Low Temperature Syntheses of Transition Metal Bronzes with an Open Structure for High Rate Energy Storage
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Low Temperature Syntheses of Transition Metal Bronzes with an Open Structure for High Rate Energy Storage X. Pétrissans1, V. Augustyn2, D. Giaume1, P. Barboux1, B. Dunn2 1 Laboratoire de Chimie de la Matière Condensée, Chimie-ParisTech, 11 rue Pierre et Marie Curie, 75005 Paris, France 2 Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, CA 90095, USA ABSTRACT Development of devices storing and delivering high-energy power such as supercapacitors is necessary to assist intermittent sources of energy. Most of the commercial systems are carbon-based, but due to their high surface charge, oxides offer a valuable alternative for high-rate energy storage. Among them, layered transition metal oxides with mixed valence properties present both good electronic and ionic conductivities suitable for application to electrochemical applications intermediate between capacitors and batteries. This work focuses on lamellar oxide bronzes based on cobalt MxCoO2 and vanadium MxV2O5 (M = H, Li, Na or K). A low temperature synthesis leads to high specific area particles (above 100 m2/g). Hydrated and anhydrous NaxCoO2 are promising cathode materials for aqueous supercapacitors, with a high capacity of more than 100 mAh/g obtained under 20 mV/s for the hydrated NaxCoO2. The MxV2O5 bronzes appear to be good candidates for organic supercapacitors, especially the LixV2O5 bronze, which shows a high stable capacity above 100 mAh/g (at 20 mV/s ie a charging time of 125 s). INTRODUCTION Supercapacitors are electrochemical devices that can deliver higher electrical power than conventional batteries. Their properties are based on the accumulation of charges at the interface between a solid and the electrolyte [1]. There is no chemical reaction or solid-state diffusion limitations resulting in a low internal resistance and high current densities. However, the amount of surface charges at the solid-electrolyte interface is limited and therefore limits the energy density (5-10 Wh/kg in commercial carbon-based systems). Due to their high surface charge, oxides offer a valuable alternative to carbon-based electrochemical capacitors for high-rate energy storage [2]. In addition to conventional charge accumulation at the double layer interface, transition metal oxides with mixed valence properties allow redox reactions in their outer layers that largely increase the capacitance value (known as pseudo-capacitance [3]). The best example is that obtained for the expensive ruthenium oxide [4]. In addition, there is a continuous transition form capacitive to faradaic processes in the bulk of the materials with high specific area such as thin films or nanoparticles. This effect can be further increased in the case of openstructure materials which present a good ionic conductivity and fast ionic intercalation and exchange properties. This work will particularly focus on cobalt oxides and vanadium bronzes presenting a lamellar-structure. Indeed, such structures can achieve good ionic mobility with good cycling re
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