Composite lithium-sulfur diaphragm as well as preparation method and application thereof

Abstract

The invention relates to the technical field of batteries, and particularly discloses a composite lithium-sulfur diaphragm as well as a preparation method and application thereof. The lithiated cellulose nanocrystals and the polyvinyl alcohol are synergistically self-assembled on the surface of the dopamine modified polypropylene diaphragm to form the composite lithium-sulfur diaphragm with a cholesteric phase ordered structure, so that the dual effects of chemical adsorption and structural barrier are realized, and the shuttle effect of polysulfide is effectively inhibited; moreover, the preparation method provided by the invention adopts aqueous preparation, low-temperature drying and self-assembly film forming processes, is environment-friendly and simple in preparation process, can directly construct a composite layer on a commercial PP diaphragm, and has a good industrial application prospect; the composite lithium-sulfur diaphragm provided by the invention has high ionic conductivity, excellent polysulfide inhibition capability and cycling stability, so that the energy density, the cycle life and the safety of the lithium-sulfur battery are remarkably improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of batteries, in particular to a composite lithium-sulfur separator and a preparation method and application thereof. BACKGROUND

[0002] With the continuous growth of global energy demand, there is an increasing demand for rechargeable batteries with low cost, high voltage and high energy density. Lithium-sulfur batteries are considered to be a potential new generation of energy storage systems due to their high theoretical energy density (about 2600 Wh·kg -1 ), and the advantages of abundant sulfur resources, low cost and environmental friendliness. They have broad application prospects in the fields of electric vehicles and grid energy storage. However, lithium-sulfur batteries have problems such as low energy density, short cycle life and poor safety, which are mainly due to the shuttle effect of soluble lithium polysulfides (LiPSs) and lithium dendrite growth during charging and discharging, resulting in loss of active material and reduction of coulombic efficiency.

[0003] To improve the performance of the battery, some studies have disclosed improvements from the positive electrode, negative electrode, electrolyte and separator. The separator, as a key component of the battery, plays an important role in inhibiting the migration of polysulfides and regulating ion conduction. The current commercial polyolefin separator (such as PP, PE) is difficult to effectively constrain the migration of polysulfides due to its non-polarity, poor wettability and lack of selective Li ＋ conduction channels. Although carbon-based coatings can improve conductivity, they have weak interactions with polar polysulfides and can easily increase the interfacial impedance. Although polymer coatings have adsorption capacity, they often hinder ion migration due to their dense structure. Therefore, there is an urgent need for a functionalized separator that has chemical adsorption, fast ion conduction and structural order.

[0004] In recent years, cellulose nanocrystals (CNC) have attracted attention due to their renewability, hydrophilicity and ability to form ordered cholesteric phase structures, providing a new approach to constructing separators with pore regulation and stable structures. However, the CNC surface is mainly composed of hydroxyl and sulfate groups, which lack effective Li ＋ conduction regulation and polysulfide fixation. Lithiumated cellulose nanocrystals (CNC-Li) prepared by chemical lithiumation modification of CNC can introduce fixed lithium ion sites on the surface, enhance the electrostatic and Lewis acid-base interactions with polysulfides, and impart selective Li⁺ conduction properties.

[0005] However, current research on CNC-Li is mainly focused on chemical functionalization, and there is still a lack of systematic exploration of its synergistic construction with self-assembled ordered structures to achieve high-performance separators. Therefore, there is still a large room for improvement in realizing new lithium-sulfur battery separators that combine structural regulation and functional optimization. SUMMARY

[0006] The present application aims to overcome the deficiencies of the prior art and provide a composite lithium-sulfur separator, a preparation method and application thereof.

[0007] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: The present application provides a composite lithium-sulfur separator, which comprises a base film and a coating layer on at least one surface of the base film, and has a chiral nematic structure. The coating layer comprises lithiumated cellulose nanocrystals and polyvinyl alcohol. The base film comprises a polydopamine modified polypropylene separator. The composite lithium-sulfur separator has diffraction peaks in the range of 1050-1150 cm -1 .

