CN111725466A

CN111725466A - Functionalized polyolefin composite diaphragm and preparation method and application thereof

Info

Publication number: CN111725466A
Application number: CN202010539605.3A
Authority: CN
Inventors: 呼微; 王宇粮; 谢海明; 朱广山
Original assignee: Northeast Normal University
Current assignee: Northeastern University China; Northeast Normal University
Priority date: 2020-06-12
Filing date: 2020-06-12
Publication date: 2020-09-29
Anticipated expiration: 2040-06-12
Also published as: CN111725466B

Abstract

The invention provides a functionalized polyolefin composite diaphragm and a preparation method and application thereof, the functionalized polyolefin composite diaphragm consists of carbon nano tubes, lithium salt, inorganic nano silicon dioxide and polyolefin components, and the preparation method of the composite diaphragm provided by the invention comprises the following steps: the preparation method comprises the steps of mixing a binder, a solution of a functionalized carbon nanotube and nano-silica to obtain a coating solution, placing a polyolefin diaphragm in an ethanol solution of lithium salt to obtain a lithium salt modified polyolefin diaphragm, uniformly coating the coating solution on the lithium salt modified polyolefin diaphragm, and drying in vacuum to obtain the functionalized carbon nanotube and lithium salt coated polyolefin diaphragm.

Description

Functionalized polyolefin composite diaphragm and preparation method and application thereof

Technical Field

The invention belongs to the field of lithium ion batteries, and particularly relates to a functionalized polyolefin composite diaphragm, a preparation method and application thereof, and application of the functionalized polyolefin composite diaphragm in a lithium ion battery.

Technical Field

Lithium Ion Batteries (LIBs) have the advantages of high working voltage, high energy density, long service life and the like, and are promising power sources for mobile equipment, smart power grids, electric vehicles and other large-scale power energy storage application power sources. However, the safety and efficiency issues inherent in the traditional materials used to make LIBs limit their further applications. LIBs are composed of four major components, a positive electrode, an electrolyte, a separator, and a negative electrode. The separator, which is an important component of LIBs and is located between the positive and negative electrodes, is an ion conductor and an electron insulator, and plays a key role in preventing short circuit by cutting off the positive and negative electrodes. Current commercially available LIBs separators are made from polyolefins such as polypropylene (PP) and Polyethylene (PE).

However, the conventional polyolefin separator is limited by surface energy, so that the wettability and retention of the electrolyte are poor, and the polyolefin separator has high crystallinity and cannot enter a crystallization region when contacting with the electrolyte, so that the wettability of the separator to the electrolyte is poor, the wetting speed is slow, and the ion conductivity and the resistance of the separator are high. In addition, thermal stability is poor due to low melting point, thermal shrinkage is likely to occur at high temperature, the thermal shrinkage is likely to cause contact between internal electrodes to cause short circuit, when the operating temperature is too high, the separator may be damaged and cause short circuit, causing explosion of LIBs, and safety and cycle performance of LIBs are seriously affected by insufficient thermal stability and electrochemical performance of the separator.

The use of modified membranes to address the safety issues arising from the use of these LIBs is a simple and effective approach. At present, the physical coating of the binder containing nano ceramic particles is one of the main ways to enhance the mechanical properties and the dimensional thermal stability of the polyolefin separator. The coating layer increases the thickness of the separator, extends the path of lithium ion transport, and the binder also decreases the lithium ion transport rate. Shi et Al convert alumina (Al)₂O₃) Carboxymethyl cellulose and styrene-butadiene rubber were mixed and coated on one side of a pure PE separator to prepare a coated separator for LIBs, with pure PECompared with the modified PE diaphragm, the thermal shrinkage rate of the modified diaphragm is kept at 65% after the modified PE diaphragm is heated at 145 ℃ for 0.5 h. Fu, etc. uses polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) as binder, uses acetone as solvent, directly hydrolyzes tetraethyl silicate to obtain nano silicon dioxide (SiO) with uniform particle size₂) The particles are coated on the PP diaphragm, the size thermal shrinkage rate of the modified PP diaphragm is kept at 63% after the modified PP diaphragm is subjected to high temperature of 145 ℃ for 0.5h, and the tensile strength, the contact angle and the electrolyte wettability are remarkably improved.

