MOF-74(M) (M = Mg(II), Fe(II), Ni(II)) frameworks to enable accelerated redox kinetics for Li-S batteries

This study presents the development of a composite electrode material for lithium-sulphur (Li-S) batteries, combining MOF-74 with carbon black and sulphur. The MOF-74 structures, incorporating Ni(II), Mg(II), and Fe(II) metal ions, were synthesized via a solvothermal method and used to encapsulate sulphur. The microporous nature of MOF-74 facilitates the physical confinement and storage of sulphur, potentially enhancing the performance of Li-S batteries. The investigation focuses on how different central metal ions in MOF-74 influence the performance of sulphur-based electrodes. Among the metal ions studied, Fe(II) and Mg(II) were selected for their low toxicity, cost-effectiveness, and availability, while Ni(II) was included for its high catalytic properties. The materials were thoroughly characterized using infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, powder X-ray diffraction, elemental analysis and X-ray photoelectron spectroscopy. The thermal stability and textural properties of the materials were assessed, showing that MOF-74(Mg) exhibited the highest stability, followed by MOF-74(Ni) and MOF-74(Fe). Nitrogen adsorption/desorption measurements indicated that the specific surface area and pore volume varied with activation temperature, impacting the material's performance. Among the tested materials, MOF-74(Ni) exhibited the strongest interaction with sulfur, as confirmed by XPS analysis. Electrochemical tests revealed that the S/MOF-74(Ni) electrode demonstrated superior stability and capacity retention with a minimal capacity fading rate of 0.001% per cycle over 200 cycles, achieving a reversible capacity of 465 mAh g^-1 and a capacity retention of 99.75%. In contrast, the S/MOF-74(Fe) electrode showed significantly reduced performance. A structure-performance correlation was established to assess sulfur interaction, electrode stability, and degradation behavior. Overall, the results highlight that MOF-74(Ni) offers the most promising performance due to its effective sulphur immobilization and superior electrochemical properties compared to MOF-74(Mg) and MOF-74(Fe).