A porous cellulose-based lithium ion battery separator and a preparation method and application thereof

Abstract

The application discloses a porous cellulose-based lithium ion battery diaphragm and a preparation method and application thereof. The preparation method of the battery diaphragm comprises the following steps: first, sheet-shaped bacterial cellulose is treated by a NaOH solution to prepare a bacterial cellulose suspension, or further grafted with chitosan to obtain a bacterial cellulose suspension grafted with chitosan; then, the bacterial cellulose suspension is suction-filtered into a film and then soaked in ethanol to obtain a porous bacterial cellulose lithium ion battery diaphragm; finally, the porous bacterial cellulose lithium ion battery diaphragm is soaked in a lithium salt solution to obtain the porous cellulose-based lithium ion battery diaphragm loaded with lithium salt. The method can improve the deposition and peeling behavior of lithium ions in the lithium ion battery, and further improve the cycle and rate performance of the battery.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of battery separator materials, and particularly relates to a porous cellulose-based lithium ion battery separator and a preparation method and application thereof. BACKGROUND

[0002] Electrolyte additives play a crucial role in lithium-ion batteries. They can significantly improve the overall performance of the battery, extend the service life of the battery, increase the energy density of the battery, and enhance the safety of the battery. Among them, lithium nitrate LiNO3 is an important additive. It can play multiple functions in the electrolyte, such as stabilizing the solid electrolyte interface (SEI) layer, reducing the occurrence of side reactions, suppressing the instability of the battery during charging and discharging, thereby improving the cycle life and rate performance of the battery.

[0003] During the charging and discharging process of the battery, LiNO3 can preferentially decompose to form a high-quality SEI layer. The SEI layer is the interface layer between the battery electrode and the electrolyte, and plays a key role in the cycle performance and safety of the battery. LiNO3 can promote the formation of Li3N inorganic components in the SEI layer, which is more stable and has better conductivity than the traditional organic SEI layer, thereby reducing side reactions during battery cycling. The high mechanical strength can also inhibit the growth and penetration of lithium dendrites, extending the service life of the battery. Secondly, the stable SEI formed by LiNO3 can also inhibit the side reactions of the battery at high voltage, reducing unnecessary side reactions between the electrolyte and the electrode material. On the positive side, the formation of CEI can inhibit the loss of active materials and reduce the generation of unstable by-products, ultimately effectively improving the stability and safety of the battery.

[0004] Although LiNO3 can effectively improve the performance of lithium-ion batteries, it faces great challenges in practical application. The most important problem is that its solubility in ester-based electrolyte is low, making it difficult for LiNO3 to maintain sufficient concentration in the electrolyte, which directly limits its function in lithium-ion batteries. In order to solve this problem, it is usually necessary to increase the amount of lithium nitrate added, but the insoluble LiNO3 crystals suspended in the electrolyte not only increase the mass transfer resistance of the solution, but also may adhere to the electrode surface to hinder the redox process of lithium ions, leading to the rise of interface impedance.

[0005] In order to overcome the problem of low solubility of LiNO3 in ester electrolytes, some researchers have tried to use a sandwich structure to load lithium nitrate between two layers of PP membranes (Reference 1: S.Stuckenberg; M.M.Bela; C.T.Lechtenfeld; M.Mense; V.Kupers; T.T.Ingber; M.Winter; M.C.Stan. Influence of LiNO(3)on theLithium Metal Deposition Behavior in Carbonate-Based Liquid Electrolytes and on the Electrochemical Performance in Zero-Excess Lithium Metal Batteries. Small 2024, 20(6), e2305203. DOI: 10.1002 / smll.202305203. Reference 2: Y.Liu; X.Qin; D.Zhou; H.Xia; S.Zhang; G.Chen; F.Kang; B.Li. A biscuit-like separator enabling high performance lithium batteries by continuous and protected releasing of NO3-in carbonate Electrolyte. Energy Storage Materials 2020, 24, 229-236. DOI: 10.1016 / j.ensm.2019.08.016.). However, this approach has a significant drawback: the LiNO3 crystals in the sandwich layer are often unevenly distributed and vary in shape and size, leading to uneven lithium ion deposition. This uneven deposition can cause local short circuits and thermal runaway in the battery, seriously affecting its safety and stability.There are also some studies on loading LiNO3 in glass fiber membranes, but the glass fiber diaphragm is too thick and the pore size is too large, which is easy to form large crystals to block the pores, and the glass fiber membrane cannot promote the slow release of LiNO3, which is not conducive to its reduction on the surface of the negative electrode (Q. Shi; Y. Zhong; M. Wu; H. Wang; H. Wang. High-capacity rechargeable batteries based on deeply cyclable lithium metal anodes. Proc Natl Acad Sci US A 2018, 115(22), 5676-5680. DOI: 10.1073 / pnas.1803634115.).

[0006] Therefore, it is necessary to find a suitable way to load LiNO3, so that it can be fixed in the electrolyte and uniformly distributed, and the diaphragm can promote the dissolution and slow release of lithium nitrate, and finally improve the performance of the battery. SUMMARY

[0007] The primary purpose of the present application is to overcome the shortcomings and deficiencies of the prior art and provide a preparation method of a porous cellulose-based lithium ion battery diaphragm.

[0008] Another purpose of the present application is to provide a porous cellulose-based lithium ion battery diaphragm prepared by the method.

[0009] Still another purpose of the present application is to provide an application of the porous cellulose-based lithium ion battery diaphragm.

