Cellulose-based separator, method for preparing the same, and lithium battery

Abstract

The application discloses a cellulose diaphragm, a preparation method thereof and a lithium battery, and relates to the technical field of battery diaphragms. In the process of forming a film by using a cellulose dispersion liquid, calcium carbonate is introduced, and the porosity and pore size are adjusted by a calcium carbonate pore forming method; acid liquid containing chitosan is used for reaction, chitosan is introduced while the calcium carbonate is removed, the strength of the diaphragm can be increased, the migration of anions can be inhibited, and the electrochemical performance of the diaphragm is improved. The prepared lithium ion battery diaphragm has a porosity of 60-70%, a tensile strength greater than 45 MPa, an ion conductivity greater than 1.7 mS cm ‑1 , and a lithium ion migration number greater than 0.5.

Description

Technical Field

[0001] This invention relates to the field of battery separator technology, and more specifically, to a cellulose-based separator, its preparation method, and a lithium battery. Background Technology

[0002] As a key component of lithium-ion batteries, the separator largely determines the battery's performance. Environmentally friendly cellulose-based separators possess numerous advantages, including excellent electrolyte wettability and thermal stability, and hold promise as a replacement for currently commercially available polyolefin separators. Among these, nanocellulose separators prepared using a papermaking wet-forming process have attracted significant attention. However, nanocellulose is typically prepared using water as the dispersion medium. During the papermaking drying process, the evaporation of water causes the fibers to stretch and shrink, resulting in a denser separator with a significantly reduced porosity, making it unsuitable for use as a lithium-ion battery separator.

[0003] Currently, the main approach to preserving the pore structure after drying is to modify the dispersion medium. For example, isopropanol / water solutions with a high isopropanol content are used as the dispersion medium, or water in the wet membrane is replaced by immersion in anhydrous ethanol. However, the porosity of membranes prepared in this way is limited, reaching a maximum that can only be achieved by pure solvent replacement. Furthermore, introducing template-based pore-forming methods can effectively improve membrane porosity. For instance, some researchers have used monodisperse polystyrene spheres as templates to dope cellulose membranes, and then removed the polystyrene spheres by immersion in a toluene bath to adjust the membrane pore size and porosity. However, current template-based pore-forming methods suffer from problems such as high porosity but decreased strength, and easy adjustment of porosity but difficulty in adjusting pore size, making it difficult to further improve membrane performance. Moreover, the process is often environmentally unfriendly and expensive.

[0004] Therefore, there is an urgent need to provide a green and environmentally friendly preparation process for cellulose membranes, while better adjusting porosity and pore size to optimize membrane performance.

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

[0006] The purpose of this invention is to provide a cellulose-based separator, its preparation method, and a lithium battery, aiming to provide a green and environmentally friendly preparation process for cellulose-based separators, while better adjusting porosity and pore size to optimize separator performance.

[0007] This invention is implemented as follows:

[0008] In a first aspect, the present invention provides a method for preparing a cellulose-based membrane, comprising:

[0009] Calcium carbonate is introduced during the film formation process using cellulose dispersion to obtain a cellulose membrane containing calcium carbonate particles.

[0010] A cellulose membrane containing calcium carbonate particles is mixed with an acidic solution containing chitosan and reacted.

[0011] In an optional embodiment, calcium carbonate is introduced during the preparation of a cellulose membrane containing calcium carbonate particles by reacting calcium salts and carbonates to generate calcium carbonate.

[0012] In an optional embodiment, the process of preparing a cellulose membrane containing calcium carbonate particles includes: mixing and dissolving calcium salt and cellulose dispersion to obtain a mixed dispersion, spraying a carbonate solution onto the surface of the mixed dispersion, stirring and reacting, and then filtering to form a membrane.