[0008] In the technical scheme of the present application, the lithiumated cellulose nanocrystals and polyvinyl alcohol (PVA) are co-assembled on the surface of the polydopamine modified PP separator. The lithiumated cellulose nanocrystals and polyvinyl alcohol spontaneously form an ordered cholesteric spiral structure (chiral nematic structure), generating a continuous network with nanoscale confined channels, which can effectively block the diffusion of polysulfides and maintain unobstructed Li ＋ conduction. The finally obtained composite lithium-sulfur separator has both chemical function and structural order, which can simultaneously inhibit polysulfide shuttling, promote selective Li ＋ ion conduction and relieve lithium dendrite growth, thereby significantly improving the cycle stability, rate performance and operation safety.

[0009] In the composite lithium-sulfur separator of the present application, the lithiumated cellulose nanocrystals impart chemical anchoring and ion channel functions, and the polyvinyl alcohol provides film forming and bonding effects. The two work synergistically in the ordered structure to achieve efficient fixation of polysulfides and rapid transport of Li⁺.

[0010] Moreover, this is the first time that a cholesteric lithiumated cellulose nanocrystal film is applied to a lithium-sulfur battery separator. The present application not only combines lithiumation chemistry with biomimetic self-assembly to construct a sustainable, scalable and multifunctional platform, but also provides a feasible new approach to realize high-performance lithium-sulfur batteries, effectively bridging renewable material design and advanced electrochemical engineering.

[0011] The present application also provides a preparation method of the above-mentioned composite lithium-sulfur separator, comprising the following steps: S1, dissolving dopamine hydrochloride in a buffer solution to prepare a solution, then immersing a polypropylene separator in the solution, washing, and drying to obtain a dopamine modified polypropylene separator; S2, preparing a suspension of lithiumated cellulose nanocrystals and polyvinyl alcohol, and performing ultrasonic treatment to obtain a treated suspension; S3. Apply the suspension treated in step S2 to the surface of the polydopamine-modified polypropylene separator from step S1, and dry it to obtain a composite lithium-sulfur separator.

[0012] This application achieves a dual effect of chemical adsorption and structural barrier by synergistically self-assembling lithium cellulose nanocrystals (CNC-Li) and polyvinyl alcohol (PVA) on the surface of dopamine-modified polypropylene (PP) membranes to form a composite lithium-sulfur membrane with an ordered cholesteric phase structure, thereby effectively suppressing the polysulfide shuttle effect.

[0013] This method employs an aqueous preparation, low-temperature drying, and self-assembly membrane formation process, which is environmentally friendly and simple to implement. It allows for the direct construction of composite layers on commercial PP membranes, demonstrating promising industrial application prospects. The composite lithium-sulfur membrane provided in this application combines the chemical lithiation modification of cellulose nanocrystals with their cholesteric phase self-assembly, achieving multifunctional performance.

[0014] Among them, the polypropylene membrane is modified, and the surface hydrophilicity of the modified composite lithium-sulfur membrane is significantly improved, and the wettability with electrolyte is good. It can reduce interfacial impedance and promote uniform lithium deposition, thereby inhibiting dendrite formation.

[0015] As a preferred embodiment of the preparation method of the composite lithium-sulfur membrane described in this application, the preparation method of the lithium-ionized cellulose nanocrystals includes the following steps: 1) The pulp board is pretreated by immersing it in alkaline solution I, and then washed until neutral to obtain the pretreated material; 2) The pretreated material is reacted in sulfuric acid solution to obtain a mixture, and then the mixture is reacted in alkaline solution II, centrifuged, and a precipitate is obtained; 3) The precipitate was dissolved in a lithium hydroxide solution to achieve lithiation, thereby obtaining a lithium-functionalized cellulose nanocrystal suspension; 4) Dialyze the lithium-functionalized cellulose nanocrystal suspension to neutral, then sonicate the lithium-functionalized cellulose nanocrystal suspension, and let it stand to obtain lithium-functionalized cellulose nanocrystals.

[0016] In the technical solution of this application, the surface sulfate groups of cellulose nanocrystal molecules are converted into lithium sulfate groups. This modification enhances the chemical affinity for polysulfides, effectively alleviates the shuttle effect, and also constructs a preferred Li group. ＋ The transmission channel improves ionic conductivity and electrolyte wettability.

