CN111725466A - A kind of functionalized polyolefin composite membrane and its preparation method and application - Google Patents

Abstract

The present invention provides a functionalized polyolefin composite diaphragm and a preparation method and application thereof. The functionalized polyolefin composite diaphragm of the present invention is composed of carbon nanotubes, lithium salts, inorganic nano-silica and polyolefin components. The present invention provides The preparation method of the composite diaphragm is as follows: the solution of the binder and the functionalized carbon nanotubes is mixed with nano-silica to obtain a coating solution, and then the polyolefin diaphragm is placed in an ethanol solution of lithium salt to obtain a lithium salt modified polymer. olefin separator, and then uniformly coat the coating liquid on the lithium salt modified polyolefin separator, and then vacuum dry to obtain functionalized carbon nanotubes and lithium salt coated polyolefin separator. The prepared separator has excellent mechanical properties. Strength, high stability, high active surface area, good electrolyte wettability, excellent lithium ion conductivity and electrochemical performance, can be well used in the field of lithium ion batteries.

Description

A kind of functionalized polyolefin composite membrane and its preparation method and application

technical field

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

technical background

Lithium-ion batteries (LIBs) have the advantages of high operating voltage, high energy density, and long service life. They are a promising power source for mobile devices and large-scale power storage applications such as smart grids and electric vehicles. However, the inherent safety and efficiency issues of traditional materials used to fabricate LIBs limit their further applications. LIBs are composed of four main components: positive electrode, electrolyte, separator, and negative electrode. The separator located between the positive and negative electrodes is an important part of LIBs, which is an ionic conductor and an electronic insulator, and plays a key role in preventing short circuits by cutting off the positive and negative electrodes. Currently commercial LIBs separators are made of polyolefins such as polypropylene (PP) and polyethylene (PE).

However, the traditional polyolefin separator is limited by the surface energy, which leads to poor wettability and retention of the electrolyte. Due to the high crystallinity of the polyolefin separator, the electrolyte cannot enter the crystalline region when it is in contact with the electrolyte, resulting in the wettability of the separator to the electrolyte. Poor wettability and slow wetting, resulting in low ionic conductivity and high electrical resistance of the separator. In addition, due to its low melting point, the thermal stability is poor, and thermal shrinkage is prone to occur at high temperatures. Thermal shrinkage can easily cause internal electrodes to contact each other and cause a short circuit. When the operating temperature is too high, the diaphragm may be damaged and cause a short circuit , causing the explosion of LIBs, and the insufficient thermal stability and electrochemical performance of the separators seriously affect the safety performance and cycling performance of LIBs.

The use of modified separators is a simple and effective approach to address the safety issues arising from the use of these LIBs. At present, one of the main ways to enhance the mechanical properties and dimensional thermal stability of polyolefin separators is by physically coating the binders containing nano-ceramic particles. But the coating layer increases the thickness of the separator, extending the path for lithium ion transport, and the binder also reduces the speed of lithium ion transport. Shi et al. prepared a coated separator for LIBs by mixing aluminum oxide (Al ₂ O ₃ ), carboxymethyl cellulose and styrene-butadiene rubber and coating one side of a pure PE separator, Compared with the unmodified PE separator, the thermal shrinkage of the modified separator remained at 65% after 0.5 h at a high temperature of 145 °C. Fu et al. used polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) as a binder and acetone as a solvent to directly hydrolyze tetraethyl silicate to obtain nano-silica (SiO ₂ ) particles with uniform particle size. Covered on the PP separator, the dimensional thermal shrinkage of the modified PP separator remains at 63% after 0.5 h at 145 °C, and the tensile strength, contact angle, and electrolyte wettability are significantly improved.

To sum up, the dimensional thermal stability of the traditional polyolefin separator and the wettability of the electrolyte are poor. Some existing inorganic coating modification methods improve the dimensional thermal stability of the separator to a certain extent, but the overall performance There is still much room and need for improvement, so a better improved method is needed to obtain a lithium-ion battery composite separator with enhanced dimensional thermal stability, good electrolyte wettability, and excellent electrochemical performance.