[0010] The purpose of the present application is achieved by the following technical solutions:

[0011] A preparation method of a porous cellulose-based lithium ion battery diaphragm, comprising the following steps:

[0012] (1) Preparation of bacterial cellulose suspension

[0013] The sheet-shaped bacterial cellulose is cut into small pieces, washed with water, and then boiled in NaOH solution for 1-1.5 h to clean the residual medium and impurities; then washed with water, neutralized with CH3COOH solution, and then washed with water to neutral, and finally fiberized into slurry with a homogenizer to obtain a bacterial cellulose suspension;

[0014] (2) Bacterial cellulose suspension grafted with chitosan

[0015] NaIO4 is added to the bacterial cellulose suspension obtained in step (1), and after deoxidation by nitrogen, the reaction is stirred in the dark at 50±2℃. After the reaction is completed, ethylene glycol is added to terminate the reaction, and an oxidized bacterial cellulose solution is obtained. Chitosan powder is added to a CH3COOH solution, and the solution is stirred to dissolve at 60-70℃ to form a transparent chitosan solution. The oxidized bacterial cellulose solution is then added to the chitosan solution, and the reaction is stirred at 50-60℃. After the reaction is completed, centrifugal washing is performed, and a bacterial cellulose suspension grafted with chitosan is obtained.

[0016] (3) Preparation of porous bacterial cellulose lithium ion battery separator

[0017] The bacterial cellulose suspension obtained in step (1) and / or the bacterial cellulose suspension grafted with chitosan obtained in step (2) is diluted with water and filtered to form a film, and a wet porous cellulose-based separator is obtained. The porous cellulose-based separator is then soaked in pure ethanol, so that the water in the porous cellulose-based separator is completely replaced by ethanol, and finally dried at 75-100℃ to obtain a porous bacterial cellulose lithium ion battery separator.

[0018] (4) Battery separator loaded with lithium salt

[0019] A lithium salt is added to ethanol, stirred to dissolve, and prepared into a lithium salt solution with a mass concentration of 0.5-2.5%. The porous bacterial cellulose lithium ion battery separator obtained in step (3) is then soaked in the lithium salt solution, taken out after soaking, and dried at 90-120℃ to obtain a porous cellulose-based lithium ion battery separator loaded with lithium salt. The lithium salt is at least one of lithium nitrate, lithium tetrafluoroborate, and lithium perchlorate.

[0020] In step (1), the sheet-shaped bacterial cellulose is cut into small pieces with a size of (2-5)*(2-5) cm. Preferably, the sheet-shaped bacterial cellulose is cut into small pieces with a size of 3*3 cm.

[0021] The concentration of the NaOH solution in step (1) is 0.05-0.2 mol / L. Preferably, the concentration of the NaOH solution is 0.1 mol / L.

[0022] The concentration of the CH3COOH solution in step (1) is 0.3±0.1% by volume.

[0023] The homogenizer in step (1) is a high-speed homogenizer with a speed of 25000-30000 rpm.

[0024] The concentration of the bacterial cellulose suspension in step (1) is 0.4-0.5% by mass.

[0025] The amount of NaIO4 added in step (2) is calculated based on its final concentration in the reaction system of 0.1-0.2 mol / L; preferably based on its final concentration in the reaction system of 0.15 mol / L.

[0026] In step (2), the stirring reaction conditions under light shielding and 50±2℃ are as follows: rotation speed 300-500 rpm / min, and stirring reaction time 2-2.5 h.

[0027] The amount of ethylene glycol used in step (2) is 5-6% of the volume of the reaction system.

[0028] The concentration of the chitosan solution in step (2) is 0.02-0.03 g / ml; preferably 0.02 g / ml.

[0029] The chitosan in step (2) is medium viscosity chitosan, with a viscosity of 200-400 mPa.s.

[0030] The concentration of the CH3COOH solution in step (2) is 2-3% by volume; preferably 2% by volume.

[0031] The stirring and dissolving time in step (2) is 1-2 h.

[0032] In step (2), the stirring reaction time at 50-60℃ is 2-3 h.

[0033] The number of centrifugal washings in step (2) is 4-6 times.

[0034] The concentration of the bacterial cellulose suspension grafted with chitosan in step (2) is 0.4-0.5% by mass.

[0035] The dilution in step (3) is 10-20 times by water.

[0036] The soaking time in step (3) is 3-4 h.

[0037] The drying in step (3) is by a drum dryer for 5-10 min.

[0038] The thickness of the porous bacterial cellulose lithium ion battery separator in step (3) is 0.8±0.1 cm.

[0039] The lithium salt in step (4) is preferably lithium nitrate.

[0040] The stirring time in step (4) is 1-2 h; preferably 1 h.

[0041] The soaking time in step (4) is 1-2 h; preferably 2 h.

[0042] The drying temperature in step (4) is preferably 110°C, and the drying is performed in a vacuum drying oven until the mass does not change significantly.

[0043] A porous cellulose-based lithium ion battery separator prepared by the method of any one of the above.

[0044] The porous cellulose-based lithium ion battery separator has a three-dimensional porous network structure, a porosity of 20% to 80% (preferably 31 to 65%), and a separator thickness of 14 μm to 100 μm (preferably 16 to 32 μm).

[0045] Use of the porous cellulose-based lithium ion battery separator in a lithium ion battery.

[0046] A lithium ion battery comprising a positive electrode, a negative electrode, the above porous cellulose-based lithium ion battery separator, and an electrolyte.

[0047] The positive electrode, the electrolyte, and the negative electrode are conventional positive electrodes, electrolytes, and negative electrodes in the art, and can be assembled into a button cell or a soft pack cell using existing conventional battery devices.

[0048] The positive electrode is composed of an active material having the ability to intercalate and deintercalate lithium, a conductive agent, a binder, and a current collector; wherein,

[0049] The active material includes, but is not limited to, at least one of lithium cobaltate, nickel-cobalt-manganese ternary material, nickel-cobalt-aluminum ternary material, lithium iron phosphate, and lithium manganate; and is preferably lithium iron phosphate;

[0050] The conductive agent includes, but is not limited to, at least one of acetylene black, carbon black, graphite, and carbon fiber; and is preferably acetylene black;

[0051] The binder includes, but is not limited to, at least one of polyvinylidene fluoride and polyacrylic acid; and is preferably polyvinylidene fluoride;

[0052] The current collector is an aluminum foil.

[0053] The method for preparing the positive electrode is as follows: a slurry of 80 wt% lithium iron phosphate, 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride after blending is coated on an aluminum foil, then dried in an oven at 60±5°C, and then cut into a positive electrode sheet.

