ABSTRACT: Capacitive energy storage is distinguished from other types of electrochemical energy storage by short charging times, the ability to deliver significantly more power than batteries and long cycle life. A key limitation to this technology, which is based on electrical double-layer capacitance, is its low energy density. For this reason, there is considerable interest in exploring materials which exhibit pseudocapacitive charge storage where the faradaic reactions that occur with transition metal oxides lead to energy densities which are many times larger than traditional double layer capacitance. With these materials there is the prospect of creating materials that exhibit both high energy density and high power density. However, the ability to identify the material characteristics which lead to pseudocapacitive responses is still at its inception.
We have used Li+ insertion in Nb2O5 as a model system in which to understand the electrochemical and structural features associated with pseudocapacitive mechanisms. Charge storage in this system arises from redox reactions as in battery materials and yet the kinetics of charge storage are determined by surface-like kinetics rather than semi-infinite diffusion. An important feature with the Nb2O5 system is that the structure does not undergo a first-order phase transition upon Li+ insertion. Another route for creating pseudocapacitive solids is through the synthesis of nanoscale redox materials. At nanoscale dimensions, electrochemical characteristics become more capacitor-like because of a larger number of surface sites for charge storage and/or suppression of phase transitions. This approach is very promising as the short diffusion distance leads to fast surface-like kinetics in addition to having charge storage through redox reactions. The guide lines presented in this paper provide a basis for developing a variety of materials systems and a new generation of energy storage devices which exhibit pseudocapacitive responses.
BIOGRAPHY: 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 and Development Center. His research interests concern the synthesis of inorganic and organic/inorganic materials, and the characterization of their electrical, optical, biological and electrochemical properties. A continuing theme in his research is the use of sol-gel methods to synthesize materials with designed microstructures and properties. His recent work on electrochemical energy storage includes three-dimensional micro batteries and pseudocapacitor materials for high rate energy storage. He has received a number of honors including a Fulbright research fellowship, invited professorships at the University of Paris, the University of Bordeaux, the University of Toulouse, Shinshu University and two awards from the Department of Energy for outstanding research in Materials Science. He is a Fellow of the American Ceramic Society, the Materials Research Society and a member of the World Academy of Ceramics. In addition to serving on the Board of Reviewing Editors at Science, he is a member of the editorial boards of Advanced Energy Materials, Solid State Ionics, Advanced Electronic Materials and Journal of the American Ceramic Society.