[0013] In an optional implementation, a carbonate solution is sprayed intermittently using an atomizing device;

[0014] And / or, the calcium salt is selected from at least one of calcium chloride and calcium nitrate;

[0015] And / or, the carbonate is selected from at least one of sodium carbonate and potassium carbonate;

[0016] And / or, the molar ratio of calcium salt to carbonate is (0.95-1.00):1;

[0017] And / or, control the mass ratio of the generated calcium carbonate to the dry weight of cellulose to be (0.5-2.0):1.

[0018] In an optional embodiment, calcium carbonate is introduced into the cellulose membrane containing calcium carbonate particles by directly adding calcium carbonate into the cellulose dispersion.

[0019] In an optional embodiment, calcium carbonate particles and cellulose dispersion are mixed and then filtered to form a film.

[0020] And / or, control the mass ratio of added calcium carbonate to cellulose dry weight to be (0.5-2.0):1;

[0021] And / or, the average particle size of calcium carbonate particles is 100 nm-5 μm.

[0022] In an optional embodiment, the acidic solution containing chitosan is a chitosan acetic acid solution; the mass fraction of chitosan in the chitosan acetic acid solution is 0.1%-2.5%, and the mass fraction of acetic acid is 0.5%-2%.

[0023] And / or, 1g of the cellulose membrane containing calcium carbonate particles corresponds to 5mg-50mg of chitosan;

[0024] And / or, control the reaction time of the cellulose membrane containing calcium carbonate particles with the acid solution containing chitosan to be 10 min-90 min.

[0025] In an optional embodiment, after the reaction with the acidic solution containing chitosan is complete, the mixture is transferred to an alkaline solution for acid-base neutralization, then transferred to an alcoholic solvent for solvent replacement, and finally dried.

[0026] The alkaline solution is at least one of ammonia solution and lithium hydroxide solution;

[0027] The alcohol solvent is ethanol.

[0028] Secondly, the present invention provides a cellulose-based membrane, which is prepared by any of the preparation methods described in the foregoing embodiments;

[0029] Cellulose membranes have a porosity of 60%-70%, and the maximum difference in pore size between the main pores is less than 400 nm.

[0030] Cellulose membranes have a tensile strength greater than 45 MPa and an ionic conductivity greater than 1.7 mS / cm. -1 The lithium-ion transference number is greater than 0.5, and the size change rate is less than 0.1% after 3 hours at 200℃.

[0031] Thirdly, the present invention provides a lithium battery comprising the cellulose-based separator described in the foregoing embodiments.

[0032] The present invention has the following beneficial effects: In the process of forming a film using cellulose dispersion, calcium carbonate is introduced, and the porosity and pore size are adjusted by calcium carbonate pore-forming method; by using an acid reaction containing chitosan, chitosan is introduced while removing calcium carbonate, which can increase the membrane strength and inhibit anion migration, thereby improving the electrochemical performance of the membrane. Attached Figure Description

[0033] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 The images show the microstructure of the membranes in the examples and comparative examples after being soaked in acetic acid solution; (a) represents comparative example 1, (b) represents example 1, (c) represents example 2, and (d) represents example 3. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0036] This invention uses nanocellulose as raw material and, based on the displacement of the dispersion medium, adjusts the porosity and pore size through the calcium carbonate pore-forming method. While removing calcium carbonate, a trace amount of chitosan is introduced to increase the membrane strength and inhibit anion migration, thereby improving the electrochemical performance of the membrane.

[0037] This invention provides a method for preparing a cellulose-based membrane, comprising the following steps:

[0038] S1. Preparation of cellulose membrane containing calcium carbonate particles

[0039] Calcium carbonate is introduced during the preparation of cellulose membrane using cellulose dispersion, so that the prepared cellulose membrane contains calcium carbonate particles, resulting in a cellulose membrane containing calcium carbonate particles.

[0040] In the process of preparing cellulose membranes containing calcium carbonate particles, there are two ways to introduce calcium carbonate: (1) calcium carbonate is introduced by reacting calcium salt and carbonate to generate calcium carbonate; (2) calcium carbonate is introduced by directly adding calcium carbonate to the cellulose dispersion.

