CN107369801B - A kind of MXene modified composite separator and its preparation method and its application in lithium-sulfur battery - Google Patents

Abstract

The invention discloses an MXene-modified composite diaphragm, a preparation method thereof, and an application in a lithium-sulfur battery. In the MXene modified composite separator of the present invention, the base film is a polyolefin separator, the modification material is a composite of MXene and polymer or inorganic particles, and the modification material is attached to one surface of the polyolefin separator. In the present invention, the A layer in the MAX is peeled off in the in-situ liquid phase, the two-dimensional material MXene is synthesized in one step, and after the MXene material is compounded with the polymer or inorganic particles, vacuum filtration is attached to the polyolefin membrane to obtain the MXene modified compound diaphragm. The MXene modified composite separator is directly placed in the lithium-sulfur battery, which enhances the ionic conductivity and electronic conductivity of the battery, and at the same time strongly adsorbs lithium polysulfides, suppresses the shuttle effect of lithium polysulfides, and greatly improves the performance and safety of the battery. sex.

Description

A kind of MXene modified composite separator and preparation method thereof and use in lithium-sulfur battery application

technical field

The invention relates to the field of synthesis and application of two-dimensional material MXene, in particular to an MXene-modified composite separator, a preparation method thereof, and an application in a lithium-sulfur battery.

Background technique

Since the beginning of the 21st century, energy and environment have become two hot issues in the contemporary era. The extensive use of fossil energy has made industrial waste gas, waste water and waste residue have a greater impact on the living environment. More and more people have begun to advocate the use of green energy. , the development and utilization of clean energy such as solar energy, wind energy and nuclear energy are gradually put on the agenda. Now an efficient energy storage device is urgently needed to store excess energy, and the battery as an efficient and convenient energy storage device has also caused extensive research. interest. From the previous lead-acid batteries to the lithium-ion batteries that are now used in all aspects of life, scientific researchers have made great progress, the field of lithium-ion batteries has achieved rapid development, and electronic devices such as mobile phones and notebook computers have also been upgraded rapidly. However, the current lithium-ion battery technology can no longer meet high-power devices such as electric vehicles, and a new type of high-energy-density battery is urgently needed to fill this gap.

Lithium-sulfur batteries have abundant and cheap materials, mainly sulfur, which is abundant in nature, and has a high theoretical energy density (1675 mAh/g), so it is widely considered to be the next-generation secondary battery with the most potential to replace lithium-ion batteries. . However, there are also many problems in the development of lithium-sulfur batteries. Among them, the discharge products are easily soluble in the electrolyte, which causes problems such as shuttle effect, poor conductivity of the cathode material sulfur and its discharge product lithium sulfide, and serious volume expansion. The battery capacity decays rapidly and the cycle stability is poor. Moreover, the shuttle effect caused by the lithium polysulfides generated during the discharge process can even cause safety problems. So far, researchers have designed various solutions to solve the above problems of lithium-sulfur batteries. The most common one is to use nano-scale cathode materials such as porous carbon materials, metal oxides, etc. to confine sulfur in the conductive framework. The strategy not only utilizes the physical and chemical adsorption of porous carbon nanomaterials to inhibit the shuttle of lithium polysulfide to the negative electrode and enhances the electrochemical performance, but also closely combines insulating sulfur with conductive materials to improve the conductivity of the positive electrode material.

In recent years, a large number of researches have been aimed at restricting the passage of lithium polysulfides through the separator. The specific methods include introducing a functional interlayer between the separator and the positive electrode, or coating a layer of material near the positive electrode to modify the traditional polymer Olefin diaphragm. For example, some researchers designed to add a layer of porous carbon interlayer between the commercial separator and the current collector, using the conductivity of the carbon layer and the effect of physically hindering lithium polysulfide, which greatly improved the utilization efficiency of cathode sulfur and the cyclability of the battery; Some researchers have coated metal oxide-modified mesoporous carbon materials or hetero-element-doped mesoporous carbons on traditional separators, using the chemical adsorption of lithium polysulfides to enhance the cycle stability and rate performance of lithium-sulfur batteries. .

