In situ visualization of sodium transport and conversion reactions of FeS ₂ nanotubes made by morphology engineering

Iron disulfide (FeS ₂ ), existing in nature as pyrite, holds great promise as a conversion-type anode material for sodium-ion batteries (SIBs), owing to its low cost and high theoretical capacity. However, the large volume expansion and the sluggish electrode reaction kinetics during conversion reactions impede its large-scale practical application in SIBs. Here, we demonstrate the utilization of morphological engineering to achieve poly-crystalline FeS ₂ nanotubes (NTs) consisting of tiny FeS ₂ crystallites. In situ transmission electron microscopy observations reveal that 1D shape can afford straight pathways for Na transport to expedite reaction kinetics, and poly-crystalline structure can buffer large volume expansion and structural strain. Furthermore, high-resolution imaging and electron diffraction were utilized to track phase evolution associated with conversion reactions in real time. We have identified an intercalation-conversion reaction mechanism from the FeS ₂ phase to the Na ₂ S + Fe phases via the intermediate NaFeS ₂ phase upon initial sodiation. Impressively, a reversible and symmetric conversion reaction between NaFeS ₂ phase and Na ₂ S + Fe phases is established during subsequent sodiation−desodiation cycles. Notably, this is the first report of FeS ₂ NTs investigated for secondary battery electrode material. This work not only provides valuable insights into sodium storage mechanism of FeS ₂ material, but also corroborates the pivotal role of morphology engineering in optimizing the microstructure of electrode materials for advanced SIBs.