[0041] When calcium carbonate is introduced using method (1), the process for preparing a cellulose membrane containing calcium carbonate particles includes: adding a certain mass of soluble calcium salt to a cellulose dispersion and stirring until completely dissolved to obtain a mixed dispersion; dissolving a certain mass of soluble carbonate to obtain a carbonate solution, and uniformly spraying the carbonate solution onto the surface of the mixed dispersion to generate CaCO3 crystals. Stirring is continued throughout the reaction to ensure complete crystallization. The resulting dispersion is then filtered to prepare a membrane.

[0042] In some embodiments, the type of nanocellulose used in preparing the cellulose dispersion is not limited, such as nanocellulose prepared from plant fibers, bacterial cellulose, or cellulose-based nanofibers obtained by processes such as electrospinning.

[0043] Soluble calcium salts are soluble calcium salts that can react with carbonate ions to form CaCO3 crystals, and soluble carbonates are soluble carbonates that can react with calcium ions to form CaCO3 crystals.

[0044] In some embodiments, the soluble calcium salt is selected from at least one of calcium chloride and calcium nitrate, and the soluble calcium salt can be any one or more of the above; the soluble carbonate is selected from at least one of sodium carbonate and potassium carbonate, and the soluble carbonate can be any one or more of the above. The molar ratio of soluble calcium salt to soluble carbonate is (0.95-1.00):1, such as 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1.00:1, etc. By adjusting the amount of nanocellulose, soluble calcium salt, and soluble carbonate, the mass ratio of the generated calcium carbonate to the dry weight of cellulose is controlled to be (0.5-2.0):1, such as 0.5:1, 0.8:1, 1.0:1, 1.3:1, 1.5:1, 1.8:1, 2.0:1, etc.

[0045] In some embodiments, an atomizing device can be used to intermittently spray the carbonate solution to ensure uniform spraying and improve the uniformity of the distribution of the generated calcium carbonate particles. Specifically, the type of atomizing device is not limited.

[0046] When introducing calcium carbonate using method (2), the required quantity and size of CaCO3 particles can be directly added to the nanocellulose dispersion and mixed evenly according to actual needs, and then filtered to form a film.

[0047] In some embodiments, the amount of calcium carbonate particles added is consistent with the theoretical amount of calcium carbonate generated in method (1), and the mass ratio of the dry weight of the added calcium carbonate to cellulose is controlled to be (0.5-2.0):1, such as 0.5:1, 0.8:1, 1.0:1, 1.3:1, 1.5:1, 1.8:1, 2.0:1, etc. The average particle size of the added calcium carbonate particles is 100nm-5μm, such as 100nm, 500nm, 1000nm, 2μm, 3μm, 4μm, 5μm, etc. By adjusting the size and amount of calcium carbonate particles, the pores formed after the calcium carbonate dissolves in step S2 are more suitable, which is beneficial to improving the electrochemical performance of the membrane.

[0048] It should be noted that in this embodiment of the invention, a cellulose membrane is formed by vacuum filtration. During the filtration process, the first deposited fibers form a substrate, and subsequent fibers are stacked on top of each other one by one. The fibers intertwine and entangle with each other through strong hydrogen bonds. Finally, after all the dispersion has been filtered, a "wet membrane" with a high water content but a three-dimensional network structure is formed on the filter membrane.

[0049] S2, Introduction of chitosan

[0050] The cellulose membrane containing calcium carbonate particles prepared in step S1 is placed in an acidic solution containing chitosan to dissolve the CaCO3 in the membrane and generate a new pore structure.

