Pi-d conjugated covalent organic framework material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of electrochemistry, and discloses a pi-d conjugated covalent organic framework material, and a preparation method thereof comprises the following steps: adding 2, 5-hydroxyterephthalic acid, 2, 3, 5, 6-tetra (amino) p-benzoquinone and nickel acetate tetrahydrate into a container, then adding N-methyl pyrrolidone, carrying out ultrasonic treatment for 0.5 h, transferring into a hydrothermal reaction kettle, carrying out a sealed reaction for 2-5 days, and carrying out vacuum drying to obtain the pi-d conjugated covalent organic framework material. And after the reaction is finished, naturally cooling to room temperature, filtering, collecting, washing and performing vacuum drying on the obtained precipitate to obtain the pi-d conjugated covalent organic framework material. The covalent organic framework material disclosed by the invention is rich in N / O chemical adsorption sites and Ni < 2 + > catalytic sites, so that shuttling of polysulfide can be effectively inhibited; on the other hand, catalytic conversion of polysulfide is promoted, the reaction kinetics of the lithium-sulfur battery is further accelerated, and compared with a traditional covalent organic framework, the conductivity is greatly improved, and the electron transmission capacity is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of electrochemistry and relates to a π-d conjugated covalent organic framework material and a preparation method and application thereof. Background Art

[0002] As people's demand for portable devices and electric vehicles continues to increase, the development of high energy density secondary batteries has become urgent. However, the energy density of lithium-ion batteries currently used has gradually failed to meet the needs of practical applications. Lithium-sulfur batteries have a capacity of 1675mAh g -1 High theoretical specific capacity and 2600Wh kg -1 The high theoretical energy density, abundant sulfur resources, low cost, and environmental friendliness provide great potential for the next generation of energy storage systems. However, sulfur and its discharge product, lithium sulfide, are insulators to both electrons and lithium ions, which seriously hinders the effective transmission of electrons and ions during the electrochemical reaction, resulting in a reduced utilization rate of the active substance sulfur and a relatively slow kinetic process. During the charge and discharge process of lithium-sulfur batteries, polysulfide intermediates will be produced at the sulfur positive electrode, which will then dissolve in the electrolyte, pass through the diaphragm, and diffuse to the negative electrode. They can directly react with the lithium metal at the negative electrode, ultimately leading to irreversible loss of active materials in the battery, and even causing certain safety hazards. Therefore, lithium-sulfur batteries have problems such as low conductivity of sulfur and discharge products, easy shuttle effect of polysulfides, and safety hazards caused by lithium dendrites, making it difficult for lithium-sulfur batteries to achieve large-scale commercial applications.

[0003] In recent decades, great progress has been made in the modification of traditional separators, mainly focusing on carbon materials, inorganic materials and polymer materials. The introduction of these modified materials provides new possibilities for improving the performance of separators. However, in practical applications, modified separators face huge challenges, including complex processes, high costs and stability. Covalent organic frameworks (COFs) are functional porous materials composed of organic ligands connected by covalent bonds. They have the advantages of low density, adjustable structure and high porosity. Therefore, they have attracted much attention worldwide, especially in the application field of batteries. They can be used for directional design around functional requirements such as rapid transfer of lithium ions / electrolytes, polysulfide suppression and electrode protection in lithium-sulfur batteries, thereby improving the performance of lithium-sulfur batteries. Summary of the invention

[0004] In view of the above technical problems, the present invention provides a π-d conjugated covalent organic framework material, which is rich in N / O chemical adsorption sites and Ni 2+The catalytic sites, on the one hand, can effectively inhibit the shuttling of polysulfides as adsorption sites; on the other hand, as catalytic sites, they promote the catalytic conversion of polysulfides and further accelerate the reaction kinetics of lithium-sulfur batteries; when the modified diaphragms are applied to lithium-sulfur batteries and symmetrical batteries, they can enhance the electron conduction capacity and improve the electrochemical performance of the battery.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a π-d conjugated covalent organic framework material, the structural formula of which is as follows:

[0007]

[0008] In a second aspect, the present invention provides a method for preparing the above-mentioned π-d conjugated covalent organic framework material, comprising the following steps: adding 2,5-hydroxyterephthalic acid, 2,3,5,6-tetra(amino)benzoquinone and nickel acetate tetrahydrate into a container, then adding N-methylpyrrolidone, transferring to a hydrothermal reactor after ultrasonic treatment for 0.5h, sealing and reacting at 100-150°C for 2-5 days, naturally cooling to room temperature after the reaction is completed, collecting the obtained precipitate by filtration, washing, and vacuum drying at 60-120°C to obtain a π-d conjugated covalent organic framework material.