SUMMARY OF THE INVENTION

The purpose of the present invention is to solve the shortcomings of the traditional polyolefin diaphragm, such as poor dimensional thermal stability and poor electrolyte wettability, the present invention provides a functionalized polyolefin composite diaphragm, a preparation method and application thereof, and obtains excellent thermal stability, The composite separator with improved mechanical strength, good electrolyte wettability and improved electrochemical performance has excellent comprehensive properties.

The invention provides a functionalized polyolefin composite diaphragm and a preparation method and application thereof. The steps are as follows:

(1) Preparation of coating solution: mix binder and solvent, reflux and stir at 70-100°C for 4-8h to obtain binder solution, mix carbon nanotubes, nano-SiO ₂ and deionized water, and add binder solution A binder solution is obtained by ultrasonic stirring to obtain a coating solution;

(2) Lithium salt-modified polyolefin diaphragm: adding lithium salt to ethanol to obtain a lithium salt ethanol solution, then vacuum drying the polyolefin diaphragm after ultrasonic cleaning with ethanol, and placing the dried polyolefin diaphragm in lithium salt ethanol After being soaked in the solution for 30-60 minutes, the lithium salt modified polyolefin diaphragm is obtained by vacuum drying.

(3) Preparation of polyolefin composite diaphragm: Lay the lithium salt-modified polyolefin diaphragm obtained in step (2) evenly on a glass plate, and use a scraper to evenly coat the coating solution of step (1) on the step (1) One side of the polyolefin separator in (2) is then vacuum-dried to obtain a polyolefin composite separator modified with carbon nanotubes, lithium salts and nano-SiO ₂ .

The carbon nanotubes described in step (1) are any one of carboxylated carbon nanotubes (CNT-COOH), sulfonated carbon nanotubes (CNT-SO ₃ ), and hydroxylated carbon nanotubes (CNT-OH);

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

The solvent described in step (1) is any one in N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), deionized water, ethanol, acetone, etc.;

The mass concentration of the carbon nanotubes in the coating solution in step (1) is 0.05-0.4%, preferably 0.1-0.3%;

The lithium salt in step (2) is lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium bisfluorosulfonimide (LiFSI), lithium trifluoromethanesulfonate (CF ₃ SO ₃ Li) , any one in lithium bis(trifluoromethanesulfonic acid) imide (LiTFSI);

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

A functionalized polyolefin composite membrane, which is prepared by the following method:

(1) Preparation of coating solution: PVA and deionized water were mixed, and a 4% mass concentration of PVA aqueous solution was obtained after refluxing and stirring at 90 °C for 4 h. After mixing carbon nanotubes, nano-SiO ₂ and deionized water, the PVA aqueous solution was added. , a coating solution with a carbon nanotube content of 0.3% was obtained by ultrasonic stirring;

(2) Preparation of lithium salt-modified polyolefin diaphragm: Lithium bis-trifluoromethanesulfonimide was added to ethanol to prepare a lithium salt ethanol solution with a mass concentration of 2%, and then the polyolefin diaphragm was placed in a 2% ethanol solution. After soaking in the lithium salt ethanol solution for 30 min, and then vacuum drying at 40 °C for 24 h to obtain the lithium salt modified polyolefin separator.

(3) Preparation of polyolefin composite diaphragm: Lay the lithium salt-modified polyolefin diaphragm obtained in step (2) evenly on a glass plate, and use a scraper to evenly coat the coating solution of step (1) on the step (1) One side of the polyolefin separator in (2) is then vacuum-dried to obtain a polyolefin composite separator modified with carbon nanotubes, lithium salts and nano-SiO ₂ .

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

The beneficial effects of the present invention

The present invention provides a method for coating a polyolefin diaphragm with functionalized carbon nanotubes and lithium salts and a method for preparing a composite film thereof. The method utilizes nano-silicon dioxide with excellent thermal stability to improve the thermal stability of the diaphragm; at the same time, The advantages of nano- _SiO2 , such as large specific surface area and small bulk density, improve the wettability of the polyolefin separator to the electrolyte, but the inorganic silica will increase the interface impedance. Furthermore, taking advantage of the outstanding electrical properties, strong mechanical strength, high stability and high specific surface area of carbon nanotubes, functionalized carbon nanotubes are coated on the positive side of the separator to improve battery performance and obtain high dimensional thermal stability, The composite polyolefin separator with excellent comprehensive properties of good electrolyte wettability, reduced interface impedance and improved electrochemical performance makes it widely used in lithium-ion batteries.