Due to the unique structure and properties of two-dimensional materials, graphene or graphene oxide also plays a great role in the modification of the separator. The researchers proved that introducing a very thin graphene oxide layer on one side of the separator can play a role in physical, The chemical doubly hinders the action of lithium polysulfides. The MXene material, one of the graphene-like materials, is a general term for two-dimensional early transition metal carbon or nitride, which is obtained by selectively etching the A layer in the MAX material. M in MAX represents early transition metals, A represents the third or fourth main group element, X represents C or N, and ene in MXene represents some groups, including -OH, -F or =O. After the exfoliation of MXene material was first reported in 2011, the material has been widely used in many fields due to its unique structure and properties. The higher electronic conductivity endows MXene materials with unique advantages in the field of electrochemistry. In 2015, it was reported that the titanium atoms in Ti ₂ CT _x - MXene (T represents the surface group) have a strong adsorption effect on lithium polysulfide, an intermediate product of lithium-sulfur batteries. Therefore, MXene materials were introduced into the field of lithium-sulfur batteries for the first time. The lithiated polymer Nafion has been confirmed to selectively permeate lithium ions and has better performance in lithium-sulfur batteries.

SUMMARY OF THE INVENTION

In order to solve the problems of poor cycle stability and low efficiency caused by the shuttle effect of lithium polysulfides in lithium-sulfur batteries, the present invention provides an MXene modified composite separator. In the MXene modified composite diaphragm, the base is a polyolefin diaphragm, the modification material is a composite of MXene material with ultra-high conductivity and strong adsorption of lithium polysulfide and polymer or inorganic particles, and the modification material is attached to the surface of one side of the polyolefin diaphragm superior.

The invention also provides the preparation method of the MXene-modified composite membrane. In this method, the A layer in MAX is exfoliated by in-situ liquid phase, and the two-dimensional material MXene is synthesized in one step. After compounding the MXene material with polymer or inorganic particles, it is attached to the polyolefin membrane by vacuum filtration. The polyolefin separator was modified to obtain the MXene modified composite separator.

The MXene material is directly coated on the commercial separator, or combined with the polymer to act on the commercial separator, which can inhibit the shuttle of lithium polysulfide to the negative electrode; the present invention utilizes the unique advantages of different polymers or inorganic particles. The excellent properties of MXene are compounded, and the separator is further modified to improve the performance of lithium-sulfur batteries.

The invention also provides the application of the MXene modified composite separator in the lithium-sulfur battery.

The present invention is realized through the following technical solutions.

A preparation method of an MXene modified composite diaphragm, comprising the following steps:

(1) Dissolve the fluoride salt in the acid solution, add MAX powder, and stir to dissolve;

(2) after the solution obtained in step (1) is centrifuged and washed until the pH value of the supernatant reaches above 6, the centrifugation is continued to collect the supernatant, and the collected supernatant is dried to obtain the MXene material;

During the centrifugal washing process, the supernatant of the first centrifugation was dark green, and as the number of centrifugations increased, the color of the supernatant deepened until it was pure black, and finally the supernatant after the pH reached above 6 was collected;

(3) The obtained MXene material is mixed with polymer or inorganic particles, dissolved in a solvent by ultrasonic, adsorbed on the surface of a polyolefin (pp) membrane by suction filtration, and dried to obtain the MXene modified composite membrane.

The polyolefin separator is a porous microporous separator, which facilitates the adhesion of MXene to the composite of polymers or inorganic particles.

Further, in step (1), the fluorine salt includes lithium fluoride or sodium fluoride.

Further, in step (1), the acid solution includes 6-12M hydrochloric acid or 6-9M sulfuric acid.

Further, in step (1), the mass ratio of the fluorine salt to the MAX powder is 1:1.

Further, in step (1), in the MAX, M is an early transition metal element, A is a group IIIA or group IVA element, and X is C or N.

Preferably, in step (1), the MAX is Ti ₃ AlC ₂ .

