Lithium-sulfur battery positive electrode side interlayer film as well as preparation method and application thereof

Abstract

The invention relates to a lithium-sulfur battery positive electrode side interlayer film and a preparation method and application thereof, the positive electrode side interlayer film comprises a CNT (at) PVDF skeleton with a three-dimensional conductive network and MXene-CoSe2 nanosheets uniformly dispersed in the CNT (at) PVDF skeleton, and the size of CoSe2 uniformly loaded on the surfaces of the nanosheets is 50-150 nm; the preparation method comprises the following steps: mixing PVDF, CNT, MXene-CoSe2 and DMF to prepare a membrane casting solution, uniformly dispersing the MXene-CoSe2 in a membrane which takes PVDF as a high polymer and is doped with CNT at the same time by using a phase inversion method, and drying to obtain the composite membrane. The material and the membrane designed by the invention have a three-dimensional conductive network, transmission of electrons and lithium ions can be accelerated, and the uniformly dispersed nanosheets can chemically adsorb polysulfide and catalyze the conversion process of the polysulfide. The shuttle effect is effectively inhibited, and the electrochemical performance of the lithium-sulfur battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery energy storage materials, and particularly relates to a separator membrane on the positive electrode side of a lithium-sulfur battery, a preparation method thereof, and an application thereof. Background Art

[0002] With the continuous development of society, energy has become an important cornerstone of human society. New energy and new technologies are constantly closely combined, and people's production and living methods have begun to shift to a new stage dominated by low-carbon and non-fossil fuels. Therefore, the development of new energy and energy storage and conversion technologies has received extensive attention from people. At present, the relatively mature traditional lithium-ion batteries are limited in many application fields due to their low capacity. The lithium-sulfur battery with elemental sulfur as the positive electrode and metallic lithium as the negative electrode is a relatively advanced battery technology at present, which has an ultra-high theoretical specific capacity (1675 mAh g -1 ), and energy density (2600 Wh kg -1 ), and is expected to become a new generation of secondary batteries. However, problems such as the poor conductivity of the active material sulfur, large volume change, and the shuttle effect of polysulfides in the battery seriously affect the performance of the lithium-sulfur battery and hinder the commercialization process of the lithium-sulfur battery. At present, the performance of lithium-sulfur batteries is improved through the following aspects: (1) doping carbon materials, which play a physical barrier role, while improving conductivity and accelerating electron transfer; (2) doping metal compound materials to enhance the adsorption and catalytic conversion ability of polysulfides. Summary of the Invention

[0003] One of the purposes of the present invention is to provide a separator membrane on the positive electrode side of a lithium-sulfur battery, which includes a CNT@PVDF skeleton with a three-dimensional conductive network and MXene-CoSe2 nanosheets uniformly dispersed therein, denoted as MXene-CoSe2-CNT@PVDF membrane.

[0004] Another purpose of the present invention is to provide a preparation method of a separator membrane on the positive electrode side of a lithium-sulfur battery. Using MXene-CoSe2, CNT, and PVDF as raw materials, the membrane is prepared by a phase inversion method. The preparation method is safe and stable, highly operable, and can be mass-produced.

[0005] The third purpose of the present invention is to provide a lithium-sulfur battery. The separator membrane of the battery uses the separator membrane on the positive electrode side. Its three-dimensional conductive network promotes the transmission of electrons and lithium ions. The MXene-CoSe2 nanosheets are uniformly dispersed in the membrane, enhancing the adsorption and catalytic conversion of polysulfides and inhibiting the shuttle effect, thereby improving the electrochemical performance of the battery.

[0006] To achieve the above purposes, the technical solutions of the present invention are as follows: A separator membrane on the positive electrode side of a lithium-sulfur battery, comprising a CNT@PVDF framework with a three-dimensional conductive network structure and MXene-CoSe2 nanosheets uniformly dispersed therein, denoted as MXene-CoSe2-CNT@PVDF membrane, which is composed of PVDF, CNT and MXene-CoSe2. The diameter of the MXene-CoSe2 nanosheets is 5-15 μm, and the size of the CoSe2 uniformly loaded on the surface of the nanosheets is 50-150 nm. The thickness of the separator membrane is 40-100 μm.