[0009] In one technical solution, the molar ratio of the 2,3,5,6-tetrakis(amino)-p-benzoquinone to 2,5-hydroxyterephthalic acid is 1:1-3.

[0010] In a third aspect, the present invention provides an application of the above-mentioned π-d conjugated covalent organic framework material on a battery separator, wherein the battery separator is provided with a mixed coating composed of the above-mentioned π-d conjugated covalent organic framework material, conductive carbon black and polyvinylidene fluoride.

[0011] In one technical solution, the mass ratio of the π-d conjugated covalent organic framework material to the conductive carbon black is 1:0.6-1.

[0012] In a fourth aspect, the present invention provides an application of the above-mentioned π-d conjugated covalent organic framework material in a lithium-sulfur battery, wherein a battery separator in the lithium-sulfur battery is provided with a mixed arrangement consisting of the above-mentioned π-d conjugated covalent organic framework material, conductive carbon black and polyvinylidene fluoride.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] 1. The π-d conjugated covalent organic framework material of the present invention contains a quinone structure and is rich in N / O chemical adsorption sites and Ni 2+On the one hand, the catalytic sites can effectively inhibit the shuttling of polysulfides as adsorption sites; on the other hand, as catalytic sites, they promote the catalytic conversion of polysulfides, further accelerating the reaction kinetics of lithium-sulfur batteries, greatly improving the electrical conductivity and electron transmission capacity compared to traditional covalent organic frameworks.

[0015] 2. The Ni-BQ COF modified diaphragm of the present invention has abundant N / O polar groups in the Ni-BQ COF structure, which is not only conducive to the infiltration of the electrolyte, but also can chemically adsorb polysulfides, effectively inhibit the shuttle effect of polysulfides, improve its cycle stability, and enhance the electrochemical performance of lithium-sulfur batteries.

[0016] 3. The Ni-BQ COF modified diaphragm of the present invention is used in lithium-sulfur batteries. The π-d conjugated structure of Ni-BQ COF effectively promotes the high delocalization of electrons and enhances its electronic conductivity. In addition, a large amount of Ni introduced into the Ni-BQ COF structure 2+ , can serve as catalytic sites to promote the catalytic conversion of polysulfides, accelerate the reaction kinetics of lithium-sulfur batteries, and enhance rate performance.

[0017] 4. The Ni-BQ COF modified diaphragm of the present invention is used in a symmetrical battery. The Ni-BQ COF modified diaphragm exhibits a higher lithium ion migration number and lithium ion diffusion coefficient, as well as better electrolyte wettability, and has more excellent electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the X-ray powder diffraction pattern of the Ni-BQ COF prepared in Example 1.

[0019] Figure 2 This is the infrared absorption spectrum of the Ni-BQ COF prepared in Example 1.

[0020] Figure 3 The nitrogen adsorption-desorption curve and pore size distribution diagram of the Ni-BQ COF prepared in Example 1.

[0021] Figure 4 X-ray powder diffraction patterns of the Ni-BQ COF prepared in Example 1 after being immersed in different solvents.

[0022] Figure 5 The X-ray powder diffraction patterns of the Ni-BQ COF prepared in Examples 2 and 3.

[0023] Figure 6 This is an electron microscope image of the Ni-BQ COF modified diaphragm prepared in Example 4, wherein Figure 5 -a is the plan view, Figure 5 -b is a cross-sectional view.

[0024] Figure 7 This is the electrochemical polarization test result of the lithium-lithium symmetric battery assembled with commercial PP separator and Ni-BQ COF modified separator in Example 5. The inset is the impedance spectrum before and after polarization.

[0025] Figure 8 This is the electrochemical impedance spectrum of a stainless steel-stainless steel symmetric cell assembled with a commercial PP separator and a Ni-BQ COF modified separator in Example 6.

