Identifying New Directions for Pseudocapacitive Materials

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 is an exciting direction which is of great interest for mobile power applications. To achieve these properties, our research has focused on pseudocapacitive materials in which ion insertion induces reversible redox reactions at or near the surface of an electrode material. For the most part, studies on pseudocapacitive materials have focused on crystalline solids where crystallographic pathways, such as open channels or sparsely occupied planes, lead to high ion mobility. Although amorphous materials may not have long-range crystallographic considerations, they do offer other interesting features such as a more open framework which can enable percolation pathways and facilitate ion transport. Our studies on sodium-ion insertion into amorphous VO₂ (a-VO₂) showed that the charge storage properties of the amorphous material were significantly better than the crystalline counterpart. The present paper reviews our recent work on amorphous systems and underscores their promise as energy storage materials. Our studies on lithium-ion insertion into reduced amorphous transition metal oxides, a-WO₂ and a-MoO₂, show that these materials exhibit box-like cyclic voltammograms and linear galvanostatic profiles. This voltage profile is reminiscent of an electrical double layer capacitor, except that with a-WO₂ and a-MoO₂, there are redox reactions which increase the specific capacity significantly. Our initial studies indicate that a-WO₂ exhibits 120 mAh/g (1-electron redox) while a-MoO₂ supports 1.3 electron redox, leading to over 280 mAh/g. A related direction has involved the effect of nanostructuring. In this study, the sodiation of nanoparticles of anatase TiO₂ produces amorphous layers of 3 to 5 nm thick. As a result, 10 nm particles of TiO₂ become completely amorphous and exhibit pseudocapacitive signatures as redox reactions occur throughout the particle. These materials exhibit specific capacities for sodium which are on the order of 200 mAh/g at high rates. Taken together, these studies suggest that short-range order should be considered a key factor in achieving charge storage.