Description of drawings

(a), (b), (c) and (d) in Figure 1 are the PE separator of Comparative Example 1, the PE@Si composite separator of Comparative Example 2 and the PE@LiSiCNT0.1 composite separator of Example 1, respectively. SEM image of the surface of the PE@LiSiCNT0.3 composite separator of Example 2;

In Figure 2 ((a), (b), (c), (d) are the PE separator of Comparative Example 1, the PE@Si composite separator of Comparative Example 2 and the PE@LiSiCNT0.1 composite separator of Example 1, respectively, Photographs of the PE@LiSiCNT0.3 composite separator of Example 2 before heat treatment, (e), (f), (g), and (h) are the PE separator of Comparative Example 1, the PE@Si composite separator of Comparative Example 2 and the examples, respectively 1. The thermal shrinkage photo of the PE@LiSiCNT composite membrane of Example 2 heat-treated at 150 °C for 30 min;

3 is the bulk impedance spectra 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;

4 is the AC impedance spectrum 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;

Figure 5 shows the battery cycles 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 at different rates performance;

Figure 6 shows the cycle performance of the PE separator of Comparative Example 1 at a charge-discharge current density of 0.2C;

Figure 7 shows the cycle performance of the PE@Si composite separator of Comparative Example 2 at a charge-discharge current density of 0.2 C;

Fig. 8 is the battery cycle performance of the PE@LiSiCNT0.1 composite separator of Example 1 at a charge-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 0.2 C.

Detailed ways

The technical solutions of the present invention will be further described in detail below with reference to specific embodiments. The purpose is to make those skilled in the art have a clearer understanding and understanding of the present application. The following specific examples should not be construed or construed to limit the scope of protection claimed in the claims of the present application to any extent.

Example 1

(1) Preparation of coating liquid:

4 g of polyvinyl alcohol (PVA) and 96 ml of deionized water were respectively added to the three-necked flask, and the mixture was refluxed and stirred at 90° C. for 4 h to obtain a PVA aqueous solution with a mass concentration of 4%. Aqueous hydroxylated carbon nanotubes (CNT-OH) were mixed with deionized water to prepare a 5% CNT-OH solution. 1 g of nano-SiO ₂ and 0.3 g of 5% CNT-OH solution were added to 13.7 g of PVA aqueous solution, sonicated and stirred for 0.5 h to obtain a coating solution with a CNT-OH content of 0.1%.

(2) Lithium salt modified polyolefin separator:

Lithium bistrifluoromethanesulfonimide (LiTFSI) was added into ethanol to prepare a LiTFSI/ethanol solution with a mass concentration of 2%. Immersion in ethanol solution for 30 min, and vacuum drying at 40 °C for 24 h to obtain a lithium salt modified polyolefin separator.

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

The lithium salt-modified polyolefin separator obtained in step (2) is laid flat on a glass plate, and the coating solution prepared in step (1) is evenly coated on one side of the PE separator obtained in step (2) with a scraper. side, and vacuum-dried to obtain the PE@LiSiCNT0.1 composite separator.

(4) Preparation of positive electrode sheet:

Take 1.6g LiFePO ₄ , 0.2g acetylene black and 0.2g PVDF respectively, dissolve them in N-methylpyrrolidone (NMP), stir evenly, apply the obtained slurry on aluminum foil paper, cut out pieces, and dry them to obtain positive electrode pieces .

(5) Assembly of the battery:

A Li//PE@LiSiCNT0.1//LiFePO ₄ half-cell was assembled from the positive electrode shell, the positive electrode sheet in step (4), the PE@LiSiCNT0.1 composite separator in step (3), lithium sheet, gasket, and spring sheet.

