CN108232085B

CN108232085B - Polyion liquid coated bacterial cellulose membrane and preparation method thereof

Info

Publication number: CN108232085B
Application number: CN201711416949.XA
Authority: CN
Inventors: 刘伟明; 蔡高宏; 何俊航; 曾柏炼
Original assignee: Shanghai Energy New Materials Technology Co Ltd
Current assignee: Shanghai Energy New Materials Technology Co Ltd
Priority date: 2017-12-25
Filing date: 2017-12-25
Publication date: 2022-03-01
Anticipated expiration: 2037-12-25
Also published as: CN108232085A

Abstract

The invention discloses a bacterial cellulose membrane and a preparation method thereof, wherein the surface of a nanofiber of the bacterial cellulose membrane is coated with a layer of polyionic liquid, and the polyionic liquid is one or a combination of more of imidazolium salts, pyrrolidine salts, piperidine salts and quaternary ammonium salt polyionic liquids; the preparation method comprises the following steps: and sequentially soaking the bacterial cellulose membrane in a polyion liquid water solution for 2-48 h, soaking in a precipitator for 30 min-24 h, soaking in a lithium salt solution for 2-48 h, and finally thinning and drying by using a hot press. The invention has the advantages that: (1) the polyion liquid coated bacterial cellulose membrane has the advantages of high mechanical strength, good thermal stability, good lyophilic property, high porosity and high ionic conductivity, and can improve the cycle stability and safety of the lithium ion battery; (2) the preparation method is simple, has low equipment requirement and is suitable for large-scale production.

Description

Polyion liquid coated bacterial cellulose membrane and preparation method thereof

Technical Field

The invention relates to a bacterial cellulose membrane and a preparation method thereof, in particular to a bacterial cellulose membrane coated by polyion liquid and a preparation method thereof, belonging to the technical field of electrochemistry.

Background

The diaphragm is a key component of the lithium ion battery and is a key factor influencing the indexes of the lithium ion battery, such as safety, capacity, service life, cost and the like. With the green, safe, long-life and high power density of lithium ion batteries, the development of high-performance separators is urgently needed.

The traditional lithium ion battery diaphragm cannot meet the performance requirements of a novel power battery due to the defects of low safety, low charging and discharging efficiency, short service life and the like. Therefore, research and development of high-performance novel power battery separators have become a great technical field of competition among countries in the world.

At present, lithium ion battery separators in the market are mainly polyolefin porous membranes, the separators are prepared from Polyethylene (PE) and polypropylene (PP) serving as raw materials, the preparation process is simple, the production efficiency is high, equipment is complex, and the manufacturing cost of the separators is increased.

Moreover, most critically, such membranes suffer from a number of disadvantages, such as:

1. the heat resistance is poor, and the battery is not high in safety because the heat shrinkage is easy to occur at high temperature (the melting temperature of PP is 165 ℃);

2. the porosity is low, so that the liquid absorption rate is low and the large-current charge and discharge are not facilitated;

3. the surface energy is low, so that the surface energy is not favorable for combination with the positive and negative plates, the interface resistance is large, and the energy density of the battery is influenced;

4. the hydrophilicity is poor, the wetting and the keeping of the electrolyte are not enough, and the migration of lithium ions and the safety of the battery are influenced.

These disadvantages make it difficult for polyolefin porous films to meet the use requirements of new power cells.