[0051] In some embodiments, the chitosan-containing acid solution is a chitosan-acetic acid solution, obtained by uniformly mixing chitosan, acetic acid, and water. The mass fraction of chitosan in the chitosan-acetic acid solution is 0.1%-2.5%, such as 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, etc.; the mass fraction of acetic acid is 0.5%-2%, such as 0.5%, 1.0%, 1.5%, 2.0%, etc. 1g of the cellulose membrane containing calcium carbonate particles corresponds to 5mg-50mg of chitosan, such as 5mg, 10mg, 20mg, 30mg, 40mg, 50mg, etc. The reaction time between the cellulose membrane containing calcium carbonate particles and the chitosan-containing acid solution is controlled to be 10min-90min, such as 10min, 30min, 50min, 70min, 90min, etc., to ensure complete dissolution of calcium carbonate.

[0052] S3, Post-processing

[0053] After the reaction with an acidic solution containing chitosan is complete, the resulting porous membrane is transferred to an alkaline solution for acid-base neutralization, followed by solvent replacement in an alcohol solvent. Finally, the porous membrane is removed and dried to obtain a high-performance cellulose-based lithium-ion battery separator product. The residual acid on the surface is removed by reacting with an alkaline solution, and then the membrane is transferred to an alcohol solvent where water is replaced with ethanol through solvent replacement, preventing excessive pore shrinkage caused by water evaporation during drying.

[0054] In some embodiments, the alkaline solution is at least one of an ammonia solution and a lithium hydroxide solution, and the alkaline solution can be any one or more of the above. The alcohol solvent can be ethanol, but is not limited thereto.

[0055] It should be noted that the preparation method of the cellulose membrane provided by the present invention has the following advantages: (1) The preparation method of the membrane provided by the present invention is simple, green and environmentally friendly, and the membrane is regenerable and degradable; (2) By controlling the number and size of CaCO3 crystals formed, the porosity and pore size of the membrane can be adjusted simultaneously to optimize the membrane performance; (3) Chitosan acetate solution precipitates chitosan molecules while dissolving CaCO3 crystals, which on the one hand makes up for the defects of nanocellulose membrane, and on the other hand strengthens the membrane through electrostatic adsorption and hydrogen bonding, greatly increasing the membrane strength. At the same time, chitosan can also inhibit the migration of lithium salt anions and increase the lithium ion migration number.

[0056] This invention also provides a cellulose-based membrane, prepared by the method provided in this invention, which has uniform and adjustable pore size, as well as good mechanical strength, excellent lithium-ion transference number, and ionic conductivity.

[0057] In some embodiments, the porosity of the cellulose membrane is 60%-70%, such as 60%, 63%, 65%, 68%, 70%, etc.; the maximum difference in pore size between the main pores is less than 400 nm, and the pore size is adjustable and uniform. The tensile strength of the cellulose membrane is greater than 45 MPa, and the ionic conductivity is greater than 1.7 mS / cm. -1 The lithium-ion transference number is greater than 0.5, and the size change rate is less than 0.1% after 3 hours at 200℃.

[0058] This invention also provides a lithium battery, including a cellulose-based separator provided in this invention. By improving the separator, the electrochemical performance of the lithium battery can be enhanced.

[0059] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0060] Example 1

[0061] This embodiment provides a method for preparing a cellulose-based membrane, the steps of which are as follows:

[0062] (1) Mix 0.02 g (dry basis weight of cellulose) of bacterial cellulose with a diameter of approximately 20-60 nm with 50 mL of water and stir until homogeneous to obtain a bacterial cellulose dispersion. Add 0.011 g of calcium chloride to the bacterial cellulose dispersion and stir until fully dissolved. Then, mix and dissolve 0.011 g of sodium carbonate with 10 mL of water and spray intermittently onto the surface of the dispersion using an atomizing device to generate CaCO3 crystals. Stir continuously during the reaction for 30 min to ensure complete crystallization. Then, prepare a film by filtration of the obtained dispersion. In this example, the mass ratio of calcium carbonate generated to the dry weight of bacterial cellulose is 0.5:1.

