Capacitive energy storage offers several attractive properties compared to batteries, including higher power, faster charging, and a longer cycle life. A key limitation to this electrochemical energy-storage approach is its low energy density and, for this reason, there is considerable interest in identifying pseudocapacitor materials where faradaic reactions are used to achieve greater charge storage. This paper reports on the electrochemical properties of Nb2O5 and establishes that crystalline phases of the material undergo fast faradaic reactions that lead to high specific capacitance in short charging times. In particular, the specific capacitance for the orthorhombic phase at infinite sweep rate reaches ≈400 F g−1, which exceeds that of birnessite MnO2 in nonaqueous electrolyte and is comparable to RuO2 at the same extrapolated rate. The specific capacitances of the orthorhombic and pseudohexagonal phases are much greater than that of the amorphous phase, suggesting that the faradaic reactions which lead to additional capacitive energy storage are associated with Li+ insertion along preferred crystallographic pathways. The ability for Nb2O5 to store charge at high rates despite its wide bandgap and low electronic conductivity is very different from what is observed with other transition metal oxides.
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