In-situ synthesis of Fe7S8 on metal sites of MOFs as high-capacity and fast-kinetics anodes for sodium ion batteries

Low cost and high theoretical capacity are the main directions for the development of anode materials for sodium-ion batteries in the future, while iron sulfide fits in with this expectation. In this paper, nano-size porous Fe7S8 anode (Fe7S8 @MC) material with its Fe and mesoporous carbon (MC) derived from the metal-organic framework (MOF) MIL-53 (Fe) for the sodium ion battery is successfully prepared by hydrothermal reaction combined with carbonization and vulcanization process. The iron ion sites are fully utilized to form carbon coated Fe7S8 basic monomers, and the monomers are connected with the carbon skeleton to form a porous nanostructure, which can not only maintain high energy storage capacity, but also ensures the stability of the electrode structure, so as to obtain stable cycling and rate performance. At the current density of 500 mA g-1, the reversible specific capacity of the obtained material is still 413.9 mAh g-1, even after 100 cycles. In particular, the reversible capacity of 390.2 mAh g-1 can still be obtained at a high current density of 1 A g-1. Simultaneously, the obtained material can recover 97% of the initial capacity after being charged and discharged at the high specific current of 1 A g-1. The total sodium storage capacity is contributed by a combination of capacitance and diffusion. In addition, the calculated result for the apparent activation energy (Ea = 30.89 kJ mol-1), the diffusion apparent activation energy (EaD = 14.41 kJ mol-1), and the sodium ion diffusion coefficient (DNa+ = 2.55 × 10-12 cm2 s-1) show that the obtained materials have rapid reaction kinetics. Therefore, synthetic materials have excellent research potential and are expected to be used in high-performance sodium ion batteries with practical efficacy.