Robust and Conductive Na₂Ti₂O_5–<i>x</i> Nanowire Arrays for High-Performance Flexible Sodium-Ion Capacitor

Hybrid capacitors, especially sodium-ion capacitors (SICs), which combine the complementary merits of high-energy batteries and high-power capacitors, have received increasing research interest and have been expected to bridge the gap between the rechargeable batteries and EDLCs. The biggest challenge for SICs is to overcome the kinetics discrepancy between the sluggish faradaic anode and the rapid nonfaradaic capacitive cathode. To boost the Na⁺ reaction kinetics, robust and conductive Na₂Ti₂O_5–<i>x</i> nanowire arrays have been constructed as an accessible and affordable SIC anode. It is found that the utilization of oxygen vacancies (OVs) can endow Na₂Ti₂O_5–<i>x</i> high electrical conductivity, introduce intercalation pseudocapacitance, and maintain the crystal structure integrity. It exhibits high reversible discharge capacity (225 mAh g^–1 at 0.5C), superior rate capability, and ultralong lifespan when utilized as self-supported and additive-free anode for SIB, remaining almost 100% capacity retention after 20 000 cycles at 25 C. When assembled as flexible hybrid SIC (4.5 cm³) with rGO/AC film cathode, a high-level energy density of 70 Wh kg^–1 at power density of 240 W kg^–1 based on active materials can be achieved, and high volumetric energy density (15.6 Wh L^–1) and power density (120 W L^–1) based on the whole packge volume can be delivered with superior cycle stability (5000 cycles, 82.5%)