A method for preparing a lithium-sulfur battery separator

Abstract

The application discloses a preparation method of a lithium-sulfur battery diaphragm and belongs to the technical field of battery diaphragms. First, a quaternary ammonium polymer PDDANO3 containing nitrate is prepared by reacting a nitrate and polydiallyldimethylammonium chloride PDDA. Then, the PDDANO3 is dissolved in water to form a solution, which is coated on one side or both sides of the diaphragm, and a functional diaphragm for a lithium-sulfur battery is obtained after drying. The quaternary ammonium polymer PDDANO3 containing nitrate obtained by the method can play the roles of inhibiting the polysulfide ion shuttle effect and lithium negative electrode dendrite growth in the lithium-sulfur battery, so that the specific capacity and cycle stability of the lithium-sulfur battery are significantly improved.

Description

TECHNICAL FIELD

[0001] The application discloses a preparation method of a lithium-sulfur battery diaphragm and belongs to the technical field of battery diaphragms. BACKGROUND

[0002] Lithium-sulfur batteries have an ultra-high theoretical energy density (2600 Wh kg -1 ), which is much higher than the energy density of the currently commercialized lithium-ion batteries, and are one of the most promising next-generation battery systems with cost-effectiveness and energy density, and have great application potential in large-scale energy storage devices and power grids due to their low cost and environmental friendliness. However, the dissolution of discharge products (polysulfides) during the charging and discharging process and the subsequent shuttle effect limit the commercial application of lithium-sulfur batteries. Specifically, the dissolution of polysulfides leads to the following problems: (1) high self-discharge rate, capacity decay, and low coulombic efficiency; (2) polysulfides can shuttle to the negative electrode surface and easily react with the lithium negative electrode, leading to dendrite and pulverization on the negative electrode side, and damage to the lithium negative electrode.

[0003] The modification of diaphragm materials is one of the simplest and most effective methods to solve the above problems of lithium-sulfur batteries. Patent CN105609690 A discloses a preparation method of a composite lithium-sulfur battery diaphragm of POSS grafted carbon nanotubes, and patent CN110890503 A discloses a fluorine and nitrogen atom co-doped porous carbon material and its application in lithium-sulfur batteries. However, these methods can only partially inhibit the shuttle effect and do not have a good protective effect on the lithium negative electrode. Therefore, it is necessary to develop a material that can inhibit the shuttle effect and dendrite growth as a functional layer of the diaphragm, so that the modified interface of the diaphragm can play different and synergistic roles on the positive electrode side and the negative electrode side in lithium-sulfur batteries. SUMMARY

[0004] The purpose of the present application is to solve the problems of soluble polysulfide intermediates migration and dendrite growth, and to provide a preparation method of a lithium-sulfur battery diaphragm. The quaternary ammonium type polymer PDDANO3 containing nitrate prepared by the method can play the role of inhibiting the shuttle effect and protecting the lithium negative electrode in the lithium-sulfur battery diaphragm, and significantly improves the specific capacity and cycle stability of the lithium-sulfur battery.

[0005] The technical solution adopted by the present application to solve the above problems is: a preparation method of a lithium-sulfur battery diaphragm, which coats a quaternary ammonium type polymer polydiallyldimethylammonium nitrate PDDANO3 containing nitrate on a conventional diaphragm of a battery;

[0006] The preparation method of the diaphragm comprises the following steps:

[0007] The PDDANO3 is dissolved in water to form a solution with a mass fraction of 0.001-50% at room temperature, the prepared solution is uniformly coated on the separator, and then naturally air-dried for 1-6 hours, and then dried in a vacuum oven at a drying temperature of 50-80°C for 12-24 hours to obtain a separator coating with a coating thickness of 50 nm-20 μm and a loading of 0.01-2 mg / cm 2 ;

[0008] The separator coating is coated on one side of the separator, or on both sides of the separator.

[0009] The functional separator with one-side coating is used to assemble a lithium-sulfur battery, and the coating faces the positive electrode or the negative electrode.

[0010] The conventional separator is a PE film, a PP film, a PP / PE two-layer film or a PP / PE / PP three-layer film.

[0011] The preparation method of the quaternary ammonium type polymer PDDANO3 containing nitrate is as follows:

[0012] The nitrate salt is dissolved in deionized water to obtain solution A; solution A is added to a polydiallyldimethylammonium chloride PDDA aqueous solution, and stirred at room temperature to form a white suspension at a stirring speed of 500-1200 r / min; after centrifugation, the supernatant is taken and rotary evaporated at a temperature of 80-100°C to obtain a solid product, which is the quaternary ammonium type cationic polymer PDDANO3 containing nitrate.

[0013] The nitrate salt is one or more of silver nitrate, mercurous nitrate and cadmium nitrate.

[0014] The content of PDDA in the polydiallyldimethylammonium chloride PDDA aqueous solution is 10-50 wt%.

[0015] The molecular weight of the polydiallyldimethylammonium chloride PDDA is 4×10 4 -3×10 6 .