In summary, the conventional polyolefin separator has poor dimensional thermal stability and poor electrolyte wettability, and the existing inorganic coating modification methods improve the dimensional thermal stability of the separator to a certain extent, but the overall performance still has a space and a need for greatly improving, so that a better improvement method is needed to obtain a lithium ion battery composite separator with enhanced dimensional thermal stability, good electrolyte wettability and excellent electrochemical performance.

Disclosure of Invention

The invention aims to overcome the defects of poor dimensional thermal stability, poor electrolyte wettability and the like of the traditional polyolefin diaphragm, and provides a functionalized polyolefin composite diaphragm, a preparation method and application thereof, so that the composite diaphragm with excellent comprehensive performance, excellent thermal stability, improved mechanical strength, good electrolyte wettability and improved electrochemical performance is obtained.

The invention provides a functionalized polyolefin composite diaphragm and a preparation method and application thereof, and the preparation method comprises the following steps:

(1) preparation of coating liquid: mixing the binder and the solvent, refluxing and stirring at 70-100 deg.C for 4-8h to obtain binder solution, mixing carbon nanotube and nano SiO₂Mixing with deionized water, adding a binder solution, and performing ultrasonic stirring to obtain a coating solution;

(2) lithium salt modified polyolefin separator: adding lithium salt into ethanol to obtain a lithium salt ethanol solution, then ultrasonically cleaning the polyolefin diaphragm in the ethanol, then drying in vacuum, soaking the dried polyolefin diaphragm in the lithium salt ethanol solution for 30-60 min, and then drying in vacuum to obtain the lithium salt modified polyolefin diaphragm.

(3) PolyalkenesPreparation of hydrocarbon composite membrane: flatly laying the lithium salt modified polyolefin diaphragm obtained in the step (2) on a glass plate, uniformly coating the coating liquid obtained in the step (1) on one side of the polyolefin diaphragm obtained in the step (2) by using a scraper, and then carrying out vacuum drying to obtain the carbon nano tube, the lithium salt and the nano SiO₂A modified polyolefin composite separator.

The carbon nano tube in the step (1) is carboxylated carbon nano tube (CNT-COOH) or sulfonated carbon nano tube (CNT-SO)₃) Any one of hydroxylated carbon nanotubes (CNT-OH);

the binder in the step (1) is polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC) or polymethyl acrylate;

the solvent in the step (1) is any one of N, N-Dimethylformamide (DMF), N-Dimethylacetamide (DMAC), deionized water, ethanol and acetone;

the mass concentration of the carbon nano tube in the coating liquid in the step (1) is 0.05-0.4%, preferably 0.1-0.3%;

the lithium salt in the step (2) is lithium hexafluorophosphate (LiPF)₆) Lithium tetrafluoroborate (LiBF)₄) Lithium bis (fluorosulfonylimide) (LiFSI), lithium trifluoromethanesulfonate (CF)₃SO₃Any one of Li) and lithium bis (trifluoromethanesulfonate) imide (LiTFSI);

the polyolefin diaphragm in the step (2) is any one of a polyethylene diaphragm (PE) and a polypropylene diaphragm (PP);

a functionalized polyolefin composite membrane is prepared by the following steps:

(1) preparation of coating liquid: mixing PVA and deionized water, refluxing and stirring at 90 deg.C for 4 hr to obtain 4% PVA water solution, mixing carbon nanotube and nano SiO₂Mixing with deionized water, adding PVA aqueous solution, and performing ultrasonic stirring to obtain a coating solution with the carbon nanotube content of 0.3%;

(2) preparing a lithium salt modified polyolefin diaphragm: adding lithium bistrifluoromethylsulfonyl imide into ethanol to prepare a lithium salt ethanol solution with the mass concentration of 2%, then placing the polyolefin diaphragm into the lithium salt ethanol solution with the mass concentration of 2%, soaking for 30min, and then carrying out vacuum drying at 40 ℃ for 24h to obtain the lithium salt modified polyolefin diaphragm.