[0054] The drying time is 5 to 8 h; and is preferably 6 h.

[0055] The size of the positive electrode sheet can be cut according to actual needs, such as assembled into a button cell R2032, and the radius of the positive electrode sheet is 0.6 cm.

[0056] The negative electrode can be made of conventional negative electrode materials in the art, such as metal lithium (sheet), or is composed of active materials capable of embedding and embedding lithium ions, conductive agents, binders and current collectors.

[0057] The negative electrode active material includes but is not limited to one or more of graphite, silicon-based materials and lithium titanate;

[0058] The conductive agent includes but is not limited to acetylene black, carbon black, graphite and carbon fiber; preferably acetylene black;

[0059] The binder includes but is not limited to carboxymethyl cellulose and butadiene rubber;

[0060] The current collector is a copper foil.

[0061] The electrolyte plays a role of conducting lithium ions in the lithium ion battery, and is usually composed of lithium salt and solvent; wherein,

[0062] The lithium salt includes but is not limited to one or more of lithium hexafluorophosphate (LiPF6), lithium bis-trifluoromethylsulfonylimide and lithium fluorosulfonylimide; preferably lithium hexafluorophosphate;

[0063] The solvent is an ester solvent, including but not limited to one or more of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (methyl ethyl carbonate, EMC) and propylene carbonate (PC); preferably ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) are mixed in a mass ratio of 1:1:1.

[0064] The concentration of lithium salt in the electrolyte is 0.5-2 mol / L; preferably 1 mol / L.

[0065] In the present application, LiNO3 loaded in the porous cellulose-based separator can gradually release in the ester electrolyte, generate a stable solid electrolyte layer SEI on the surface of the negative electrode, reduce the occurrence of side reactions between the electrolyte and the electrode surface, and further reduce the interfacial mass transfer resistance of lithium ions, improve the deposition / exfoliation process of lithium ions, and improve the cycle and rate performance of the battery.

[0066] The present application has the following advantages and effects relative to the prior art:

[0067] 1. The present application provides a preparation method of a porous cellulose-based lithium ion battery separator capable of releasing lithium nitrate, which can load LiNO3, and affect the release of lithium nitrate, the transfer of lithium ions and the formation process of SEI on different cellulose-based functional groups, and finally effectively improve the performance of the battery.

[0068] 2. To address the disadvantage of uneven loading of traditional LiNO3, the present invention achieves uniform distribution of LiNO3 in porous cellulose-based battery separators through the interaction between characteristic functional groups on different celluloses and LiNO3, thereby reducing crystal agglomeration and lowering the impact of polarization.

[0069] 3. In order to address the defect of low porosity of conventional cellulose-based diaphragms, the present invention modifies and grafts cellulose to prepare a high-porosity cellulose-based battery diaphragm. While achieving LiNO3 coating, it increases the absorption of electrolyte, thereby promoting the rapid conduction of lithium ions.

[0070] 4. The sustained release of LiNO3 by the porous cellulose-based diaphragm prepared in the present invention keeps the concentration of LiNO3 in the electrolyte in a saturated state, thereby further affecting the reaction process at the electrode / electrolyte interface, promoting the formation of inorganic components in SEI, improving the deposition / stripping behavior of lithium ions, and ultimately improving the cycle and rate performance of the battery.

[0071] 5. The present invention prepares porous cellulose-based battery separators using three different cellulose raw materials (bacterial cellulose, bacterial cellulose grafted with chitosan, and TEMPO-oxidized cellulose). It is found that nanoscale cellulose raw materials can form separators with higher strength and more uniform pore size distribution.

[0072] 6. In the present invention, a porous cellulose-based lithium-ion battery membrane is used to load LiNO3, and the solvent ethanol is used to dissolve LiNO3. It is found that when water is replaced as the solvent for LiNO3, the porosity and liquid absorption rate of the obtained bacterial cellulose membrane will decrease; the LiNO3 concentration is 0.5% to 2.5%. If the concentration is too low, the load in the membrane will be too small, and LiNO3, as a sacrificial additive, will be difficult to maintain its effect for a long time; if the load is too large, the pores of the membrane will be blocked, resulting in increased battery polarization and affecting the transfer of lithium ions.

[0073] 7. The present invention soaks the porous cellulose membrane in a LiNO3 solution and adopts a heating in-situ recrystallization loading method to compound LiNO3 in the porous cellulose-based lithium-ion battery separator, wherein the soaking time is 1 to 2 hours, and the heating temperature during drying after removal is 90 to 120°C. If the heating temperature is too low, the solvent cannot be completely removed, thereby inducing side reactions and reducing battery performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 This is the SEM image of the porous bacterial cellulose membrane without LiNO3 loading in Example 1.

[0075] Figure 2 This is the SEM image of the porous bacterial cellulose membrane grafted with chitosan without LiNO3 loading in Example 6.

[0076] Figure 3 SEM image of the oxidized bacterial cellulose membrane with 0.5% loading of LiNO3 in Comparative Example 6.

[0077] Figure 4 SEM image of the bacterial cellulose membrane grafted with chitosan with 1.5% loading of LiNO3 in Example 8.

[0078] Figure 5 SEM image of the bacterial cellulose membrane grafted with chitosan with 3% loading of LiNO3 in Comparative Example 2. DETAILED DESCRIPTION

[0079] The present application is further described in detail by the following Examples. The Examples do not limit the scope of the present application, unless otherwise specified. The reagents, methods, and apparatuses used in the present application are those conventional in the art, unless otherwise specified. The test methods in the following Examples, unless otherwise specified, are generally carried out according to the conventional experimental conditions. The reagents and raw materials used in the present application are commercially available, unless otherwise specified.

[0080] In the present application, all percentages (%) are mass percentages (wt%) unless otherwise specified.

[0081] The sheet-like bacterial cellulose involved in the Examples in the present application is commercially available from Hainan Yide Food Co., Ltd. (obtained by fermenting a commercial bacterial cellulose fermentation strain, i.e., Xylophilus sp., for 5-7 days, and the thickness of the membrane is 3-20 mm).

