Nitrogen-doped carbon-encapsulation of Fe ₃O ₄ for increased reversibility in Li ⁺ storage by the conversion reaction

One great challenge in designing anode materials for lithium-ion batteries is to satisfy the concurrent requirements for good capacity retention, high rate performance and low first cycle losses. We report here the design and synthesis of a nitrogen-doped carbon encapsulated Fe ₃O ₄ composite which performed very well in all these areas. The composite with the optimized carbon content not only showed a high reversible capacity of ∼850 mA h g ^-1 for 50 cycles at 100 mA g ^-1, but was also able to maintain a stable cycling performance at a twenty-fold increase in current density to 2000 mA g ^-1. More importantly, the composite significantly lowered the irreversible capacity loss in the first cycle compared with other iron oxide anodes reported in the literature. Characterization of the electrode/electrolyte interface indicated the presence of a protective solid electrolyte interface (SEI) layer in which chemically stable LiF and FeF ₃ were the major constituents. Thus, it is believed that the N-doped carbon coating had effectively modified the surface chemistry at the anode/electrolyte interface to increase the columbic efficiency of cycling and to reduce the secondary reactions in the first cycle of use.