[0017] Among them, lithium-ion nanocrystals introduce fixed lithium-ion sites, and the lithium-ion sites on the surface of the lithium-ion nanocrystals can interact with Li... ＋ Forming dynamic coordination channels to improve Li ＋ Selective mobility significantly improves ion conduction performance and cycling stability.

[0018] Furthermore, lithium-ionized cellulose nanocrystals contain abundant polar groups, which can achieve strong fixation with polysulfides through Lewis acid-base interactions and electrostatic adsorption, reducing the loss of active materials and improving coulombic efficiency and capacity retention.

[0019] In a preferred embodiment of the preparation method of the composite lithium-sulfur membrane described in this application, the alkaline solution I in step 1) includes a sodium hydroxide solution, wherein the mass concentration of sodium hydroxide in the sodium hydroxide solution is 4-5 wt%. And / or, In step 2), the alkaline solution II includes a lithium hydroxide solution, wherein the mass concentration of lithium hydroxide in the lithium hydroxide solution is 6-7 wt%; and the mass concentration of sulfuric acid in the sulfuric acid solution is 64-66 wt%. And / or, In step 3), the mass concentration of lithium hydroxide solution in the lithium hydroxide solution is 1.9~2.2 M.

[0020] This application obtains lithium-ionized cellulose nanocrystals (CNC-Li) through alkaline pretreatment, sulfuric acid hydrolysis, and lithiation reaction. The surface of the lithium-ionized cellulose nanocrystals contains fixed lithium ion sites and exhibits selective Li-ionization. ＋ Conductivity and polysulfide adsorption capacity.

[0021] In a preferred embodiment of the preparation method of the composite lithium-sulfur membrane described in this application, in step 2), the pretreated material is reacted with sulfuric acid solution at a temperature of 45~50℃ for a reaction time of 45~50 min. And / or, in step 2), the mixture is reacted with an alkaline solution at a temperature of 60~65℃ for 5~6 hours.

[0022] In a preferred embodiment of the preparation method of the composite lithium-sulfur membrane described in this application, in step 3), the precipitate is dissolved in a lithium hydroxide solution at a temperature of 30~35℃ to achieve lithiation.

[0023] In a preferred embodiment of the preparation method of the composite lithium-sulfur membrane described in this application, the conditions for ultrasonic treatment in step 4) are as follows: The ultrasound frequency is 40 kHz, and the ultrasound duration is 5-10 min.

[0024] In a preferred embodiment of the preparation method of the composite lithium-sulfur membrane described in this application, the buffer solution in step S1 includes Tris-HCl buffer solution.

[0025] In a preferred embodiment of the preparation method of the composite lithium-sulfur membrane described in this application, in step S2, the mass concentration of lithium cellulose nanocrystals in the suspension of lithium cellulose nanocrystals and polyvinyl alcohol is 1~1.2 wt%, and the mass concentration of polyvinyl alcohol is 0.1~0.11 wt%.

[0026] This application uses lithium cellulose nanocrystals and lithium cellulose nanocrystals of the above-mentioned mass concentration to form a suspension. During the drying process, an ordered cholesteric phase structure is formed, and the resulting coating has the dual functions of ion selective conduction and polysulfide inhibition.

[0027] Furthermore, by using dopamine-modified self-polymerization to form an adhesive interface layer on the surface of the polypropylene membrane, the adhesion and interface stability between the coating and the base membrane are enhanced, ensuring the integrity of the membrane layer.

[0028] This application also provides the application of the above-mentioned composite lithium-sulfur separator in the preparation of high-performance lithium-sulfur batteries.

[0029] The composite lithium-sulfur separator provided in this application combines high ionic conductivity, excellent polysulfide suppression capability, and cycle stability, which can significantly improve the energy density, cycle life, and safety of lithium-sulfur batteries.