Example 2

(1) Preparation of coating liquid:

Put 4g of PVA and 96ml of deionized water in a three-necked flask, respectively, and stir at 90 °C for 4 hours to obtain a 4% mass concentration of PVA aqueous solution. The aqueous CNT-OH and deionized water were made into a 5% CNT-OH solution, and the 1 g of nano-SiO ₂ and 0.9 g of 5% CNT-OH solution were added to 13.1 g of PVA aqueous solution, sonicated and stirred for 0.5 h to obtain a coating solution with a CNT-OH content of 0.3%.

(2) Lithium salt modified polyolefin separator:

Lithium bistrifluoromethanesulfonimide (LiTFSI) was added to ethanol to prepare a LiTFSI/ethanol solution with a mass concentration of 2%. The PE diaphragm was ultrasonically cleaned with ethanol, dried in vacuum at 40°C and placed in LiTFSI/ethanol solution. Soak for 30 min and vacuum dry at 40 °C for 24 h to obtain a lithium salt-modified polyolefin separator.

(3) Preparation of PE@LiSiCNT0.3 composite separator:

The lithium salt-modified polyolefin diaphragm obtained in step (2) is evenly spread on a glass plate, and the coating solution prepared in step (1) is evenly coated on one side of the PE diaphragm with a scraper, and vacuum-dried, The PE@LiSiCNT0.3 composite separator was obtained.

(4) Preparation of positive electrode sheet:

Take 1.6g LiFePO ₄ , 0.2g acetylene black and 0.2g PVDF respectively, dissolve them in NMP, stir evenly, apply the obtained slurry on aluminum foil paper, cut out, and dry to obtain a positive electrode sheet.

(5) Assembly of the battery:

Take the PE@LiSiCNT0.3 composite diaphragm in step (3), take the positive electrode sheet prepared in step (4), and combine the positive electrode shell, the positive electrode sheet in step (4), the PE@LiSiCNT0.3 composite diaphragm in step (3), Li//PE@LiSiCNT0.3//LiFePO ₄ half-cells were assembled from lithium sheets, spacers, and shrapnel.

Comparative Example 1

A Li//PE//LiFePO ₄ half-cell was assembled using a commercial polyolefin separator and the same LiFePO ₄ positive electrode sheet as in Example 1-2.

Comparative Example 2

(1) Place 4g of PVA and 96ml of deionized water in a three-necked flask, stir at 90°C for 4h under reflux to obtain a PVA aqueous solution with 4% mass concentration, add 1g of nano _- SiO to 14g of PVA aqueous solution, ultrasonically and stir for 0.5h to obtain 1wt% _SiO2 nanocoating solution.

(2) ultrasonically cleaning the PE diaphragm with ethanol, vacuum drying at 40° C., and coating the coating solution of step (1) on one side of the PE diaphragm to obtain a PE@Si composite diaphragm.

(3) Using the same LiFePO ₄ positive electrode sheet and the PE@Si composite separator of step (2) as in Example 1-2, a half-cell of Li//PE@Si//LiFePO ₄ was assembled

Performance Testing

1. Thickness, porosity and electrolyte 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 Absorption rate.

2. Electrolyte static contact angle test 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.

3. SEM test 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.

4. The thermal shrinkage rate test 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.

5. The ionic conductivity test 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.

6. AC impedance test 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.

7. Battery cycle performance test 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 tests and conclusions are as follows:

1. Membrane thickness, porosity, electrolyte absorption rate test: measure the thickness of each membrane with a screw micrometer, and measure three times to get the average value; cut the diaphragm into a size of 2*2cm, weigh its quality W _dry , and then Soak the diaphragm for 2 hours in a closed container with n-butanol, take out the n-butanol wiped off the surface of the diaphragm with filter paper and call its quality W _wet , according to the formula liquid absorption = (W _wet -W _dry )/ρv*100 (W _dry represents the quality of the diaphragm before absorbing n-butanol, W _we t represents the quality of the diaphragm after absorbing the electrolyte. ρ is the density of n-butanol, v is the volume of the diaphragm) to calculate the porosity; cut the diaphragm into a size of 2*2cm, Weigh its mass W _dry , then soak the diaphragm in a closed container filled with electrolyte for 2h, take out the diaphragm and weigh its mass W _wet , according to the formula liquid absorption rate=(W _we tW _dry )/W _dry *100(W _dry The mass of the diaphragm before absorbing the electrolyte, W _we t represents the mass of the diaphragm after absorbing the electrolyte), and the electrolyte absorption rate of each diaphragm is calculated.

