A slurry, separator and method of making the same to accelerate lithium polysulfide conversion

Abstract

The application discloses a slurry for accelerating conversion of lithium polysulfide, a diaphragm and a preparation method thereof. The slurry comprises the following components in parts by weight: iron phytate 1-3 parts; Super-P 1-3 parts; adhesive PVDF 0.5-1.5 parts; N, N-dimethylformamide DMF 40-120 parts. The diaphragm comprises a diaphragm base material and the slurry coated on the diaphragm base material, solves the problem of slow lithium polysulfide conversion kinetics in the prior art, and has excellent rate performance and cycle performance.

Description

Technical Field

[0001] This invention relates to the field of lithium batteries, specifically to a slurry, a separator, and a method for preparing the same for accelerating the conversion of lithium polysulfides. Background Technology

[0002] Lithium-sulfur batteries are a type of lithium battery that uses elemental sulfur as the positive electrode and metallic lithium as the negative electrode. Because lithium-sulfur battery materials have a very high theoretical capacity, far exceeding that of lithium-ion batteries such as lithium cobalt oxide used by mainstream lithium-ion battery manufacturers, and also offer advantages such as environmental friendliness and lower cost, sulfur-based materials have become a popular research direction in lithium-based batteries.

[0003] Although lithium-sulfur batteries have many advantages, the intermediate product lithium polysulfide generated during charging and discharging readily shuttles from the sulfur cathode to the lithium anode through the separator, forming an insulating layer on the lithium anode. This reduces the contact between the lithium anode and the separator, hindering lithium-ion transport and lowering coulombic efficiency. Furthermore, lithium polysulfide is a byproduct of the sulfur cathode active material; its loss from the cathode reduces the amount of active material, decreasing sulfur utilization and leading to a capacity decrease. This effect is known as the "shuttle effect." The shuttle effect in lithium-sulfur batteries significantly depletes their capacity, impacting their practical applications. The separator plays a crucial role in suppressing the shuttle effect; existing commercial separators suffer from slow lithium polysulfide conversion kinetics and a pronounced shuttle effect in lithium-sulfur battery applications. Summary of the Invention

[0004] This invention provides a slurry, a diaphragm, and a method for preparing lithium polysulfide to accelerate lithium conversion, which solves the problem of slow lithium-sulfur conversion kinetics in the prior art.

[0005] To solve this technical problem, the present invention provides the following technical solution:

[0006] A slurry for accelerating the conversion of lithium polysulfides, the slurry comprising the following components in parts by weight:

[0007] 1-3 parts of ferrous phytate;

[0008] Super-P 1-3 servings;

[0009] 0.5-1.5 parts PVDF adhesive;

[0010] N,N-Dimethylformamide (DMF) 40-120 parts.

[0011] Preferably, the slurry comprises the following components in parts by weight:

[0012] 2 parts ferrous phytate;

[0013] Two servings of Super-P;

[0014] 1 part PVDF adhesive;

[0015] 80 parts of N,N-dimethylformamide (DMF).

[0016] Preferably, the preparation method of the phytate iron includes the following steps: 2.4g of sodium phytate is fully dissolved in 40mL of H2O by ultrasonication to prepare a sodium phytate solution; then, under continuous stirring, 80mL of H2O solution containing 4.54g of Fe(NO3)3·9H2O is gradually added dropwise to the sodium phytate solution, then the precipitate is collected by centrifugation, then washed, irrigated with ethanol and H2O several times, and finally dried in an oven to obtain the final product.

[0017] This solution also provides a separator for accelerating lithium polysulfide conversion, comprising a separator substrate and a coating applied to the separator substrate, the coating being obtained by applying the aforementioned slurry.

[0018] Preferably, the membrane substrate is selected from commercial Celgard 2500 membranes.

[0019] Preferably, the coating is applied to one side of the diaphragm substrate.

[0020] Preferably, the coating is applied to both sides of the diaphragm substrate.

[0021] The above-mentioned method for preparing a membrane that accelerates lithium polysulfide conversion includes the following steps:

[0022] Ferric phytate, Super-P and PVDF binder are added to N,N-dimethylformamide DMF and stirred to form a uniform slurry; the uniform slurry is then coated on one or both sides of the diaphragm substrate and finally dried in air.

[0023] Preferably, the drying process is carried out at 50-60°C.

[0024] Preferably, the preparation method of the phytate iron includes the following steps: 2.4g of sodium phytate is fully dissolved in 40mL of H2O by ultrasonication to prepare a sodium phytate solution; then, under continuous stirring, 80mL of H2O solution containing 4.54g of Fe(NO3)3·9H2O is gradually added dropwise to the sodium phytate solution, then the precipitate is collected by centrifugation, then washed, irrigated with ethanol and H2O several times, and finally dried in an oven to obtain the final product.

