Ilmenite Nanotubes for High Stability and High Rate Sodium-Ion Battery Anodes.

To solve the problem of large volume change and low electronic conductivity of earth-abundant ilmenite used in rechargeable Na-ion batteries (SIBs), an anode of tiny ilmenite FeTiO nanoparticle embedded carbon nanotubes (FTO⊂CNTs) has been successfully proposed. By introducing a TiO shell on metal-organic framework (Fe-MOF) nanorods by sol-gel deposition and subsequent solid-state annealing treatment of these core-shell Fe-MOF@TiO, such well-defined FTO⊂CNTs are obtained. The achieved FTO⊂CNT electrode has several distinct advantages including a hollow interior in the hybrid nanostructure, fully encapsulated ultrasmall electroactive units, flexible conductive carbon matrix, and stable solid electrolyte interface (SEI) of FTO in cycles. FTO⊂CNT electrodes present an excellent cycle stability (358.8 mA h g after 200 cycles at 100 mA g) and remarkable rate capability (201.8 mA h g at 5000 mA g) with a high Coulombic efficiency of approximately 99%. In addition, combined with the typical NaV(PO) cathode to constitute full SIBs, the assembled FTO⊂CNT//NaV(PO) batteries are also demonstrated with superior rate capability and a long cycle life.