Referring to Table 1, the thickness of the PE@LiSiCNT composite film obtained in Examples 1-2 is about 22 μm, which meets the requirement that the thickness of the LIB separator is less than 25 μm. Compared with the PE separator and the PE@Si composite separator, the PE@LiSiCNT composite membrane obtained in Example 1-2 has improved electrolyte absorption rate and porosity. Among them, CNT-OH has a large specific surface area and good affinity for the electrolyte, which improves the porosity and liquid absorption of the composite membrane. The PE@LiSiCNT0.1 composite separator and the PE@LiSiCNT0.3 composite separator contain lithium salts, which can further promote the electrolyte wettability of the composite separators of Examples 1-2.

2. Electrolyte static contact angle test: Cut the diaphragm samples of the examples and comparative examples into strips of a certain size, stick the test samples flat on the surface of the glass plate, and use about 1.2 μl of each drop of the electrolyte to test and test. The contact angle values were recorded and averaged over multiple measurements. Referring to Table 2, the electrolyte contact angles of PE@LiSiCNT0.1 composite separator and PE@LiSiCNT0.3 composite separator are better than PE separator and PE@Si composite separator.

3. Morphology observation: After spraying gold on the diaphragm sample, observe the surface morphology of the diaphragm under an electron scanning microscope.

Referring to Figure 1 of the specification, the surface of the PE@Si composite separator is coated with nano-SiO ₂ , and there are abundant pore structures between the nano-SiO _2. The CNT-OH on the surface of the PE@LiSiCNT0.1 and PE@LiSiCNT0.3 composite separators is Three-dimensional network-like morphology exists, forming a large number of pore structures.

4. Thermal stability test: The diaphragm samples of the examples and comparative examples were cut into a size of 2*2 cm, heated in an electric heating constant temperature blast drying oven at 150°C for 30 minutes, and the dimensional changes of the diaphragms were observed.

Referring to Table 3 and Figure 2 of the description, the thermal shrinkage rates of PE separator and PE@Si composite separator, PE@LiSiCNT0.1 composite separator, and PE@LiSiCNT0.3 composite separator are 74%, 36%, 17%, and 14%, respectively. . Compared with the heat shrinkage rate of the pure PE separator (74%), the thermal shrinkage rate of the composite separator is significantly improved. This is mainly because the SiO and CNT _- OH coatings on the surface of the composite separator improve the thermal stability of the separator, while the melting point of LiTFSI in the PE@LiSiCNT composite separator is 236 °C, and the LiTFSI in the pores of the separator is effectively reduced at a high temperature of 150 °C The thermal shrinkage of the separator, so the thermal stability of the PE@LiSiCNT composite separator is significantly improved.

5. Ionic conductivity test: Cut the diaphragm sample into a circle with a diameter of 1.6 cm, fully infiltrate the cut diaphragm in the electrolyte in an argon-filled glove box, take it out and place it between two stainless steel electrodes to assemble A CR2032 button battery. Conductivity was tested by Chenhua CHI660E electrochemical workstation, using the frequency range of 10mHz-1MHz at room temperature. The ionic conductivity was calculated according to σ=L/(R S) (σ is the ionic 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 Figure 3 of the description, the resistance values of the PE separator, PE@Si composite separator, PE@LiSiCNT0.1 composite separator, and PE@LiSiCNT0.3 composite separator are 1.84, 2.06, 1.72 and 1.69Ω, respectively. The ionic conductivities are 1.01×10-3, 1.21×10-3, 1.66×10-3 and 1.69×10-3S cm-1, respectively. The PE@LiSiCNT composite separator has low resistance and high ionic conductivity.

