A monatomic catalyst modified separator for lithium-sulfur battery and a preparation method and application thereof

Abstract

The application discloses a monatomic catalyst modified diaphragm for a lithium-sulfur battery and a preparation method and application thereof. The composite material takes nitrogen-doped graphene composite defect cerium hydroxide (NC@Ce(OH)4) as a carrier, anchors high-dispersed cobalt monatomic (SACo) through oxygen defects to form an NC@Ce(OH)4@SACo composite material. The composite material is coated on a commercial diaphragm to prepare a functionalized diaphragm, which is applied to a lithium-sulfur battery. The cobalt monatomic and the defect cerium hydroxide in the diaphragm can synergistically act, on one hand, to catalyze the rapid conversion of lithium polysulfide and inhibit the shuttle effect, and on the other hand, to guide the uniform deposition of lithium and inhibit the dendrite growth. The lithium-sulfur battery adopting the diaphragm shows high specific capacity and excellent cycle life. The application has a simple preparation process and provides a new idea for the development of high-performance lithium-sulfur batteries.

Description

Technical Field

[0001] This invention belongs to the field of electrochemistry and new energy materials technology, specifically relating to a method for preparing a functionalized separator for lithium-sulfur batteries, particularly a composite separator that suppresses lithium polysulfide shuttle and lithium dendrite growth by constructing a single-atom catalytic regulation layer, its preparation method, and its application in long-life lithium-sulfur batteries. Background Technology

[0002] Lithium-sulfur (Li-S) batteries are known for their extremely high theoretical specific capacity (1675 mAh g⁻¹). -1 ) and theoretical energy density (2600 Whkg) -1 With its abundant sulfur resources, low cost, and environmental friendliness, lithium-sulfur batteries are considered one of the most promising next-generation high-energy-density energy storage systems. However, the commercialization of lithium-sulfur batteries still faces several serious challenges: (1) the dissolution and shuttle effect of intermediate lithium polysulfides (LiPSs) in the electrolyte leads to irreversible loss of active materials and low coulombic efficiency; (2) the insulation of the sulfur cathode and its discharge end products (Li2S / Li2S2), as well as the huge volume change (about 80%) during charging and discharging, leads to slow reaction kinetics and low utilization of active materials; (3) the uncontrollable growth of lithium dendrites in the lithium metal anode during cycling not only consumes electrolyte and active lithium, but may also puncture the separator and cause a short circuit in the battery, bringing serious safety hazards.

[0003] Currently, researchers have proposed various strategies to address these issues. For example, on the positive electrode side, various carbon materials or polar metal compounds are used to adsorb and catalyze the conversion of LiPSs to suppress the shuttle effect. On the negative electrode side, three-dimensional current collectors or artificial solid electrolyte interphase (SEI) films are constructed to guide uniform lithium deposition and suppress dendrite growth. However, these strategies often only solve a single problem and usually come at the cost of sacrificing the overall energy density of the battery.

[0004] Single-atom catalysts (SACs) have shown great potential in catalyzing the conversion of LiPSs due to their high atom utilization and tunable electronic structure. However, most current SACs have low metal loading (typically <3 wt%), and are prone to aggregation at high loadings, leading to a reduction in active sites and a decrease in catalytic performance. Therefore, developing a multifunctional material capable of stably loading high concentrations of metal single atoms and simultaneously addressing the problems of both positive and negative electrodes is crucial for promoting the practical application of lithium-sulfur batteries. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a single-atom catalyst-modified membrane and its preparation method that can simultaneously suppress lithium polysulfide shuttle and lithium dendrite growth.

[0006] Another object of the present invention is to provide the application of the above-described modified separator in the construction of high-capacity, long-cycle-life lithium-sulfur batteries.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides a single-atom catalyst composite material for lithium-sulfur batteries, characterized in that the composite material uses nitrogen-doped graphene as a conductive substrate, on which oxygen-deficient cerium hydroxide nanoparticles are loaded, and highly dispersed transition metal single atoms are anchored through the oxygen defects; wherein the mass loading of the transition metal single atoms is 1% to 10%.