[0063] (2) Chitosan, acetic acid, and water are mixed to obtain a chitosan-acetic acid solution, wherein the mass fraction of chitosan is 0.5% and the mass fraction of acetic acid is 1%. The membrane prepared by filtration in step (1) is placed in 100 mL of the chitosan-acetic acid solution (i.e., 1 g of the cellulose membrane containing calcium carbonate particles corresponds to approximately 0.01 g of chitosan), allowing the CaCO3 in the membrane to dissolve and generate a new pore structure. After sufficient reaction, the membrane is transferred to a 1% ammonia solution for acid-base neutralization for 20 min, and then transferred to anhydrous ethanol for solvent replacement for 12 h. Finally, the porous membrane is removed and dried to obtain the high-performance cellulose-based lithium-ion battery separator.

[0064] Example 2

[0065] This embodiment provides a method for preparing a cellulose-based membrane, the steps of which are as follows:

[0066] (1) Mix 0.02g of bacterial cellulose with a diameter of approximately 20-60nm with 50mL of water and stir until homogeneous to obtain a bacterial cellulose dispersion. Add 0.022g of calcium chloride to the bacterial cellulose dispersion and stir until fully dissolved. Then dissolve 0.022g of sodium carbonate and spray it intermittently onto the surface of the dispersion using an atomizing device to generate CaCO3 crystals. Stir continuously during the reaction for 30min to ensure complete crystallization. The resulting dispersion is then filtered to prepare a membrane. In this embodiment, the mass ratio of calcium carbonate generated to the dry weight of bacterial cellulose is 1:1.

[0067] (2) Chitosan, acetic acid, and water are mixed to obtain a chitosan-acetic acid solution, wherein the mass fraction of chitosan is 0.5% and the mass fraction of acetic acid is 1%. The membrane prepared by filtration in step (1) is placed in 100 mL of the chitosan-acetic acid solution (i.e., 1 g of the cellulose membrane containing calcium carbonate particles corresponds to approximately 0.01 g of chitosan), allowing the CaCO3 in the membrane to dissolve and generate a new pore structure. After sufficient reaction, the membrane is transferred to a 1% ammonia solution for acid-base neutralization for 20 min, and then transferred to anhydrous ethanol for solvent replacement for 12 h. Finally, the porous membrane is removed and dried to obtain the high-performance cellulose-based lithium-ion battery separator.

[0068] Example 3

[0069] This embodiment provides a method for preparing a cellulose-based membrane, the steps of which are as follows:

[0070] (1) Mix 0.02g of bacterial cellulose with a diameter of approximately 20-60nm with 50mL of water and stir until homogeneous to obtain a bacterial cellulose dispersion. Add 0.044g of calcium chloride to the bacterial cellulose dispersion and stir until fully dissolved. Then, mix and dissolve 0.044g of sodium carbonate with 10mL of water and spray intermittently onto the surface of the dispersion using an atomizing device to generate CaCO3 crystals. Stir continuously during the reaction for 30min to ensure complete crystallization. The resulting dispersion is then filtered to prepare a membrane. In this embodiment, the mass ratio of calcium carbonate generated to the dry weight of bacterial cellulose is 2:1.

[0071] (2) Chitosan, acetic acid, and water are mixed to obtain a chitosan-acetic acid solution, wherein the mass fraction of chitosan is 0.5% and the mass fraction of acetic acid is 1%. The membrane prepared by filtration in step (1) is placed in 100 mL of the chitosan-acetic acid solution (i.e., 1 g of the cellulose membrane containing calcium carbonate particles corresponds to approximately 0.01 g of chitosan), allowing the CaCO3 in the membrane to dissolve and generate a new pore structure. After sufficient reaction, the membrane is transferred to a 1% ammonia solution for acid-base neutralization for 20 min, and then transferred to anhydrous ethanol for solvent replacement for 12 h. Finally, the porous membrane is removed and dried to obtain the high-performance cellulose-based lithium-ion battery separator.