[0007] The present invention also provides a method for preparing a separator membrane on the positive electrode side of a lithium-sulfur battery, which includes mixing MXene-CoSe2, CNT, PVDF and DMF to prepare a casting solution, uniformly dispersing MXene-CoSe2 in a membrane with PVDF as the polymer and doped with CNT by the phase inversion method, and then drying to obtain the MXene-CoSe2-CNT@PVDF membrane.

[0008] The preparation method specifically includes the following steps: Step 1: Preparation of MXene-CoSe2 nanosheets: Dissolve MXene in methanol, add Co(NO3)2·6H2O and stir for 1-4 h, then add 2-methylimidazole methanol solution and continue to stir for 1-4 h. Let it stand for 20-28 h, wash and dry to obtain MXene-ZIF-67, and then transfer it to a tubular furnace and carry out selenization under argon protection to obtain MXene-CoSe2 nanosheets; Step 2: Preparation of MXene-CoSe2-CNT@PVDF membrane: Add DMF, MXene-CoSe2, CNT and PVDF to a threaded bottle in sequence, heat and stir to obtain a casting solution, use an automatic coater to scrape the casting solution on a glass plate to form a film, place it in a gel bath for phase inversion for 12-24 h, take it out and dry to obtain the MXene-CoSe2-CNT@PVDF membrane.

[0009] In Step 1, the mass ratio of MXene, methanol, Co(NO3)2·6H2O and 2-methylimidazole is 1-2:1000-2000:20-40:50-80.

[0010] In Step 1, the selenium source is selenium powder, and the mass ratio of MXene-ZIF-67 to selenium powder is 1-3.

[0011] In Step 1, the conditions for tubular furnace selenization are: the selenization temperature is 500-800 °C, the selenization time is 2-4 h, and the heating rate is 2-5 °C min -1 .

[0012] In Step 2, the mass ratio of DMF, MXene-CoSe2, CNT, and PVDF is 25-50:1-2:1-2:1-2.

[0013] In Step 2, the heating and stirring temperature is 60-80 °C, and the heating and stirring time is 6-24 h.

[0014] In Step 2, the thickness of the liquid film obtained by the automatic coater is 200-400 μm.

[0015] In Step 2, the drying conditions are: drying temperature 60-80 °C, and drying time 6-24 h.

[0016] The present invention also provides a lithium-sulfur battery, and the separator film on the positive electrode side of the battery uses the above-mentioned separator film material for the positive electrode side of the lithium-sulfur battery.

[0017] The beneficial effects of the present invention are as follows: For the separator film on the positive electrode side of the lithium-sulfur battery of the present invention, in the separator film, CNT and PVDF are cross-linked with each other (CNT@PVDF), forming the spatial structure and conductive network inside the film. MXene-CoSe2 nanosheets are uniformly dispersed inside the film, increasing the specific surface area, facilitating the infiltration of the electrolyte, fully exposing more active sites, and being able to adsorb polysulfides to a greater extent. At the same time, it promotes the redox reaction kinetics of polysulfides. The three-dimensional conductive network and MXene-CoSe2 nanosheets enhance the transport of electrons and lithium ions. The MXene-CoSe2 nanosheets prepared by high-temperature selenization have excellent conductivity, and the granular CoSe2 effectively increases the active sites. Therefore, MXene-CoSe2 nanosheets can effectively fix sulfur species and play a catalytic role in the redox reaction. Therefore, when such materials are used as the battery separator material, the purpose of improving the cycle performance and rate performance of the lithium-sulfur battery is achieved.

[0018] The preparation method of the present invention has high operability, is safe and stable, and can be mass-produced. Description of the Drawings

[0019] Figure 1 Digital photo of the separator film on the positive electrode side of the lithium-sulfur battery prepared in Example 1.