In recent years, developed countries and regions in europe and america compete with the development of high-performance power battery separators. The nano-fiber membrane is prepared by utilizing high-temperature resistant polymers (such as polyethylene terephthalate, polyimide, aromatic polyamide, poly (p-phenylene benzobisoxazole) and the like) through an electrostatic spinning process, so that the lithium ion battery diaphragm with high porosity, high heat resistance, good lyophilic property, high ionic conductivity and long service life can be obtained. Composite membranes based on polyethylene terephthalate membranes and coated with ceramic particles, developed by Degussa, germany, exhibit excellent heat resistance with closed cell temperatures up to 220 ℃. High-performance diaphragm developed by EVONIC company in Germany and specially used for power battery

The film has the advantages of safety temperature of 210 ℃, heat shrinkage rate of less than 1% (200 ℃, 24h), obviously improved wettability and excellent thermal stability and chemical stability. The Japanese emperor technical products company Limited develops the aramid nano-fiber which can be produced in large scale in 2013, has uniform fiber size and good heat resistance and oxidation resistance, and has no need of using the aramid nano-fiberThe form of the woven plate is applied to the manufacturing of the lithium ion battery diaphragm.

Similarly, the high-temperature resistant polymer is used for preparing the lithium ion battery diaphragm, so that the manufacturing cost is high, and the price of the lithium ion battery cannot be reduced.

The Bacterial Cellulose (BC) is high-purity cellulose synthesized by bacteria under a certain condition, the molecular main chain of the BC is composed of glucose molecules, high-strength nano fibers are formed through strong hydrogen bonds, and the BC has a large number of polar groups and a three-dimensional micro-nano porous structure. The bacterial cellulose membrane has the advantages of high porosity, high water retention rate, high heat resistance, excellent mechanical property, good biocompatibility and the like, is applied to the fields of biomedicine, environmental protection, energy materials and the like, and also has wide commercial application prospect in the aspect of lithium ion battery diaphragms. Jiang kujing and the like adopt an improved hot-pressing process to prepare an ultrathin bacterial cellulose membrane (CN104157815B), the membrane (dry membrane) can be used as a lithium ion battery membrane, but the porosity of the ultrathin bacterial cellulose membrane is low, so that the conductivity of the ultrathin bacterial cellulose membrane is lower than that of a commercial membrane. And then, people in Jiang Peak landscape and the like obtain an inorganic coated bacterial cellulose porous film (CN106450115A) by coating inorganic nano particles on the bacterial cellulose nano fibers, and the film has the characteristics of high elastic modulus, good lyophilic property, high conductivity and good thermal stability, and can improve the performance and safety of the lithium ion battery in the aspects of power batteries, large-scale energy storage and the like. The nitrate and the nano oxide particles are absorbed into the bacterial cellulose membrane by a soaking method (CN106531931A), and then the nitrate is decomposed by a step-by-step hot pressing and drying method to obtain the metal oxide-cellulose composite membrane.

When the bacterial cellulose membrane is used as a lithium ion battery diaphragm, the hydroxyl on the surface of the cellulose membrane can be matched with lithium ions, so that the dissociation capability of the lithium ions is reduced, and the ion conductivity is reduced; and when the cellulose membrane is used for a high-performance lithium ion battery with lithium metal as an electrode, active groups on the surface of the cellulose membrane may react with metal lithium, so that the safety of the battery is reduced and the long-term service life of the battery is prolonged. Therefore, it is necessary to treat the surface of the bacterial cellulose membrane while introducing an element that promotes lithium ion transport to improve the ionic conductivity of the cellulose membrane.

Disclosure of Invention

It is a first object of the present invention to provide a bacterial cellulose membrane having high porosity, imbibition rate and ionic conductivity and excellent high temperature dimensional stability.

The second purpose of the invention is to provide a simple and efficient method for preparing the bacterial cellulose membrane with low cost.

In order to achieve the first object, the invention adopts the following technical scheme:

the bacterial cellulose membrane is characterized in that the surface of the nanofiber of the bacterial cellulose membrane is coated with a layer of polyion liquid.

The bacterial cellulose membrane is characterized in that the polyion liquid is one or a combination of more of imidazolium salts, pyrrolidine salts, piperidine salts and quaternary ammonium salt polyion liquid.