[0082] Example 1

[0083] 1. Preparation of porous cellulose-based lithium ion battery separator using bacterial cellulose as raw material

[0084] (1) Preparation of bacterial cellulose suspension: cut the sheet-like bacterial cellulose into small pieces of 3*3 cm, wash with water, and then completely boil in an excess amount of 0.1 M NaOH solution for 1-1.5 h to clean residual media and impurities. After washing with distilled water for several times, neutralize with 0.3±0.1% (v / v) CH3COOH solution, and finally wash the bacterial cellulose with distilled water until the pH value reaches neutral (pH=7±0.5); then use a high-speed homogenizer to fiberize the wet small pieces of bacterial cellulose (about 30-35 g) into a slurry at a speed of 25000-30000 rpm to obtain a bacterial cellulose suspension with a concentration of 0.4-0.5 wt%.

[0085] (2) Preparation of porous bacterial cellulose lithium ion battery separator: The porous bacterial cellulose lithium ion battery separator was prepared by vacuum filtration method. The three-dimensional porous network was formed by the interlaced stacking of cellulose fibers. 6-7 g of bacterial cellulose suspension obtained in step (1) was diluted with water to 80-100 g, and then poured into a sand core funnel for filtration into a film. Subsequently, the wet porous cellulose-based separator was placed in excess pure ethanol for 3-4 h to completely replace the water in the porous cellulose-based separator with ethanol. Then the wet porous cellulose-based separator was dried at 75-100 °C with a roller dryer for 5-10 min to obtain the desired porous bacterial cellulose lithium ion battery separator, with a radius of 0.8 cm.

[0086] (3) Battery separator loaded with lithium nitrate: 0.18 g of LiNO3 was dissolved in 10 g of pure ethanol, stirred for 1 h until the solution was clear, and the porous bacterial cellulose lithium ion battery separator prepared in step (2) was soaked therein for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until the mass showed no significant change, to obtain the porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate.

[0087] 2. Assembly of button cell

[0088] The positive electrode was prepared by coating a slurry of 80 wt% lithium iron phosphate, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride on an aluminum foil, then drying in an oven at 60 °C for 6 h, and then cutting into a positive electrode sheet with a radius of 0.6 cm and a specific capacity of 150 mAh / g; the electrolyte was 60 μL of 1 M LiPF6-EC / DMC / EMC (in an argon atmosphere, first mix ethylene carbonate (EC), methyl ethyl carbonate (EMC) and dimethyl carbonate (DMC) in a mass ratio of 1:1:1, then add lithium salt lithium hexafluorophosphate (LiPF6) to dissolve and mix uniformly to prepare a 1 M LiPF6 solution); the negative electrode was a lithium metal sheet with a radius of 0.75 cm and a thickness of 0.5 mm. The separator dried in step (3) was assembled into a button cell R2032 together with the positive electrode sheet, the negative electrode and the electrolyte. The entire assembly process was carried out in an argon-filled glove box.

[0089] Example 2

[0090] 1. Preparation of porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate

[0091] (1) Preparation of bacterial cellulose suspension: The method was the same as in Example 1.

[0092] (2) Preparation of porous bacterial cellulose lithium ion battery separator: The method was the same as in Example 1.

[0093] (3) Battery separator loading with lithium nitrate: 0.36 g of LiNO3 was dissolved in 10 g of absolute ethanol, stirred for 1 h until the solution was clear, the porous bacterial cellulose lithium ion battery separator prepared in step (2) was immersed in it for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until there was no significant change in mass, to obtain the porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate.

[0094] 2. Assemble the coin cell

[0095] Assemble the coin cell in the same way as the coin cell in Example 1.

[0096] Example 3

[0097] 1. Preparation of porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate

[0098] (1) Preparation of bacterial cellulose suspension: the method is the same as in Example 1.

[0099] (2) Preparation of porous bacterial cellulose lithium ion battery separator: the method is the same as in Example 1.

[0100] (3) Battery separator loading with lithium nitrate: 0.54 g of LiNO3 was dissolved in 10 g of absolute ethanol, stirred for 1 h until the solution was clear, the porous bacterial cellulose lithium ion battery separator prepared in step (2) was immersed in it for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until there was no significant change in mass, to obtain the porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate.

[0101] 2. Assemble the coin cell

[0102] Assemble the coin cell in the same way as the coin cell in Example 1.

[0103] Example 4

[0104] 1. Preparation of porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate

[0105] (1) Preparation of bacterial cellulose suspension: the method is the same as in Example 1.

[0106] (2) Preparation of porous bacterial cellulose lithium ion battery separator: the method is the same as in Example 1.

[0107] (3) Battery separator loading lithium nitrate: 0.72 g LiNO3 was dissolved in 10 g of pure ethanol, stirred for 1 h until the solution was clear, the porous bacterial cellulose lithium ion battery separator prepared in step (2) was immersed therein for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until the mass showed no significant change, to obtain the porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate.

[0108] 2. Assemble the button cell

[0109] The button cell was assembled in the same manner as the button cell in Example 1.

[0110] Example 5

[0111] 1. Preparation of porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate

[0112] (1) Preparation of bacterial cellulose suspension: the method was the same as in Example 1.

[0113] (2) Preparation of porous bacterial cellulose lithium ion battery separator: the method was the same as in Example 1.

[0114] (3) Battery separator loading lithium nitrate: 0.72 g LiNO3 was dissolved in 10 g of pure ethanol, stirred for 1 h until the solution was clear, the porous bacterial cellulose lithium ion battery separator prepared in step (2) was immersed therein for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until the mass showed no significant change, to obtain the porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate.

[0115] 2. Assemble the button cell

[0116] The button cell was assembled in the same manner as the button cell in Example 1.

[0117] Example 6

[0118] 1. Preparation of porous cellulose-based lithium ion battery separator using bacterial cellulose grafted with chitosan as raw material

[0119] (1) Preparation of bacterial cellulose suspension: the method was the same as in Example 1.