6. AC impedance test: Cut the diaphragm sample into a circle with a diameter of 1.6 cm, fully infiltrate the cut diaphragm in the electrolyte in a glove box filled with argon gas, take it out and place it between the negative electrode of the lithium sheet and the positive electrode of LiFePO ₄ assembled into a button battery. The resistance is tested by Chenhua CHI660E electrochemical workstation, and the frequency range of 10mHz-1MHz is used at room temperature.

Referring to Table 6 and Figure 4 of the description, the interface resistances of the PE separator and the PE@Si composite separator, the PE@LiSiCNT0.1 composite separator, and the PE@LiSiCNT0.3 composite separator are 181, 202, 130, and 128Ω, respectively. The impedance of PE@LiSiCNT composite separator is significantly lower than that of PE separator battery.

7. Electrical performance test

The charge-discharge performance and coulombic efficiency of the battery were tested by Xinwei battery test system (BTS-4000) at room temperature. In the examples and comparative examples, a lithium sheet is used as the negative electrode, LiFePO ₄ is used as the positive electrode, the separator sample is placed in the middle and the coating surface is close to the positive electrode side, and a button battery is assembled, under constant current-constant voltage mode (0.2C The cycle performance of the battery was measured in the mode of charging to 4.3V and then discharging to 3.0V. Similarly, the rate performance of the battery assembled with the separator was charged to 4.2V and discharged to 3V when the charge-discharge current density was 0.2C, 0.5C, 1.0C, 2.0C and 0.2C.

Referring to Figure 5 of the description, at the charge-discharge current densities at 0.2C, 0.5C, 1.0C, 2.0C and 0.2C, compared with the PE separator and the PE@Si composite separator, the discharge capacity of the PE@LiSiCNT composite separator was Significantly improved. When the current density reaches 2C, the discharge capacities of the PE separator, PE@Si composite separator, PE@LiSiCNT0.1 composite separator, and PE@LiSiCNT0.3 composite separator are 105, 113, 137, and 138 mAh/g, respectively. This indicates that the Li transport in the PE@ ^LiSiCNT composite films is less affected by the current density. The high current density makes the interface polarized and affects Li+ transport. The PE film has serious interfacial polarization under high rate conditions, which affects the Li ⁺ transport speed; while the low impedance and efficient ion transport of the PE@LiSiCNT composite separator improves the battery. stability.

Referring to Figures 6, 7, 8, and 9 in the description, the discharge capacities of the PE separator, PE@Si composite separator, PE@LiSiCNT0.1 composite separator, and PE@LiSiCNT0.3 composite separator are 143 and 148 respectively after 250 cycles of charge and discharge. , 163, and 164 mAh/g, and the discharge capacity of PE@LiSiCNT composite separators was significantly higher than that of PE and PE@Si composite separators during cycling. The discharge capacity of the PE@LiSiCNT composite separator is higher because the electrophilic solution of CNT-OH and its network-like structure of CNT-OH endow the composite separator with high porosity; in addition, Li ⁺ can pass through the ports or sidewall openings of CNTs and carbon The nanotube layer effectively diffuses to stable positions on the surface of CNTs and inside individual CNTs, and CNTs can enrich and store Li ⁺ , buffer the loss and accumulation of Li ⁺ , and reduce the resistance of the separator, thereby improving the lithium ion conductivity and high transport efficiency of the separator. At the same time, the introduction of LiTFSI provides more lithium sources, improves the ion transport performance, effectively reduces the impedance and polarization of the battery, and then obtains high discharge specific capacity and stable cycle performance, and improves the electrochemical performance.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: The technical solutions described in the foregoing embodiments can still be modified, or some or all of the technical features thereof can be equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the technical solutions of the embodiments of the present invention. scope.

