Multifunctional modified diaphragm for lithium-sulfur battery and preparation method

Abstract

The invention belongs to the technical field of chemical power sources, and particularly relates to a multifunctional modified diaphragm for a lithium-sulfur battery and a preparation method. According to the modified diaphragm for the lithium-sulfur battery and the preparation process of the modified diaphragm, an organic-inorganic composite modified layer with extremely low loading capacity is constructed on the surface of a commercial diaphragm through an ultrasonic atomization spraying method, and the modified layer is prepared from a single-ion conductor polymer Li-Nafion and polyoxometallate lithium phosphotungstate. Wherein the lithium phosphotungstate has excellent oxidation-reduction capability and can effectively catalyze the conversion of polysulfide, and the structure of the lithium phosphotungstate can effectively conduct lithium ions and effectively improve the conduction kinetics of the lithium ions. Besides, Li-Nafion is of a nonporous structure and has an electronegative functional group, so that polysulfide ions can be effectively blocked through coulomb repulsion, Li < + > can be quickly conducted through the action of the internal functional group, and Li < + > flux is increased. The diaphragm modified layer has a good effect, and the practicability of the lithium-sulfur battery can be enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical power sources, and particularly relates to a multifunctional modified separator for lithium-sulfur batteries and a preparation method thereof. Technical Background

[0002] Lithium-sulfur batteries have a relatively high theoretical specific capacity (1675 mAh g -1 ) and energy density (2600 Wh kg -1 ), and are low in cost, low in toxicity and environmentally friendly. In the future, they are expected to be used in fields such as electric vehicles, rail transit, large-scale energy storage systems, aerospace, etc. to meet various needs of national defense, military and civilian use. However, polysulfides (Li 2 S n ) intermediates generated at the positive electrode of lithium-sulfur batteries and slow redox kinetics lead to the shuttle effect, ultimately causing irreversible loss of active substances, low Coulomb efficiency, poor battery cycle stability, and adverse effects such as pulverization of the lithium negative electrode and growth of lithium dendrites. In order to suppress the shuttle effect, people mostly adopt physical barrier, chemical adsorption and catalytic schemes to inhibit the shuttle of polysulfide ions. However, a single scheme strategy has little effect on suppressing the shuttle effect.

[0003] By constructing a modified layer on the separator on the positive electrode side to solve the above problems, the process is simple, the processability is strong, the cost is low, and it is conducive to the commercialization of lithium-sulfur batteries. Traditional commercial separators are mostly made of polypropylene (PP), polyethylene (PE) or PP / PE / PP (PEP) composite materials. Although their processes are mature and the cost is low, they have micron-sized holes on their surfaces and poor wettability to electrolytes, and cannot inhibit the diffusion of polysulfides in the electrolyte, resulting in low utilization rate of active substances and low Coulomb efficiency. Therefore, it is necessary to propose a simple surface modification strategy and preparation process for commercial separators to effectively overcome the problems existing in lithium-sulfur batteries.

[0004] Based on the above problems, it is necessary to design a composite modified separator of lithiated polyoxometalate and Li-Nafion polymer to solve the technical problems. Summary of the Invention

[0005] The concept of the present invention is to utilize the synergistic effect of the single-ion conductor polymer Li-Nafion and lithium phosphotungstate of polyoxometalate to inhibit the shuttle of polysulfide ions, improve the utilization rate of positive electrode active substances, improve the Coulomb efficiency and reduce the loss of active substances.

[0006] The present invention provides a design and preparation process of a multifunctional modified separator for lithium-sulfur batteries to solve the problem of polysulfide shuttle in lithium-sulfur batteries.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a method for preparing a modified separator layer for a lithium-sulfur battery, comprising the following steps:

[0009] (1) Take lithium compound A and add it to the Nafion solution, and obtain a Li-Nafion solution after heating and stirring;

[0010] The lithium compound A is any one or a mixture of two or more of lithium hydroxide, lithium chloride, lithium fluoride, lithium nitrate, and lithium carbonate; the concentration of the lithium compound A in the reaction solution is 0.5 to 4 mg / ml;

[0011] (2) Mix the Li-Nafion solution and the solvent in a volume ratio of 1:9 to 1:1 and stir to obtain precursor solution A;

[0012] The solvent is any one or a mixture of two or more of water, ethanol, n-propanol, isopropanol, methanol, ethyl acetate, chloroform, N-methylpyrrolidone, or N,N-dimethylformamide;

[0013] (3) Take a polyoxometalate and lithium compound B and dissolve them in deionized water in a molar ratio of 1:9 to 1:1, stir to dissolve, and add ammonia water until the pH of the solution is 3 to 10 to obtain precursor solution B;