[0120] (2) Bacterial cellulose suspension grafted with chitosan: 100 mL of the bacterial cellulose suspension prepared in step (1) was placed in a conical flask, and NaIO4 was added to a final concentration of 0.15 mol·L -1The oxidized bacterial cellulose was obtained after deoxygenation with nitrogen, stirring at 300-500 rpm / min for 2-2.5 h in the dark at 50±2 °C, and then adding 5-6 mL of ethylene glycol to terminate the reaction. 10 g of chitosan powder was dispersed in 500 mL of 2% (v / v) CH3COOH solution, and stirred at 60-70 °C for 1-2 h to form a transparent chitosan solution. The oxidized bacterial cellulose solution was added to the chitosan solution, and continuously stirred at 50-60 °C for 2-3 h, and then washed by a centrifuge for 4-6 times to obtain a modified bacterial cellulose suspension grafted with chitosan at a concentration of 0.4-0.5 wt%; wherein the viscosity of the chitosan was medium viscosity 200-400 mPa.s (purchased from Macklin Reagent Co., Ltd.) (Note: when the viscosity was 200 mPa.s, the molecular weight of the chitosan was usually around 100-200 thousand; when the viscosity was 400 mPa.s, the molecular weight of the chitosan was usually around 200-400 thousand).

[0121] (3) Preparation of porous bacterial cellulose lithium-ion battery separator: The bacterial cellulose grafted with chitosan lithium-ion battery separator was prepared by vacuum filtration method, and a three-dimensional porous network was formed by the interlaced stacking of cellulose fibers. 6-7 g of the bacterial cellulose grafted with chitosan suspension obtained in step (1) was diluted with water to 80-100 g, and then poured into a sand core funnel for filtration into a film. Subsequently, the wet separator was placed in excess pure ethanol for 3-4 h to completely replace the water in the separator with ethanol. Then the separator was dried at 75-100 °C for 5-10 min by a roller dryer to obtain the desired porous bacterial cellulose lithium-ion battery separator grafted with chitosan, and the radius of the film was 0.8 cm.

[0122] (4) Battery separator loaded with lithium nitrate: 0.18 g of LiNO3 was dissolved in 10 g of pure ethanol, and stirred for 1 h until the solution was clear. The porous bacterial cellulose lithium-ion battery separator grafted with chitosan prepared in step (3) was soaked therein for 2 h, and then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until the mass did not change significantly to obtain the bacterial cellulose lithium-ion battery separator grafted with chitosan loaded with lithium nitrate.

[0123] 2. Assembly of a coin cell battery

[0124] The coin cell battery was assembled according to the manner of assembling the coin cell battery in Example 1.

[0125] Example 7

[0126] 1. Preparation of porous bacterial cellulose lithium-ion battery separator grafted with chitosan

[0127] (1) Preparation of bacterial cellulose suspension: the method was the same as in Example 6.

[0128] (2) Bacterial cellulose suspension grafted with chitosan: Method same as example 6.

[0129] (3) Preparation of porous bacterial cellulose lithium-ion battery separator: Method same as example 6.

[0130] (4) Battery separator loaded with lithium nitrate: 0.36 g of LiN03was dissolved in 10 g of absolute ethanol, stirred for 1 h until the solution was clear, the porous and chitosan grafted bacterial cellulose lithium-ion battery separator prepared in step (3) was immersed in it for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until there was no significant change in mass, to obtain the chitosan grafted bacterial cellulose lithium-ion battery separator loaded with lithium nitrate.

[0131] 2. Assemble the button cell

[0132] The button cell was assembled in the same way as the button cell in example 1.

[0133] Example 8

[0134] 1. Preparation of porous and chitosan grafted bacterial cellulose lithium-ion battery separator

[0135] (1) Preparation of bacterial cellulose suspension: Method same as example 6.

[0136] (2) Bacterial cellulose suspension grafted with chitosan: Method same as example 6.

[0137] (3) Preparation of porous bacterial cellulose lithium-ion battery separator: Method same as example 6.

[0138] (4) Battery separator loaded with lithium nitrate: 0.54 g of LiN03was dissolved in 10 g of absolute ethanol, stirred for 1 h until the solution was clear, the porous and chitosan grafted bacterial cellulose lithium-ion battery separator prepared in step (3) was immersed in it for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until there was no significant change in mass, to obtain the chitosan grafted bacterial cellulose lithium-ion battery separator loaded with lithium nitrate.

[0139] 2. Assemble the button cell

[0140] The button cell was assembled in the same way as the button cell in example 1.

[0141] Example 9

[0142] 1. Preparation of porous and chitosan grafted bacterial cellulose lithium-ion battery separator

[0143] (1) Preparation of bacterial cellulose suspension: Method same as example 6.

[0144] (2) Bacterial cellulose suspension grafted with chitosan: Method same as Example 6.

[0145] (3) Preparation of porous bacterial cellulose lithium-ion battery separator: Method same as Example 6.

[0146] (4) Battery separator loaded with lithium nitrate: 0.72 g of LiN03was dissolved in 10 g of absolute ethanol, stirred for 1 h until the solution was clear, the porous and chitosan grafted bacterial cellulose lithium-ion battery separator prepared in step (3) was immersed in it for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until there was no significant change in mass, to obtain the chitosan grafted bacterial cellulose lithium-ion battery separator loaded with lithium nitrate.

[0147] 2. Assemble the button cell

[0148] The button cell was assembled in the same way as the button cell in Example 1.

[0149] Example 10

[0150] 1. Preparation of porous bacterial cellulose lithium-ion battery separator grafted with chitosan

[0151] (1) Preparation of bacterial cellulose suspension: Method same as Example 6.

[0152] (2) Bacterial cellulose suspension grafted with chitosan: Method same as Example 6.

[0153] (3) Preparation of porous bacterial cellulose lithium-ion battery separator: Method same as Example 6.