[0026] Fig. 9 This is the rate performance diagram of the lithium-sulfur battery assembled with a commercial PP separator and a Ni-BQ COF modified separator in Example 7.

[0027] Fig.10 This is the long cycle performance diagram of the lithium-sulfur battery assembled with a commercial PP separator and a Ni-BQ COF modified separator in Example 7. DETAILED DESCRIPTION

[0028] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention. Unless otherwise specified, the technical means used in the examples are conventional means known to those skilled in the art. The test methods in the following examples are conventional methods unless otherwise specified.

[0029] Example 1

[0030] 2,5-Hydroxyterephthalic acid (HBC, 1mmol), 2,3,5,6-tetra(amino)benzoquinone (TABQ, 0.5mmol) and excess nickel acetate tetrahydrate (NiOAc﹒4H2O, 1.5mmol) were added to a 10mL glass bottle, followed by N-methylpyrrolidone (NMP, 10ml) and ultrasonication for 0.5h; the glass bottle was transferred to a Teflon-lined stainless steel hydrothermal autoclave, sealed, heated at 120℃ for 72h, and cooled naturally to room temperature after the reaction. The resulting black precipitate was collected by filtration and repeatedly washed with DMF and methanol, and then the product was vacuum dried at 80℃ for 12h to obtain the π-d conjugated covalent organic framework material Ni-BQ COF.

[0031] The X-ray powder diffraction spectrum of the π-d conjugated Ni-BQ COF material synthesized in Example 1 is as follows: Figure 1 As shown in the figure, there are three obvious characteristic peaks at 7.4°, 8.4° and 11.3°, corresponding to the (100), (010) and (110) crystal planes, respectively, indicating that the Ni-BQ COF material synthesized in Example 1 is a covalent organic framework material with good crystallinity.

[0032] The infrared absorption spectrum of the π-d conjugated Ni-BQ COF material synthesized in Example 1 is shown in FIG. Figure 2 As shown. It can be seen that the NH peak at 3381 and the -OH peak at 3270 in the infrared spectrum of Ni-BQ COF disappeared significantly, indicating that the reaction can be fully carried out through a simple one-step solvothermal method. In addition, it can be clearly observed that the -OH peak at 1654 cm -1 and 1570cm -1 The absorption peaks of can be attributed to the stretching vibration peaks of C=O and the connecting group C=N, respectively, further indicating the successful synthesis of Ni-BQ COF.

[0033] The nitrogen adsorption-desorption curve of the π-d conjugated Ni-BQ COF material synthesized in Example 1 is as follows: Figure 3 shown. Figure 3 -a shows a typical type I isotherm, indicating that Ni-BQ COF is a microporous material with a specific surface area of ​​51.45 m 2 g -1 . Figure 3 -b The pore size distribution diagram shows that the pore size is mainly distributed at 1.088nm, which is close to the theoretical pore size (1.1nm), further fully proving the successful construction of the periodic microporous structure of Ni-BQ COF.

[0034] The π-d conjugated Ni-BQ COF material prepared in Example 1 was subjected to X-ray powder diffraction test after being soaked in different solvents. The results are as follows: Figure 4 Compared with the XRD pattern of the initially synthesized Ni-BQ COF material, the PXRD spectrum of Ni-BQ COF after immersion has almost no change, indicating that Ni-BQ COF has stable chemical stability.

[0035] Example 2

[0036] 2,5-Hydroxyterephthalic acid (HBC, 1mmol), 2,3,5,6-tetra(amino)benzoquinone (TABQ, 0.5mmol) and excess nickel acetate tetrahydrate (NiOAc﹒4H2O, 2.0mmol) were added to a 10mL glass bottle, followed by N-methylpyrrolidone (NMP, 10ml) and ultrasonication for 0.5h; the glass bottle was transferred to a Teflon-lined stainless steel hydrothermal autoclave, sealed, heated at 120℃ for 72h, and cooled naturally to room temperature after the reaction. The resulting black precipitate was collected by filtration and repeatedly washed with DMF and methanol, and then the product was vacuum dried at 80℃ for 12h to obtain the π-d conjugated covalent organic framework material Ni-BQ COF.