[0016] The molar ratio of the nitrate salt to the polydiallyldimethylammonium chloride PDDA is 1:0.5-2.0.

[0017] The lithium-sulfur battery is characterized in that the negative electrode of the lithium-sulfur battery is a metal lithium negative electrode, and the metal lithium negative electrode is a metal lithium sheet, a lithium foil, lithium particles or lithium powder; or the negative electrode of the lithium-sulfur battery is an alloy negative electrode containing metal lithium, and the alloy negative electrode containing metal lithium is a sheet, a foil, particles or powder of a metal lithium alloy.

[0018] The application has the following beneficial effects: the method uses a novel quaternary ammonium salt polymer containing nitrate, polydiallyldimethylammonium nitrate (PDDANO3), to modify the diaphragm, and a functional diaphragm for lithium-sulfur batteries is designed and prepared, which has strong cationic main chains, and the nitrate is beneficial to the sulfur positive electrode and the lithium negative electrode. Due to the strong cationic main chain for anchoring polysulfides and the nitrate for promoting the conversion rate of polysulfides, the influence of polysulfides can be effectively reduced, and the lithium ion migration capacity can be improved, and the utilization rate of active substances can be greatly improved. In addition, the nitrate anion can promote the in-situ formation of a stable solid electrolyte interface (SEI) on the surface of the lithium negative electrode, inhibit the generation of lithium dendrites, and avoid the uncontrollable consumption of lithium nitrate in the electrolyte, thereby long-term protecting the lithium negative electrode. The reaction between soluble polysulfides and lithium can be further effectively inhibited by passivating lithium and eliminating the reaction between soluble polysulfides and lithium. Based on the above multiple effects, the polymer as a functional coating layer of the diaphragm can effectively improve the overall performance of the lithium-sulfur battery. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 NMR spectrum of prepared PDDANO3

[0020] In the figure: (a) is the NMR spectrum of PDDA, and (b) is the NMR spectrum of the quaternary ammonium polymer containing nitrate prepared in Example 1;

[0021] The X-axis is the chemical shift, and the unit is ppm; the Y-axis is the peak intensity, and the unit is a.u.

[0022] Figure 2 Infrared spectrum of prepared PDDANO3

[0023] In the figure: the X-axis is the wave number, and the unit is cm -1 ; the Y-axis is the transmittance of the signal, and the unit is %.

[0024] Figure 3 Adsorption effect of prepared PDDANO3 on polysulfides.

[0025] In the figure: a is the polysulfide solution with or without PDDANO3 before standing, b is the UV spectrum of the solution after standing for two hours, and c is the polysulfide solution with or without PDDANO3 after standing for two hours;

[0026] is the polysulfide solution, and is the polysulfide solution added with PDDANO3 prepared in the example;

[0027] The X-axis is the wavelength, and the unit is nm; the Y-axis is the intensity of the signal, and the unit is a.u.

[0028] Figure 4The electrochemical performance chart of Li-Li symmetrical battery containing the PDDANO3 modified separator prepared in the example

[0029] In the figure: The battery cycle curve of PDDANO3 modified separator, and the battery cycle curve of unmodified separator;

[0030] The X axis is the test time, in h; and the Y axis is the voltage, in V.

[0031] Figure 5 The cycle performance chart of lithium-sulfur battery containing the PDDANO3 modified separator prepared in the example

[0032] In the figure:▲● are the battery discharge specific capacity curves of PDDANO3 modified separator prepared in the example and unmodified separator, respectively; and △○ are the battery coulomb efficiency curves of PDDANO3 modified separator prepared in the example and unmodified separator, respectively.

[0033] In the figure: the X axis is the cycle number, in 1; the left Y axis is the discharge specific capacity, in mAh / g; and the right Y axis is the coulomb efficiency, in %. DETAILED DESCRIPTION

[0034] The application will be further described below in conjunction with the examples and the drawings.

[0035] The test materials used in the examples of the application are all conventional test materials in the art, and can be purchased through commercial channels.

[0036] Example 1

[0037] (1) Silver nitrate was dissolved in deionized water to obtain a 20wt% solution, which was then quickly added to 20wt% PDDA (molecular weight 400000), and stirred at room temperature at a stirring speed of 1200r / min. The molar ratio of silver ions to chloride ions was 1:1. After 30min of reaction, centrifugal separation was performed, and the supernatant was obtained, which was evaporated at 80℃ to obtain a solid, which was taken out and ground into a powder. Finally, the collected quaternary ammonium cationic polymer (PDDANO3) solid containing nitrate was obtained.

[0038] Figure 1 and Figure 2NMR and IR spectra of the quaternary ammonium cationic polymer containing nitrate synthesized in Example 1 and polydiallyldimethylammonium chloride (PDDA) respectively. The NMR spectrum shows that the polymer main chain structure is maintained before and after anion exchange, and due to the influence of nitrate, all peak shapes are shifted, indicating that the anion exchange reaction is carried out. At the same time, the surface chemical identification of the reaction product by infrared spectrum shows that the characteristic peaks of the polymer main chain and nitrate exist, so it can be determined that NO3 - is fixed on the cationic polymer main chain of PDDA.

