum tri-sec-butoxide, aluminum tert-butoxide, and aluminum trimethoxide.

14. In paragraph 11, A method for producing a porous film, characterized in that the metal alkoxide of the first step is contained in an amount of 0.01 to 5 wt% based on the total mass of the reaction solution.

15. In paragraph 11, A method for producing a porous film, characterized in that the organic solvent of the first step is selected from the group consisting of ethanol, isopropyl alcohol, butanol, NMP (N-Methyl-2-Pyrrolidone), and acetonitrile.

16. In paragraph 11, A method for manufacturing a porous film, characterized in that the first step comprises dispersing the nanocellulose fibers in the organic solvent at a concentration of 0.05 to 2 wt%.

17. In paragraph 11, A method for producing a porous film, characterized in that the nanocellulose fibers of the first step have a width of 5 to 500 nm and a length of 100 nm to 10 ㎛.

18. In paragraph 11, The reaction catalyst of the second step is an acid catalyst selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, and acetic acid, or A method for producing a porous film, characterized in that the basic catalyst is at least one selected from the group consisting of sodium hydroxide, lithium hydroxide, barium hydroxide, and ammonium hydroxide.

19. In paragraph 18, A method for producing a porous film, characterized in that the reaction catalyst is contained in an amount of 1 to 30 wt% based on the total mass of the reaction solution.

20. In paragraph 11, A method for manufacturing a porous film, characterized in that the metal oxide of the second step is compounded in the form of particles on the surface of the nanocellulose fibers.

21. In paragraph 20, A method for manufacturing a porous film, characterized in that the particle diameter of the metal oxide is 50 to 500 nm.

22. In paragraph 11, A method for manufacturing a porous film, characterized in that the metal oxide of the second step is chemically bonded to the surface of the nanocellulose fiber and coated.

23. In paragraph 22, A method for manufacturing a porous film, characterized in that the metal oxide is coated to a thickness of 2 to 50 nm.

24. In paragraph 11, A method for producing a porous film, characterized in that the third step comprises washing the metal oxide-nanocellulose through centrifugation, vacuum filtration or osmotic filtration.

25. In paragraph 11, A method for manufacturing a porous film, characterized in that the cleaning solvent of the third step is selected from the group consisting of methanol, ethanol, isopropyl alcohol, butanol, NMP (N-Methyl-2-Pyrrolidone), acetonitrile, and water.

26. In paragraph 11, A method for producing a porous film, characterized in that the metal oxide-nanocellulose of the fourth step is dispersed in an amount of 0.01 to 5 wt% based on the total mass of the dispersion.

27. In paragraph 11, A method for manufacturing a porous film, characterized in that the organic solvent of the fourth step is selected from the group consisting of acetone, methanol, ethanol, isopropyl alcohol, butanol, NMP (N-Methyl-2-Pyrrolidone), and acetonitrile.

28. In paragraph 11, A method for producing a porous film, characterized in that the dispersion of the fourth step further comprises at least one additive selected from the group consisting of acrylic, polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyvanillin oxalate (PVO), styrene butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyurethane (PU), and polyvinylidene fluoride (PVDF).

29. In paragraph 11, A method for manufacturing a porous film, characterized in that the film forming process of the fourth step is a vacuum filtration, coating or casting process.

30. A separator for a secondary battery, characterized by including a porous film according to Article 9.

31. An electrolyte for an all-solid-state battery, characterized by including a porous film according to Article 9.

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