(3) Preparing a polyolefin composite diaphragm: flatly laying the lithium salt modified polyolefin diaphragm obtained in the step (2) on a glass plate, uniformly coating the coating liquid obtained in the step (1) on one side of the polyolefin diaphragm obtained in the step (2) by using a scraper, and then carrying out vacuum drying to obtain the carbon nano tube, the lithium salt and the nano SiO₂A modified polyolefin composite separator.

The invention also provides application of the functionalized polyolefin composite membrane in a lithium ion battery.

The invention has the advantages of

The invention provides a method for coating a polyolefin diaphragm with a functionalized carbon nanotube and a lithium salt and a preparation method of a composite film thereof, wherein the method utilizes nano-silica with excellent thermal stability to improve the thermal stability of the diaphragm; at the same time, nano SiO₂The specific surface area of (a) is large, the bulk density is small, and the like, the wettability of the polyolefin separator to the electrolyte is improved, but the interface resistance is increased by the inorganic silica. And then by utilizing the remarkable characteristics of excellent electrical property, strong mechanical strength, high stability, high specific surface area and the like of the carbon nano tube, the functionalized carbon nano tube is coated on the positive electrode side of the diaphragm, the battery performance is improved, and the composite polyolefin diaphragm with excellent comprehensive performance, high size thermal stability, good electrolyte wettability, reduced interface impedance and improved electrochemical performance is obtained, so that the composite polyolefin diaphragm can be applied to lithium ion batteries in a wider range.

Drawings

In FIG. 1, (a), (b), (c), and (d) are respectively surface scanning electron microscope images of the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, the PE @ LiSiCNT0.1 composite separator of example 1, and the PE @ LiSiCNT0.3 composite separator of example 2;

in fig. 2 ((a), (b), (c), (d) are photographs before heat treatment of the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, the PE @ lisicnt0.1 composite separator of example 1, and the PE @ lisicnt0.3 composite separator of example 2, respectively, and (e), (f), (g), and (h) are heat-shrinkage photographs of the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, and the PE @ LiSiCNT composite separators of example 1 and example 2, respectively, after heat treatment at 150 ℃ for 30 min;

FIG. 3 is a bulk impedance plot of the PE membrane of comparative example 1, the PE @ Si composite membrane of comparative example 2, the PE @ LiSiCNT0.1 composite membrane of example 1, and the PE @ LiSiCNT0.3 composite membrane of example 2;

FIG. 4 is an AC impedance plot of the PE membrane of comparative example 1, the PE @ Si composite membrane of comparative example 2, the PE @ LiSiCNT0.1 composite membrane of example 1, and the PE @ LiSiCNT0.3 composite membrane of example 2;

fig. 5 shows the cell cycle performance at different rates for the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, and the PE @ liscnt0.1 composite separator of example 1, and the PE @ liscnt0.3 composite separator of example 2;

fig. 6 is the battery cycle performance of the PE separator of comparative example 1 at a charge and discharge current density of 0.2C;

fig. 7 shows the battery cycle performance of the PE @ Si composite separator of comparative example 2 at a charge and discharge current density of 0.2C;

fig. 8 is the battery cycle performance of the PE @ lisicnt0.1 composite separator of example 1 at a charge and discharge current density of 0.2C;

fig. 9 shows the battery cycle performance of the PE @ lisicnt0.3 composite separator of example 2 at a charge-discharge current density of 0.2C.

Detailed Description

The technical solution of the present invention will be further described in detail with reference to specific examples. For the purpose of providing a more clear understanding and appreciation for the same by those skilled in the art. The following specific examples should not be construed or interpreted as limiting the scope of the claims of the present application in any way.

Example 1

(1) Preparation of coating liquid:

4g of polyvinyl alcohol (PVA) and 96ml of deionized water were added to a three-necked flask, respectively, and stirred under reflux at 90 ℃ for 4 hours to obtain a 4% mass concentration PVA aqueous solution. Aqueous hydroxylated carbon nanotubes (CNT-OH) were mixed with deionized water to make a 5% CNT-OH solution. 1g of nano SiO₂And 0.3g of 5% CNT-OH solution was added to 13.7g of aqueous PVA solution, sonicated and stirred for 0.5h to give a CNT-OH content of 0.1% of coating liquid.

