The prospect of developing materials with the energy density of batteries and the power density and cycle life of electrical double-layer capacitors is an exciting direction that has yet to be realized. With these materials, there is the promise of achieving charging in minutes to storage levels comparable to battery electrode materials. One pathway which offers the possibility for achieving this combination of properties is pseudocapacitance.
An underlying assumption with pseudocapacitive materials is that charge-storage kinetics are not characterized by semi-infinite diffusion. Instead, the ion diffusion length l and the diffusion coefficient D are considered to be related by l << (Dt)1/2, where t is diffusion time. There has been considerable success in creating nanoscale materials which exhibit pseudocapacitive properties. One reason for this is that when the thickness of the material is very small, i.e. less than the characteristic diffusion length, diffusion of redox reactions is governed by thin-layer electrochemistry. Under these conditions, faradaic reactions occur within a finite diffusion space and redox kinetics begin to resemble capacitive processes. We consider such materials to be ‘extrinsic pseudocapacitors’ in that materials which exhibit battery-type behavior as bulk solids, change their electrochemical characteristics and exhibit pseudocapacitive responses when reduced to nanoscale dimensions. This mechanism change has been associated with decreasing ionic and electronic diffusion distances, suppressing phase transitions, and increasing the number of available surface sites for Li+. In this paper, we will review extrinsic pseudocapacitive behavior for several nanoscale materials systems including MoO2, LiMn2O4 and MoS2 and show how their fast kinetics and pseudocapacitive responses arise from thin layer electrochemistry considerations. Pseudocapacitive materials fill an important gap in the energy storage field, namely having solids that possess the energy density of battery materials with the power density of capacitive materials. It is certain that interest in these materials will continue to expand as their unique combination of properties will meet the needs of several of the expected growth areas for 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 and Development Center. His research interests concern the synthesis of inorganic and organic/inorganic materials and 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 batteries and pseudocapacitor materials. Among the honors he has received are a Fulbright research fellowship, the Orton Lectureship from the American Ceramic Society, awards from the Department of Energy and invited professorships in France, Japan and Singapore. He is a Fellow of the Materials Research Society, the American Ceramic Society and a member of the World Academy of Ceramics. In addition to the Board of Reviewing Editors at Science, he is a member of the editorial boards of the Journal of the American Ceramic Society, Advanced Energy Materials, Solid State Ionics and Energy Storage Materials.