Free-Standing Sandwich-Structured Flexible Film Electrode Composed of Na2Ti3O7 Nanowires@CNT and Reduced Graphene Oxide for Advanced Sodium-Ion Batteries

A free-standing flexible anode material for sodium storage with sandwich-structured characteristics was fabricated by modified vacuum filtration, consisting of stacked layers of Na2Ti3O7 nanowires@ carbon nanotubes (NTO NW@CNT) and graphene oxide. The NTO NWs have a larger specific surface area for Na+ insertion/extraction with shortened ion diffusion pathways, accelerating the charge transfer/collection kinetics. The added CNTs both facilitate the uniform dispersion of the nanowires and nanotubes and also contribute to the connectivity of the nanowires, improving their conductivity. More importantly, the unique sandwichlike layered-structured film not only provides large numbers of electron-transfer channels and promotes the reaction kinetics during the charging and discharging process but also ensures the structural stability of the NTO NWs and the electrode. Electrochemical measurements suggest that this rationally designed structure endows the electrode with a high specific capacity and excellent cycling performance. A satisfactory reversible capacity as high as 92.5 mA h g(-1) was achieved after 100 cycles at 2C; subsequently, the electrode also delivered 59.9 mA h g(-1) after a further 100 cycles at 5C. Furthermore, after the rate performance test, the electrode could be continuously cycled for 100 cycles at a current density of 0.2C, which demonstrated that durable cyclic capacity with a high reversible capacity of 114.1 mA h g(-1) was retained. This novel and low-cost fabrication procedure is readily scalable and provides a promising avenue for potential industrial applications.