[0037] Example 3

[0038] 2,5-Hydroxyterephthalic acid (HBC, 1mmol), 2,3,5,6-tetra(amino)benzoquinone (TABQ, 0.5mmol) and excess nickel acetate tetrahydrate (NiOAc﹒4H2O, 2.5mmol) were added to a 10mL glass bottle, followed by N-methylpyrrolidone (NMP, 10ml) and ultrasonication for 0.5h; the glass bottle was transferred to a Teflon-lined stainless steel hydrothermal autoclave, sealed, heated at 120℃ for 72h, and cooled naturally to room temperature after the reaction. The resulting black precipitate was collected by filtration and repeatedly washed with DMF and methanol, and then the product was vacuum dried at 80℃ for 12h to obtain the π-d conjugated covalent organic framework material Ni-BQ COF.

[0039] The X-ray powder diffraction patterns of the Ni-BQ COF prepared in Examples 2 and 3 are shown in Figure 5 As shown. It can be seen that the prepared Ni-BQ COF has good crystallinity.

[0040] Example 4

[0041] 40 mg of the Ni-BQ COF material synthesized in Example 1 was mixed with 40 mg of conductive carbon black and 20 mg of polyvinylidene fluoride (5 wt%) in an agate mortar, and NMP was added to adjust the viscosity of the slurry. Then the prepared mixed slurry was coated on the PP diaphragm, and the coated diaphragm was placed in a 50°C vacuum oven overnight. After drying, a manual cutting machine was used to cut the diaphragm into a circular shape. Finally, the cut diaphragm was transferred to a glove box for use.

[0042] The Ni-BQ COF modified PP diaphragm prepared in Example 4 was observed by electron microscope to obtain a plan view and a cross-sectional view. The results are as follows: Figure 5 As shown. Figure 6 -a shows a dense layer of Ni-BQ COF material; Figure 6 -b It can be seen that the thickness of the Ni-BQ COF modification layer on the membrane surface is about 15 μm. This dense Ni-BQ COF layer plays an important role in inhibiting polysulfide shuttling.

[0043] Example 5

[0044] The commercial PP separator and the Ni-BQ COF modified PP separator prepared in Example 4 were assembled with metal lithium sheets to form symmetrical cells for electrochemical polarization tests. The results are as follows: Figure 7 According to the electrochemical polarization results, the ion migration number of the commercial PP separator was calculated to be 0.32, while the ion migration number of the Ni-BQ COF modified separator was 0.76, which was significantly higher than that of the commercial PP separator. The main reason is that the regular pores of Ni-BQ COF play an important role in ion transfer.

[0045] Example 6

[0046] The commercial PP separator and the Ni-BQ COF modified PP separator prepared in Example 4 were assembled into symmetrical cells with stainless steel sheets, and electrochemical impedance tests were performed. The results are as follows: Figure 8 As shown. The ionic conductivity of the commercial PP separator is calculated to be 0.49 mS cm -1 , while the ionic conductivity of the PP membrane modified with Ni-BQ COF is 1.7 mS cm -1 , which is significantly higher than that of the commercial PP separator, indicating that Ni-BQ COF can effectively promote the rapid transport of ions.

[0047] Example 7

[0048] The S@SP composite pole piece was used as the positive electrode (traditional melting preparation method), the metal lithium sheet was used as the negative electrode, and the PP separator modified with Ni-BQ COF prepared in Example 2 was used. When assembling the battery, the side coated with the Ni-BQ COF coating was facing the S@SP positive pole piece, and the electrolyte composition was 1.0M bistrifluoromethanesulfonyl imide lithium LiTFSI (DOL: DME = 1 / 1, 0.1MLiNO3), and a CR2032 battery shell was selected. After assembly, it was compacted on a tablet press and the electrochemical performance test was performed after standing for a certain period of time. A lithium-sulfur battery was assembled with a commercial PP separator for test control.

[0049] The lithium-sulfur battery assembled with the commercial PP separator and the Ni-BQ COF modified separator in Example 4 was electrochemically tested at different current densities. The battery rate performance results are shown in Fig. 9 As shown. It can be seen that when the current density is 0.5, 1, 2 and 4C, the discharge capacity of the battery assembled with the commercial PP separator is 623.68, 532.91, 503.86 and 459.77 mAh g -1 However, the discharge capacities of the batteries assembled with Ni-BQ COF modified separators were as high as 926.36, 851.41, 771.78 and 720.17 mAh g -1 At the same current density, the rate performance of the Ni-BQ COF modified separator assembled battery is significantly higher than that of the commercial PP separator, which shows that the Ni-BQ COF modified separator has very excellent rate performance. This is because a large amount of Ni is introduced into the separator in Ni-BQ COF. 2+ , can serve as catalytic sites to promote the catalytic conversion of polysulfides, accelerate the reaction kinetics of lithium-sulfur batteries, and enhance rate performance.

