Three-dimensional mesoporous γ-Fe2O3@carbon nanofiber network as high performance anode material for lithium- and sodium-ion batteries.

Electrode materials that can function well in both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) are desirable for electrochemical energy storage applications, especially under high rate. In this work, a three-dimensional (3D) mesoporous γ-Fe2O3@carbon nanofiber (γ-Fe2O3@CNF) mat has been successfully synthesized by sol-gel based electrospinning and carbonization. It delivers a specific capacity of 820 mAh g-1 at 0.5 C after 250 cycles, 430 mAh g-1 at 6 C after 1000 cycles, and 222 mAh g-1 at ultrahigh rate of 60 C for LIBs, while for SIBs it delivers a specific capacity of 360 mAh g-1 at 1 C after 1000 cycles and 130 mAh g-1 at 60 C. Besides, the result of ex situ microstructure examination shows the polycrystalline nature of γ-Fe2O3 nanoparticle still exists in LIB even after 1000 cycles, while it vanishes in SIB, suggesting that the relatively larger volume expansion occurred during Na+ insertion/deinsertion, resulting in pulverization of the particles. The CNFs maintained their pristine 3D network structure after the charge/discharge, which demonstrated the critical role of a robust conductive electrode in promoting fast Li+/Na+ transportation. More importantly, they act as an electrical bridge between Li+/Na+ and γ-Fe2O3 nanoparticles, therefore suppressing the cell impedance growth and γ-Fe2O3 volume expansion, resulting in the enhancement in both cyclic and rate capability.