Further, in step (1), the stirring and dissolving is carried out at 30-45 °C for 12-36 h, and the MAX is exfoliated in situ by stirring and dissolving to synthesize MXene.

Further, in step (2), in the MXene, M is an early transition metal element, X is C or N, and ene includes -OH, -F or =O.

Preferably, in step (2), the MXene is Ti ₃ C ₂ .

Further, in step (2), the rotational speed of the centrifugation is all 3000rpm.

Further, in step (2), the time for the continued centrifugation is 0.5-1 h.

Further, in step (3), the solid-to-liquid ratio of the MXene material to the solvent is 0.09-0.1 mg/mL.

Further, in step (3), the polymer includes one or more of perfluorosulfonic acid (Nafion) and cetyltrimethylammonium bromide (CTAB).

Further, in step (3), the inorganic particles include one or more of silicon dioxide and titanium dioxide.

The introduction of the cation selective permeability material Nafion will help to selectively permeate lithium ions while physically hindering lithium polysulfides; while the introduction of CTAB or inorganic particles is conducive to regulating the interlayer spacing, which is conducive to the transport of lithium ions, and at the same time Composite with conductive material MXene enhances electronic conductivity.

Further, in step (3), the mass percentage of the polymer relative to MXene is 10-25%.

Further, in step (3), the mass percentage of the inorganic particles relative to MXene is 10-30%.

Further, in step (3), the process of ultrasonic dissolving in the solvent is ultrasonic dissolving in an argon atmosphere, and the ultrasonic time is more than 0.5h.

Further, in step (3), the solvent includes one or more of water and ethanol.

Further, in step (3), the suction filtration is vacuum suction filtration.

Further, in steps (2) and (3), the drying is vacuum drying at normal temperature.

An MXene-modified composite membrane prepared by any of the above-mentioned preparation methods, the base is a polyolefin membrane, and the modification material is an MXene material with ultra-high conductivity and strong adsorption of lithium polysulfide and a polymer or inorganic particle. The composite, the modification material is attached to the surface of one side of the polyolefin membrane.

The application of the MXene-modified composite separator in a lithium-sulfur battery, the MXene-modified composite separator is directly placed in the lithium-sulfur battery, and the MXene-modified composite separator is not attached to a composite of MXene materials and polymers or inorganic particles. The object side is close to the negative terminal of the battery.

MXene materials have advantages: (1) good electronic conductivity (up to 5000S/cm), comparable to graphene; (2) good hydrophilicity, the surface of transition metal atoms in the early synthesis process will have more hydrophilic Water-based groups, mainly including -OH, -F or =O, make the material more soluble in water and common organic solvents; (3) The unique two-dimensional lamellar structure can make lithium, sodium, potassium, and ammonium ions and DMSO, CTAB, N ₂ H ₄ and other organics are intercalated into the interlayer to adjust the interlayer spacing; (4) In lithium-sulfur batteries, the early transition metal atoms in MXene have a good adsorption effect on the intermediate product lithium polysulfide. It can effectively inhibit the shuttle effect and greatly improve the cycle stability of lithium-sulfur batteries; (5) It has good mechanical properties.

Due to the excellent properties of MXene materials, a defect-free, highly electronically conductive thin layer that can effectively block lithium polysulfide is formed between the traditional polyolefin separator and the cathode material, so that the MXene-modified composite separator is installed The lithium-sulfur battery has achieved a significant improvement in cycle stability and specific capacity.

Compared with the prior art, the present invention has the following advantages and technical effects:

(1) The MXene powder prepared by the present invention is monolithic, micron-scale, and has few defects, and the synthesis method is simple and efficient. Instead of using HF, fluorine salt is used, and the safety is far better than the traditional hydrofluoric acid etching method.