(2) Lithium salt modified polyolefin separator:

adding lithium bistrifluoromethylsulfonyl imide (LiTFSI) into ethanol to prepare a LiTFSI/ethanol solution with the mass concentration of 2%, ultrasonically cleaning a PE diaphragm by using ethanol, drying the PE diaphragm in vacuum at 40 ℃, soaking the PE diaphragm in the LiTFSI/ethanol solution for 30min, and drying the PE diaphragm in vacuum at 40 ℃ for 24h to obtain the lithium salt modified polyolefin diaphragm.

(3) Preparation of a PE @ LiSiCNT0.1 composite membrane:

and (3) flatly laying the lithium salt modified polyolefin diaphragm obtained in the step (2) on a glass plate, uniformly coating the coating liquid prepared in the step (1) on one side of the PE diaphragm obtained in the step (2) by using a scraper, and drying in vacuum to obtain the PE @ LiSiCNT0.1 composite diaphragm.

(4) Preparing a positive plate:

1.6g of LiFePO were respectively taken₄0.2g of acetylene black and 0.2g of PVDF are dissolved in N-methylpyrrolidone (NMP), the mixture is stirred uniformly, the obtained slurry is coated on aluminum foil paper, and the positive plate is obtained after cutting and drying.

(5) Assembling the battery:

assembling the positive electrode shell, the positive electrode sheet obtained in the step (4), the PE @ LiSiCNT0.1 composite diaphragm obtained in the step (3), a lithium sheet, a gasket and a shrapnel into Li// PE @ LiSiCNT0.1// LiFePO₄And (4) half cell.

Example 2

(1) Preparation of coating liquid:

respectively placing 4g of PVA and 96ml of deionized water in a three-neck flask, carrying out reflux stirring at 90 ℃ for 4 hours to obtain a PVA aqueous solution with the mass concentration of 4%, preparing aqueous CNT-OH and deionized water into a CNT-OH solution with the mass concentration of 5%, and preparing 1g of nano SiO₂And 0.9g of 5% CNT-OH solution were added to 13.1g of PVA aqueous solution, sonicated and stirred for 0.5h to give a coating solution with a CNT-OH content of 0.3%.

(2) Lithium salt modified polyolefin separator:

(3) Preparation of a PE @ LiSiCNT0.3 composite membrane:

and (3) flatly laying the lithium salt modified polyolefin diaphragm obtained in the step (2) on a glass plate, uniformly coating the coating liquid prepared in the step (1) on one side of the PE diaphragm by using a scraper, and drying in vacuum to obtain the PE @ LiSiCNT0.3 composite diaphragm.

(4) Preparing a positive plate:

1.6g of LiFePO were respectively taken₄0.2g of acetylene black and 0.2g of PVDF are dissolved in NMP, evenly stirred, coated on aluminum foil paper, cut into pieces and dried to obtain the positive plate.

(5) Assembling the battery:

taking the PE @ LiSiCNT0.3 composite diaphragm in the step (3), taking the positive plate prepared in the step (4), and assembling the positive shell, the positive plate in the step (4), the PE @ LiSiCNT0.3 composite diaphragm in the step (3), a lithium plate, a gasket and an elastic sheet into Li// PE @ LiSiCNT0.3// LiFePO₄And (4) half cell.

Comparative example 1

The same LiFePO as in examples 1-2 was used with a commercial polyolefin separator₄The positive plate is assembled into Li// PE// LiFePO₄And (4) half cell.

Comparative example 2

(1) Placing 4g of PVA and 96ml of deionized water in a three-neck flask, refluxing and stirring at 90 ℃ for 4h to obtain a PVA aqueous solution with the mass concentration of 4%, and placing 1g of nano SiO₂Adding into 14g of PVA water solution, ultrasonic stirring for 0.5h to obtain 1 wt% SiO₂And (3) nano coating liquid.

