Investigation of Lamellar Oxides for High Rate Energy Storage

Due to their large surface charge, oxides offer a valuable alternative to carbon-based supercapacitors for high-rate energy storage. In addition to conventional charge accumulation at the double layer interface, transition metal oxides with mixed valence properties may allow some redox reactions at their boundary that largely increase the capacitance value [1]. This effect can be further increased in the case of layered materials which present a good ionic conductivity and fast ionic intercalation and exchange properties. We thus study layered oxides or hydroxides based on Fe, Co, Ni. To increase the capacity, low temperature synthesis or fast precipitation have been developed to obtain nanoparticles with high specific surface area. This abstract will particularly focus on the NaxCoO2 structure. For specific x values, this phase can be electronically conductive, and therefore is a good candidate for supercapacitor applications. It is widely used as conductive additive in Ni-MH positive electrode [2,3,4]. Nanoparticles of NaxCoO2 with size ranging between 30 and 100 nm and high specific area (above 100 m/g) can be synthesized by rapid precipitation in alkaline oxidizing media. They are highly reactive and yield fast ionic exchange with protons in neutral solution or with lithium in LiOH to convert to HCoO2 or LiCoO2 phase respectively. A large effort has been devoted to understanding the effect of electrode formulation on the properties. The intrinsic electrochemical properties of the materials have been tested on minute amounts of materials deposited onto vitreous carbon following the method described in [5] (see Fig. 1), whereas thick pastes of the material mixed with a current collector, carbon, binder and a porogen were used to study the extrinsic effects of the active specific area, electronic conductivity and diffusion of the electrolyte (Fig. 2) (method described in ref [6]).