(2) The present invention introduces MXene materials into the field of lithium-sulfur battery separators. On the one hand, the high conductivity of MXene materials greatly enhances the electronic conductivity of lithium-sulfur batteries. Adsorption and the unique two-dimensional structure of MXene, the cation-selective permeability of Nafion, and the regulation of interlayer spacing characteristics of CTAB or inorganic particles make the composite separator better improve the lithium-sulfur battery under the synergistic effect of physical and chemical dual barriers. cycle performance.

(3) The preparation method of the present invention has the advantages of simple operation, low time consumption, low energy consumption and low cost, and is favorable for large-scale industrial production.

(4) The composite membrane obtained by simple vacuum filtration of the present invention has the common characteristics of pp, MXene and polymer, has abundant pores and excellent conductivity on one side, and enhances the ionic conductivity and electronic conductivity of the battery. At the same time, it strongly adsorbs lithium polysulfides, preventing lithium polysulfides from shuttling to the negative electrode and causing lithium dendrites, which greatly improves the performance and safety of the battery.

Description of drawings

FIG. 1 is a SEM image of the raw material Ti ₃ AlC ₂ powder used in the examples of the present invention.

FIG. 2 is a SEM image of the Ti ₃ C ₂ material prepared in Example 1. FIG.

FIG. 3 is the XRD pattern of the Ti ₃ C ₂ material prepared in Example 1. FIG.

FIG. 4 is a TEM image of the Ti ₃ C ₂ material prepared in Example 1. FIG.

FIG. 5 is a SEM image of the Nafion@pp composite membrane prepared in Example 4. FIG.

FIG. 6 is a SEM image of the Ti ₃ C ₂ @Nafion modified composite membrane prepared in Example 6. FIG.

7 is a graph showing the cycle performance of the Ti ₃ C ₂ @SiO ₂ modified composite separator in Example 7 applied to a lithium-sulfur battery.

Figure 8 shows the cycle performance of traditional polyolefin separator, Nafion@pp composite separator and Ti ₃ C ₂ @Nafion modified composite separator applied in lithium-sulfur batteries, respectively.

Figure 9 shows the cycle performance of the Ti ₃ C ₂ @CTAB modified composite separator applied to Li-S batteries.

Detailed ways

The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings, but the present invention is not limited thereto.

In the specific embodiment of the present invention, the Al layer in Ti ₃ AlC ₂ is peeled off by in-situ liquid phase, the two-dimensional material Ti ₃ C ₂ is synthesized in one step, and then the Ti ₃ C ₂ material is compounded with polymer or inorganic particles, and the vacuum The suction filtration was attached to the polyolefin membrane, and the polyolefin membrane was modified by simple suction filtration to obtain a Ti ₃ C ₂ modified composite membrane.

The SEM image of the raw material Ti ₃ AlC ₂ powder used in the specific embodiment of the present invention is shown in FIG. 1 , and it can be seen from FIG. 1 that the raw material Ti ₃ AlC ₂ has a three-dimensional bulk structure.

Example 1

Lithium fluoride and dilute hydrochloric acid liquid phase exfoliation of monolayer Ti ₃ C ₂

(1) Dissolve 2 g of LiF in 20 ml of 6M hydrochloric acid, stir to dissolve;

(2) Take 2 grams of Ti ₃ AlC ₂ powder, slowly add it to the solution in step (1) within 10 minutes, and stir at 45°C for 12 hours to dissolve;

(3) The solution obtained in step (2) was centrifuged and washed 10 times (3000rpm, 5min each time) to make the pH of the supernatant reach 6.2, and then centrifuged for 1 hour (3000rpm), and the supernatant was collected. The solution was stored under an argon atmosphere to obtain an aqueous solution of Ti ₃ C ₂ (0.1 mg/ml).

The obtained Ti ₃ C ₂ aqueous solution (0.1 mg/ml) was vacuum-dried at room temperature to obtain a Ti ₃ C ₂ material.

The SEM image of the prepared Ti ₃ C ₂ material is shown in FIG. 2 . It can be seen from FIG. 2 that the prepared Ti ₃ C ₂ material is a single-layer material.

