Fe 7 Se 8 encapsulated in N-doped carbon nanofibers as a stable anode material for sodium ion batteries.

Transition metal chalcogenides especially Fe-based selenides for sodium storage have the advantages of high electric conductivity, low cost, abundant active sites, and high theoretical capacity. Herein, we proposed a facile synthesis of Fe 7 Se 8 embedded in carbon nanofibers (denoted as Fe 7 Se 8 -NCFs). The Fe 7 Se 8 -NCFs with a 1D electron transfer network can facilitate Na + transportation to ensure fast reaction kinetics. Moreover, Fe 7 Se 8 encapsulated in carbon nanofibers, Fe 7 Se 8 -NCFs, can effectively adapt the volume variation to keep structural integrity during a continuous Na + insertion and extraction process. As a result, Fe 7 Se 8 -NCFs present improved rate performance and remarkable cycling stability for sodium storage. The Fe 7 Se 8 -NCFs exhibit practical feasibility with a reasonable specific capacity of 109 mA h g -1 after 200 cycles and a favorable rate capability of 136 mA h g -1 at a high rate of 2 A g -1 when coupled with Na 3 V 2 (PO 4 ) 3 to assemble full sodium ion batteries.