(001) Facet-Dominated Hierarchically Hollow Na2Ti3O7 as a High-Rate Anode Material for Sodium-Ion Capacitors.

Sodium-ion capacitors (SICs) have shown great potential to combine the merits of high-power capability of traditional capacitors and high energy capability of batteries. However, the sluggish kinetics and inferior stability of conventional sodium-ion storage anode materials are major challenges for the practical utilization of SICs. In this work, interconnected urchin-like hollow Na2Ti3O7 (Na2Ti3O7-IcUH) chains were designed and prepared by a simple one-step template-assisting method. Through a variety of controlled experiments, we explored how to effectively engineer the crystal-oriented growth and string the urchin-like spheres together. Benefiting from its urchin-like hollow structure and fully exposed (001) facet, the resulting Na2Ti3O7-IcUH exhibits a superior rate capability of 96.2 mA h g-1 at 5 A g-1. Meanwhile, the interconnected three-dimensional primary structure endows Na2Ti3O7-IcUH with excellent cyclic stability (15% capacity loss at 5 A g-1 after 2000 cycles). By coupling with commercial active carbon, the assembled SIC successfully demonstrates a energy density of 134.3 W h kg-1 at a power density of 125 W kg-1 and 38.2 W h kg-1 at a high-power density of 2500 W kg-1, as well as a superior capacity retention of 75% after 2000 cycles at 2 A g-1 within 1-4 V.