Oxygen-vacancy-rich TiO2-coated carbon nanofibers for fast sodium storage in high-performance sodium-ion hybrid capacitors

Sodium-ion hybrid capacitors (SIHCs) are an emerging energy storage device with various potential applications due to their combined merits of batteries and supercapacitors . However, this device still suffers from the slow kinetics of the anode, which severely restricts the rate performance. Herein, we demonstrate that the rate performance of a carbon-nanofiber-based anode can be remarkably enhanced by coating a thin layer of oxygen-vacancy-rich TiO 2 . The as-prepared material (TiO 2 @PBC) is used as an anode in a sodium-ion battery, which delivers a reversible capacity of 180.2 mA h g −1  at 10 A g −1 with a capacity retention of 110.7% after 20,000 cycles, outperforming those without TiO 2 coating. Furthermore, the TiO 2 @PBC is assembled into a SIHC with activated carbon as the cathode, which shows an energy density of 91.0 W h kg −1  at a power density of 165 W kg −1 and retaining an energy density of 43.4 W h kg −1  at an ultra-high power density of 13 kW kg −1 . The excellent sodium storage performance of TiO 2 @PBC is attributed to the three-dimensionally cross-linked conductive network, the open ion-transportation channels, and more importantly, the TiO 2 coating with high content of oxygen vacancies which expands the lattice and thus enables fast ion transport.