The XRD pattern of the prepared Ti ₃ C ₂ material is shown in Figure 3. It can be seen from Figure 3 that the characteristic peaks of aluminum in the material have disappeared, indicating that the aluminum in Ti ₃ AlC ₂ is completely etched.

The TEM image of the prepared Ti ₃ C ₂ material is shown in FIG. 4 , and it can be seen from FIG. 4 that the size of the prepared nanosheets is 0.5-1 μm.

Example 2

Lithium fluoride and dilute sulfuric acid liquid phase exfoliation of monolayer Ti ₃ C ₂

(1) Dissolve 5 g of LiF in 50 ml of 9M sulfuric acid, stir and dissolve under ice bath;

(2) Take 5 grams of Ti ₃ AlC ₂ powder, slowly add it to the solution of step (1) within 10 minutes, and stir at 30°C for 24 hours to dissolve;

(3) The obtained solution was centrifuged and washed with water (3000rpm, 5min each time) to make the pH of the supernatant reach 6.3, and the centrifugation was continued for 0.5 hours (3000rpm), and the supernatant was collected and vacuum-dried at room temperature to obtain Ti ₃ C ₂ Material.

The SEM image, XRD image and TEM image of the Ti ₃ C ₂ material prepared in this example are shown in Fig. 2, Fig. 3 and Fig. 4, respectively. The prepared Ti ₃ C ₂ material is a single layer with a nanosheet size of 0.5-1 μm. There are few lamellar materials, and the characteristic peaks of aluminum in the materials have disappeared, indicating that high-quality Ti ₃ C ₂ nanosheets can still be obtained after changing the type of acid and processing conditions.

Example 3

Lithium fluoride and concentrated hydrochloric acid liquid phase exfoliation of monolayer Ti ₃ C ₂

(1) Dissolve 5 g of LiF in 50 ml of 12M concentrated hydrochloric acid, stir and dissolve under ice bath;

(2) Take 5 grams of Ti ₃ AlC ₂ powder, slowly add it to the solution of step (1) within 10 minutes, and stir at 35°C for 36 hours to dissolve;

(3) The obtained solution was centrifuged and washed with water (3000rpm, 5min each time) to make the pH of the supernatant reach 6.1, and the centrifugation was continued for 0.5 hours (3000rpm), and the supernatant was collected and vacuum-dried at room temperature to obtain Ti ₃ C ₂ Material.

The SEM, XRD and TEM images of the prepared Ti ₃ C ₂ material are shown in Figure 2, Figure 3 and Figure 4, respectively. The prepared Ti ₃ C ₂ material is a single-layer material with a nanosheet size of 0.5-1 μm, and The characteristic peaks of aluminum in the material have disappeared.

Example 4

Synthesis of Nafion@pp Composite Separator

(1) Dissolve commercial Nafion solution in ethanol to prepare 0.05mg/mL Nafion solution;

(2) 40 mL of the above solution was vacuum-filtered onto pp (polyolefin membrane), and vacuum-dried at room temperature for 12 hours to obtain Nafion@pp composite membrane.

The SEM image of the prepared Nafion@pp composite separator is shown in Figure 5. Nafion forms a dense and defect-free film on the pp substrate.

Example 5

Synthesis of Ti ₃ C ₂ @CTAB Modified Composite Separator

(1) Dissolve 0.4 mg of CTAB powder in ethanol to prepare a 0.1 mg/mL CTAB solution;

(2) According to the mass ratio of Ti ₃ C ₂ and CTAB 4:1, take 16 mL of the Ti ₃ C ₂ solution of Example 1 and the above CTAB solution, and ultrasonically dissolve it for 12 hours in an argon atmosphere;

(3) Vacuum filtration of the mixed solution obtained in step (2) onto a commercial polyolefin membrane, and vacuum drying at room temperature to obtain a Ti ₃ C ₂ @CTAB modified composite membrane.

