Micro-supercapacitors (MSCs) are accompanying the popularity of connected objects, but their energy storage still needs to be improved against a backdrop of increasing miniaturisation. Vanadium nitride, for example, is used in large supercapacitors, but it is incorporated in powder form, which is incompatible with miniaturisation. Researchers at the Institut d'électronique, de microélectronique et de nanotechnologies (IEMNCNRS/Université polytechnique Hauts-de-France/Université de Lille/Centrale Lille), the Institut des matériaux Jean Rouxel (IMNCNRS/Université de Nantes) and the Catalysis and Solid State Chemistry Unit (UCCSCNRS/Université d'Artois/Université de Lille/Centrale Lille) have elucidated the charge storage process in vanadium nitride, after using this material for the first time in MSCs. The team was supported by the Electrochemical Energy Storage Network (RS2E) and the Renatech network.

The researchers used an industrial microelectronics process: magnetron cathode sputtering. A metallic vanadium pebble is subjected to ionised gases and then condensed on a silicon substrate. The material delivers an electrical storage capacity per unit volume (> 700 F cm^-3) four times greater than that of nanoporous carbon electrodes. This MSC retains more than 80 % of its initial performance after 50,000 charge/discharge cycles, whereas most MSCs wear out after 10,000 cycles. Using analyses carried out in part at the Soleil synchrotron, the researchers showed that this performance was due to the presence of a tiny layer of vanadium oxide, which forms naturally when the electrodes are in contact with the air. It is this layer that stores the electrical charges, without involving the rest of the material, which therefore deteriorates much more slowly.

Study of the charge storage mechanism in vanadium nitride integrated as a micro-supercapacitor electrode. (c) IEMN