(2) And (2) ultrasonically cleaning the PE diaphragm by using ethanol, drying the PE diaphragm in vacuum at 40 ℃, and coating the coating liquid obtained in the step (1) on one side of the PE diaphragm to obtain the PE @ Si composite diaphragm.

(3) The same LiFePO as in example 1-2 was used₄And (3) assembling the positive plate and the PE @ Si composite diaphragm obtained in the step (2) into Li// PE @ Si// LiFePO₄Half cell of

Performance testing

1. The thickness, porosity, electrolyte absorption rate of the PE membrane of comparative example 1, the PE @ Si composite membrane of comparative example 2, the PE @ lisicnt0.1 composite membrane of example 1, and the PE @ lisicnt0.3 composite membrane of example 2.

2. Electrolyte static contact angle tests of the PE membrane of comparative example 1, the PE @ Si composite membrane of comparative example 2, the PE @ lisicnt0.1 composite membrane of example 1, and the PE @ lisicnt0.3 composite membrane of example 2.

3. SEM tests of the PE membrane of comparative example 1, the PE @ Si composite membrane of comparative example 2, and the PE @ lisicnt0.1 composite membrane of example 1, the PE @ lisicnt0.3 composite membrane of example 2.

4. Thermal shrinkage tests were conducted on the PE membrane of comparative example 1, the PE @ Si composite membrane of comparative example 2, the PE @ lisicnt0.1 composite membrane of example 1, and the PE @ lisicnt0.3 composite membrane of example 2.

5. Ion conductivity tests were performed on the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, and the PE @ liscnt 0.1 composite separator of example 1, and the PE @ liscnt 0.3 composite separator of example 2.

6. Ac impedance tests of the PE membrane of comparative example 1, the PE @ Si composite membrane of comparative example 2, the PE @ lisicnt0.1 composite membrane of example 1, and the PE @ lisicnt0.3 composite membrane of example 2.

7. Cell cycle performance tests of the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, the PE @ lisicnt0.1 composite separator of example 1, and the PE @ lisicnt0.3 composite separator of example 2.

The detailed testing and conclusions are as follows:

1. testing the film thickness, porosity and electrolyte absorption rate: measuring the thickness of each film by using a micrometer screw, and measuring for three times to obtain an average value; the diaphragm was cut to a size of 2 x 2cm and its mass W was weighed_drySoaking the diaphragm in n-butanol in a sealed container for 2 hr, taking out, wiping off n-butanol on the surface of the diaphragm with filter paper, and weighing the mass W_wetAccording to the formula (W)_wet-W_dry)/ρv*100(W_dryDenotes the mass of the membrane before absorption of n-butanol, W_wet represents the mass of the separator after absorbing the electrolyte. Rho is the n-butanol density, v is the diaphragm volume) to calculate the porosity; the diaphragm was cut to a size of 2 x 2cm and its mass W was weighed_dryThen will separateSoaking the membrane in a closed container filled with electrolyte for 2h, taking out the membrane, and weighing the mass W of the membrane_wetAccording to the formula (W)_wet-W_dry)/W_dry*100(W_dryDenotes the mass of the separator before the absorption of the electrolyte, W_wet represents the mass of the separator after absorbing the electrolyte), the electrolyte absorption rate of each separator was calculated.

Referring to table 1, the PE @ LiSiCNT composite film obtained in examples 1-2 had a thickness of about 22 μm, which meets the requirement that the LIB membrane thickness is less than 25 μm. The PE @ LiSiCNT composite membrane obtained in the embodiment 1-2 has improved electrolyte absorptivity and porosity compared with PE diaphragms and PE @ Si composite diaphragms. The CNT-OH has large specific surface area and good affinity to electrolyte, and improves the porosity and the liquid absorption rate of the composite diaphragm. The PE @ lisicnt0.1 composite separator and the PE @ lisicnt0.3 composite separator contain a lithium salt, and the electrolyte wettability of the composite separators of examples 1 to 2 can be further improved.