Example 6

Synthesis of _Ti3C2 @ _Nafion Modified Composite Separator

(1) Take 20 mL of the Ti ₃ C ₂ solution prepared in Example 1 and 4 mL of the Nafion solution prepared in Example 3, and mix and sonicate for 1 h in an argon environment to obtain a mixed solution;

(2) vacuum filtration of the mixed solution obtained in step (1) onto a commercial polyolefin membrane, and vacuum drying at room temperature to obtain a Ti ₃ C ₂ @Nafion modified composite membrane;

The SEM image of the prepared Ti ₃ C ₂ @Nafion modified composite separator is shown in Figure 6. It can be seen from Figure 6 that Nafion and Ti ₃ C ₂ are well mixed together to form a dense and defect-free film.

Example 7

Synthesis of Ti ₃ C ₂ @SiO ₂ Modified Composite Separator

(1) Take 200 mL of the Ti ₃ C ₂ solution prepared in Example 1; take 2 mg of nano-SiO ₂ and dissolve it in 20 mL of water by ultrasonic to obtain a SiO ₂ solution with a concentration of 0.1 mg/mL;

(2) Mixing the Ti ₃ C ₂ solution and the SiO ₂ solution in step (1) and ultrasonicating for 0.5 h in an argon atmosphere to obtain a mixed solution;

(3) Vacuum filtration of the obtained mixed solution onto a commercial polyolefin membrane, and vacuum drying at room temperature to obtain a Ti ₃ C ₂ @SiO ₂ modified composite membrane.

The performance diagram of Ti ₃ C ₂ @SiO ₂ modified composite separator applied to lithium-sulfur battery is shown in Figure 7. It can be seen from Figure 7 that the introduction of inorganic particles SiO ₂ increases the interlayer spacing of MXene, which is beneficial to the transport of lithium ions , the specific capacity and cycle performance of the battery have been further improved, with an average capacity decay of 0.27% per cycle.

Example 8

Traditional polyolefin separators used in lithium-sulfur batteries

(1) Take 70mg of sublimated sulfur and 20mg of conductive black, grind for 0.5h and heat at 155℃ for 10h in a vacuum environment;

(2) After grinding the powder obtained in step (1) for 0.5h, add 125 mg of 8wt% polyvinylidene fluoride (PVDF) solution (solvent is N-methyldipyrrolidone) as a binder, and apply after grinding for 0.5h Dry on aluminum foil at 60°C;

(3) Punching the sheet obtained in step (2) into a Φ12 specification as a battery positive electrode material; punching a traditional polyolefin separator into a Φ16 specification as a battery separator to assemble a button-type lithium-sulfur battery.

Example 9

Ti ₃ C ₂ @Nafion modified composite separator for lithium-sulfur batteries

(1) Take 70mg of sublimated sulfur and 20mg of conductive black, grind for 0.5h and heat at 155℃ for 10h in a vacuum environment;

(2) After grinding the powder obtained in step (1) for 0.5h, add 125 mg of 8wt% PVDF solution (solvent is N-methyldipyrrolidone), grind for 0.5h, coat on aluminum foil, and dry at 60°C;

(3) Punching the sheet obtained in step (2) into Φ12 specification as a battery positive electrode material; punching the Ti ₃ C ₂ @Nafion modified composite diaphragm synthesized in Example 6 into Φ16 specification as a battery diaphragm, and without adhering to it. One side of Ti ₃ C ₂ @Nafion is close to the negative electrode to assemble a coin-type lithium-sulfur battery.

Figure 8 shows the cycle performance diagram of the traditional polyolefin separator, Nafion@pp composite separator and Ti ₃ C ₂ @Nafion modified composite separator respectively applied to lithium-sulfur batteries. It can be seen from Figure 8 that Ti ₃ C ₂ @Nafion The modified composite separator effectively limits the shuttle effect of lithium polysulfide, which can improve the cycle performance of lithium-sulfur batteries. , which can better improve the cycle performance of the battery.

By utilizing the synergistic effect of the strong adsorption of lithium polysulfides by _Ti3C2 and the selective permeation of lithium _ions by Nafion, they work together in the lithium-sulfur battery to improve the performance of the lithium-sulfur battery.

