"Creating materials for high rate energy storage"
The prospect of developing energy storage materials with the energy density of batteries and the power density and cycle life of electrical double-layer capacitors (EDLCs) is an exciting direction that has yet to be achieved. With such materials there is the promise of charging in minutes, much faster than batteries, with charge storage levels comparable to battery electrode materials, much higher than EDLCs. Our research has focused on so-called pseudocapacitive materials in which lithium-ion insertion induces reversible redox reactions at or near the surface of an electrode material. With certain solids, these faradaic reactions are sufficiently rapid so that the electrochemical features are similar to those of an electrical double-layer capacitor, but with significantly higher levels of energy storage because of the redox reactions. In recent years, using Nb2O5 as a model system, we have established several criteria for identifying materials that exhibit pseudocapacitor-like properties and retain high energy density at high rates of charge/discharge. In addition, when battery materials are reduced to nanoscale dimensions, they may begin to exhibit pseudocapacitive characteristics because of the large number of surface sites or because phase transitions are suppressed. In research to date, we have identified several electrode materials that retain high capacity for lithium at high charging rates and in this presentation we review how the criteria are met with Nb2O5 and other transition metal oxide compositions. These results provide guidelines for developing a new generation of electrochemical energy storage devices that possess high energy density at high rates of operation.