[0014] The polyoxometalate is any one or a mixture of two or more of phosphotungstic acid, silicotungstic acid, phosphomolybdic acid, and silicomolybdic acid;

[0015] The lithium compound B is any one or a mixture of two or more of lithium hydroxide, lithium chloride, lithium fluoride, lithium nitrate, and lithium carbonate;

[0016] (4) Perform a hydrothermal reaction on precursor solution B at 90 to 150 °C, cool to room temperature after sufficient reaction, filter by suction, and dry to obtain lithiated polyoxometalate;

[0017] (5) Take lithium phosphotungstate powder and add it to the above precursor solution A, and ultrasonically disperse it until completely dispersed to obtain precursor solution C; the concentration of lithium phosphotungstate in precursor solution C is 1 to 10 mg / ml;

[0018] (6) Take precursor solution C, and use ultrasonic atomization spraying technology to spray it on the surface of the separator to form an organic-inorganic composite modified layer, and dry it to obtain a modified separator.

[0019] Preferably, in step (1), the Nafion mass fraction in the Nafion solution is 5% to 15%; the reaction temperature is 60 to 100 °C, and the stirring time is 12 to 24 h.

[0020] Preferably, in the step (2), the solvent is any one or a mixture of two or more of N,N-dimethylformamide, N-methylpyrrolidone, and chloroform, and the volume ratio of the Li-Nafion solution to the solvent is 1:1 to 1:9.

[0021] Preferably, in the step (4), the hydrothermal reaction time is 3 to 12 h, the drying temperature is 60 to 120 °C, and the drying time is 8 to 24 h.

[0022] Preferably, in the step (6), the separator used is a PP / PE / PP composite separator.

[0023] Preferably, the process parameters of the ultrasonic atomization spraying in the step (6) are as follows: the ultrasonic power is 2.00 W, the liquid outlet speed of the nozzle is 0.2 to 1.0 ml / min, and the heating temperature of the vacuum spraying platform is 50 to 120 °C.

[0024] Furthermore, the thickness of the ultrasonic atomization spraying is 100 nm to 2 μm.

[0025] Preferably, in the step (6), the drying temperature of the modified separator is 50 to 120 °C, and the drying time is 8 to 24 h.

[0026] In a second aspect, the present invention provides a modified separator for a lithium-sulfur battery, which is prepared by the method described in the first aspect.

[0027] In a third aspect, the present invention provides an application of the modified separator for a lithium-sulfur battery described in the second aspect, which is used as a separator for a lithium-sulfur battery.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] For the modified separator for a lithium-sulfur battery and its preparation process of the present invention, an organic-inorganic composite modified layer with an extremely low loading amount is constructed on the surface of a commercial separator by ultrasonic atomization spraying. The modified layer is made of a single-ion conductor polymer Li-Nafion and lithium phosphotungstate, a polyoxometalate. Among them, lithium phosphotungstate has excellent redox ability, effectively catalyzes the conversion of polysulfides, and its structure can effectively conduct lithium ions, effectively improving the lithium-ion conduction kinetics. In addition, Li-Nafion has a non-porous structure and negatively charged functional groups, which can effectively block polysulfide ions through Coulomb repulsion, and it can quickly conduct Li + , increasing Li +Flux. The Coulomb repulsion of the negatively charged sulfonic acid groups in Li-Nafion blocks polysulfide ions and promotes the desolvation of lithium ions through the oxygen sites of the sulfonic acid groups. Lithiated polyoxometalates have excellent adsorption and catalytic capabilities, and the dense oxygen sites inside provide rich lithium ion conduction paths, which can effectively block and adsorb and catalyze polysulfides and effectively conduct lithium ions, improving the utilization rate of active substances from multiple angles, and increasing battery capacity, Coulomb efficiency, and long-cycle capacity retention rate. Description of the Drawings

[0030] Figure 1 SEM images of the separator surface;

[0031] (a) Example 1; (b) Example 2; (c) Comparative example;

[0032] Figure 2 Ionic transference number of the separator;

[0033] (a) Example 1; (b) Example 2; (c) Comparative example;

[0034] Figure 3 Long-cycle performance of the lithium-sulfur batteries prepared in Examples 1-2 and the comparative example at a current density of 0.5C. Detailed Description of the Invention

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0036] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0037] Unless otherwise specifically stated, all kinds of raw materials, reagents, instruments, and equipment used in the present invention can be obtained through the market or can be prepared by existing methods.

[0038] Example 1

[0039] (1) Lithium hydroxide was added to the Nafion solution to form a reaction solution. The mass concentration of lithium hydroxide was 2.5 mg / ml, and it was stirred at 60 °C for 12 h to obtain the Li-Nafion solution, which was cooled to room temperature for standby.

