CN102516585B - Biomass cellulose porous composite diaphragm used for lithium ion secondary cell - Google Patents

Abstract

The invention discloses a cellulose porous membrane able to be used for a lithium ion secondary cell diaphragm. Two side surfaces of the membrane are coated with sodium alginate, a fluorine-containing polymer, polyaryletherketone, polyimide, polynorbornene or inorganic nanoparticles and other structure enhanced and interface stable components. The membrane has thickness of 15-100 micrometers, and air permeability of 2-500s. The upper and lower surfaces as well as inner pores of the membrane are symmetrically and uniformly distributed, with an average pore size of 20-200 nanometers and tensile strength of 50-250MPa. The invention also discloses a preparation method of the cellulose porous membrane. The membrane of the invention can be used as a lithium ion cell diaphragm and has good heat resistance, and even if at a temperature of 150DEG C, a cell short circuit phenomenon cannot occur. Therefore, the cell diaphragm provided in the invention is especially suitable for large capacity and power lithium ion cells. In addition, employment of biomass cellulose that has the largest output in the nature as the raw material for producing the lithium ion cell diaphragm has the advantages of low cost, sustainability and environmental protection.

Description

Biomass Cellulose Porous Composite Separator for Lithium-ion Secondary Batteries

technical field

The invention relates to a biomass cellulose porous membrane.

The present invention also relates to a method for preparing the above-mentioned cellulose porous membrane.

The present invention also relates to the application of the above-mentioned cellulose porous membrane in lithium-ion secondary batteries.

Background technique

Lithium-ion secondary batteries have achieved great development in the past ten years due to their advantages of high specific capacity, high voltage, small size, light weight, and no memory. However, for lithium-ion secondary batteries using liquid electrolytes, in Sometimes, lithium-ion batteries are prone to safety hazards such as smoke, fire, explosion, and even personal injury. As a result, high-capacity and power lithium-ion batteries have not been widely used. Therefore, improving the safety performance of lithium-ion batteries is the key to developing lithium-ion secondary batteries. The key to the battery. Now commonly used battery separators such as polyethylene (PE) and polypropylene (PP) have a melting temperature lower than 160°C (for example, the self-closing temperature of PE diaphragm is 135-140°C, and the self-closing temperature of PP diaphragm is about 160°C). In some cases, such as the external temperature is too high, the discharge current is too large or the thermal inertia of the electrolyte heating process, even if the current is interrupted, the temperature of the battery may continue to rise, so the separator may be completely destroyed and cause the battery to short circuit, causing the battery to explode or catch fire. In addition, the tensile strength of uniaxially stretched PE separators and PP separators in the transverse direction is much lower than that in the longitudinal direction, and there is a danger of membrane rupture in the case of battery stacks or accidental impacts. Therefore, the safety of using PE diaphragms and PP diaphragms is low.

There are many factors for the increase of internal heat and temperature of high-capacity and high-power batteries, so it is particularly important to improve the high-temperature resistance of batteries. The safety performance of PE diaphragm and PP diaphragm can no longer meet this demand, so polymer diaphragm materials with better heat resistance are needed. Compared with PE and PP, natural cellulose has higher heat-resistant temperature and mechanical strength. Therefore, the present invention uses a large amount of natural and low-cost biomass cellulose, combined with advanced nano-manufacturing technology, to prepare low-cost high-temperature-resistant High-performance battery separator material with the advantages of low cost and environmental friendliness.

CN1215505A has set forth that cellulose is used for the separator of alkaline battery after amine oxide solution spinning, and JP10003898A, JP2005317495A, JP2005239028A etc. have reported that porous membranes such as cellulose and cellulose acetate are used for lithium ion battery separator, and the separator prepared by the above method are all asymmetric membranes. The fibrous porous membrane prepared by the invention belongs to a symmetrical membrane, has uniform pore structure and distribution, is convenient to prepare, is suitable for mass production, and has high heat resistance, and is especially suitable for lithium-ion power battery separators.

Contents of the invention

The object of the present invention is to provide a kind of biomass cellulose porous membrane.

Another object of the present invention is to provide a method for preparing a cellulose composite porous membrane.

In order to achieve the above object, a cellulose nanofiber composite membrane provided by the present invention is composed of 20-500 nanofibers in diameter, and the surfaces on both sides of the membrane are coated with sodium alginate, fluoropolymer, polyaryletherketone, Polyimide, polynorbornene or inorganic nanoparticles and other components that enhance the structure and stabilize the interface, the film thickness is 15-100 microns, the film air permeability is 2-500 seconds; the surface and internal pores of the film are distributed symmetrically and uniformly, The average pore diameter is 20-200 nanometers, and the tensile strength is 50-250 MPa.

