Intercalation Pseudocapacitance: A Route Towards Oxide Supercapacitors
Capacitive energy storage offers a number of attractive features including high power capability, fast response times, and long-term cycling. Commercial technology uses carbon-based electrochemical capacitors in which energy storage by double layer processes leads to high power density, but low energy density. The interest in using pseudocapacitor-based materials for electrochemical capacitors is that the energy density associated with faradaic reactions is much greater, by at least an order of magnitude, than the electrical double layer capacitance of carbon electrodes. One key criterion is that fast faradaic reactions are required and for this reason oxide pseudocapacitors have largely been those materials which exhibit surface or near surface redox reactions. Our results show that Nb2O5 undergoes fast faradaic reactions through an intercalation pseudocapacitance mechanism in which lithium ion insertion occurs in the bulk of the material. The principal benefit realized from this mechanism is that high levels of charge storage are achieved within short periods of time because there are no limitations from solid-state diffusion. For example, Nb2O5 exhibits an energy density in excess of 100 mAh/g at a charging rate of 60C. While Nb2O5 may not be the optimum material for energy storage devices, it does provide a model system for addressing some of the outstanding issues regarding pseudocapacitive energy storage. We have identified some of the characteristics associated with intercalation pseudocapacitance and expect that other oxide systems are also capable of achieving comparable levels of high rate energy storage.
Bruce Dunn is the Nippon Sheet Glass Professor of Materials Science and Engineering at UCLA. Prior to joining UCLA, he was a staff scientist at the General Electric Research Laboratory. His research interests include the synthesis of inorganic and organic/inorganic materials, and the characterization of their electrical, optical, biochemical and electrochemical properties. A continuing theme in his research is the use of sol-gel methods to create materials with designed microstructures and properties. His recent work on electrochemical energy storage includes pseudocapacitive energy storage materials and three-dimensional batteries.