[0008] Furthermore, the transition metal is cobalt, and the mass loading of cobalt single atoms is 3% to 6%, preferably 5%. When the cobalt single atom loading exceeds 6%, agglomerated phases of cobalt nanoparticles appear in the composite material. By controlling the loading at 3% to 6%, the oxygen defect sites of cerium hydroxide in the support (NC@Ce(OH)4) can be used to achieve stable anchoring of cobalt single atoms, completely avoiding agglomeration problems and ensuring that each cobalt single atom can participate in the reaction as an effective active site. A loading of 5% can provide more cobalt single atom active sites, which can more efficiently catalyze the rapid conversion of lithium polysulfides (LiPSs), further suppress the shuttle effect, reduce irreversible loss of active materials, and lay the foundation for high specific capacity of the battery. With a loading of 5%, the cobalt single atoms are uniformly dispersed in the support, which can form denser and more uniformly distributed lithiophilic sites, guide the uniform flow of lithium ions and promote the uniform nucleation and deposition of lithium metal on the negative electrode surface, avoiding dendrite growth caused by excessively high local lithium concentration.

[0009] On the other hand, a method for preparing the single-atom catalyst composite material as described above is characterized by comprising the following steps: S1: Graphene is dispersed in water, a surfactant and a cerium source are added, and after ultrasonic dispersion, a precipitant containing nitrogen is added. After stirring, hydrothermal reaction, filtration, washing and drying, nitrogen-doped graphene composite defect cerium hydroxide carrier material is obtained. S2: Disperse the carrier material in a solvent, add a transition metal salt, stir to allow it to be fully adsorbed, and then evaporate the solvent to obtain the precursor; S3: The precursor is heat-treated in a reducing atmosphere, and after cooling, the single-atom catalyst composite material is obtained.

[0010] Furthermore, in step S1, the cerium source is cerium nitrate, and the nitrogen-containing precipitant is ammonia; the hydrothermal reaction temperature is 120-180℃, and the time is 6-18 hours.

[0011] Furthermore, in step S2, the transition metal salt is one or more of nitrates, sulfates, acetates, carbonates, and chlorides.

[0012] Furthermore, in step S3, the reducing atmosphere is a hydrogen / argon mixture; the heat treatment temperature is 500-800℃, and the holding time is 1-4 hours.

[0013] On the other hand, the present invention also provides a functionalized separator for lithium-sulfur batteries, which is a modification layer formed by coating one or both sides of a commercial base film with a composite material of transition metal single atoms as described above. On the one hand, its abundant polar sites (defective cerium hydroxide, transition metal single atoms, nitrogen-doped carbon) can strongly adsorb LiPSs and catalyze their rapid conversion, suppressing the shuttle effect; on the other hand, its uniformly distributed lithiophilic sites (transition metal single atoms) can guide the uniform flow of lithium ions and promote the uniform nucleation and deposition of lithium, effectively suppressing the growth of lithium dendrites.

[0014] Furthermore, the commercial base film is a polypropylene diaphragm, a polyethylene film, or a composite film of both.

[0015] The present invention also provides a method for preparing the functionalized membrane as described above, comprising: dispersing the above-described transition metal single-atom catalyst composite material in a solvent to form a slurry, coating the slurry onto a commercial base membrane by vacuum filtration, and drying the membrane to obtain the final product.

[0016] The application of the aforementioned single-atom catalyst composite material or the aforementioned functionalized separator in lithium-sulfur batteries, wherein the composite material or separator is used to suppress the shuttle effect of lithium polysulfides and / or suppress dendrite growth of lithium metal anode, so as to improve the capacity, coulombic efficiency and cycle life of lithium-sulfur batteries.

[0017] The beneficial effects of this invention are as follows: High loading and high dispersion: By utilizing the abundant oxygen vacancies in defective cerium hydroxide (Ce(OH)4) as anchoring sites, stable loading of high-quality fractional metal single atoms was successfully achieved without significant agglomeration, thus maximizing the exposure of active sites.

[0018] Dual Functionality: The prepared NC@Ce(OH)4@SACo composite material and its modified separator can simultaneously solve the two core problems of shuttle effect in lithium-sulfur battery cathode and dendrite growth in anode, achieving dual benefits from a single material.

[0019] Excellent performance: The lithium-sulfur battery assembled using this modified separator exhibits excellent electrochemical performance: high discharge specific capacity (1058 mAh g⁻¹ at 1C). -1 It also boasts excellent cycle life (63.6% capacity retention after 400 cycles at 3C, with a single-cycle capacity decay rate as low as 0.09%).