The invention also provides a method for preparing a nanofiber film, which is characterized in that the cellulose solution is spun by electrospinning or wet spinning to obtain a cellulose non-woven film.

Wherein, the mass percentage of the cellulose solution is 1-20%, and the solvent is N-tertiary amine oxide or lithium chloride/DMAc system;

Among them, the inner diameter of the spinning needle for electrospinning is 0.8-2.0 mm, the voltage is 100 V-30 kV, the distance between the needle and the receiving electrode is 10-30 cm, and the flow rate of the spinning solution is greater than 0.1 ml/hour.

The present invention also provides a method for preparing cellulose composite membrane, which is characterized in that sodium alginate, fluoropolymer, polyaryletherketone, polyimide, polynorbornene are coated on both sides of the nanofiber membrane Enhanced components such as alkenes or inorganic nanoparticles.

Among them, sodium alginate, fluoropolymer, polyaryletherketone, polyimide, and polynorbornene are solutions with a mass percentage of 1-10%, and the solvents are water, acetone, tetrahydrofuran, N, N-dimethyl one or two of N, N-dimethylacetamide;

Among them, the inorganic nanoparticles are nano-silica, zirconia, alumina or lithium metaaluminate and other inorganic nanoparticles, inorganic nanoparticles and sodium alginate, fluoropolymer, polyaryletherketone, polyimide The mass percentage of the equal polymer is 0-9:10-1.

The cellulose porous membrane prepared by the present invention belongs to a symmetrical membrane, has uniform pore structure and distribution, is convenient to prepare, is suitable for mass production, and has high heat resistance, and can be used as a lithium-ion battery separator. The battery will not be short-circuited, so the porous cellulose membrane provided by the invention can be used in high-capacity and power batteries.

Description of drawings

FIG. 1 is an electron micrograph of the cellulose porous membrane in Example 2.

Detailed ways

The symmetric cellulose porous membrane provided by the present invention is characterized in that the upper and lower surfaces of the membrane and the internal pores are distributed symmetrically and evenly, the pore diameter is adjustable, the tensile strength is high, and more importantly, the heat resistance of the membrane is good. The diaphragm of the secondary battery will not short-circuit the battery even at 150°C.

The method for preparing the cellulose porous membrane of the present invention is to firstly use electrospinning to nano-spin the cellulose solution to obtain a cellulose nanofiber membrane, and the surfaces of both sides of the membrane are coated with sodium alginate, fluoropolymer, poly Reinforced components such as aryl ether ketone, polyimide, polynorbornene or inorganic nanoparticles.

Cellulose porous film of the present invention can be used in lithium ion secondary battery, and this battery comprises electrode group and nonaqueous electrolytic solution, and electrode group and nonaqueous electrolytic solution are sealed in battery case, and electrode group comprises positive pole, negative pole and separator, The separator used therein is the cellulose porous membrane of the present invention.

The battery membrane provided by the invention has excellent chemical stability, high temperature resistance, good permeability and high tensile strength because the natural cellulose porous membrane with good high temperature resistance is used as the base material. The battery separator obtained in the embodiment of the present invention will not rupture when heated to a high temperature of 150° C.; the thermal shrinkage rate of the battery separator at 150° C. is less than 0.5%, which is much smaller than the 3% and 5% thermal shrinkage rates in the prior art. The puncture strength is greater than that of the battery separator in the prior art, the surface and internal pores of the membrane are evenly distributed, the pore diameter and porosity both meet the requirements of electrical conductivity, and have suitable and excellent air permeability. The lithium-ion secondary battery using the battery diaphragm provided by the invention will not short-circuit even at a high temperature of 150° C., so the battery diaphragm provided by the invention can be used in high-capacity and power batteries.

Example 1

3.0 g of cellulose and 8.0 g of lithium chloride were added to 89.0 g of N,N-dimethylacetamide and stirred at 25° C. for 24 hours to slowly dissolve to obtain a uniform cellulose solution (3% by mass). Then store the cellulose solution in a refrigerator at 0-5°C. Another 3.0 ml of cellulose solution was taken out for electrospinning, the needle diameter was 1.6 mm, the spinning voltage was 25 kV, the height from the needle tip to the receiving plate was 10 cm, and electrospinning was performed for 4 hours to obtain cellulose with a thickness of 85 microns. nanofibrous membrane. After soaking the film in 3% sodium alginate aqueous solution for 10 minutes, take it out, and after drying in the air, place the film in a roller press with a pressure of 2 MPa for 2 minutes to obtain a composite cellulose nanocomposite with a thickness of 40 microns. Fiber membrane.