[0030] Compared with the prior art, this application has the following beneficial effects: This application provides a composite lithium-sulfur separator, its preparation method, and its application. The composite lithium-sulfur separator is formed by the synergistic self-assembly of lithium-ion cellulose nanocrystals (CNC-Li) and polyvinyl alcohol (PVA) onto the surface of a dopamine-modified polypropylene (PP) separator, creating a cholesteric-phase ordered structure. This achieves a dual effect of chemical adsorption and structural barrier, effectively suppressing the polysulfide shuttle effect. Furthermore, the preparation method employs an aqueous preparation, low-temperature drying, and self-assembly film formation process, which is environmentally friendly and simple. The composite layer can be directly constructed on commercial PP separators, showing promising industrial application prospects. The composite lithium-sulfur separator provided by this application combines high ionic conductivity, excellent polysulfide suppression capability, and cycle stability, significantly improving the energy density, cycle life, and safety of lithium-sulfur batteries. Attached Figure Description

[0031] Figure 1 A diagram showing the chemical reaction results of lithiation of cellulose nanocrystals; Figure 2 Fourier transform infrared spectra of cellulose nanocrystals before and after lithiation; Figure 3 The circular dichroism chromatogram of lithium-ionized cellulose nanocrystals is shown. Figure 4 SEM image of the composite lithium-sulfur membrane; Figure 5The diagrams show the AC impedance and ionic conductivity results for different electrolytes. Figure 6 The graph shows the long-term cycle stability results of button cells using different separators. Detailed Implementation

[0032] To better illustrate the purpose, technical solution, and advantages of this application, the following description will be provided in conjunction with the accompanying drawings and specific embodiments.

[0033] In the following embodiments, unless otherwise specified, the experimental methods used are conventional methods, and the materials and reagents used are commercially available unless otherwise specified, and the raw materials used in each parallel experiment are the same.

[0034] Example 1, a composite lithium-sulfur separator and a method of making the same This embodiment provides a method for preparing a composite lithium-sulfur separator, including the following steps: 1. Preparation of lithium-ionized cellulose nanocrystals (CNC-Li): 1) First, the pulp board is soaked in a 4.0 wt% sodium hydroxide solution and treated at room temperature for 24 hours. Then, it is thoroughly washed with deionized water until neutral and vacuum dried to obtain the pretreated material. 2) The pretreated material was reacted with 64 wt% sulfuric acid at 50°C with vigorous stirring for 45 minutes, followed by the addition of a large amount of deionized water to terminate the acid hydrolysis reaction. Next, the resulting mixture was continuously stirred in a 7% lithium hydroxide solution at 65°C for 5 hours, and then centrifuged (8000 rpm, 10 minutes each time) to obtain a precipitate. 3) The precipitate was dissolved in a 2 M lithium hydroxide solution and stirred at room temperature for 24 hours to achieve lithiation, thereby obtaining a lithium-functionalized cellulose nanocrystal suspension; 4) Dialyze the obtained lithium-ionized cellulose nanocrystal suspension to neutral with deionized water, then sonicate the suspension at 40 kHz for 10 minutes to disperse the nanoparticles and obtain a homogeneous colloidal suspension. Allow the suspension to stand for more than 24 hours to allow larger particles to settle, finally obtaining a 1 wt% lithium-ionized cellulose nanocrystal (CNC-Li) suspension.

[0035] 2. Preparation of composite lithium-sulfur membrane (CNC-Li / PP@PDA composite membrane), including: 1) First, the polypropylene (PP) membrane was hydrophilically modified. Dopamine hydrochloride was dissolved in Tris–HCl buffer (pH 8.5) and stirred for 5 minutes to prepare a solution with a concentration of 2 mg·mL⁻¹. The PP membrane was immersed in this solution and allowed to stand for 2 hours. Then, the membrane was removed and rinsed three times with deionized water to remove unreacted dopamine and buffer residue. It was then laid flat in a petri dish and dried at room temperature to obtain the dopamine-modified polypropylene membrane. 2) Prepare a suspension containing 1 wt% CNC-Li and 0.1 wt% polyvinyl alcohol (PVA), stir thoroughly, and then sonicate in a water bath for 10 seconds. Take 200 μL of the obtained suspension and uniformly coat it onto the surface of the dopamine-modified PP membrane, and dry it under ambient conditions. Then place it in a vacuum oven at 60°C for more than 8 hours to obtain a lithium-modified cellulose nanocrystal self-assembled composite lithium-sulfur membrane (PDCL) with a chiral nematic structure.