Table 1 Thickness, porosity and electrolyte 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 Absorption rate

diaphragm Electrolyte Absorption Rate (%) Porosity(%) Diaphragm thickness (μm) 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@LiSiCNT0.1 composite separator of Example 1, and the PE@LiSiCNT0.3 composite separator of Example 2

diaphragm Comparative Example 1 Comparative Example 2 Example 1 Example 2 Electrolyte Contact(°) 54.8 18.8 14.8 13.3

Table 3 Thermal shrinkage data 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

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

Table 4 The ionic conductivity 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

Table 5 AC impedance data 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

diaphragm Comparative Example 1 Comparative Example 2 Example 1 Example 2 Impedance(Ω) 181 202 130 128

Claims (10)

1. a functionalized polyolefin composite diaphragm, is characterized in that, it is prepared by the following method: (1) Preparation of coating solution: Mix the binder and the solvent at 70-100°C under reflux and stir for 4-8 hours to obtain a binder solution, mix the carbon nanotubes, nano-silica and the solvent, and then add the binder The solution is obtained by ultrasonic stirring to obtain a coating solution; (2) Lithium salt-modified polyolefin diaphragm: adding lithium salt to ethanol to obtain a lithium salt ethanol solution, then vacuum drying the polyolefin diaphragm after ultrasonic cleaning with ethanol, and placing the dried polyolefin diaphragm in lithium salt ethanol After soaking in the solution for 30-60min, vacuum drying is performed to obtain a lithium salt modified polyolefin diaphragm; (3) Preparation of polyolefin composite diaphragm: Lay the lithium salt-modified polyolefin diaphragm obtained in step (2) evenly on a glass plate, and use a scraper to evenly coat the coating solution of step (1) on the step (1) One side of the polyolefin separator in (2) is vacuum dried to obtain a polyolefin composite separator modified with carbon nanotubes, lithium salts and nano-silica. 2 . A functionalized polyolefin composite membrane according to claim 1 , wherein the carbon nanotubes in step (1) are carboxylated carbon nanotubes, sulfonated carbon nanotubes, and hydroxylated carbon nanotubes. 3 . any of the . 3. a kind of functionalized polyolefin composite membrane according to claim 1, is characterized in that, the binder described in step (1) is polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose or polyacrylic acid Any one or any combination of methyl esters. 4. a kind of functionalized polyolefin composite membrane according to claim 1 is characterized in that, the solvent described in step (1) is N,N-dimethylformamide, N,N-dimethylacetamide , any one of deionized water, ethanol, and acetone. 5 . The functionalized polyolefin composite membrane according to claim 1 , wherein the carbon nanotubes in step (1) have a mass concentration of 0.05-0.4% in the coating solution. 6 . 6 . The functionalized polyolefin composite membrane according to claim 5 , wherein the carbon nanotubes in step (1) have a mass concentration of 0.1-0.3% in the coating solution. 7 . 7. A functionalized polyolefin composite diaphragm according to claim 1, wherein the lithium salt in step (2) is lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bisfluorosulfonimide, trifluoromethane Any of lithium sulfonate and lithium bis(trifluoromethanesulfonate)imide. 8 . The functionalized polyolefin composite membrane according to claim 1 , wherein the polyolefin in step (2) is any one of polyethylene or polypropylene. 9 . 9. a kind of functionalized polyolefin composite membrane according to claim 1 is characterized in that, it is prepared by the following method: (1) Preparation of coating solution: Mix PVA and deionized water, and stir at 90°C for 4 hours to obtain a PVA aqueous solution with a concentration of 4% by mass. After mixing carbon nanotubes, nano-silica and deionized water, add PVA An aqueous solution, and a coating solution with a carbon nanotube content of 0.3% is obtained by ultrasonic stirring; (2) Preparation of lithium salt-modified polyolefin diaphragm: Lithium bis-trifluoromethanesulfonimide was added to ethanol to prepare a lithium salt ethanol solution with a mass concentration of 2%, and then the polyolefin diaphragm was placed in a 2% ethanol solution. After soaking in the lithium salt ethanol solution for 30 min, and then vacuum drying at 40 °C for 24 h to obtain the lithium salt modified polyolefin separator. (3) Preparation of polyolefin composite diaphragm: Lay the lithium salt-modified polyolefin diaphragm obtained in step (2) evenly on a glass plate, and use a scraper to evenly coat the coating solution of step (1) on the step (1) One side of the polyolefin separator in (2) is then vacuum-dried to obtain a polyolefin composite separator modified with carbon nanotubes, lithium salts and nano-silica ₂ . 10. The application of the functionalized polyolefin composite membrane of claim 1 in a lithium ion battery.

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