[0050] The lithium-sulfur battery assembled with the commercial PP separator and the Ni-BQ COF modified separator in Example 4 was subjected to long cycle tests at a current density of 0.5C. The results are as follows: Fig.10 When using a commercial PP separator, the initial capacity at 0.5C current density is 679.00 mAh g -1 , the capacity of the commercial PP separator was 506.3 mAh g after 200 cycles. -1 The capacity decay of the commercial PP separator is serious, with a capacity decay rate of 0.13% per cycle. In contrast, it can be clearly seen that the initial capacity of the battery using the Ni-BQ COF modified separator at a current density of 0.5C is 880.21mAh g -1 , maintaining 688.18 mAh g after 200 cycles -1 The capacity retention rate is 78.2%, and the capacity decay rate per cycle is only 0.10%, which is significantly higher than that of commercial PP separators, further verifying the superiority of Ni-BQ COF modified separators. The excellent long-cycle stability is due to the fact that the Ni-BQ COF material acts as a physical and chemical barrier as a coating on the separator, and the effective adsorption of polysulfides has made a great contribution to the performance improvement of lithium-sulfur batteries.

[0051] In summary, the π-d conjugated Ni-BQ COF material of the present invention is used as a separator modification layer. On the one hand, the π-d conjugated structure effectively promotes the high delocalization of electrons and enhances its electronic conductivity; on the other hand, a large amount of Ni 2+ , can serve as catalytic sites to promote the catalytic conversion of polysulfides, accelerate the reaction kinetics of lithium-sulfur batteries, and enhance rate performance. On the other hand, the abundant N / O polar groups in the Ni-BQ COF structure can chemically adsorb polysulfides, effectively inhibit the shuttle effect of polysulfides, and improve the cycle stability of lithium-sulfur batteries.

[0052] The embodiments described above are only preferred embodiments of the present invention and are only used to explain the present invention, not to limit the scope of implementation of the present invention. For those skilled in the art, other implementation methods can certainly be easily made by replacement or modification based on the technical contents disclosed in this specification. Therefore, all changes and improvements made on the principles of the present invention should be included in the scope of the patent application of the present invention.

Claims

1. A π-d conjugated covalent organic framework material, characterized in that: Its structural formula is as follows:

2. The method for preparing the π-d conjugated covalent organic framework material according to claim 1, characterized in that: The following steps are involved: 2,5-Hydroxyterephthalic acid, 2,3,5,6-tetra(amino)benzoquinone and nickel acetate tetrahydrate are added into a container, and then N-methylpyrrolidone is added. After ultrasonic treatment for 0.5h, the mixture is transferred into a hydrothermal reactor and reacted at 100-150°C for 2-5 days in a sealed state. After the reaction is completed, the mixture is naturally cooled to room temperature. The obtained precipitate is collected by filtration, washed, and vacuum dried at 60-120°C to obtain a π-d conjugated covalent organic framework material.

3. The preparation method according to claim 2, characterized in that: The molar ratio of the 2,3,5,6-tetra(amino)-p-benzoquinone to the 2,5-hydroxyterephthalic acid is 1:1-3.

4. Application of a π-d conjugated covalent organic framework material in a battery separator, characterized in that: The battery separator is provided with a mixed coating consisting of the π-d conjugated covalent organic framework material according to claim 1, conductive carbon black and polyvinylidene fluoride.

5. The use according to claim 4, characterized in that: The mass ratio of the π-d conjugated covalent organic framework material to the conductive carbon black is 1:0.6-1.

6. Application of a π-d conjugated covalent organic framework material in a lithium-sulfur battery, characterized in that: A mixed coating consisting of the π-d conjugated covalent organic framework material according to claim 1, conductive carbon black and polyvinylidene fluoride is arranged on the battery separator in the lithium-sulfur battery.

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