[0036] The chemical reaction results of lithiation of cellulose nanocrystals are shown in the figure below. Figure 1 As shown, the lithiation process involves placing cellulose nanocrystals prepared by sulfuric acid hydrolysis in a 2M LiOH solution and stirring at room temperature for 24 hours.

[0037] Fourier transform infrared spectra of cellulose nanocrystals before and after lithiation are shown below. Figure 2 As shown.

[0038] from Figure 2 Mid-Fourier transform infrared (FTIR) spectroscopy reveals a significant difference in the characteristic absorption peaks between CNC and CNC-Li, indicating a chemical structural change in the cellulose nanocrystals during lithiation. Specifically, CNC-Li exhibits a significant difference in characteristic absorption peaks at approximately 3400 cm⁻¹. -1 The intensity of the –OH stretching vibration peak at 1050–1150 cm⁻¹ is significantly weakened and redshifted, indicating that some hydroxyl groups have coordinated with Li⁺, weakening the vibrational energy of the O–H bond. Furthermore, the intensity of the peak at 1050–1150 cm⁻¹ is significantly weakened. -1 Within this range, a slight shift in the position of the C–O–C stretching vibration peak indicates that the oxygen atom in the ether bond or the oxygen atom in the sulfate half-ester group forms a coordinate bond with Li⁺. Meanwhile, CNC-Li exhibits a peak position at approximately 1220 cm⁻¹. -1 and 810 cm -1 The decrease in absorption peak intensity or change in peak shape corresponding to the –OSO3⁻ group further proves that the sulfate group participates in lithium-ion binding. In summary, the red shift and weakening of the –OH peak, and the shifts in the C–O and –OSO3⁻ vibrational peaks all indicate that Li⁺ was successfully introduced into the CNC surface and formed a stable coordination structure through the hydroxyl and sulfate group oxygen atoms, thus confirming the effective lithiation of cellulose nanocrystals.

[0039] The circular dichroism chromatogram of lithium-ionized cellulose nanocrystals is as follows:Figure 3 As shown.

[0040] Figure 3 The strong CD signal in the image, consistent with the structural color in the inset, indicates that the lithium-treated cellulose nanocrystals can still self-assemble into a chiral nematic (cholesterol) phase on a macroscopic scale. This preserved ordered chiral structure is of great significance for constructing functionalized membranes with controllable pore size, anisotropic ion transport channels, and interactions with polysulfides.

[0041] SEM image of the composite lithium-sulfur membrane as shown below Figure 4 As shown.

[0042] SEM results showed that lithium-ionized cellulose nanocrystals (CNC-Li) formed a dense and uniform coating on the surface of the composite separator, exhibiting a clear orientational arrangement. This indicates that CNC-Li maintains good self-assembly ability and morphological stability after lithiation, which helps to construct continuous ion conduction pathways and improve the wettability and structural integrity of the separator.

[0043] Example 2, a composite lithium-sulfur separator and a method of making the same Compared with Example 1, the difference in Example 2 is that the mass concentration of lithium hydroxide solution in step 3) of the lithium hydroxide solution in the preparation process of lithium cellulose nanocrystals (CNC-Li) is 1.9 M, and the remaining steps are the same as in Example 1.

[0044] Example 3, a composite lithium-sulfur separator and a method of making the same Compared with Example 1, the difference in Example 3 is that the mass concentration of lithium hydroxide solution in step 3) of the lithium cellulose nanocrystal (CNC-Li) preparation process is 2.2 M, while the remaining steps are the same as in Example 1.

[0045] Example 4, a composite lithium-sulfur separator and a method of making the same Compared with Example 1, the difference in Example 4 is that in the preparation of the composite lithium-sulfur membrane, step 2) involves preparing a suspension containing 1.2 wt% CNC-Li and 0.1 wt% polyvinyl alcohol (PVA), while the remaining steps are the same as in Example 1.