[0020] Simple process: The preparation method is simple, the conditions are mild, it is easy to scale up production, and it has broad application prospects. Attached Figure Description

[0021] Figure 1 This refers to the preparation process of the modified diaphragm; Figure 2 XRD patterns of the prepared NC@Ce(OH)4, NC@Ce(OH)4@SACo3% and NC@Ce(OH)4@SACo5% materials; Figure 3 The EDS elemental analysis results for the NC@Ce(OH)4@SACo5% material prepared in Example 2 are shown below. Figure 4 HR-TEM images of NC@Ce(OH)4@SACo5% material prepared in Example 2; Figure 5 Electrochemical performance diagrams of Li-Li symmetric cells prepared from NC@Ce(OH)4, NC@Ce(OH)4@SACo3% and NC@Ce(OH)4@SACo5% materials; Figure 6 Electrochemical performance diagrams of Li-Cu half-cells for the prepared NC@Ce(OH)4, NC@Ce(OH)4@SACo3% and NC@Ce(OH)4@SACo5% materials; Figure 7 The full-cell performance diagrams of the prepared NC@Ce(OH)4, NC@Ce(OH)4@SACo3% and NC@Ce(OH)4@SACo5% materials are shown. Figure 8 SEM image of the prepared NC@Ce(OH)4@SACo8% modified membrane. Detailed Implementation

[0022] The present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings. The following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention.

[0023] Example 1: Preparation of NC@Ce(OH)4@SACo3% composite material Weigh 140 mL of deionized water, 14 mL of graphene dispersion (containing approximately 100 mg of graphene), and 1.1 mL of polyethylene glycol 600, mix them, and sonicate for 1 hour.

[0024] Add 0.131 g of cerium nitrate hexahydrate (Ce(NO3)3·6H2O) and continue sonication for 1 hour.

[0025] Slowly add 3 mL of ammonia water (25-28%) and stir magnetically for 3 hours.

[0026] The mixture was transferred to a 100 mL hydrothermal reactor lined with polytetrafluoroethylene and reacted at 120 °C for 12 hours.

[0027] After cooling, the product was filtered and washed until the filtrate was neutral. The resulting gel-like product was freeze-dried for 24 hours to obtain the NC@Ce(OH)4 carrier.

[0028] Weigh 200 mg of the above NC@Ce(OH)4 carrier, disperse it in 10 mL of anhydrous ethanol, add 10 mg of cobalt nitrate (Co(NO3)2), and stir magnetically for 12 hours.

[0029] Heat and stir the mixture at 60°C until it is completely evaporated to dryness.

[0030] The dried precursor powder was placed in a tube furnace and heated at 5°C for 5 min under a 5% H2 / Ar mixed atmosphere. -1 The temperature was increased to 600℃ at a certain rate, held for 2 hours, and then naturally cooled to room temperature to obtain the final product NC@Ce(OH)4@SACo composite material.

[0031] Preparation of NC@Ce(OH)4@SACo3% modified separator Take 16 mg of the NC@Ce(OH)4@SACo3% composite material prepared in Example 1, disperse it in 20 mL of ethanol, and sonicate for 20 minutes to form a uniform slurry. Fix a commercial Celgard 2400 membrane (cut to a suitable size) onto a vacuum filtration device. First, use a dropper to draw 2 mL of slurry and drip it onto the membrane for filtration. Then, draw 3 mL of slurry for a second coating, ensuring a uniform coating. Place the coated membrane in a vacuum oven at 60°C and dry for 12 hours to obtain the modified membrane.

[0032] Example 2: Preparation of NC@Ce(OH)4@SACo5% composite material The preparation method mentioned in this embodiment is the same as that in Example 1, except that the amount of cobalt nitrate added needs to be increased so that the cobalt content in the final composite material is 5%, while other processes remain unchanged.

[0033] Preparation of NC@Ce(OH)4@SACo5% modified separator Take 16 mg of the NC@Ce(OH)4@SACo5% composite material prepared in Example 2, disperse it in 20 mL of ethanol, and sonicate for 20 minutes to form a uniform slurry. Fix a commercial Celgard 2400 membrane (cut to a suitable size) onto a vacuum filtration device. First, use a dropper to draw 2 mL of slurry and drip it onto the membrane for filtration. Then, draw 3 mL of slurry for a second coating, ensuring a uniform coating. Place the coated membrane in a vacuum oven at 60°C and dry for 12 hours to obtain the modified membrane.

[0034] Comparative Example 1: Preparation of NC@Ce(OH)4 composite material The preparation steps are the same as those in Example 1, but without cobalt loading and thermal reduction.

