Fluorine-doped porous carbon-decorated Fe3O4-FeF2 composite versus LiNi0.5Mn1.5O4 towards a full battery with robust capability

A new fluorine-doped porous carbon-decorated Fe3O4-FeF2 composite, referred to as Fe3O4-FeF2@CFx, was prepared for the first time. The formation mechanism is discussed, and a new concept of introducing double layers of FeF2 and CFx into the oxide-based anode is presented for lithium ion batteries. Varying the amount of fluorine precursor, derivatives of Fe3O4@CFx and FeF2@CFx were further obtained, allowing an original analysis of their electrochemical behaviors. As-prepared Fe3O4-FeF2@CFx can deliver a high capacity of 718 mAh g−1 at 50 mA g−1. Under a hash rate of 1600 mAg−1, the capacity of Fe3O4-FeF2@CFx (around 338 mAh g−1) is higher than that (200 mAh g−1) of FeF2@CFx. Further, its capacity retention of 97% over 100 cycles is much better than the 59.4% observed for Fe3O4@CFx. The positive effect of the CFx layer on the electronic conductivity and ionic diffusion ability was confirmed. The role of FeF2 in the stabilization of the structure of CFx and Fe3O4 is also discussed. Further, a new battery composed of Fe3O4-FeF2@CFx/LiNi0.5Mn1.5O4 with a robust rate capability was assembled and delivered a reversible capacity of 565 mAh g−1 (vs. anode) at 100 mA g−1 with a high potential of 3.3 V and a capacity retention of 81.5% over 50 cycles.