[0046] Example 5, a composite lithium-sulfur separator and a method of making the same Compared with Example 1, the difference in Example 5 is that in the preparation of the composite lithium-sulfur membrane, step 2) involves preparing a suspension containing 1 wt% CNC-Li and 0.11 wt% polyvinyl alcohol (PVA), while the remaining steps are the same as in Example 1.

[0047] Comparative Example 1 Compared with Example 1, the difference in Comparative Example 1 is that lithium-ionized cellulose nanocrystals were not used, but un-lithiated cellulose nanocrystals were used instead of lithium-ionized cellulose nanocrystals, and the composite membrane (PDCS) was prepared directly according to step 2. The remaining steps were the same as in Example 1.

[0048] Comparative Example 2 Compared to Example 1, Comparative Example 2 was PP & PDA (modified with polydopamine only).

[0049] Preparation method: Dopamine hydrochloride was dissolved in Tris-HCl buffer (pH 8.5) and stirred for 5 minutes to prepare a solution with a concentration of 2 mg·mL⁻¹. The PP membrane was then immersed in this solution and allowed to stand for 2 hours. The membrane was then removed and rinsed three times with deionized water to remove unreacted dopamine and buffer residue. It was then placed flat in a petri dish and dried at room temperature, followed by drying in a vacuum oven at 60°C for at least 8 hours. The difference from Example 1 is that a lithium-ionized cellulose nanocrystal / PVA coating was not added on top.

[0050] Test Example 1, AC impedance and ionic conductivity of different electrolytes The AC impedance and ionic conductivity of four types of separators (PP, PDCS, PP&PDA, and PDCL, Example 1) were tested (all assembled into a 2032 button cell).

[0051] Test experiment: Electrochemical tests were conducted using a standard CR2032 push-button battery.

[0052] 1M LiTFSI was dissolved in a DOL:DME (1:1, V:V) / 0.2 M LiNO3 electrolyte solvent. The electrolyte volume for commercially available PP separator-assembled batteries was 80 μL, while for batteries with other separators, the electrolyte volume was 200 μL. Ionic conductivity was measured using AC impedance spectroscopy, with a frequency range of 0.1 Hz to 100 kHz and an amplitude of 10 mV, using a 2032 coin cell (stainless steel gasket / separator / stainless steel gasket) filled with electrolyte.

[0053] The results are as follows: AC impedance diagrams and ionic conductivity of different electrolytes, such as Figure 5 As shown.

[0054] The bulk resistivity of four types of separators—PP (commercial separator), PDCS, PP&PDA, and PDCL (Example 1)—were 4.36Ω, 12.63Ω, 5.12Ω, and 5.65Ω, respectively. Although the composite lithium-sulfur separator had a higher resistance than the PP separator, its thickness was greater. Calculations using the formula σ=L / RS yielded ionic conductivities of 2.92 mS / cm for the four types. -11.21 mS cm -1 2.35 mS cm -1 2.99 mScm -1 The significantly improved ionic conductivity of the lithium-ion cellulose nanocrystal composite membrane indicates that the lithium-ion cellulose nanocrystals form continuous ion conduction channels after self-assembly into a chiral nematic arrangement, thereby enhancing lithium-ion conduction efficiency. The composite membranes prepared in Examples 2-5 have similar effects to the composite membrane in Example 1.

[0055] Test Example 2, long-term cycling stability experiment of coin cells using different separators Two types of separators, PP and PDCL (Example 1), were applied to lithium-sulfur batteries to observe the long-term cycle stability of the lithium-sulfur batteries.

[0056] Specific experiment: The lithium-sulfur batteries were subjected to constant current charge-discharge tests at 1.1V to 2.8V using the Blue Electric test system. The test current was 0.5C. After the assembled batteries were placed for 8 hours, they were activated with a current of 0.1C for 3 cycles before being subjected to constant current charge-discharge tests.

[0057] Long-term cycle stability of button batteries using PP and PDCL separators, such as Figure 6 As shown.