[0039] The adsorption of PDDA NO3 to polysulfide is more directly observed by static adsorption experiment. It can be seen from Figure 3 that after standing for 12 h, the solution containing PDDA NO3 becomes colorless, while the solution containing PDDA still has part of the yellow color, indicating that the chemical adsorption of PDDA NO3 is stronger. In the UV-vis spectrum, only the S x 2- characteristic peak intensity of (4≤x≤8) disappears, further reflecting the effective chemical adsorption of PDDA NO3 to polysulfide.

[0040] The adsorption of PDDA NO3 to polysulfide is more directly observed by static adsorption experiment. It can be seen from Figure 1 that after standing for 12 h, the solution containing PDDA NO3 becomes colorless, indicating that the chemical adsorption of PDDA NO3 is stronger. In the UV-vis spectrum, only the S x 2- characteristic peak intensity of (4≤x≤8) disappears, further reflecting the effective chemical adsorption of PDDA NO3 to polysulfide.

[0041] (2) The PDDA NO3 prepared in step (1) is dissolved in water to prepare a solution with a mass fraction of 2.5%, and the prepared solution is uniformly coated on both sides of the separator, naturally air-dried for 6 h, and then the coated separator is dried in a vacuum oven at 60°C for 24 h to prepare a PDDA NO3 modified separator.

[0042] Application Example

[0043] The modified separator prepared in the example is applied to a lithium-sulfur battery, the electrolyte is a traditional lithium-sulfur electrolyte, and a Celgard 2400pp separator without modification is used as a blank control.

[0044] It can be seen from Figure 4 that the voltage polarization of the lithium symmetrical battery using the separator is significantly reduced, and the capacity retention rate is increased by 10% at 1 mA cm 2The PDDANO3 coating can continuously protect the lithium negative electrode by improving ionic conductivity and forming a stable SEI layer, thereby improving the cycle stability of the lithium negative electrode. Figure 5 The cycle performance diagram of the PDDANO3 modified separator prepared in the application in a lithium-sulfur battery. The specific capacity and cycle performance of the PDDANO3 modified separator battery are obviously higher than those of the unmodified separator, proving that the improvement of the electrochemical performance is due to the synergistic effect of the strong cationic main chain of the polymer and the nitrate anion. In addition, the separator designed in the application has a first discharge specific capacity of 1436 mAh g -2 and a capacity retention rate (50 cycles) of 77% in a lithium-sulfur battery with a sulfur loading of 2 mg cm -1 , and has good application effect.

Claims

1. A method of making a lithium-sulfur battery separator, characterized by The method coats a quaternary ammonium type polymer polydiallyldimethylammonium nitrate PDDANO3 on a regular separator of a battery; The preparation method of the separator comprises the following steps: The PDDANO3 is dissolved in water to prepare a solution with a mass fraction of 0.001-50% at room temperature, the prepared solution is uniformly coated on the diaphragm, and then naturally air-dried for 1-6 h, and then dried in a vacuum oven, with a drying temperature of 50-80 °C and a drying time of 12-24 h, to obtain a diaphragm coating with a coating thickness of 50 nm-20 μm and a loading of 0.01-2 mg / cm 2 . The separator coating is coated on one side of the separator, or on both sides of the separator; The functional separator with the coating on one side is assembled into a lithium-sulfur battery, and the coating faces the positive electrode or the negative electrode; The regular separator is a PE film, a PP film, a PP / PE double-layer film or a PP / PE / PP three-layer film.

2. The method of claim 1, wherein the method further comprises The preparation method of the quaternary ammonium type polymer PDDANO3 comprises the following steps: dissolving the nitrate in deionized water to obtain solution A; adding the solution A into a polydiallyldimethylammonium chloride PDDA aqueous solution, stirring at room temperature to form a white suspension, and stirring at a speed of 500-1200 r / min; centrifuging, taking the supernatant, and rotary evaporating at a temperature of 80-100 ℃ to obtain the quaternary ammonium type cationic polymer PDDANO3 containing nitrate; The nitrate is one or more of silver nitrate, mercurous nitrate and cadmium nitrate; The content of PDDA in the polydiallyldimethylammonium chloride PDDA aqueous solution is 10-50 wt%; The polydiallyldimethylammonium chloride PDDA has a molecular weight of 4 x 10 4 ~ 3 x 10 6 ; The molar ratio of the nitrate to the polydiallyldimethylammonium chloride PDDA is 1:0.5-2.

0.

3. The method of claim 1, wherein the method further comprises The negative electrode of the lithium-sulfur battery is a metal lithium negative electrode, which is a metal lithium sheet, a lithium foil, lithium particles or lithium powder; or the negative electrode of the lithium-sulfur battery is an alloy negative electrode containing metal lithium, which is an alloy sheet, foil, particle or powder containing metal lithium.

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