[0040] (2) Prepare precursor solution A by mixing Li-Nafion solution and organic solvent N,N-dimethylformamide (DMF) in a volume ratio of 1:9.

[0041] (3) Dissolve phosphotungstic acid and lithium chloride in deionized water in a molar ratio of 1:3, stir to dissolve, and add ammonia water until the pH of the solution is 7 to prepare precursor solution B;

[0042] (4) Add precursor solution B to the inner liner of a polytetrafluoroethylene hydrothermal reactor, place it in a hydrothermal reactor, and put it into a forced-draft oven for hydrothermal reaction. The hydrothermal reaction temperature is 90 °C, and the hydrothermal reaction time is 3 h. After the solution is cooled to room temperature, filter it by suction, and vacuum-dry the product at 90 °C to obtain lithium phosphotungstate.

[0043] (5) Add lithium phosphotungstate powder to precursor solution A. The mass concentration of lithium phosphotungstate is 2 mg / ml, and stir until completely dissolved to prepare precursor solution C.

[0044] (6) Spray precursor solution C on the surface of the PEP separator using ultrasonic atomization spraying technology. Vacuum-dry it at 60 °C for 24 h to obtain modified separator A. Ultrasonic atomization spraying process parameters: ultrasonic power 2.00 W, nozzle liquid outlet speed 0.2 ml / min, base film size 7×14 cm, vacuum spraying platform heating temperature 50 °C, spraying times 30 times. The SEM image of the surface of modified separator A is shown in the appendix Figure 1 , the ion transference number is 0.72, see the appendix Figure 2 . Assemble a lithium-sulfur button battery. At a current density of 0.5 C, the initial discharge capacity is 1148.8 mAh / g, the capacity after 200 cycles is 661.4 mAh / g, and the average Coulombic efficiency is 91.9%, see the appendix Figure 3 .

[0045] Example 2

[0046] (1) Add lithium hydroxide to Nafion solution to form a reaction solution. The mass concentration of lithium hydroxide is 2.5 mg / ml, stir at 60 °C for 12 h to obtain Li-Nafion solution, and cool it to room temperature for standby.

[0047] (2) Prepare precursor solution A by mixing Li-Nafion solution and organic solvent N,N-dimethylformamide (DMF) in a volume ratio of 1:9.

[0048] (3) Dissolve phosphotungstic acid and lithium chloride in deionized water in a molar ratio of 1:3, stir to dissolve, and add ammonia water until the pH of the solution is 7 to prepare precursor solution B;

[0049] (4) Add the precursor solution B into the inner liner of a polytetrafluoroethylene hydrothermal reactor, load it into the hydrothermal reactor, and place it in a forced-draft oven for hydrothermal reaction. The hydrothermal reaction temperature is 90 °C, and the hydrothermal reaction time is 3 h. After the solution is cooled to room temperature, perform suction filtration, and vacuum-dry the product at 90 °C to obtain lithium phosphotungstate.

[0050] (5) Take lithium phosphotungstate powder and add it to the precursor solution A. The mass concentration of lithium phosphotungstate is 2 mg / ml, stir until completely dissolved to obtain the precursor solution C.

[0051] (6) Take the precursor solution C and spray it on the surface of the PEP separator using ultrasonic atomization spraying technology. Vacuum-dry it at 60 °C for 24 h to obtain the modified separator B. Ultrasonic atomization spraying process parameters: ultrasonic power 2.00 W, nozzle liquid outlet speed 0.2 ml / min, base film size 7×14 cm, vacuum spraying platform heating temperature 50 °C, spraying times 50 times. The SEM image of the surface of the modified separator B is shown in the appendix Figure 1 , the ion transference number is 0.84, see the appendix Figure 2 . Assemble a lithium-sulfur button battery. At a current density of 0.5 C, the initial discharge capacity is 1219.9 mAh / g. After 200 cycles, the capacity is 812.7 mAh / g, and the average Coulombic efficiency is 99.5%, see the appendix Figure 3 .

[0052] Comparative example

[0053] Take the PEP separator without any treatment for comparative experiments. The SEM image of the surface of the PEP separator is shown in the appendix Figure 1 , the ion transference number is 0.62, see the appendix Figure 2 . Assemble a lithium-sulfur button battery. At a current density of 0.5 C, the initial discharge capacity is 1104.2 mAh / g. After 200 cycles, the capacity is 578.9 mAh / g, and the average Coulombic efficiency is 85.6%, see the appendix Figure 3 .