[0154] (4) Battery separator loaded with lithium nitrate: 0.72 g of LiN03was dissolved in 10 g of absolute ethanol, stirred for 1 h until the solution was clear, the porous and chitosan grafted bacterial cellulose lithium-ion battery separator prepared in step (3) was immersed in it for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110 °C until there was no significant change in mass, to obtain the chitosan grafted bacterial cellulose lithium-ion battery separator loaded with lithium nitrate.

[0155] 2. Assemble the button cell

[0156] The button cell was assembled in the same way as the button cell in Example 1.

[0157] Comparative Example 1

[0158] 1. Preparation of porous bacterial cellulose lithium-ion battery separator loaded with lithium nitrate

[0159] (1) Preparation of bacterial cellulose suspension: Method same as Example 1.

[0160] (2) Preparation of porous bacterial cellulose lithium ion battery separator: Method same as example 1.

[0161] (3) Battery separator loaded with lithium nitrate: 1.08 g of LiNO3 was dissolved in 10 g of absolute ethanol, stirred for 1 h until the solution was clear, the porous bacterial cellulose lithium ion battery separator obtained in the preparation of step (2) was immersed in it for 2 h, then removed and placed on a glass plate, dried in a vacuum oven at 110 °C until there was no significant change in mass, obtaining the porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate.

[0162] 2. Assembly of the coin cell

[0163] The coin cell was assembled in the same way as the coin cell of example 1.

[0164] Comparative example 2

[0165] 1. Preparation of porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate

[0166] (1) Preparation of bacterial cellulose suspension: Method same as example 6.

[0167] (2) Bacterial cellulose suspension grafted with chitosan: Method same as example 6.

[0168] (3) Preparation of porous bacterial cellulose lithium ion battery separator: Method same as example 6.

[0169] (4) Battery separator loaded with lithium nitrate: 1.08 g of LiNO3 was dissolved in 10 g of absolute ethanol, stirred for 1 h until the solution was clear, the porous bacterial cellulose lithium ion battery separator grafted with chitosan obtained in the preparation of step (3) was immersed in it for 2 h, then removed and placed on a glass plate, dried in a vacuum oven at 110 °C until there was no significant change in mass, obtaining the porous bacterial cellulose lithium ion battery separator grafted with chitosan loaded with lithium nitrate.

[0170] 2. Assembly of the coin cell

[0171] The coin cell was assembled in the same way as the coin cell of example 1.

[0172] Comparative example 3

[0173] 1. Preparation of porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate

[0174] (1) Preparation of bacterial cellulose suspension: Method same as example 1.

[0175] (2) Preparation of porous bacterial cellulose lithium ion battery separator: Method same as example 1.

[0176] (3) Battery separator loading lithium nitrate: 0.18 g LiNO3 was dissolved in 10 g deionized water, stirred for 1 h until the solution was clear, the porous bacterial cellulose lithium ion battery separator prepared in step (2) was immersed therein for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110°C until the mass did not change significantly, to obtain the porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate.

[0177] 2. Assembling a button cell

[0178] The button cell was assembled in the same manner as the button cell in Example 1.

[0179] Comparative Example 4

[0180] 1. Preparation of a lithium ion battery separator

[0181] 0.18 g LiNO3 was dissolved in 10 g pure ethanol, stirred for 1 h until the solution was clear, a commercial polypropylene PP lithium ion battery separator (purchased from Kuraray Co., Ltd., the thickness of the film was 14-18 μm) was immersed therein for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110°C until the mass did not change significantly, to obtain the lithium ion battery separator.

[0182] 2. Assembling a button cell

[0183] The button cell was assembled in the same manner as the button cell in Example 1.

[0184] Comparative Example 5

[0185] 1. Preparation of a porous bacterial cellulose lithium ion battery separator loaded with lithium nitrate

[0186] (1) Preparation of a bacterial cellulose suspension: the method was the same as in Example 1.

[0187] (2) Preparation of a porous bacterial cellulose lithium ion battery separator: the method was the same as in Example 1.

[0188] (3) Battery separator loading lithium nitrate: the method was the same as in Example 1.

[0189] 2. Preparation of an electrolyte

[0190] In an argon atmosphere, first, vinyl carbonate (EC), methyl ethyl carbonate (EMC) and dimethyl carbonate (DMC) were mixed in a mass ratio of 1:1:1, then lithium salt lithium hexafluorophosphate (LiPF6) was added to dissolve and mix uniformly, to obtain an electrolyte with a lithium salt concentration of 1M. LiNO3 was added to the above electrolyte in a weight ratio of 0.5%.

[0191] 3. Assembling a button cell

[0192] A button cell was assembled in the same manner as the button cell in Example 1.

[0193] Comparative Example 6

[0194] 1. Porous oxidized bacterial cellulose lithium ion battery separator loaded with lithium nitrate

[0195] (1) Preparation of bacterial cellulose suspension: the method was the same as in Example 1.

[0196] (2) Preparation of oxidized bacterial cellulose: 100 mL of the bacterial cellulose suspension prepared in step (1) was placed in a conical flask, and NaIO4 was added at a concentration of 0.15 mol·L -1 . After deoxygenation with nitrogen, the reaction was stirred at 300-500 rpm / min under light shielding and at 50±2°C for 2-2.5 h to obtain oxidized bacterial cellulose. Finally, 5-6 mL of ethylene glycol was added to terminate the reaction.

[0197] (3) Preparation of porous oxidized bacterial cellulose lithium ion battery separator: a vacuum suction filtration method was used to prepare the oxidized bacterial cellulose lithium ion battery separator, and a three-dimensional porous network was formed by the interlaced stacking of cellulose fibers. 6-7 g of the oxidized bacterial cellulose suspension prepared in step (2) was diluted with water to 80-100 g, and then poured into a sand core funnel to filter into a film. Subsequently, the wet separator was placed in ethanol for 3-4 h to completely replace the water in the separator with ethanol. Then the separator was dried at 75-100°C with a roller dryer for 5-10 min to obtain the desired porous oxidized bacterial cellulose lithium ion battery separator, with a radius of 0.8 cm.