2. And (3) testing the static contact angle of the electrolyte: the separator samples of examples and comparative examples were cut into strips of a certain size, the test strips were smoothly stuck to the surface of a glass plate, contact angle values were measured and recorded with about 1.2. mu.l per drop of the electrolyte, and the values were averaged by a plurality of measurements. Referring to table 2, the electrolyte contact angles of the PE @ lisicnt0.1 composite membrane and the PE @ lisicnt0.3 composite membrane were superior to those of the PE membrane and the PE @ Si composite membrane.

3. And (3) appearance observation: and (3) spraying gold on the diaphragm sample, and observing the surface appearance of the diaphragm under an electron scanning microscope.

Referring to the attached figure 1 of the specification, the surface of the PE @ Si composite diaphragm is provided with nano SiO₂Coating of nano SiO₂The composite diaphragm has rich pore structures, and CNT-OH on the surfaces of the PE @ LiSiCNT0.1 and the PE @ LiSiCNT0.3 composite diaphragms exists in a three-dimensional network shape, so that a large number of pore structures are formed.

4. And (3) testing thermal stability: example and comparative example diaphragm samples were cut into 2 x 2cm size, heated in an electrically heated constant temperature forced air drying oven at 150 ℃ for 30min, and the diaphragm was observed for dimensional change.

Referring to Table 3 and accompanying drawing 2 of the specification, PE diaphragm and PE @ Si composite diaphragm, PE @ LiSiCNT0.1 composite diaphragm, PE@ lisicnt0.3 composite separator, the thermal shrinkage rates were 74%, 36%, 17%, and 14%, respectively. Compared with the heat shrinkage rate (74%) of a pure PE diaphragm, the heat shrinkage rate of the composite diaphragm is obviously improved. This is mainly the SiO of the surface of the composite diaphragm₂And the CNT-OH coating improves the thermal stability of the diaphragm, the LiTFSI melting point in the PE @ LiSiCNT composite diaphragm is 236 ℃, and the LiTFSI in the diaphragm pores effectively reduces the thermal shrinkage of the diaphragm at the high temperature of 150 ℃, so that the thermal stability of the PE @ LiSiCNT composite diaphragm is obviously improved.

5. And (3) ion conductivity test: cutting a diaphragm sample into a wafer with the diameter of 1.6cm, fully soaking the cut diaphragm in electrolyte in a glove box filled with argon, taking out the diaphragm and placing the diaphragm between two stainless steel poles to assemble the CR2032 type button cell. The conductivity was tested by electrochemical workstation type chenhua CHI660E using a frequency range of 10mHz to 1mHz at room temperature. The ion conductivity was calculated from σ ═ L/(R S) (σ is the ion conductivity, L is the thickness of the separator, R is the bulk resistance of the separator, and S is the effective contact area of the separator).

Referring to table 5 and accompanying figure 3 of the specification, the resistance values of the PE diaphragm and the PE @ Si composite diaphragm, the PE @ liscnt0.1 composite diaphragm, and the PE @ liscnt0.3 composite diaphragm are 1.84, 2.06, 1.72, and 1.69 Ω, respectively. The ionic conductivities were 1.01X 10-3, 1.21X 10-3, 1.66X 10-3 and 1.69X 10-3S cm-1, respectively. The PE @ LiSiCNT composite membrane has low resistance and high ionic conductivity.

6. And (3) testing alternating current impedance: cutting a diaphragm sample into a wafer with the diameter of 1.6cm, fully soaking the cut diaphragm in electrolyte in a glove box filled with argon, taking out the diaphragm and placing the diaphragm in a lithium plate cathode and LiFePO₄And a button battery is assembled between the positive electrodes. The resistance was tested by electrochemical workstation model Chenghua CHI660E using a frequency range of 10mHz-1MHz at room temperature.

Referring to table 6 and accompanying fig. 4 of the specification, the interface resistances of the PE membrane and the PE @ Si composite membrane, the PE @ liscnt0.1 composite membrane, and the PE @ liscnt0.3 composite membrane were 181, 202, 130, and 128 Ω, respectively. Compared with a PE diaphragm battery, the PE @ LiSiCNT composite diaphragm has obviously reduced impedance.

