X-ray diffraction (XRD) was used to determine the crystal structure of the prepared samples; Fig. 1a displays the XRD crystal diffraction peaks of MIL-101(Fe), and the main diffraction peaks of the MIL-101(Fe) sample appear at 2.8°, 3.3°, 8.4°, and 9.1°, as has been previously reported in the literature[39], thus proving the successful synthesis of highly crystalline MIL-101(Fe). Figure 1b shows the XRD of MIL-88B(Fe), with the main diffraction peaks occurring at 9.1°, 10.4°, 16.7°, 18.8°, and 21.1°, corresponding to the crystal planes [002], [101], [103], [202] and [211], in agreement with those previously reported[37], which is confirmed the successful synthesis of MIL-88B(Fe) material. The XRD of MIL-53(Fe), shown in Fig. 1c, demonstrates the effective synthesis of highly crystalline MIL-53, with prominent diffraction peaks at 9.4°, 12.5°, 17.9°, and 25.1°, which coincide with those previously reported[40]. Figure 1d shows the crystal structures of MIL-101-NO2-0.1, MIL-101-NO2-0.15, MIL-101-NO2-0.2, and MIL-101-NO2-0.25 materials are also consistent with MIL-101(Fe). As shown in Fig. 2a-d, the MIL-101(Fe) material has a regular octahedral crystal form with a particle size of approximately 1 µm. MIL-88B particles have a needle-like shape with a length of around 200 nm. MIL-53(Fe) has an irregular octahedral shape with a particle size of around 0.8-1 µm. The MIL-101-NO2-0.25 material has an octahedral crystal morphology and a particle size of around 1 µm as seen in Fig. 2e.
The functional groups and molecular structures of the materials can be analyzed by FT-IR. Figure 3a-d shows the FT-IR plots of the iron-based MOFs with different introductions, The IR absorption peaks at 1602 cm− 1 can be seen as the asymmetric vibration of C = O, the skeletal vibration of the aromatic ring at 1508 cm− 1, the symmetric vibration of C = O at 1380 cm− 1, the stretching vibration of C-O at 1020 cm− 1, the out-of-plane bending vibration of C-H in the benzene ring at 740 cm− 1, and the stretching vibration of Fe-O at 550 cm− 1. In contrast, the stretching vibration of the carboxylic acid group at 1414 cm− 1 in the IR spectrum of terephthalic acid shifts to a lower wave, demonstrating the successful coordination of the carbonyl group to the metal ion, and the stretching vibration of the benzene ring skeleton at 773 cm− 1 in comparison to MIL-101(Fe) shifts to a lower wave at 740 cm− 1. The IR of MIL-88B and MIL-53 signature peaks resemble those of MIL-101. Figure 3d shows that a distinct IR absorption peak can be seen at 1538 cm− 1 as the characteristic peak of nitro, and there is a significant increase in the peak of the characteristic peak as the amount of nitro introduced increases, which proves the successful introduction of nitro ligand[41]. And by comparing the IR spectra of MIL-101(Fe) with MIL-101-NO2-0.25, it can be found that the introduction of the nitro ligand shifts the position of the characteristic peak of the Fe-O stretching vibration towards the high wave number by 4.7 cm− 1, This demonstrates that the nitro-functionalized MOFs metal nodes Fe-O clusters have lowered electron density[42].
To analyze the adsorption capacity of lithium polysulfide, Fig. 4a shows that the iron-based MOFs were added to the previously prepared Li2S6 solution and left to stand. After 30 minutes, the color of the solution with the addition of MIL-101-NO2-0.25 material has faded significantly to a pale yellow compared to the Li2S6 solution used as a reference. After 2 hours, the solution of MIL-101-NO2-0.25 had faded, however, the solution of MIL-101(Fe) was only very light yellow, while the solutions of MIL-88B(Fe) and MIL53(Fe) were still more obviously yellow. After 12 hours, the solution of MIL-101 (Fe) became clear and transparent, and the MIL-88B (Fe) solution had pale yellow color. The MIL-53 (Fe) solution had a more pronounced yellow color. This can visually indicate that the MIL-101-NO2-0.25 is more effective for the adsorption of Li2S6. To strengthen the conclusions, ultraviolet-visible spectroscopy (UV-Vis) was used to test the content of S62− in the solution after 12 h of adsorption. Figure 4b shows that the solution of MIL-101-NO2-0.25 has the lowest intensity of the characteristic absorption peak of S62−, indicating that the MIL-101-NO2-0.25 has the best adsorption effect on Li2S6. Figure 4(c-f) displays the average pore size of 6.03 nm and the specific surface area of 139.26 m2g− 1 for MIL-53(Fe) material. The specific surface area of MIL-88B(Fe) material is 181.76 m2g− 1 with an average pore size of 4.007 nm. The specific surface area of MIL-101(Fe) material is 577.21 m2g− 1 with an average pore size of 2.33 nm. The specific surface area of MIL-101-NO2-0.25 material is 653.06 m2g− 1 with an average pore size of 1.07 nm. The specific surface area of the MIL-101-NO2-0.25 is 653.06 m2 g− 1 with an average pore size of 1.07 nm. Compared to MIL-101(Fe), the nitro-functionalized MOF has an increased and average surface area and the material is more capable of blocking polysulfides. Based on the analysis of the pore size distribution curves, MIL-101-NO2-0.25 has the smallest average pore size and can effectively block polysulfides, making it an excellent candidate for use in lithium-sulfur battery diaphragm modifications.