[0198] (4) Loading of lithium nitrate on the battery separator: 0.18 g of LiNO3 was dissolved in 10 g of pure ethanol and stirred for 1 h until the solution was clear. The porous oxidized bacterial cellulose lithium ion battery separator prepared in step (3) was soaked in it for 2 h, then taken out and placed on a glass plate, and dried in a vacuum drying oven at 110°C until there was no significant change in mass, to obtain the porous oxidized bacterial cellulose lithium ion battery separator loaded with lithium nitrate.

[0199] 2. Assembly of button cell

[0200] A button cell was assembled in the same manner as the button cell in Example 1.

[0201] Effect example

[0202] 1. Scanning electron microscope observation

[0203] The scanning electron microscope (SEM) observation results of the porous bacterial cellulose lithium ion battery separator (unloaded with LiNO3) prepared in Example 1 Step (2) and the porous bacterial cellulose lithium ion battery separator grafted with chitosan (unloaded with LiNO3) prepared in Example 6 Step (3) are shown in Figure 1 and Figure 2 It can be seen that the pores of the bacterial cellulose separator grafted with chitosan are more abundant, thus being able to absorb more electrolyte and promote the migration of lithium ions.

[0204] The SEM image of the porous oxidized bacterial cellulose lithium ion battery separator (the loading amount of LiNO3 is 0.5%) prepared in Comparative Example 6 Step (4) is shown in Figure 3 Compared with Figure 1 , it can be seen that the pore size of the oxidized bacterial cellulose is significantly reduced, showing a more dense surface.

[0205] The SEM images of the lithium nitrate-loaded grafted chitosan bacterial cellulose lithium ion battery separators (the loading amounts of LiNO3 are 1.5% and 3%, respectively) prepared in Example 8 and Comparative Example 2 Step (4) are shown in Figure 4 and Figure 5 It can be seen that too high concentration of LiNO3 will block the pores of the bacterial cellulose separator, thus increasing the difficulty of lithium ion conduction and more easily causing polarization in the battery.

[0206] 2. Performance test

[0207] The porous cellulose-based lithium ion battery separators prepared in Examples 1-10 and the lithium ion battery separators in Comparative Examples 1-6 were measured and compared in thickness, porosity and liquid absorption rate tests, and cycle performance and coulombic efficiency tests were performed, with three replicates in the experiment, and the specific test methods are as follows:

[0208] ① Test method of porosity: immerse the separator in n-butanol for 1 h to fully soak, and then calculate the porosity according to the formula:

[0209]

[0210] In the formula, W b and W c are the wet weight and dry weight of the separator, respectively, and p b and p c are the densities of n-butanol and the separator, respectively.

[0211] ② Test method of liquid absorption rate: weigh the mass of the separator before and after absorbing the electrolyte, and divide the mass after absorption by the mass before absorption.

[0212] ③ Test method of cycle performance and coulombic efficiency:

[0213] The assembled button cell was charged at 0.2C constant current to 3.8V at 25℃, at this time the button cell was full charged, the charge capacity of the first cycle was recorded as the initial charge capacity; the button cell was rested for 2min, then discharged at 0.5C constant current to the discharge cut-off voltage 2.5V, this was a cycle of charge and discharge process, the discharge capacity of the first cycle was recorded as the initial discharge capacity. The button cell was tested by the above method for cycle charge and discharge, the discharge capacity after each cycle was recorded, until the discharge capacity of the button cell was attenuated to 80% of the discharge capacity of the first cycle, the cycle number at this time was used to represent the cycle performance of the button cell. The higher the cycle number, the better the cycle performance of the button cell. The coulombic efficiency was the ratio of the discharge capacity to the charge capacity.

[0214] The results are shown in Table 1 below.

[0215] Table 1

[0216]

[0217]

[0218] Note: In the table, the concentration of LiNO3(%) is the mass ratio concentration of LiNO3 in the membrane, that is, the mass ratio of lithium nitrate loaded in the membrane to the mass of the membrane.

[0219] Result analysis:

[0220] From Examples 1-4 and Comparative Example 3, it can be seen that loading LiNO3 in the porous bacterial cellulose separator can effectively improve the cycle performance of the battery, because LiNO3 can be slowly released in the electrolyte, always maintaining a saturated state, so as to preferentially decompose on the negative electrode surface to form a stable SEI mainly composed of inorganic components, promoting the deposition / stripping behavior of lithium ions, and reducing the side reaction between the electrolyte and the electrode.

[0221] From Examples 1-5 and Examples 6-10, it can be seen that the porosity and liquid absorption rate of the bacterial cellulose separator grafted with chitosan are improved, so more electrolyte can be stored, and the conduction of lithium ions is further promoted, thereby improving the cycle efficiency of the battery. In addition, it can be seen that when the concentration of LiNO3 is 1.5%, the porosity and liquid absorption rate of the separator decrease less, and the formed SEI is the most stable, and the cycle of the battery is the highest. It shows that too low concentration of LiNO3 is not enough to maintain the formation of stable SEI layer, and too high concentration of LiNO3 will block the pores of the porous separator, reducing the conduction of lithium ions. But when the concentration of LiNO3 continues to increase to 3%(Comparative Examples 1 and 2), excessive lithium nitrate will block the pores, because lithium nitrate itself is not soluble in the electrolyte, thereby reducing the cycle performance of the battery.

[0222] As can be seen from Example 1 and Comparative Example 3, the porosity and liquid absorption of the bacterial cellulose separator obtained by using water as the solvent of LiNO3 in Comparative Example 1 are both reduced, because the polarity of water is stronger than that of ethanol, and the spacing between cellulose chains is reduced during the drying process, thereby forming a denser structure, which can result in reduced absorption of electrolyte and increased transmission impedance of lithium ions.

[0223] As can be seen from Example 1, 6 and Comparative Example 3, 4, the porous structure can effectively load LiNO3 and play a role in slow release, while the commercial PP separator has only a single layer of pores and cannot load LiNO3 by physical coating, which indicates that the porous cellulose-based separator can be used as an effective slow-release carrier for insoluble additives.

