Mesopore-Induced Ultrafast Na -Storage in T-Nb O /Carbon Nanofiber Films toward Flexible High-Power Na-Ion Capacitors.

Hybrid Na-ion capacitors (NICs) are receiving considerable interest because they combine the merits of both batteries and supercapacitors and because of the low-cost of sodium resources. However, further large-scale deployment of NICs is impeded by the sluggish diffusion of Na in the anode. To achieve rapid redox kinetics, herein the controlled fabrication of mesoporous orthorhombic-Nb O (T-Nb O )/carbon nanofiber (CNF) networks is demonstrated via in situ SiO -etching. The as-obtained mesoporous T-Nb O (m-Nb O )/CNF membranes are mechanically flexible without using any additives, binders, or current collectors. The in situ formed mesopores can efficiently increase Na -storage performances of the m-Nb O /CNF electrode, such as excellent rate capability (up to 150 C) and outstanding cyclability (94% retention after 10 000 cycles at 100 C). A flexible NIC device based on the m-Nb O /CNF anode and the graphene framework (GF)/mesoporous carbon nanofiber (mCNF) cathode, is further constructed, and delivers an ultrahigh power density of 60 kW kg at 55 Wh kg (based on the total weight of m-Nb O /CNF and GF/mCNF). More importantly, owing to the free-standing flexible electrode configuration, the m-Nb O /CNF//GF/mCNF NIC exhibits high volumetric energy and power densities (11.2 mWh cm , 5.4 W cm ) based on the full device, which holds great promise in a wide variety of flexible electronics.