Lithium intercalation mechanism into FeF₃·0.5H₂O as a highly stable composite cathode material

The growing demand for lithium-ion batteries (LIBs) requires investigation of high-performance electrode materials with the advantages of being environmentally friendly and cost-effective. In this study, a nanocomposite of open-pyrochlore-structured FeF₃·0.5H₂O and reduced graphene oxide (RGO) is synthesized for use as a high-performance cathode in LIBs, where RGO provides high electrical conductivity to the composite material. The morphology of the composite shows that FeF₃·0.5H₂O spheres are embedded into RGO layers and high-resolution TEM image shows that those spheres are composed of primary nanoparticles with a size of ~5 nm. The cycling performance indicates that the composite electrode delivers an initial high discharge capacity of 223 mAh g^-1 at 0.05 C, a rate capability up to a high C-rate of 10 C (47 mAh g^-1) and stable cycle performance at 0.05 C (145 mAh g^-1 after 100 cycles) and 0.2 C (93 mAh g^-1 after 100 cycles) while maintaining high electrochemical reversibility. Furthermore, the responsible electrochemical reaction is investigated using in-situ XRD and synchrotron-based X-ray absorption spectroscopy (XAS), and the XRD results show that FeF₃·0.5H₂O transitions to an amorphous-like phase through a lithiation process. However, a reversible oxidation change of Fe³⁺ ↔ Fe²⁺ is identified by the XAS results.