7. Electrical Performance testing

At room temperatureThe charge-discharge performance and the coulombic efficiency of the battery are tested by a Xinwei battery test system (BTS-4000). In the examples and comparative examples, a lithium sheet was used as the negative electrode, LiFePO₄When the membrane sample is the positive electrode, the membrane sample is placed in the middle, the coating surface is close to one side of the positive electrode, the button cell is assembled, the cell is charged to 4.3V in a constant current-constant voltage mode (0.2C charging and discharging), and then the cycle performance of the cell is measured in a mode of discharging to 3.0V. Similarly, the battery assembled by the diaphragm has the rate performance when the battery is charged to 4.2V and discharged to 3V when the charge-discharge current density is 0.2C, 0.5C, 1.0C, 2.0C and 0.2C.

Referring to the attached figure 5 of the specification, under the charge-discharge current densities of 0.2C, 0.5C, 1.0C, 2.0C and 0.2C, the discharge capacity of the PE @ LiSiCNT composite diaphragm is obviously improved compared with that of a PE diaphragm and a PE @ Si composite diaphragm. When the current density reaches 2C, the discharge capacities of the PE diaphragm, the PE @ Si composite diaphragm, the PE @ LiSiCNT0.1 composite diaphragm and the PE @ LiSiCNT0.3 composite diaphragm are respectively 105 mAh/g, 113 mAh/g, 137 mAh/g and 138 mAh/g. This indicates that Li⁺Transport in the PE @ LiSiCNT composite film is less affected by current density. The interface polarization generated by high current density affects Li + transmission, and the interface polarization phenomenon generated by the PE film under the condition of high multiplying power is serious, which affects Li⁺The transmission speed of (2); and the low impedance and high-efficiency ion transmission of the PE @ LiSiCNT composite membrane improve the stability of the battery.

Referring to the attached figures 6, 7, 8 and 9 of the specification, after the cyclic charge and discharge for 250 times, the discharge capacities of the PE diaphragm, the PE @ Si composite diaphragm, the PE @ LiSiCNT0.1 composite diaphragm and the PE @ LiSiCNT0.3 composite diaphragm are 143, 148, 163 and 164mAh/g respectively, and the discharge capacity of the PE @ LiSiCNTT composite diaphragm is obviously improved compared with that of the PE and PE @ Si composite diaphragms in the cyclic process. The PE @ LiSiCNT composite membrane has higher discharge capacity because the CNT-OH electrophilic electrolyte and the CNT-OH network structure endow the composite membrane with high porosity; in addition Li⁺Can effectively diffuse to the stable position on the surface of the CNT and the inside of the single CNT through the opening of the port or the side wall of the CNT and the carbon nano tube layer, and the CNT can store Li in an enrichment way⁺Buffer Li⁺While reducing the impedance of the separator, thereby improving the lithium ion conductivity and high transmission efficiency of the separator. Simultaneous LitThe FSI is introduced to provide more lithium sources, improve the ion transmission performance, effectively reduce the impedance and polarization of the battery, further obtain high discharge specific capacity and stable cycle performance, and improve the electrochemical performance.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Table 1 thickness, porosity, electrolyte absorption rate of the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, and the PE @ liscnt0.1 composite separator of example 1, the PE @ liscnt0.3 composite separator of example 2

Diaphragm	Electrolyte absorption rate (%)	Porosity (%)	Diaphragm thickness (mum)
				Comparative example 1	70	48	15±2
Comparative example 2	105	60	20±2
				Example 1	102	67	22±2
Example 2	110	69	22±2

Table 2 electrolyte static contact angles of the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, the PE @ liscnt0.1 composite separator of example 1, and the PE @ liscnt0.3 composite separator of example 2

Diaphragm	Comparative example 1	Comparative example 2	Example 1	Example 2
					Contact of electrolyte solution (°)	54.8	18.8	14.8	13.3

Table 3 thermal shrinkage data for the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, the PE @ liscnt0.1 composite separator of example 1, and the PE @ liscnt0.3 composite separator of example 2