[0035] Preparation of NC@Ce(OH)4 modified membranes 16 mg of the NC@Ce(OH)4 composite material prepared in Comparative Example 1 was dispersed in 20 mL of ethanol and ultrasonically treated for 20 minutes to form a uniform slurry. A commercial Celgard 2400 membrane (cut to a suitable size) was fixed on a vacuum filtration device. 2 mL of slurry was first dripped onto the membrane using a dropper and then filtered. Another 3 mL of slurry was then applied for a second coating, ensuring a uniform coating. The coated membrane was then dried in a vacuum oven at 60 °C for 12 hours to obtain the modified membrane.

[0036] Comparative Example 2 Preparation of NC@Ce(OH)4@SACo8% composite material The preparation method mentioned in this embodiment is the same as that in Example 1, except that the amount of cobalt nitrate added needs to be increased so that the cobalt content in the final composite material is 8%, while other processes remain unchanged.

[0037] Preparation of NC@Ce(OH)4@SACo 8% modified separator 16 mg of the NC@Ce(OH)4@SACo8% composite material prepared in Comparative Example 2 was dispersed in 20 mL of ethanol and ultrasonically treated for 20 minutes to form a uniform slurry. A commercial Celgard 2400 membrane (cut to a suitable size) was fixed on a vacuum filtration device. 2 mL of slurry was first dripped onto the membrane using a dropper and then filtered. Another 3 mL of slurry was then applied for a second coating, ensuring uniformity. The coated membrane was dried in a 60℃ vacuum oven for 12 hours to obtain the modified membrane. SEM images of the NC@Ce(OH)4@SACo8% modified membrane (…) Figure 8 It can be seen that when the load exceeds 6%, a clustering phenomenon occurs.

[0038] Material analysis, Figure 2 The images show the XRD patterns of the prepared NC@Ce(OH)4, NC@Ce(OH)4@SACo3% and NC@Ce(OH)4@SACo5% materials. XRD analysis verifies that within a cobalt single-atom loading range of 1.5%-5% (e.g., 3% and 5%), cobalt is stably anchored on the NC@Ce(OH)4 support in single-atom form, with no cobalt nanoparticle agglomeration. This provides the structural basis for the high catalytic activity of the composite materials (inhibiting lithium polysulfide shuttle and guiding uniform lithium deposition). Figure 3 The presence of cobalt in the EDS further confirms that cobalt exists in the material in a single-atom form. Figure 4 As can be seen, the crystal plane stripes of cerium hydroxide in the figure are clear and uniformly distributed, indicating that it has good crystallinity and provides stable oxygen defect sites for the anchoring of cobalt single atoms. At a cobalt single atom loading of 5%, no lattice stripes of cobalt nanoparticles were observed in the image. The typical interplanar spacing of cobalt elemental, such as (111), is about 0.20 nm. If agglomeration exists, corresponding stripes will appear, further proving that cobalt is highly dispersed in the support in the form of single atoms, which is consistent with the conclusion of no cobalt agglomeration in XRD analysis.

[0039] Assembly and electrochemical testing of lithium-sulfur batteries Positive electrode preparation: The prepared materials from the examples and comparative examples were mixed with sulfur powder at a mass ratio of 1:3, and sulfur was loaded using a melt diffusion method (heat treatment at 155℃ for 12 hours) to prepare sulfur-containing composite positive electrode materials. The active material, conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in N-methylpyrrolidone (NMP) at a mass ratio of 7:2:1 and stirred to form a homogeneous slurry. The slurry was coated onto carbon-coated aluminum foil, vacuum dried at 60℃ for 12 hours, and then punched into electrode sheets with a diameter of 12 mm. The sulfur surface loading was approximately 1.0-1.5 mg / cm². -2 .

[0040] Battery Assembly: CR2032 coin cells were assembled in an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm). The positive electrode used to prepare the sulfur-loaded sample was the working electrode, lithium metal sheets were used as the counter and reference electrodes, and the modified separators prepared in the examples and comparative examples were used as separators. The electrolyte was a DOL / DME solution (v:v = 1:1) containing 1.0 M LiTFSI and 0.2 M LiNO3, with approximately 60 μL of electrolyte used per cell.

[0041] Electrochemical testing: Electrochemical performance was tested using the Xinwei Battery Testing System. Test results are as follows: Figure 5-7 As shown in the figure, the battery assembled using the materials of this invention exhibits high specific capacity and excellent cycle stability.

