Aligned Arrays of Na₂Ti₃O₇ Nanobelts and Nanowires on Carbon Nanofiber as High-Rate and Long-Cycling Anodes for Sodium-Ion Hybrid Capacitors

Sodium-ion capacitors (SICs) have attracted extensive attentions due to their integration of high-energy battery and high-power capacitor as well as the naturally abundant sodium resource. A major challenge of current SICs is to achieve high rate performance and long-cycle stability of the battery-type anode. Herein, fast sodium storage is achieved from sodium titanate (Na₂Ti₃O₇) arrays that are uniformly grown on highly conductive carbon nanofiber networks with a high mass loading of 5.6 mg cm⁻². Nanowires and nanobelts of Na₂Ti₃O₇ are both synthesized, and their Na-ion storage properties are compared. Both arrays can be used as binder-free and flexible electrodes, but the nanobelts exhibit higher specific capacity and better rate performance than the nanowires with similar mass loading. The difference between two types of nanostructures is ascribed to their different kinetics in ion/charge transport, according to the electrochemical impedance data. SIC full devices consisting of the Na₂Ti₃O₇ nanobelt anode and biomass-derived porous carbon cathode are constructed, which show pretty high specific energy and power performance.