Example 2

Add 5.0 g of cellulose to 95 g of N-methylmorpholine-N-oxide and stir at 25° C. for 3 hours to slowly dissolve to obtain a uniform cellulose solution (5% by mass). Then store the cellulose solution in a refrigerator at 0-5°C. Another 3.0 ml of cellulose solution was taken out for electrospinning, the needle diameter was 1.6 mm, the spinning voltage was 25 kV, the height from the needle tip to the receiving plate was 10 cm, and electrospinning was performed for 4 hours to obtain cellulose with a thickness of 85 microns. nanofibrous membrane. The membrane was soaked in a DMF solution with 2.7% silica nanoparticles and 0.3% vinylidene fluoride and hexafluoropropylene copolymer dissolved in it for 10 minutes. After drying, the membrane was placed on a roller with a pressure of 2 MPa. Stay in the press for 2 minutes to obtain a regenerated cellulose nanofiber membrane with a thickness of 40 microns.

Example 3

Add 5.0 g of cellulose to 95 g of N-methylmorpholine-N-oxide and stir at 25° C. for 3 hours to slowly dissolve to obtain a uniform cellulose solution (5% by mass). Then store the cellulose solution in a refrigerator at 0-5°C. Another 3.0 ml of cellulose solution was taken out for electrospinning, the needle diameter was 1.6 mm, the spinning voltage was 25 kV, the height from the needle tip to the receiving plate was 10 cm, and electrospinning was performed for 4 hours to obtain cellulose with a thickness of 85 microns. nanofibrous membrane. The film was soaked in a DMF solution of 2.4% silica nanoparticles and 0.6% polyetherimide (Ultem1000) for 10 minutes, taken out, and after drying, the film was placed on a roller with a pressure of 2 MPa Stay in the press for 2 minutes to obtain a regenerated cellulose nanofiber membrane with a thickness of 40 microns.

Comparative example 1

The commercial polyolefin separator Celgard 2400 was used as a comparison to further clarify the advantages of the cellulose nanofiber separator described in the present invention.

Characterize the diaphragm performance in the above-mentioned Examples 1-3 and Comparative Example 1:

Infrared spectroscopy: Fourier transform infrared spectroscopy (Nicolet iN10) was used to characterize the chemical structure of the films.

Scanning electron microscope: use a cold field emission scanning electron microscope (S-4800) to observe the surface and cross-sectional morphology of the membrane, the size and arrangement of nanofibers, and the size of some pores.

Air permeability: Gurley 4110N air permeability meter (USA) was used to measure the air permeability of film samples.

Membrane thickness: use a micrometer (accuracy: 0.01 mm) to test the thickness of the cellulose nanofiber membrane, randomly take 5 points on the sample, and take the average value.

Porosity: Using the following test method, soak the cellulose nanofiber membrane in n-butanol for 2 hours, and then calculate the porosity according to the formula: $p=\frac{m_a/{\mathrm{\ρ}}_a}{(m_a/{\mathrm{\ρ}}_a)+(m_p/{\mathrm{\ρ}}_p)}$

Among them, ρ _a and ρ _p are the density of n-butanol and the dry density of the fiber membrane, ma _{and mp} are the mass of n-butanol absorbed by the membrane and the mass of the fiber membrane itself.

Tensile strength: The tensile strength and elongation of the cellulose nanofiber membrane were tested by the plastic tensile test method of GB1040-79.

The obtained results are listed in Table 1. As can be seen from the results in table 1, the cellulose nanofiber non-woven fabric membrane prepared by the method provided by the invention has high porosity, gas permeability and mechanical strength, which meets the requirements of the lithium-ion battery diaphragm to the aperture, from the examples From the test results of 1-3 and Comparative Example 1, it can be seen that the commercialized polyolefin separator has poor shrinkage resistance and transverse tensile strength.

Test battery performance

1) Preparation of positive electrode

First, mix 5.75 grams of positive electrode active material LiCoO ₂ and 0.31 grams of conductive agent acetylene black, and then add 6.39 grams of 5% polyvinylidene fluoride (PVDF) solution (solvent is N-methyl-2-pyrrolidone) , stirred to form a uniform positive electrode slurry.

The slurry was uniformly coated on an aluminum foil, then dried at 120°C, rolled, and punched to obtain a circular positive electrode sheet with a radius of 12 mm and a thickness of 80 microns, which contained 17.6 mg of active ingredient LiCO ₂ .

2) Preparation of negative electrode

4.74 grams of negative electrode active material natural graphite, 0.10 grams of conductive agent acetylene black are mixed homogeneously, and then adding 2.55 grams of mass fraction is 10% polyvinylidene fluoride (PVDF) solution (solvent is N-methyl-2-pyrrolidone), Stir to form a uniform negative electrode slurry.

