(Invited) High Rate Energy Storage Based on Pseudocapacitance in Oxide Materials

Abstract

The growing interest in using pseudocapacitor-based materials for electrochemical energy storage 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. The reversible redox reactions that characterize pseudocapacitance occur at or near the surface of an electrode material and are fast enough so that the device's electrochemical features are similar to those of a carbon-based capacitor, but with significantly higher energy density. This paper will review recent research directed at identifying various electrochemical and structural characteristics which provide insight regarding pseudocapacitive materials and properties. One key feature associated with pseudocapacitance is that the rate of charge storage is determined by surface-like kinetics rather than semi-infinite diffusion as occurs with battery materials. In this regard, the presence of two-dimensional pathways in the structure of the oxide material seems to be favorable for obtaining a pseudocapacitive response. Another important consideration with this mechanism is that the structure does not undergo a first-order phase transition upon Li+ insertion. However, even with materials that exhibit such phase transitions, nanostructuring the material can effectively suppress the structural change and lead to attractive properties. In the case of nanoscale MoO2, we showed that pseudocapacitive properties are developed which enable the material to operate at high charge/discharge rates without decreasing the level of charge storage. Morphology is another parameter that can be used to develop pseudocapacitive properties in oxides. Mesoporous materials, which possess an interconnected pore network that provides electrolyte access to thin (<15 nm) redox-active walls, lead to a pseudocapacitive response while two-dimensional nanosheets of transition metal oxides exhibit surface-controlled kinetics indicative of pseudocapacitive behavior. The ensemble of these results suggests that we can expect an increasing number of materials to be developed that retain high energy density at charge/discharge rates which are well above those of battery materials.