High‐Performance Supercapacitors Based on Nanocomposites of Nb2O5 Nanocrystals and Carbon Nanotubes

Compared with batteries, supercapacitors often deliver signifi cantly higher power densities with longer cycling life but lower energy densities. Developing supercapacitors with improved energy density therefore becomes a highly attractive topic. Generally, energy densities of supercapacitors ( E ) are determined by E = 1⁄2 CV 2 , where C is the cell capacitance and V is the cell potential; higher cell voltage and capacitance lead to higher energy density. [ 3 ] Porous carbon, the most commonly used electrode material, possesses a double layer capacitance of 100 ∼ 150 F g − 1 in organic electrolyte, while transition metal oxides may exhibit signifi cantly higher pseudo-capacitances. Designing asymmetric supercapacitors consisting of a carbon cathode and a transition-metal-oxide anode is therefore considered as the most effective solution. [ 4 ] For instance, asymmetric cells based on Li 4 Ti 5 O 12 anode and activated carbon cathode can achieve an energy density of 40 Wh kg − 1 in organic electrolyte system, [ 5 , 6 ] which is much higher than those of carbon-based symmetric devices. Seeking better anode materials with high specifi c capacitance, low working potential, and long cycling stability therefore becomes a main theme of the fi eld. Compared with Li 4 Ti 5 O 12 with a working potential of 1.5 V and a specifi c capacity of 140 mA h g − 1 , [ 7 ] niobium pentoxide (Nb 2 O 5 ) exhibits a higher capacity ( ∼ 200 mA h g − 1 ) and a