Example 10

_Ti3C2 @CTAB _- modified composite separator for lithium-sulfur batteries

(1) Mix 70 mg of sublimated sulfur and 20 mg of Ketjen black, grind for 0.5 hours, and heat at 155°C for 10 hours in a vacuum environment;

(2) After grinding the powder obtained in step (1) for 0.5 hours, add 125 mg of 8wt% PVDF solution (solvent is N-methyldipyrrolidone), grind for 0.5 hours, coat on aluminum foil, and dry at 60°C;

(3) The sheet obtained in step (2) was punched into Φ12 specifications as a battery positive electrode material; the Ti ₃ C ₂ @ CTAB modified composite separator synthesized in Example 4 was punched into Φ 16 as a lithium-sulfur battery separator, and was not used as a lithium-sulfur battery separator. _The side with _Ti3C2 @CTAB attached is close to the negative electrode to assemble the coin-type lithium-sulfur battery.

Figure ₉ shows the cycle performance of the _Ti3C2 @CTAB modified composite separator applied to a lithium _- sulfur battery. It can be seen from Figure ₉ that the interlayer spacing of _Ti3C2 is regulated by intercalating CTAB between _Ti3C2 layers to It achieves the effect of improving the conductivity of lithium ions, and at the same time, Ti ₃ C ₂ inhibits the shuttle of lithium polysulfides, and works together in the lithium-sulfur battery to improve the specific capacity and cycle performance of the battery, with an average capacity attenuation of 0.303% per cycle.

Claims (3)

1. a preparation method of MXene modified composite diaphragm, is characterized in that, comprises the steps: (1) Dissolve the fluoride salt in the acid solution, add MAX powder, and stir to dissolve; the fluoride salt includes lithium fluoride or sodium fluoride; the acid solution includes 6-12M hydrochloric acid or 6-9M sulfuric acid; The mass ratio of the fluorine salt to the MAX powder is 1:1; in the MAX, M is an early transition metal element, A is a group IIIA or group IVA element, and X is C or N; the stirring and dissolving is at 30-45 ℃ Stir for 12~36h; (2) After centrifuging and washing the solution obtained in step (1) until the pH value of the supernatant reaches 6 or more, continue to centrifuge and collect the supernatant, and dry the collected supernatant to obtain the MXene material; in the MXene, M is Early transition metal elements, X is C or N, and ene includes -OH, -F or =O; the rotational speed of the centrifugation is all 3000rpm; the time for the continued centrifugation is 0.5~1h; the drying is carried out at room temperature vacuum drying; (3) The obtained MXene material is mixed with polymer or inorganic particles, dissolved in a solvent by ultrasonic, adsorbed on the surface of the polyolefin diaphragm by suction filtration, and dried to obtain the MXene modified composite diaphragm; the MXene material and the solvent are mixed; The solid-liquid ratio is 0.09-0.1 mg/mL; the polymer includes one or more of perfluorosulfonic acid and cetyltrimethylammonium bromide; the inorganic particles include one of silicon dioxide and titanium dioxide. more than one species; the mass percentage of the polymer relative to MXene is 10% to 25%; the mass percentage of the inorganic particles relative to MXene is 10% to 30%; the process of ultrasonically dissolving in a solvent is as follows: Ultrasonic for more than 0.5h in an argon atmosphere; the solvent includes one or more of water and ethanol; the suction filtration is vacuum suction filtration; and the drying is vacuum drying at room temperature. 2. a kind of MXene modified composite diaphragm made by the preparation method of claim 1, is characterized in that, the base is a polyolefin diaphragm, and the modification material is a composite of MXene and polymer or inorganic particles, and the modification material is attached to the polyolefin. on the surface of one side of the olefin diaphragm. 3. The application of a MXene-modified composite separator in a lithium-sulfur battery according to claim 2, wherein the MXene-modified composite separator is directly placed in the lithium-sulfur battery, and the MXene-modified composite separator is not attached with MXene The composite side of the material with polymer or inorganic particles is close to the negative electrode of the battery.

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