[0020] Figure 2 Cross-sectional scanning electron microscope image of the separator film on the positive electrode side of the lithium-sulfur battery prepared in Example 1.

[0021] Figure 3 Transmission electron microscope image of MXene-CoSe2 prepared in Example 1.

[0022] Figure 4 Rate performance of the lithium-sulfur battery containing the separator film on the positive electrode side of the lithium-sulfur battery prepared in Example 1.

[0023] Figure 5 The cycling performance of a lithium-sulfur battery containing the separator film on the positive electrode side of the lithium-sulfur battery prepared in Example 1. Specific implementation

[0024] The present invention will be further illustrated below in conjunction with examples, but not limited thereto.

[0025] In the following examples: (1) Scanning electron microscope (SEM) test: The instrument model of the scanning electron microscope is NOVA NanoSEM450.

[0026] (2) Transmission electron microscope (TEM) test: The instrument model of the transmission electron microscope is JEM-F200. (3) Lithium-sulfur battery performance test: The instrument model LAND CT2100A, Wuhan Blue Electric Co., Ltd., test parameters: charge-discharge voltage threshold 1.7 - 2.8 V, charge-discharge rate: 0.2, 0.5, 1, 2 and 4 C. Example

[0027] Dissolve 10 mg of MXene and 291 mg of cobalt nitrate hexahydrate in 20 ml of methanol, and ultrasonically dissolve for 60 min under argon protection. Quickly pour 20 ml of a methanol solution of dimethylimidazole (657 mg) into the above solution, stir at room temperature for 2 h, and let stand for 24 h. Centrifuge and freeze-dry for two days to obtain MXene-ZIF-67. Place selenium powder in the upstream porcelain boat of the tube furnace and MXene-ZIF-67 in the downstream porcelain boat, ensuring that the mass ratio of the two is 2. Under an argon atmosphere, heat to 800 °C at a heating rate of 2 °C min -1 and hold for 3 h to obtain MXene-CoSe2 powder. Then add 250 mg of MXene-CoSe2 and 250 mg of CNT to 7 ml of DMF. Subsequently, add 250 mg of PVDF to the above solution and stir at 60 °C for 12 h. Cast the solution into a 375 μm liquid film on a glass plate, and then place it in deionized water and let stand for 24 h to obtain a flexible MXene-CoSe2-CNT@PVDF membrane with a membrane thickness close to 100 μm, Figure 1 and its physical diagram.

[0028] (1) Scanning electron microscope test: The test results are as Figure 2 shown, the internal structure of the membrane material presents a network structure, there are pore structures, and nanosheets are evenly dispersed inside.

[0029] (2) Transmission electron microscope test: The test results are as Figure 3As shown, CoSe2 is uniformly loaded on the surface of MXene, and the particle size is 50 - 150 nm.

[0030] (3)Lithium-sulfur battery performance test: The lithium-sulfur battery containing the separator film is subjected to charge-discharge cycle testing. The lithium-sulfur button battery is assembled in a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm). The battery uses the C / S electrode as the positive electrode, a lithium sheet as the negative electrode, Celgard 2325 as the separator, and the electrolyte is a solution of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in DOL / DME (volume ratio 1:1), and 1 wt% lithium nitrate (LiNO3) is added, with a dosage of 70 μL. The separator prepared in the present invention is placed between the positive electrode and the Celgard 2325 separator, and the diameter of each layer is 16 mm. The charge-discharge performance of the battery is tested using a BlueTEC battery test system. The results are as Figure 4 shown. In the rate test, at current densities of 0.2, 0.5, 1, 2, and 4 C, the capacities of the battery are 1115, 1026, 947, 833, and 689 mAh g -1 respectively. When the current density is restored to 0.2 C, the capacity is restored to 1047 mAh g -1 . As Figure 5 shown, in the long-term cycling test at a current density of 2 C, a high initial capacity of 1014 mAh g -1 is exhibited. After operating for 700 cycles at 2 C, a capacity of 587 mAh g -1 is maintained, corresponding to a decay rate of only 0.053% per cycle, and the battery has excellent electrochemical performance.