[0072] Example 4

[0073] This embodiment provides a method for preparing a cellulose-based membrane, the steps of which are as follows:

[0074] (1) Mix 0.02g of bacterial cellulose with a diameter of approximately 20-60nm with 50mL of water and stir until homogeneous to obtain a bacterial cellulose dispersion. Add 0.022g of calcium chloride to the bacterial cellulose dispersion and stir until fully dissolved. Then, mix and dissolve 0.022g of sodium carbonate with 10mL of water and spray intermittently onto the surface of the dispersion using an atomizing device to generate CaCO3 crystals. Stir continuously during the reaction for 30min to ensure complete crystallization. The resulting dispersion is then filtered to prepare a membrane. In this embodiment, the mass ratio of calcium carbonate generated to the dry weight of bacterial cellulose is 1:1.

[0075] (2) Chitosan, acetic acid, and water were mixed to obtain a chitosan-acetic acid solution, wherein the mass fraction of chitosan was 1% and the mass fraction of acetic acid was 1%. The membrane prepared by filtration in step (1) was placed in 100 mL of the chitosan-acetic acid solution (i.e., 1 g of cellulose membrane containing calcium carbonate particles corresponds to approximately 0.02 g of chitosan) to dissolve the CaCO3 in the membrane and generate a new pore structure. After sufficient reaction, the membrane was transferred to a 1% ammonia solution for acid-base neutralization for 20 min, and then transferred to anhydrous ethanol for solvent replacement for 12 h. Finally, the porous membrane was removed and dried to obtain the high-performance cellulose-based lithium-ion battery separator.

[0076] Example 5

[0077] The only difference from Example 2 is that in step (1), the mass ratio of calcium carbonate production to bacterial cellulose dry weight is controlled to be 0.8:1. Specifically, the amount of bacterial cellulose is kept constant, while the amounts of calcium chloride and sodium carbonate are adjusted, and the concentration of the bacterial cellulose dispersion is kept consistent.

[0078] Example 6

[0079] The only difference from Example 2 is that in step (1), the mass ratio of calcium carbonate production to bacterial cellulose dry weight is controlled to be 1.5:1. Specifically, the amount of bacterial cellulose is kept constant, while the amounts of calcium chloride and sodium carbonate are adjusted, and the concentration of the bacterial cellulose dispersion is kept consistent.

[0080] Example 7

[0081] The only difference from Example 2 is that the mass fraction of chitosan in the chitosan acetate solution is 0.8%, so that 1g of cellulose membrane containing calcium carbonate particles corresponds to approximately 0.016g of chitosan.

[0082] Example 8

[0083] The only difference from Example 2 is that the mass fraction of chitosan in the chitosan acetate solution is 1.2%, so that 1g of cellulose membrane containing calcium carbonate particles corresponds to approximately 0.024g of chitosan.

[0084] Comparative Example 1

[0085] The only difference from Example 2 is that calcium chloride and sodium carbonate are not added in step (1), and the bacterial cellulose dispersion is directly prepared into a membrane by vacuum filtration. The specific steps of step (2) are the same as in Example 1, as follows: chitosan, acetic acid and water are mixed to obtain a chitosan-acetic acid solution, wherein the mass fraction of chitosan is 0.5% and the mass fraction of acetic acid is 1%. The membrane prepared by vacuum filtration in step (1) is placed in 100 mL of chitosan-acetic acid solution. Then the membrane is transferred to a 1% ammonia solution for acid-base neutralization for 20 min, and then transferred to anhydrous ethanol for solvent replacement for 12 h. Finally, the porous membrane is taken out and dried to obtain the required comparison membrane.

[0086] Comparative Example 2

[0087] The only difference from Example 2 is that step (2) does not react with the chitosan acetate solution. That is, step (1) is the same as in Example 2, and the steps of step (2) are as follows: the membrane prepared by vacuum filtration is transferred into anhydrous ethanol for solvent replacement (the replacement time is the same as in Example 2), and finally the porous membrane is taken out and dried to obtain the required comparison membrane.