The negative electrode slurry is evenly coated on the copper foil, then dried at 120°C, rolled, and punched to obtain a circular negative electrode sheet with a radius of 14 mm and a thickness of 70 microns, which contains 11.9 mg of active ingredient Natural graphite.

3) Preparation of batteries with the membrane of the present invention

The positive electrode obtained above, the negative electrode and the diaphragm are laminated successively and packed into a button battery (battery model 2032), and the films are respectively commercialized cellulose nanofiber membranes in Examples 1-3 and Comparative Example 1. Polyolefin membrane.

About 150 mg of an electrolyte solution containing 1 mole of lithium hexafluorophosphate (LiPF ₆ ) in a mixed solvent (ethylene carbonate: methyl ethyl carbonate (EC/EMC) volume ratio of 1:1) was injected into the above battery, and followed by Aging in a conventional method, and sealing the aluminum case of the battery to obtain a lithium-ion secondary battery.

4) Battery high temperature performance test

The test method is as follows: charge the battery at 1C to 100% state of charge, place it in an oven, and the temperature of the oven rises from room temperature to 150°C at a rate of 5°C/min. If the battery voltage drops greater than 0.2 volts, it is considered a short circuit.

5) Battery life test

The test method is as follows: at 25±5°C, the battery is charged and discharged 250 times, and the remaining power is recorded. The higher the remaining power, the longer the battery life.

The cellulose porous membrane prepared in Examples 1-4 and the commercial separator in Comparative Example 1 were used to make a battery, and the high temperature resistance and life of the battery were tested according to the above test method. The results obtained are listed in Table 2.

It can be seen from the results in Table 2 that the lithium-ion battery prepared by using the cellulose nanofiber membrane of the present invention as a battery separator has better safety performance and longer service life.

Table 1

Table 2

Claims (3)

1. a biomass cellulose nanofibers composite membrane, it is characterized in that: base material is adopt electrostatic spinning law technology to carry out to biomass cellulose the Gradient distribution cellulose non-woven film that diameter that spinning obtains is 20 nanometer ~ 20 micron, the both side surface of film is coated with sodium alginate, fluoropolymer, polyaryletherketone, polyimide, polynorbornene and/or inorganic nano-particle, film thickness is 15 ~ 100 microns, and film air penetrability is 2 ~ 500 seconds; Film surface and internal holes are distributed symmetrically and evenly, mean pore size is 20 ~ 200 nanometers, and tensile strength is 50 ~ 250 MPas;

Wood pulp, jute pulp or cotton pulp are as the initial feed of biomass cellulose;

Adopt electrostatic spinning technique to carry out spinning to biomass fiber cellulose solution, obtain the Gradient distribution cellulose non-woven film that diameter is 20 nanometer ~ 20 micron, the large fiber of micron order is for improving physical strength, and nanofiber is used for uniform electric field intensity and improves absorbent;

Wherein, the mass percent of biomass fiber cellulose solution is 1-20%, and solvent is N-tertiary amine oxide class or lithium chloride/DMAc system,

Wherein, the spinning syringe needle internal diameter of electrostatic spinning is 0.8-2.0 millimeter, and voltage is 100 volts-30 kilovolts, and syringe needle is 10-30 centimetre with the distance accepting electrode, and spinning solution flow is greater than 0.1 ml/hour.

2. a kind of biomass cellulose nanofibers composite membrane according to claim 1, is characterized in that being coated with sodium alginate, fluoropolymer, polyaryletherketone, polyimide in the both side surface of cellulose membrane, polynorbornene and/or inorganic nano-particle;

Wherein, sodium alginate, fluoropolymer, polyaryletherketone, polyimide, the solution of polynorbornene to be mass percent be 1-10%, solvent is water, acetone, tetrahydrofuran (THF), DMF, one or both among N,N-dimethylacetamide;

Wherein, fluoropolymer is vinylidene fluoride-hexafluoropropylene copolymer (P (VDF-HFP)), vinylidene-chlorotrifluoroethylcopolymer copolymer (P (VDF-CTFE)) and vinylidene-trifluoro-ethylene copolymer P (VDF-TFE), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene copolymer (ETFE), ethene trichlorine fluoride copolymers (ECTFE);

Wherein, inorganic nano-particle is nano silicon, zirconium dioxide, alchlor or lithium niobate inorganic nano-particle, inorganic nano-particle and sodium alginate, fluoropolymer, polyaryletherketone, and polyimide polymer mass ratio is 0 ~ 9: 10 ~ 1.

3. the application of biomass cellulose nanofibers composite membrane according to claim 1 in lithium-ion secondary cell.