The morphology and electrochemical properties of porous FeO@C and FeS@C nanofibers as stable and high-capacity anodes for lithium and sodium storage.

Among an enormous variety of electrode materials for lithium and sodium storage, transition metal-oxides/sulfides stand out on account of their widespread availability and high theoretical charge capacity. However, these anodes still undergo poor capacity retention and limited cycle life. Herein, we present a simple approach to synthesize one-dimensional (1D) porous FeO@C and FeS@C nanofibers in which ultra-small active nanoparticles are first distributed in the internal porous carbon matrix and further encapsulated in the external nano-carbon walls. The 1D porous nano-architecture effectively alleviates the pulverization or aggregation induced by huge volume changes during cycling as well as provides a short ion/electron diffusion path in the crystal. Furthermore, the internal porous carbon matrix and the external nano-carbon layers keep the structural and mechanical stability of the entire electrode. The as-synthesized FeO@C and FeS@C nanofibers show high specific capacities, robust cycling stability as well as desirable rate capability for LIBs and SIBs. Simultaneously, the FeS@C nanofibers achieve better lithium and sodium storage properties due to good electrical property and fast ion diffusion kinetics compared with FeO@C nanofibers. This novel architecture design may open an avenue to seeking out high performance electrodes for advanced energy storage.