[0031] The above examples are only one of the specific implementation manners of the present invention, and the description thereof is relatively detailed and specific, but this should not be construed as a limitation on the scope of the present invention. It must be pointed out that without departing from the concept of the present invention, obvious substitution forms made by those skilled in the art all belong to the protection scope of the present invention.

Claims

1. A lithium-sulfur battery positive electrode side separator film, characterized in that: The invention comprises a CNT@PVDF skeleton with a three-dimensional conductive network structure and MXene-CoSe2 nanosheets uniformly dispersed therein, which is recorded as a MXene-CoSe2-CNT@PVDF membrane. The diameter of the MXene-CoSe2 nanosheets is 5-15 μm, the size of the CoSe2 uniformly loaded on the surface of the nanosheets is 50-150 nm, and the thickness of the interlayer membrane is 40-100 μm.

2. A method for preparing a positive electrode side separator film of a lithium-sulfur battery according to claim 1, characterized in that: The method includes mixing MXene-CoSe2, CNT, PVDF and DMF to prepare a casting solution, using a phase inversion method to uniformly disperse MXene-CoSe2 in a membrane with PVDF as a polymer and doped with CNT, and then drying to obtain a MXene-CoSe2-CNT@PVDF membrane.

3. The method for preparing the positive electrode side separator film of a lithium-sulfur battery according to claim 2, characterized in that: The specific steps include: Step 1: Preparation of MXene-CoSe2 nanosheets: Dissolve MXene in methanol, add Co(NO3)2·6H2O and stir for 1-4h, then add 2-methylimidazole methanol solution, continue stirring for 1-4h, let stand for 20-28h, wash and dry to obtain MXene-ZIF-67, then transfer to a tube furnace and perform selenization under argon protection to obtain MXene-CoSe2 nanosheets; Step 2: Preparation of MXene-CoSe2-CNT@PVDF membrane: Add DMF, MXene-CoSe2, CNT and PVDF to a silk-mouth bottle in sequence, heat and stir to obtain a casting solution, use an automatic coating machine to scrape the casting solution onto a glass plate to prepare a membrane, place it in a gel bath for phase inversion for 12-24 hours, take it out and dry it to obtain a MXene-CoSe2-CNT@PVDF membrane.

4. The method for preparing a positive electrode side separator film of a lithium-sulfur battery according to claim 3, characterized in that: In the step 1, the mass ratio of MXene, methanol, Co(NO3)2·6H2O and 2-methylimidazole is 1-2:1000-2000:20-40:50-80.

5. The method for preparing a positive electrode side separator film of a lithium-sulfur battery according to claim 3, characterized in that: In the step 1, the selenium source is selenium powder, and the mass ratio of MXene-ZIF-67 to selenium powder is 1-3.

6. The method for preparing a positive electrode side separator film of a lithium-sulfur battery according to claim 3, characterized in that: In the step 1, the selenization temperature is 500-800° C., and the selenization time is 2-4 h.

7. The method for preparing a lithium-sulfur battery positive electrode side separator film according to claim 3, characterized in that: In the step 2, the mass ratio of DMF, MXene-CoSe2, CNT and PVDF is 25-50:1-2:1-2:1-2; the heating and stirring temperature is 60-80 ° C, and the heating and stirring time is 6-24 h.

8. The method for preparing a positive electrode side separator film of a lithium-sulfur battery according to claim 3, characterized in that: In the step 2, the thickness of the liquid film obtained by the automatic coating machine is 200-400 μm.

9. A lithium-sulfur battery positive electrode side separator film according to claim 3, characterized in that: In the step 2, the drying conditions are: drying temperature 60-80°C, and drying time 6-24 h.

10. A lithium-sulfur battery, characterized in that: The separator film of the battery adopts the lithium-sulfur battery separator film material according to claim 1.

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