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

Abstract 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 TiO2. The as-prepared material (TiO2@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 TiO2 coating. Furthermore, the TiO2@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 TiO2@PBC is attributed to the three-dimensionally cross-linked conductive network, the open ion-transportation channels, and more importantly, the TiO2 coating with high content of oxygen vacancies which expands the lattice and thus enables fast ion transport.