[0088] Comparative Example 3

[0089] The only difference from Example 2 is that the chitosan acetate solution in step (2) is replaced with an acetic acid solution. That is, step (1) is the same as in Example 2, and step (2) is as follows: The membrane prepared by vacuum filtration is placed in 100 mL of a 1% (w / w) aqueous acetic acid solution to dissolve the CaCO3 in the membrane and generate a new pore structure. After sufficient reaction, the membrane is transferred to a 1% (w / w) ammonia solution for acid-base neutralization for 20 min, and then transferred to anhydrous ethanol for solvent replacement for 12 h. Finally, the porous membrane is removed and dried to obtain the desired comparative membrane.

[0090] Comparative Example 4

[0091] The only difference from Example 2 is that in step (1), the mass ratio of calcium carbonate production to bacterial cellulose dry weight is controlled to be 0.2:1. Specifically, the amount of bacterial cellulose is kept constant, while the amounts of calcium chloride and sodium carbonate are adjusted, and the concentration of the bacterial cellulose dispersion is kept consistent.

[0092] Comparative Example 5

[0093] The only difference from Example 2 is that in step (1), the mass ratio of calcium carbonate production to bacterial cellulose dry weight is controlled at 3.0:1. Specifically, the amount of bacterial cellulose is kept constant, while the amounts of calcium chloride and sodium carbonate are adjusted, and the concentration of the bacterial cellulose dispersion is kept consistent.

[0094] Comparative Example 6

[0095] The only difference from Example 2 is that the mass fraction of chitosan in the chitosan acetate solution is 0.05%, so that 1g of cellulose membrane containing calcium carbonate particles corresponds to approximately 0.002g of chitosan.

[0096] Comparative Example 7

[0097] The only difference from Example 2 is that the mass fraction of chitosan in the chitosan acetate solution is 2.6%, so that 1g of cellulose membrane containing calcium carbonate particles corresponds to approximately 0.058g of chitosan.

[0098] Figure 1 The images show the microstructure of the diaphragm products obtained in the examples and comparative examples, where (a) represents Comparative Example 1, (b) represents Example 1, (c) represents Example 2, and (d) represents Example 3. Figure 1 As can be seen, the membrane prepared in Example 1 had almost no calcium carbonate residue after acetic acid treatment, while the membranes prepared in Examples 2 and 3 had obvious nano-sized particles attached after acetic acid treatment, and the pore size of the membranes gradually increased with the increase of the initial calcium carbonate ratio.

[0099] Experimental Example 1

[0100] The physical and electrochemical properties of various cellulose-based lithium-ion battery separators prepared in Examples 1-8 and Comparative Examples 1-7 were compared, and the results are shown in Table 1.

[0101] The ionic conductivity and lithium-ion transference number were tested after the battery separator was further assembled into a battery: R2032 was used as the standard test battery. Cut lithium iron phosphate positive electrode sheets (LF0609), the prepared battery separator, and graphite negative electrode sheets (SM0206) were concentrically stacked and placed in an R2032 battery case. 30 μL of lithium-ion battery electrolyte (model LFP01, lithium hexafluorophosphate, dissolved as EC (ethylene carbonate):DMC (dimethyl carbonate) = 1:1 (volume ratio)) was added, and the battery was then sealed under pressure to obtain the lithium-ion battery required for testing. The positive electrode sheet, negative electrode sheet, battery case, and electrolyte were all purchased from KELOD Technology Co., Ltd.

[0102] The ionic conductivity was measured by assembling a coin cell with a separator in a structure of negative electrode shell / steel sheet / separator / steel sheet / spring sheet / positive electrode shell, and then performing electrochemical impedance spectroscopy (EIS) using an electrochemical workstation at a frequency of 0.1 MHz–0.1 Hz. The ionic conductivity of the separator (…) σ ,mS·cm -1 It is calculated using the following formula:

[0103] ;

[0104] in, (cm) is the thickness of the diaphragm. (cm) 2 ( ) is the contact area between the steel electrode and the diaphragm. (Ω) represents the diaphragm resistance obtained from the Nyquist plot.