In order to achieve the second objective, the invention adopts the following technical scheme:

the method for preparing the bacterial cellulose membrane is characterized by comprising the following steps:

(1) placing the bacterial cellulose membrane into the polyion liquid water solution, and soaking for 2-48 hours to enable the bacterial cellulose membrane to adsorb the polyion liquid;

(2) putting the bacterial cellulose membrane obtained in the step (1) into a precipitator, and soaking for 30 minutes to 24 hours to obtain a bacterial cellulose membrane coated with polyion liquid;

(3) putting the bacterial cellulose membrane coated with the polyion liquid obtained in the step (2) into a lithium salt solution, and soaking for 2-48 hours to obtain the bacterial cellulose membrane coated with the polyion liquid after anion exchange;

(4) and (4) pressing the anion-exchanged polyion liquid coated bacterial cellulose membrane obtained in the step (3) to be thin by using a hot press, and then drying to obtain a clean polyion liquid coated bacterial cellulose membrane.

The method described above, wherein in the step (1), the concentration of the aqueous solution of the polyionic liquid is 0.001% to 5%.

The method described above is characterized in that, in the step (2), the precipitant is a mixture of one or more of acetone, ethanol, methanol, and isopropanol.

In the step (3), the lithium salt is one or a mixture of more of lithium perfluoroalkyl sulfonate, lithium perfluoroalkyl sulfonyl imide, lithium perfluoroalkyl sulfonyl methyl and lithium hexafluorophosphate, and the concentration of the lithium salt is 0.01 to 10%.

The invention has the advantages that:

(I) polyion liquid coating bacterial cellulose membrane

The composite material can be used as a lithium ion battery diaphragm, has the advantages of high mechanical strength, good thermal stability, good lyophilic property, high porosity and high ionic conductivity, and can improve the cycle stability and safety of the lithium ion battery.

The polyion liquid is loaded on the surface of the nanofiber, so that the combination of cellulose hydroxyl and lithium ions can be overcome, the polyion liquid can promote the dissociation of the lithium ions and corresponding anions, the conductivity and the rapid migration capability of the lithium ions in the electrolyte can be improved, and the polyion liquid is suitable for serving as a diaphragm of a novel high-power lithium ion battery.

The bacterial cellulose with low price and rich yield is used for loading the polyion liquid, so that the manufacturing cost is reduced, and further the price of the lithium ion battery can be reduced.

(II) preparation method

The preparation method is simple, has low equipment requirement and is suitable for large-scale production.

Drawings

FIG. 1(a) is a microstructure view of a BC film;

FIG. 1(b) is a microstructure of a BC @ PILs1-1.0 film;

FIG. 2 is a graph comparing electrochemical windows of BC @ PILs1 membranes to BC membranes;

FIG. 3 is a graph of ionic conductivity of BC @ PILs1 films as a function of polyionic liquid solution concentration;

fig. 4 is a graph comparing the battery charge and discharge performance of BC @ PILs1-1.0 films with BC films and commercial PP films;

FIG. 5 is a microstructure view of a BC @ PILs2-0.05 film;

FIG. 6 is a graph of ionic conductivity of BC @ PILs2 films as a function of polyionic liquid solution concentration;

FIG. 7 is a microstructure view of a BC @ PILs3-2.0 film;

FIG. 8 is a graph of ionic conductivity of BC @ PILs3 films as a function of polyionic liquid solution concentration;

FIG. 9 is a microstructure view of a BC @ PILs4-0.2 film;

fig. 10 is a graph of ionic conductivity of BC @ PILs4 membranes as a function of polyionic liquid solution concentration.