[0042] Li-Li symmetric cells: The overpotential of the NC@Ce(OH)4 composite material reached over 200 mV after 100 h, indicating poor surface voltage stability. However, the cells prepared with separators modified with NC@Ce(OH)4@SACo3% and NC@Ce(OH)4@SACo5% still had an overpotential of less than 100 mV after 200 h.

[0043] Li-Cu half-cell: The cell assembled with NC@Ce(OH)4@SACo5% materials has extremely stable coulombic efficiency, which basically remains above 98% within 100 cycles.

[0044] The full cell assembled with NC@Ce(OH)4@SACo 5% materials has an initial discharge specific capacity of 1058 mA hg. -1 In contrast, NC@Ce(OH)4 in Comparative Example 1 only had a concentration of 850 mA hg. -1 After 100 cycles, the discharge specific capacity of NC@Ce(OH)4@SACo5% remained at 848 mA hg. -1 It is 266 mA hg higher than the control ratio NC@Ce(OH)4. -1 .

[0045] The battery assembled with NC@Ce(OH)4@SACo 5% material has an initial discharge specific capacity of 1058 mA hg. -1 In contrast, the NC@Ce(OH)4@SACo8% in Comparative Example 2 was only 926 mA hg. -1 This is because aggregation reduces catalytic performance. After 100 cycles, the discharge specific capacity of NC@Ce(OH)4@SACo8% remained at 765 mA hg. -1 .

[0046] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A single-atom catalyst composite material for lithium-sulfur batteries, characterized in that, The composite material uses nitrogen-doped graphene as a conductive substrate, on which oxygen-deficient cerium hydroxide nanoparticles are loaded, and highly dispersed transition metal single atoms are anchored through the oxygen defects; wherein the transition metal is cobalt, and the mass loading of cobalt single atoms is 3% to 6%.

2. The single-atom catalyst composite material for lithium-sulfur batteries according to claim 1, characterized in that, The mass loading of cobalt single atoms is 5%.

3. A method for preparing a single-atom catalyst composite material for lithium-sulfur batteries as described in claim 1 or 2, characterized in that, Includes the following steps: S1: Graphene is dispersed in water, a surfactant and a cerium source are added, and after ultrasonic dispersion, a precipitant containing nitrogen is added. After stirring, hydrothermal reaction, filtration, washing and drying, nitrogen-doped graphene composite defect cerium hydroxide carrier material is obtained. S2: Disperse the carrier material in a solvent, add a transition metal salt, stir to allow it to be fully adsorbed, and then evaporate the solvent to obtain the precursor; S3: The precursor is heat-treated in a reducing atmosphere, and after cooling, the single-atom catalyst composite material is obtained.

4. The method according to claim 3, characterized in that, In step S1, the cerium source is cerium nitrate, and the precipitant is ammonia water; the hydrothermal reaction temperature is 120-180℃, and the time is 6-18 hours.

5. The method according to claim 3, characterized in that, In step S2, the transition metal salt is one or more of nitrates, sulfates, acetates, carbonates, and chlorides.

6. The method according to claim 3, characterized in that, In step S3, the reducing atmosphere is a hydrogen / argon mixture; the heat treatment temperature is 500-800℃, and the holding time is 1-4 hours.

7. A functionalized separator for lithium-sulfur batteries, characterized in that, It is a modification layer formed by coating one or both sides of a commercial base film with the single-atom catalyst composite material for lithium-sulfur batteries as described in claim 1 or 2.

8. The functionalized diaphragm according to claim 7, characterized in that, The commercial base film is a polypropylene diaphragm, a polyethylene film, or a composite film of both.

9. A method for preparing the functionalized membrane as described in claim 7 or 8, characterized in that, include: The single-atom catalyst composite material for lithium-sulfur batteries as described in claim 1 or 2 is dispersed in a solvent to form a slurry, which is then coated onto a commercial base film by vacuum filtration and dried to obtain the final product.

10. The application of the single-atom catalyst composite material for lithium-sulfur batteries as described in claim 1 or 2, or the functionalized separator as described in claim 7 or 8, in lithium-sulfur batteries, characterized in that... The composite material or separator is used to suppress the shuttle effect of lithium polysulfides and / or suppress dendrite growth of lithium metal anodes, so as to improve the capacity, coulombic efficiency and cycle life of lithium-sulfur batteries.

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