[0224] As can be seen from Example 1 and Comparative Example 5, by directly adding LiNO3 to the electrolyte, LiNO3 cannot be completely dissolved and can only exist in the form of crystals in the electrolyte, which can result in aggregation on the electrode surface and cause polarization, inhibit the conduction of lithium ions and reduce the cycle performance of the battery.

[0225] As can be seen from Example 1, 6 and Comparative Example 6, only oxidizing the bacterial cellulose without grafting chitosan can damage the degree of polymerization of cellulose to some extent, reduce the size of cellulose and form a denser separator, which can reduce the absorption of electrolyte and increase the mass transfer impedance of lithium ions, and ultimately reduce the cycle performance.

[0226] The above examples are preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement methods and are included in the protection scope of the present application.

Claims

1. A method for preparing a porous cellulose-based lithium-ion battery separator, characterized in that: The steps include: (1) Preparation of bacterial cellulose suspension The bacterial cellulose flakes are cut into small pieces, washed with water, and then completely boiled in a NaOH solution for 1 to 1.5 hours to clean the residual medium and impurities; then rinsed with water, a CH3COOH solution is added to neutralize it, and then washed with water until neutral, and finally fiberized into a slurry using a homogenizer to obtain a bacterial cellulose suspension; (2) Bacterial cellulose suspension grafted with chitosan NaIO4 is added to the bacterial cellulose suspension obtained in step (1), and after deoxygenation with nitrogen, the mixture is stirred and reacted under the conditions of light shielding and 50±2°C. After the reaction is completed, ethylene glycol is added to terminate the reaction to obtain an oxidized bacterial cellulose solution; chitosan powder is added to a CH3COOH solution, and the mixture is stirred and dissolved at 60-70°C to form a transparent chitosan solution; the oxidized bacterial cellulose solution is then added to the chitosan solution, and the mixture is stirred and reacted at 50-60°C. After the reaction is completed, the mixture is centrifuged and washed to obtain a bacterial cellulose suspension grafted with chitosan; (3) Preparation of porous bacterial cellulose lithium-ion battery separator The bacterial cellulose suspension obtained in step (1) and / or the bacterial cellulose suspension grafted with chitosan obtained in step (2) is diluted with water and filtered to form a membrane to obtain a wet porous cellulose-based membrane; then the membrane is immersed in pure ethanol so that the water in the porous cellulose-based membrane is completely replaced by ethanol, and finally dried at 75-100° C. to obtain a porous bacterial cellulose lithium-ion battery membrane; (4) Battery separator loaded with lithium salt Adding lithium salt to ethanol, stirring and dissolving, and preparing a lithium salt solution with a mass concentration of 0.5 to 2.5%; then immersing the porous bacterial cellulose lithium ion battery separator obtained in step (3) in the lithium salt solution, taking it out after immersion, and drying it at 90 to 120° C. to obtain a porous cellulose-based lithium ion battery separator loaded with lithium salt; wherein the lithium salt is lithium nitrate; The soaking time in step (4) is 1 to 2 hours.

2. The method according to claim 1, wherein: The concentration of the bacterial cellulose suspension in step (1) is 0.4-0.5% by mass; The concentration of the bacterial cellulose suspension grafted with chitosan in step (2) is 0.4-0.5% by mass.

3. The method according to claim 1, wherein: The concentration of the chitosan solution in step (2) is 0.02-0.03 g / ml; The chitosan described in step (2) is a medium-viscosity chitosan with a viscosity of 200 to 400 mPa.s.

4. The method according to claim 1, wherein: The concentration of the NaOH solution in step (1) is 0.05 to 0.2 mol / L; The amount of NaIO4 added in step (2) is calculated based on its final concentration in the reaction system being 0.1 to 0.2 mol / L; The amount of ethylene glycol used in step (2) accounts for 5-6% of the volume of the reaction system; The dilution described in step (3) is to dilute the solution 10 to 20 times with water.

5. The method according to claim 1, wherein: The homogenizer described in step (1) is a high-speed homogenizer with a rotation speed of 25,000 to 30,000 rpm; In step (2), the stirring reaction conditions under the conditions of shielding from light and 50±2°C are: rotation speed 300-500 rpm / min, stirring reaction time 2-2.5h; The stirring and dissolving time in step (2) is 1 to 2 hours; In step (2), the stirring reaction time at 50-60° C. is 2-3 h; The number of centrifugal washings in step (2) is 4 to 6 times; The soaking time in step (3) is 3 to 4 hours; The drying in step (3) is performed by using a drum dryer for 5 to 10 minutes; The stirring time in step (4) is 1 to 2 hours.

6. A porous cellulose-based lithium-ion battery separator, characterized in that: It is prepared by the method according to any one of claims 1 to 5.

7. Use of the porous cellulose-based lithium ion battery separator according to claim 6 in lithium ion batteries.

8. A lithium-ion battery, characterized in that: The invention comprises a positive electrode, a negative electrode, the porous cellulose-based lithium-ion battery separator according to claim 6 and an electrolyte.

9. The lithium-ion battery according to claim 8, wherein: The positive electrode is composed of an active material capable of inserting and extracting lithium ions, a conductive agent, a binder, and a current collector; wherein, The active material includes at least one of lithium cobalt oxide, nickel-cobalt-manganese ternary material, nickel-cobalt-aluminum ternary material, lithium iron phosphate and lithium manganese oxide; The conductive agent includes at least one of acetylene black, carbon black, graphite and carbon fiber; The binder includes at least one of polyvinylidene fluoride and polyacrylic acid; The current collector is aluminum foil.

10. The lithium-ion battery according to claim 8, wherein: The electrolyte consists of lithium salt and solvent; wherein, The lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide and lithium fluorosulfonyl imide; The solvent includes one or more of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate and propylene carbonate; The concentration of lithium salt in the electrolyte is 0.5-2 mol / L.

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