Mesopore‐Induced Ultrafast Na+‐Storage in T‐Nb2O5/Carbon Nanofiber Films toward Flexible High‐Power Na‐Ion Capacitors

AbstractHybrid 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‐Nb2O5 (T‐Nb2O5)/carbon nanofiber (CNF) networks is demonstrated via in situ SiO2‐etching. The as‐obtained mesoporous T‐Nb2O5 (m‐Nb2O5)/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‐Nb2O5/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‐Nb2O5/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−1 at 55 Wh kg−1 (based on the total weight of m‐Nb2O5/CNF and GF/mCNF). More importantly, owing to the free‐standing flexible electrode configuration, the m‐Nb2O5/CNF//GF/mCNF NIC exhibits high volumetric energy and power densities (11.2 mWh cm−3, 5.4 W cm−3) based on the full device, which holds great promise in a wide variety of flexible electronics.