Diaphragm	Comparative example 1	Comparative example 2	Example 1	Example 2
					Thermal shrinkage (%)	74	36	17	14

Table 4 ion conductivities of the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, and the PE @ liscnt 0.1 composite separator of example 1 and the PE @ liscnt 0.3 composite separator of example 2

Table 5 ac impedance data for the PE separator of comparative example 1, the PE @ Si composite separator of comparative example 2, the PE @ liscnt0.1 composite separator of example 1, and the PE @ liscnt 0.3 composite separator of example 2

Diaphragm	Comparative example 1	Comparative example 2	Example 1	Example 2
					Impedance (omega)	181	202	130	128

Claims

1. A functionalized polyolefin composite membrane is characterized by being prepared by the following method:

(1) preparation of coating liquid: mixing a binder and a solvent, stirring at 70-100 ℃ for 4-8h under reflux to obtain a binder solution, mixing the carbon nano tube, the nano silicon dioxide and the solvent, adding the binder solution, and performing ultrasonic stirring to obtain a coating solution;

(2) lithium salt modified polyolefin separator: adding a lithium salt into ethanol to obtain a lithium salt ethanol solution, ultrasonically cleaning a polyolefin diaphragm in the ethanol, then drying the polyolefin diaphragm in vacuum, soaking the dried polyolefin diaphragm in the lithium salt ethanol solution for 30-60 min, and drying in vacuum to obtain a lithium salt modified polyolefin diaphragm;

(3) preparing a polyolefin composite diaphragm: and (3) flatly laying the lithium salt modified polyolefin diaphragm obtained in the step (2) on a glass plate, uniformly coating the coating liquid obtained in the step (1) on one side of the polyolefin diaphragm obtained in the step (2) by using a scraper, and then carrying out vacuum drying to obtain the carbon nano tube, lithium salt and nano silicon dioxide modified polyolefin composite diaphragm.

2. The functionalized polyolefin composite membrane according to claim 1, wherein the carbon nanotubes in step (1) are any one of carboxylated carbon nanotubes, sulfonated carbon nanotubes and hydroxylated carbon nanotubes.

3. The functionalized polyolefin composite membrane according to claim 1, wherein the binder in step (1) is any one or a combination of any several of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose or polymethyl acrylate.

4. The functionalized polyolefin composite membrane according to claim 1, wherein the solvent in step (1) is any one of N, N-dimethylformamide, N-dimethylacetamide, deionized water, ethanol and acetone.

5. The functionalized polyolefin composite membrane according to claim 1, wherein the mass concentration of the carbon nanotubes in the coating solution in the step (1) is 0.05-0.4%.

6. The functionalized polyolefin composite membrane according to claim 5, wherein the mass concentration of the carbon nanotubes in the coating solution in the step (1) is 0.1-0.3%.

7. The functionalized polyolefin composite separator according to claim 1, wherein the lithium salt in step (2) is any one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis-fluorosulfonylimide, lithium trifluoromethanesulfonate and lithium bis (trifluoromethanesulfonate) imide.

8. The functionalized polyolefin composite membrane according to claim 1, wherein the polyolefin in the step (2) is any one of polyethylene or polypropylene.

9. The functionalized polyolefin composite membrane according to claim 1, which is prepared by the following method:

(1) preparation of coating liquid: mixing PVA and deionized water, stirring at 90 ℃ for 4 hours under reflux to obtain a PVA aqueous solution with the mass concentration of 4%, mixing carbon nano tubes, nano-silica and deionized water, adding the PVA aqueous solution, and performing ultrasonic stirring to obtain a coating solution with the carbon nano tube content of 0.3%;

(3) Preparing a polyolefin composite diaphragm: flatly laying the lithium salt modified polyolefin diaphragm obtained in the step (2) on a glass plate, uniformly coating the coating liquid obtained in the step (1) on one side of the polyolefin diaphragm obtained in the step (2) by using a scraper, and then carrying out vacuum drying to obtain the carbon nano tube, the lithium salt and the nano silicon dioxide₂A modified polyolefin composite separator.

10. The use of the functionalized polyolefin composite separator of claim 1 in a lithium ion battery.