[0105] The lithium-ion transport number is obtained by combining potentiostatic polarization and EIS measurements, and the calculation formula is as follows:

[0106] ;

[0107] in This indicates the lithium-ion transference number after the diaphragm is immersed in the electrolyte; and These represent the initial current and steady-state current measured by the constant potential polarization method, respectively. and These represent the initial interface resistance and the steady-state interface resistance obtained through EIS testing, respectively. This represents the step potential difference (10 mV).

[0108] Table 1 Performance testing of cellulose-based battery separators in the examples and comparative examples.

[0109]

[0110] The above experiments revealed that the high-performance cellulose-based lithium-ion battery separator prepared by this invention can simultaneously adjust the porosity and pore size of the separator by controlling the number and size of CaCO3 crystals formed. Simultaneously, the chitosan acetate solution dissolves the CaCO3 crystals while precipitating chitosan molecules, which can compensate for the defects of the nanocellulose separator and strengthen the separator through electrostatic adsorption and hydrogen bonding, significantly increasing the separator strength. Furthermore, chitosan can inhibit the migration of lithium salt anions and increase the lithium-ion transference number. Comparative experiments showed that the separator strength and lithium-ion transference number were lower with only calcium carbonate pore-forming; and the porosity and ionic conductivity were lower with only chitosan added. Only a two-step treatment combined can comprehensively improve the separator performance.

[0111] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a cellulose-based membrane, characterized in that, include: Calcium carbonate is introduced during the film formation process using cellulose dispersion to obtain a cellulose membrane containing calcium carbonate particles. The mass ratio of introduced calcium carbonate to cellulose dry weight is controlled to be (0.5-2.0):1; The cellulose membrane containing calcium carbonate particles is mixed and reacted with an acidic solution containing chitosan; the acidic solution containing chitosan is a chitosan-acetic acid solution; the mass fraction of chitosan in the chitosan-acetic acid solution is 0.1%-2.5%, and the mass fraction of acetic acid is 0.5%-2%; 1g of the cellulose membrane containing calcium carbonate particles corresponds to 5mg-50mg of chitosan. In the process of preparing the cellulose membrane containing calcium carbonate particles, calcium carbonate is introduced by reacting calcium salt and carbonate to generate calcium carbonate. The process of preparing the cellulose membrane containing calcium carbonate particles includes: mixing and dissolving calcium salt and cellulose dispersion to obtain a mixed dispersion, spraying a carbonate solution onto the surface of the mixed dispersion, stirring and reacting, and then filtering to form a membrane; After the reaction with the acidic solution containing chitosan is complete, it is transferred to an alkaline solution for acid-base neutralization, then transferred to an alcoholic solvent for solvent replacement, and finally dried.

2. The preparation method according to claim 1, characterized in that, The carbonate solution is sprayed intermittently using an atomizing device; And / or, the calcium salt is selected from at least one of calcium chloride and calcium nitrate; And / or, the carbonate is selected from at least one of sodium carbonate and potassium carbonate; And / or, the molar ratio of the calcium salt to the carbonate is (0.95-1.00):

1.

3. The preparation method according to claim 1, characterized in that, The reaction time between the cellulose membrane containing calcium carbonate particles and the acidic solution containing chitosan is controlled to be 10 min-90 min.

4. The preparation method according to claim 1, characterized in that, The alkaline solution is at least one of ammonia solution and lithium hydroxide solution; The alcohol solvent is ethanol.

5. A cellulose-based membrane, characterized in that, It is prepared by the preparation method according to any one of claims 1-4; The porosity of the cellulose membrane is 60%-70%; The cellulose membrane has a tensile strength greater than 45 MPa and an ionic conductivity greater than 1.7 mS / cm. -1 The lithium-ion transference number is greater than 0.5, and the size change rate is less than 0.1% after 3 hours at 200℃.

6. A lithium battery, characterized in that, Includes the cellulose membrane of claim 5.

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