[0025] Compared with the prior art, the present invention has the following advantages: This solution is the first to modify the membrane with phytic acid, the preparation process is simple and easy to scale up; phytic acid is rich in twisted six-coordinate polar Fe-OP centers, which is beneficial for capturing polysulfides. At the same time, the Fe-OP centers of phytic acid can form surface Fe-S bonds with polysulfides, which can promote the polysulfide conversion kinetics process and thus obtain better electrochemical performance. Attached Figure Description

[0026] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0027] Figure 1 This is a schematic diagram of the molecular structure of iron phytate.

[0028] Figure 2 The images show the SEM structural characterization of a commercially available separator (Celgard 2500), a carbon-coated separator (carbon@polyolefin), and the carbon-iron phytate@polyolefin separator of the present invention; wherein, (a) represents the commercially available Celgard 2500 polyolefin separator, (b) represents the carbon@polyolefin separator, and (c) represents the carbon-iron phytate@polyolefin separator of the present invention.

[0029] Figure 3 Electrochemical impedance spectroscopy for carbon-coated membranes (carbon@polyolefin) and carbon-ferric phytate@polyolefin membranes of the present invention.

[0030] Figure 4 Tafel slope diagrams for carbon-coated separators (carbon@polyolefin) and the carbon-ferric phytate@polyolefin separator of the present invention.

[0031] Figure 5 Rate performance graphs for commercially available separators (Celgard 2500), carbon-coated separators (carbon@polyolefin), and the carbon-ferric phytate@polyolefin separator of this invention.

[0032] Figure 6 The graph shows the cycling performance of a commercially available membrane (Celgard 2500), a carbon-coated membrane (carbon@polyolefin), and the carbon-ferric phytate@polyolefin membrane of this invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments. The illustrative embodiments and descriptions of this invention are only used to explain this invention and are not intended to limit this invention.

[0034] Example 1

[0035] Preparation method of phytate iron: Dissolve 2.4g of sodium phytate in 40mL of H2O by ultrasonication. Then, while stirring continuously, gradually add 80mL of H2O solution containing 4.54g of Fe(NO3)3.9H2O to the sodium phytate solution. Centrifuge to collect the precipitate, wash it several times with ethanol and H2O, and finally dry it in an oven at 80℃.

[0036] Membrane preparation method: 0.2 g ferric phytate, 0.2 g Super-P and 0.1 g PVDF binder were added to 8 mL N,N-dimethylformamide (DMF) and stirred for 8 hours to form a homogeneous slurry. The homogeneous slurry was then coated onto one side of a commercial Celgard 2500 membrane (PP) and finally dried overnight in air at 50°C.

[0037] Example 2

[0038] Preparation method of phytate iron: Dissolve 2.4g of sodium phytate in 40mL of H2O by ultrasonication. Then, while stirring continuously, gradually add 80mL of H2O solution containing 4.54g of Fe(NO3)3.9H2O to the sodium phytate solution. Centrifuge to collect the precipitate, wash it several times with ethanol and H2O, and finally dry it in an oven at 80℃.

[0039] Membrane preparation method: 0.1 g ferric phytate, 0.1 g Super-P and 0.05 g PVDF binder were added to 40 mL N,N-dimethylformamide (DMF) and stirred for 8 hours to form a homogeneous slurry. The homogeneous slurry was then coated onto one side of a commercial Celgard 2500 membrane (PP) and finally dried overnight in air at 60°C.

[0040] Example 3

[0041] Preparation method of phytate iron: Dissolve 2.4g of sodium phytate in 40mL of H2O by ultrasonication. Then, while stirring continuously, gradually add 80mL of H2O solution containing 4.54g of Fe(NO3)3.9H2O to the sodium phytate solution. Centrifuge to collect the precipitate, wash it several times with ethanol and H2O, and finally dry it in an oven at 80℃.

[0042] Membrane preparation method: 0.3 g ferric phytate, 0.3 g Super-P and 1.5 g PVDF binder were added to 120 mL N,N-dimethylformamide (DMF) and stirred for 8 hours to form a homogeneous slurry. The homogeneous slurry was then coated onto one side of a commercial Celgard 2500 membrane (PP) and finally dried overnight in air at 55°C.

[0043] Comparative Example 1

[0044] 0.2 g of Super-P and 0.1 g of PVDF binder were added to 8 mL of N,N-dimethylformamide (DMF) and stirred for 8 hours to form a homogeneous slurry. The homogeneous slurry was then coated onto one side of a commercial Celgard 2500 membrane (PP) and finally dried overnight in air at 50°C to obtain a carbon-coated polyolefin membrane.