[0058] CNC-Li chiral nematic layers significantly improved the charge-discharge performance of the PP separator. The increased ionic conductivity of the composite separator resulted in a higher initial specific discharge capacity for the lithium-sulfur battery. More importantly, the lithium-sulfur battery using the composite separator exhibited reduced capacity degradation after 500 stable cycles, significantly improving its cycle performance. The composite separators prepared in Examples 2-5 showed similar effects to the composite separator in Example 1.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A composite lithium-sulfur separator, characterized in that, The composite lithium-sulfur membrane includes a base membrane and a coating on at least one surface of the base membrane, and the composite lithium-sulfur membrane has a chiral nematic structure. The coating comprises lithium-ionized cellulose nanocrystals and polyvinyl alcohol; The base membrane comprises a polydopamine-modified polypropylene membrane; The composite lithium-sulfur separator has a thickness of 1050~1150 cm⁻¹ -1 It has diffraction peaks within the range.

2. The method for preparing the composite lithium-sulfur separator as described in claim 1, characterized in that, Includes the following steps: S1. Dissolve dopamine hydrochloride in buffer solution to prepare a solution, then immerse the polypropylene membrane in the solution, wash and dry to obtain the polydopamine-modified polypropylene membrane. S2. Prepare a suspension of lithium-ionized cellulose nanocrystals and polyvinyl alcohol, and subject it to ultrasonic treatment to obtain the treated suspension. S3. Apply the suspension treated in step S2 to the surface of the polydopamine-modified polypropylene separator from step S1, and dry it to obtain a composite lithium-sulfur separator.

3. The method for preparing the composite lithium-sulfur separator as described in claim 2, characterized in that, The method for preparing the lithium-ionized cellulose nanocrystals includes the following steps: 1) The pulp board is pretreated by immersing it in alkaline solution I, and then washed until neutral to obtain the pretreated material; 2) The pretreated material is reacted in sulfuric acid solution to obtain a mixture, and then the mixture is reacted in alkaline solution II, centrifuged, and a precipitate is obtained; 3) The precipitate was dissolved in a lithium hydroxide solution to achieve lithiation, thereby obtaining a lithium-functionalized cellulose nanocrystal suspension; 4) Dialyze the lithium-functionalized cellulose nanocrystal suspension to neutral, then sonicate the lithium-functionalized cellulose nanocrystal suspension, and let it stand to obtain lithium-functionalized cellulose nanocrystals.

4. The method for preparing the composite lithium-sulfur separator as described in claim 3, characterized in that, In step 1), the alkaline solution I includes a sodium hydroxide solution, and the sodium hydroxide solution has a sodium hydroxide concentration of 4-5 wt%. And / or, In step 2), the alkaline solution II includes a lithium hydroxide solution, wherein the mass concentration of lithium hydroxide in the lithium hydroxide solution is 6-7 wt%; and the mass concentration of sulfuric acid in the sulfuric acid solution is 64-66 wt%. And / or, In step 3), the mass concentration of lithium hydroxide solution in the lithium hydroxide solution is 1.9~2.2 M.

5. The method for preparing the composite lithium-sulfur separator as described in claim 3, characterized in that, In step 2), the pretreated material is reacted with sulfuric acid solution at a temperature of 45-50°C for 45-50 minutes. And / or, in step 2), the mixture is reacted with an alkaline solution at a temperature of 60~65℃ for 5~6 hours.

6. The method for preparing the composite lithium-sulfur separator as described in claim 3, characterized in that, In step 3), the precipitate is dissolved in a lithium hydroxide solution at a temperature of 30~35℃ to achieve lithiation.

7. The method for preparing the composite lithium-sulfur separator as described in claim 3, characterized in that, In step 4), the conditions for ultrasonic treatment are as follows: The ultrasound frequency is 40 kHz, and the ultrasound duration is 5-10 min.

8. The method for preparing the composite lithium-sulfur separator as described in claim 2, characterized in that, In step S1, the buffer solution includes Tris–HCl buffer.

9. The method for preparing the composite lithium-sulfur separator as described in claim 2, characterized in that, In step S2, the mass concentration of lithium cellulose nanocrystals in the suspension of lithium cellulose nanocrystals and polyvinyl alcohol is 1~1.2 wt%, and the mass concentration of polyvinyl alcohol is 0.1~0.11 wt%.

10. The application of the composite lithium-sulfur separator as described in claim 1 in the preparation of high-performance lithium-sulfur batteries.

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