The lithium-sulfur cell was constructed through cyclic voltammetry (CV) testing of the modified diaphragm and the common diaphragm, with a voltage range of 1.7 V-2.8 V (vs. Li+/Li) and a specified sweep rate of 0.1 mVs− 1. As shown in Fig. 5a, taking MIL-101-NO2-0.25 as an example, two distinct reduction peaks can be seen located at 2.33 V and 2.03 V, corresponding to the reduction of sulfur monomer to soluble polysulfide (Li2Sn, 4 ≤ n ≤ 8), followed by reduction to Li2S[43], respectively The oxidation peak at 2.38 V represents the conversion of Li2S to polysulfide before being oxidized to S8[44]. MIL-101-NO2-0.25 separators have good reversibility and the voltage difference between redox peaks is 0.07 V. In conclusion, it demonstrates that the MIL-101-NO2-0.25 separators have stronger polysulfide adsorption and better electrical conductivity in the cell. Figure 5b displays the electrochemical impedance spectra of batteries with MOFs and Celgard separators. Both batteries display the normal mid- to high-frequency semicircles and low-frequency lines. The diameter of the semicircle is smaller for the battery with the MIL-101-NO2-0.25 modified separator, reflecting the lower charge-transfer resistance, as compared to the conventional battery. Figure 5c displays the discharge capacities corresponding to the high and low discharge voltage plateaus as well as the voltage plateaus (QH and QL)[45] of various diaphragms at 0.5 C. MIL-101-NO2-0.25 separator have the lowest polarization potential (223.5 mV) where QH and QL are involved in the slow conversion of S8 to soluble polysulfides and the conversion of soluble polysulfides to insoluble substances respectively. the value of QL/QH can be used to evaluate the ability of the material to catalyze the redox reaction of polysulfides, a large QL value means that more polysulfides are involved in the discharge reaction with a weaker shuttle effect and a fast kinetic reaction process. In Fig. 5d, the QL/QH value of the MIL-101-NO2-0.25 separator is 2.2, which is higher than that of the other separators.
The rate performance test results of the Li-S battery assembled with the MIL-101-NO2-0.25 separator at the current density of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C and then back to 0.1 C (five cycles for each current density) are shown in Fig. 6a. The discharge-specific capacities of MIL-101-NO2-0.25 are 1224.9 mAhg− 1, 1068.9 mAhg− 1, 984.3 mAhg− 1, 899.8 mAhg− 1, and 818.1 mAhg− 1, and the current density can be returned to 1087.6 mAhg− 1 when returning to 0.1 C, which is proved that MIL-101-NO2-0.25 separator has good rate performance. The cycling performances of batteries with different separators at 0.5 C are compared in Fig. 6b. The initial discharge capacity of MIL-101(Fe) separators diaphragm is 984.5 mAhg− 1, MIL-101-NO2-0.25 separators is 1051.5 mAh g− 1, and the commercial separators is 659.4 mAh g− 1. After 250 cycles, the specific capacity of the MIL-101-NO2-0.25 separator-equipped cells is still 908 mAh g− 1, which is higher than pure MIL-101(Fe) (620 mAh g− 1) and commercial separators (399.6 mAh g− 1). This shows that after the introduction of the MIL-101(Fe) modified nitro group, compared with the MIL-101-NO2-0.25 material before modification, the cycle performance of the lithium-sulfur battery is greatly improved. To examine the impact of a changed diaphragm on the electrochemical performance of lithium-sulfur batteries, Fig. 6c displays the charge and discharge curves of lithium-sulfur batteries assembled with different separators respectively at the current density of 0.2 C and the potential of 1.7–2.8 V. During the discharge of the lithium-sulfur battery, two separate discharge plateaus correspond to two reduction reactions. The first high platform corresponds to the conversion of singlet sulfur to the long-chain polysulfide Li2Sn (4 ≤ n ≤ 8) and the second platform corresponds to the conversion of Li2Sn (4 ≤ n ≤ 8) to the short-chain LiS2/Li2S. The first discharge capacity can reach 1319.4 mAh g− 1, and the attenuation is not obvious after 100 cycles, indicating that the Li-S battery with MIL-101-NO2-0.25 separator has good stability. To further characterize the stability of the MIL-101-NO2-0.25 separator, we assembled Li||Li symmetric cells with different separators. Figure 6d is the stability test of symmetrical batteries with different separators, the lithium-sulfur cell with MIL-101-NO2-0.25 modified separators can be cycled stably for 1000 hours without large voltage fluctuations throughout and can maintain an overpotential of 11 mV at the end. The lithium-sulfur cell with MIL-101(Fe) material modification starts to show a significant increase in the overpotential after 650 hours and polarization is obvious. For the commercial separators, significant polarization could already be noticed after 300 hours. According to the study of the above results, adding the electron-absorbing group nitro can increase the MOF material's affinity for lithium ions, which in turn encourages the uniform deposition of lithium.