[0045] A schematic diagram of the molecular structure of ferrous phytate is shown below. Figure 1 As shown, the microstructure of the commercial Celgard 2500 polyolefin membrane, the carbon-ferric phytate@polyolefin membrane prepared in Example 1, and the carbon@polyolefin membrane prepared in Comparative Example 1 were characterized as follows: Figure 2 As shown, among them, compared with the purely commercial Celgard 2500 polyolefin membrane ( Figure 2 a) with carbon-coated polyolefin separator ( Figure 2 (b) In contrast, the carbon-ferric phytate@polyolefin membrane prepared in Example 1 of this application has uniformly dispersed ferric phytate particles on its surface, and the membrane has a large number of pores.

[0046] The AC impedance and Tafel slope of the carbon-coated membrane and the membrane of this invention were tested, and the test results are as follows: Figure 3-4 As shown; by Figure 3 It can be seen that, compared with carbon-coated membranes, the membrane prepared by this invention has a smaller electrochemical impedance value, indicating a smaller charge transfer impedance, which is more conducive to the conversion of lithium polysulfides; from Figure 4 It can be seen that, at the same potential, the exchange current density of this scheme is greater, which also proves that the iron phytate in this scheme can reduce the conversion of lithium polysulfides.

[0047] This invention also tested the rate capability and long-term charge-discharge stability of lithium-ion batteries using three types of separators: a commercial separator (Celgard 2500), a carbon-coated separator, and the carbon-iron phytate@polyolefin separator of this invention. The test results are as follows: Figure 5-6 As shown. Using a carbon / sulfur mixture as the positive electrode and lithium metal as the negative electrode, and employing these three different separators, button batteries were assembled, and their rate performance and long-term electrochemical energy storage behavior were tested. Rate performance was determined by... Figure 5 It can be seen that the lithium-sulfur battery using the separator described in this invention still maintains a discharge capacity of 560 mA·h / g even at a 5C rate, which is far higher than the capacity of batteries assembled with carbon-coated separators (245 mA·h / g) and commercial separators (100 mA·h / g). Meanwhile, as Figure 6As shown, long-term charge-discharge tests indicate that at a 1C rate, the initial discharge specific capacity of the lithium-sulfur battery using the separator described in this invention is 1003 mA·h / g, while that of the commercial separator (Celgard 2500) is only 460 mA·h / g. Furthermore, after 800 consecutive charge-discharge cycles, the capacity decay rate is only 0.058%, while the capacity decay of batteries assembled with carbon-coated separators and commercial separators is rapid. Therefore, the separator described in this invention exhibits better rate performance and long-term stability than both the commercial separator (Celgard 2500) and the carbon-coated separator when used in lithium-ion batteries. This demonstrates that the separator prepared in this invention possesses excellent rate performance and cycle performance.

[0048] The above results indicate that iron phytate is rich in twisted six-coordinate polar Fe-OP centers, which are beneficial for capturing polysulfides. At the same time, the Fe-OP centers of iron phytate can form surface Fe-S bonds with polysulfides, which can promote the polysulfide conversion kinetics and thus obtain better electrochemical performance.

[0049] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A membrane for accelerating the conversion of lithium polysulfides, characterized in that, The invention includes a diaphragm substrate and a coating applied to the diaphragm substrate, the coating being obtained by applying a slurry that accelerates the conversion of lithium polysulfides; The slurry comprises the following components in parts by weight: 1-3 parts of ferrous phytate; Super-P 1-3 servings; 0.5-1.5 parts PVDF adhesive; 40-120 parts of N,N-dimethylformamide (DMF).

2. The membrane for accelerating lithium polysulfide conversion according to claim 1, characterized in that, The membrane substrate is selected from commercial Celgard 2500 membranes.

3. The membrane for accelerating lithium polysulfide conversion according to claim 1, characterized in that, The coating is applied to one side of the diaphragm substrate.

4. The membrane for accelerating lithium polysulfide conversion according to claim 1, characterized in that, The coating is applied to both sides of the diaphragm substrate.

5. The method for preparing a membrane for accelerating lithium polysulfide conversion according to any one of claims 1-4, characterized in that, Includes the following steps: Ferric phytate, Super-P and PVDF binder are added to N,N-dimethylformamide DMF and stirred to form a uniform slurry; the uniform slurry is then coated on one or both sides of the diaphragm substrate and finally dried in air.

6. The method for preparing a membrane for accelerating lithium polysulfide conversion according to claim 5, characterized in that, The drying process is carried out at 50-60℃.

7. The method for preparing the separator for accelerating lithium polysulfide conversion according to claim 5, characterized in that, The preparation method of the phytate includes the following steps: 2.4 g of sodium phytate is fully dissolved in 40 mL of H2O by ultrasonication to prepare a sodium phytate solution; then, under continuous stirring, 80 mL of H2O solution containing 4.54 g of Fe(NO3)3·9H2O is gradually added dropwise to the sodium phytate solution, then the precipitate is collected by centrifugation, then washed, and rinsed with ethanol and H2O several times, and finally dried in an oven to obtain the final product.

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