Detailed Description

In order to make the technical solutions of the present invention more comprehensible, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

Example 1

Preparing 100mL of 1.0% polydiene dimethyl ammonium chloride aqueous solution (polyion liquid aqueous solution), soaking 4g of bacterial cellulose membrane (wet membrane) in the aqueous solution for 24 hours, slowly stirring (facilitating the polydiene dimethyl ammonium chloride to diffuse into the bacterial cellulose membrane), taking out, slightly cleaning the surface with deionized water, then placing the mixture into 50mL of acetone (precipitator) to be soaked for 12 hours to ensure that polydiene dimethyl ammonium chloride is completely precipitated and coated on the surface of the nano-fiber, then placing the mixture into 50mL of lithium bis (trifluoromethane sulfonyl) imide ethanol solution with the concentration of 0.5 percent to be soaked for 24 hours, slowly stirring (being beneficial to the exchange of bis (trifluoromethane sulfonyl) imide ions and chloride ions), finally utilizing a hot press to thin a wet film at the temperature of 60 ℃, drying the wet film for 24 hours at the temperature of 80 ℃, the polyion liquid coated bacterial cellulose membrane is obtained and is marked as BC @ PILs1-1.0 membrane.

The main chemical components of the BC @ PILs1-1.0 film are bacterial cellulose and quaternary ammonium salt polyionic liquid.

The microstructure of the BC @ PILs1-1.0 film prepared by the method is shown in figure 1(b), and the BC @ PILs1-1.0 film has a three-dimensional micro-nano porous structure.

The microstructure of the pure bacterial cellulose membrane (BC membrane) is shown in FIG. 1 (a).

Comparing fig. 1(a) and fig. 1(b), it can be seen that the polyion liquid coating does not significantly increase the diameter of the nanofiber of the bacterial cellulose membrane, and has little influence on the porosity of the bacterial cellulose membrane, and can still maintain a high imbibition rate.

The separator (BC @ PILs1 film, BC film) is assembled with a lithium sheet and a stainless Steel Sheet (SS) to form a Li/separator/SS structure, and the electrochemical stability of the structure is tested, and the test result is shown in figure 2.

As shown in FIG. 2, the electrochemical window of the BC membrane is about 4.6V, the electrochemical windows of the BC @ PILs1 membrane reach more than 4.8V, the BC @ PILs1 membrane has a higher electrochemical window than the BC membrane, and the loaded polyion liquid can reduce the action of hydroxyl on the surface of cellulose and metal lithium, so that the electrochemical stability of the bacterial cellulose membrane is improved.

We assembled a membrane (BC @ PILs1 film, commercial PP film) between stainless Steel Sheets (SS) into a SS/membrane/SS structure and tested the ionic conductivity of the structure, the results of which are shown in fig. 3.

As can be seen from fig. 3, the BC @ PILs1 film has higher ionic conductivity than the commercial PP film, and the ionic conductivity of the commercial PP film is improved by coating the fiber surface with the polyion liquid.

A diaphragm (BC @ PILs1-1.0 film, commercial PP film and BC film) is mixed with a lithium sheet and lithium iron phosphate (LiFePO)₄) Assemble Li/diaphragm/LiFePO₄The structure is tested for specific capacity and high rate performance, and the test result is shown in figure 4.

As can be seen from fig. 4, the battery using the polyion liquid to coat the bacterial cellulose membrane has significantly improved battery capacity and high-rate charge and discharge performance, and is superior to BC membrane and commercial PP membrane.

Example 2

Preparing 50mL of 0.05% poly [ 3-ethyl-1-vinyl imidazole bromide ] aqueous solution, soaking 2g of bacterial cellulose membrane (wet membrane) in the solution for 12 hours, slowly stirring, taking out, slightly cleaning the surface with deionized water, then soaking in 30mL of ethanol for 24 hours to ensure that the poly [ 3-ethyl-1-vinyl imidazole bromide ] is completely precipitated and coated on the surface of the nanofiber, then soaking in 40mL of 1.0% lithium trifluoromethanesulfonate ethanol solution for 12 hours, slowly stirring, finally pressing the wet membrane to be thin at 70 ℃ by using a hot press, and drying at 60 ℃ for 24 hours to obtain the polyion liquid coated bacterial cellulose membrane, which is marked as BC PILs2-0.05 membrane, wherein the appearance of the polyion liquid coated bacterial cellulose membrane is shown in figure 5.

The main chemical components of the BC @ PILs2-0.05 membrane are bacterial cellulose and imidazolium salt polyionic liquid.