[0054] The modified separators for lithium-sulfur batteries in Examples 1 and 2 and their preparation processes. A composite layer is set on the separator through ultrasonic atomization spraying technology. The composite layer is made of organic Li-Nafion and inorganic lithium phosphotungstate particles. Among them, Li-Nafion has a non-porous structure and negatively charged functional groups, which can effectively block polysulfide ions through Coulomb repulsion, and it can quickly conduct Li through the action of internal functional groups + , increasing the Li + flux. Introduce inorganic nanoparticle lithium phosphotungstate into the Li-Nafion layer. Utilize its excellent redox ability to accelerate the conversion of polysulfides, and the terminal oxygen atoms of the W-O octahedrons constituting its outer structure can act as conductors of Li +Lewis basic sites, so the terminal oxygen of the W-O octahedron within the structure can form a three-dimensional interconnected Li + conduction channels, enabling the rapid transport of lithium ions within the structure and accelerating the catalytic redox kinetics.

Claims

1. A method for preparing a modified diaphragm layer for a lithium-sulfur battery, characterized in that: The specific steps include: (1) adding lithium compound A to a Nafion solution and mixing to obtain a Li-Nafion solution; The lithium compound A is any one of lithium hydroxide, lithium chloride, lithium fluoride, lithium nitrate, and lithium carbonate, or a mixture of two or more thereof; the concentration of the lithium compound A in the reaction solution is 0.5 to 4 mg / ml; (2) mixing the Li-Nafion solution and the solvent in a volume ratio of 1:9 to 1:1 and stirring to prepare a precursor solution A; The solvent is any one of water, ethanol, n-propanol, isopropanol, methanol, ethyl acetate, chloroform, N-methylpyrrolidone or N,N-dimethylformamide, or a mixture of two or more thereof; (3) dissolving a polyoxometalate and a lithium compound B in a molar ratio of 1:9 to 1:1 in deionized water, stirring and dissolving, and adding ammonia water until the pH of the solution is 3 to 10 to prepare a precursor solution B; The polyoxometalate is any one of phosphotungstic acid, silicotungstic acid, phosphomolybdic acid and silicomolybdic acid, or a mixture of two or more thereof; The lithium compound B is any one of lithium hydroxide, lithium chloride, lithium fluoride, lithium nitrate, and lithium carbonate, or a mixture of two or more thereof; (4) subjecting the precursor solution B to a hydrothermal reaction at 90-150° C., cooling to room temperature after sufficient reaction, filtering and drying to obtain a lithiated polyoxometalate; (5) adding lithium phosphotungstate to the above-mentioned precursor solution A, and performing ultrasonication until it is completely dispersed to obtain a precursor solution C; the concentration of lithium phosphotungstate in the precursor solution C is 1 to 10 mg / ml; (6) Precursor solution C is sprayed on the surface of the diaphragm using ultrasonic atomization spraying technology to form an organic-inorganic composite modified layer, and then dried to obtain a modified diaphragm.

2. The method for preparing a modified diaphragm layer for a lithium-sulfur battery according to claim 1, characterized in that: In step (1), the mass fraction of Nafion in the Nafion solution is 5% to 15%; the mixing reaction temperature is 60 to 100° C., and the mixing and stirring time is 12 to 24 hours.

3. The method for preparing a modified diaphragm layer for a lithium-sulfur battery according to claim 1, characterized in that: In the step (2), the solvent is any one of N,N-dimethylformamide, N-methylpyrrolidone and chloroform, or a mixture of two or more thereof, and the volume ratio of the Li-Nafion solution to the solvent is 1:1 to 1:

9.

4. The method for preparing a modified diaphragm layer for a lithium-sulfur battery according to claim 1, characterized in that: In step (4), the hydrothermal reaction time is 3 to 12 hours, the drying temperature is 60 to 120° C., and the drying time is 8 to 24 hours.

5. The method for preparing a modified diaphragm layer for a lithium-sulfur battery according to claim 1, characterized in that: The diaphragm used in step (6) is a PP / PE / PP composite diaphragm.

6. The method for preparing a modified diaphragm layer for a lithium-sulfur battery according to claim 1, characterized in that: The ultrasonic atomization spraying process parameters in step (6) are as follows: ultrasonic power 2.00 W, nozzle liquid outlet speed 0.2-1.0 ml / min, vacuum spraying platform heating temperature 50-120° C.

7. The method for preparing a modified diaphragm layer for a lithium-sulfur battery according to claim 6, characterized in that: The ultrasonic atomization spraying thickness is 100nm-2μm.

8. The method for preparing a modified diaphragm layer for a lithium-sulfur battery according to claim 1, characterized in that: The modified diaphragm in step (6) is dried at a temperature of 50 to 120° C. for a drying time of 8 to 24 hours.

9. A modified diaphragm for lithium-sulfur battery, prepared by the method according to any one of claims 1 to 8.

10. Use of the modified diaphragm for lithium-sulfur battery according to claim 9 as a diaphragm for lithium-sulfur battery.

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