Interface engineering of FeOF/FeF2 heterostructure for ultrastable Li-ion/ Na-ion storage

Iron fluoride is a prospective cathode material for Li-ion batteries (LIBs) and Na-ion batteries (SIBs) due to their high theoretical capacity and working voltage, whereas the practical applicability is inhibited by its slow reaction kinetics and poor cycling stability. Herein, the FeOF/FeF2 heterostructure is constructed to address the aforementioned issues and obtain ultrastable Li-ion/Na-ion storage. The FeOF/FeF2 heterostructure possesses a builtin electric field, oxygen vacancies and a homo-tetragonal phase, which can considerably improve the charge transfer kinetics, increase the ion storage sites and strengthen the structural stability. As expected, the FeOF/FeF2 cathode delivers remarkable cycle stability: 134.7 mAh g-1 after 1000 cycles at 1000 mA g-1 for LIBs, and 124.8 mAh g-1 after 200 cycles at 500 mA g-1 for SIBs. Furthermore, the Li-ion/Na-ion storage mechanism of the FeOF/FeF2 heterostructure during the electrochemical process is revealed through in-situ X-ray diffraction and ex-situ characterizations. This method of constructing heterostructure opens a way for other conversion materials to achieve high-performance LIBs/SIBs.