The ionic conductivity of the BC @ PILs2 film is shown in figure 6.

As can be seen from fig. 6, the performance of the BC @ PILs2 film was substantially equivalent to the performance of the BC @ PILs1 film in example 1.

Example 3

Preparing 200mL of 2.0% poly [1- (4-benzyl) -3-n-butylimidazolium tetrafluoroborate ] aqueous solution, soaking 8g of a bacterial cellulose wet film in the solution for 48 hours, slowly stirring, taking out, slightly washing the surface with deionized water, then soaking in 100mL of methanol for 24 hours to completely precipitate and coat the poly [1- (4-benzyl) -3-n-butylimidazolium tetrafluoroborate ] on the surface of the nanofiber, then soaking in 100mL of 5.0% tris (trifluoromethylsulfonyl) methyllithium ethanol solution for 24 hours, slowly stirring, finally pressing the wet film to be thin by a hot press at 60 ℃, and drying at 70 ℃ for 24 hours to obtain the polyion liquid coated bacterial cellulose film, which is marked as BC @ PILs3-2.0 film, and the appearance of the polyion liquid coated bacterial cellulose film is shown in figure 7.

The main chemical components of the BC @ PILs3-2.0 membrane are bacterial cellulose and imidazolium salt polyionic liquid.

The ionic conductivity of the BC @ PILs3 film is shown in figure 8.

As can be seen from fig. 8, the performance of the BC @ PILs3 film was substantially equivalent to the performance of the BC @ PILs1 film in example 1.

Example 4

Preparing 150mL of 0.2% polymethacryloxyethyltrimethyl ammonium tetrafluoroborate aqueous solution, placing 5g of bacterial cellulose wet film in the solution to be soaked for 36 hours, slowly stirring, taking out, slightly cleaning the surface by deionized water, then placing in 70mL of methanol to be soaked for 12 hours, completely precipitating and coating the polymethacryloxyethyltrimethyl ammonium tetrafluoroborate on the surface of the nanofiber, then placing in 80mL of 0.05% lithium hexafluorophosphate ethanol solution to be soaked for 12 hours, slowly stirring, finally pressing the wet film at 70 ℃ by using a hot press, and drying at 80 ℃ for 24 hours to obtain a polyion liquid coated bacterial cellulose film, which is marked as BC @ PILs4-0.2 film, wherein the appearance of the film is shown in figure 9.

The main chemical components of the BC @ PILs4-0.2 film are bacterial cellulose and quaternary ammonium salt polyionic liquid.

The ionic conductivity of the BC @ PILs4 film is shown in figure 10.

As can be seen from fig. 10, the performance of the BC @ PILs4 film was substantially equivalent to the performance of the BC @ PILs1 film in example 1.

It should be noted that the above-mentioned embodiments do not limit the present invention in any way, and all technical solutions obtained by using equivalent alternatives or equivalent variations fall within the protection scope of the present invention.

Claims

1. A method for preparing a bacterial cellulose membrane used as a lithium ion battery diaphragm is characterized in that the surface of a nanofiber of the bacterial cellulose membrane is coated with a layer of polyion liquid, wherein the polyion liquid is one or the combination of two of imidazolium salt and quaternary ammonium salt polyion liquid, and the method comprises the following steps:

2. The method according to claim 1, wherein in step (1), the concentration of the polyionic liquid in the aqueous solution is 0.001-5%.

3. The method of claim 1, wherein in step (2), the precipitating agent is a mixture of one or more of acetone, ethanol, methanol, and isopropanol.

4. The method according to claim 1, wherein in step (3), the lithium salt is a mixture of one or more of lithium perfluoroalkyl sulfonate, lithium perfluoroalkyl sulfonyl imide, lithium perfluoroalkyl sulfonyl methide and lithium hexafluorophosphate.

5. The method of claim 4, wherein the lithium salt is present at a concentration of 0.01% to 10%.