N°30 I quarterly I july 2013 Focus | 21 w and the electrolyte,” explains Danielle Gonbeau of the IPREM4 in Pau. Degradation of the solvents and salts contained in the battery’s electrolyte occurs at the interfaces, forming layers a few nanometers thick. While these deposits are beneficial in certain cases, in others they impede performance. Using cutting-edge techniques like X-ray photoelectron spectroscopy (XPS), the IPREM has acquired in-depth knowledge of the chemical species involved in the phenomenon, which is essential for understanding and slowing down the mechanisms of battery ageing. To meet user’s needs, batteries should also be able to deliver a burst of power for only a few seconds. Applications are as diverse as the emergency door mechanism on the Airbus A380, “stop and start” car engines that turn off automatically at red lights, or wireless screw guns. Supercapacitors could be the answer. Weighing between a few grams and several hundred kilos, they consist of two porous carbon electrodes separated by a membrane and immersed in a liquid electrolyte containing positive and negative ions. The ions accumulate in the carbon pores on both sides of the membrane, creating a difference in potential and thus an electric current. Researchers focus on increasing the energy density of supercapacitors. “They could take over the battery of electric cars to recover energy from braking or to deliver the power needed for acceleration, thus extending the battery’s lifetime,” explains Patrice Simon of the CIRIMAT5 in Toulouse. Delivering a power of 100 megawatts per kilogram—the equivalent of 10 high-speed TGV trains traveling at top speed—but only for a few milliseconds: that is the performance that researchers hope to achieve with SMES (Superconducting Magnetic Energy Storage). The underlying principle is quite simple. Energy is stored in the form of a magnetic field generated by the circulation of an electric current within a short-circuited coil, made of a superconducting material to prevent energy loss. The coil is then discharged when desired by connecting it to the load. Yet for a SMES system to function, it has to be cooled to about 4 Kelvin (–269°C) for conventional superconductors. The technology therefore entails a bulky, costly installation, which limits its scope of application. In Grenoble, the Institut Néel and G2Elab design SMES devices that operate at higher temperatures (20 Kelvin, or -253°C), allowing for smaller cooling units and considerably improved performances. The potential uses are of interest in both civil and military applications, especially for pulse / transient energy sources. An electromagnetic launcher, for example, would be able to propel a load at much greater speeds than that attained with conventional chemical propulsion. This could be useful for placing a microsatellite into low orbit, or launching an artillery shell. More generally, this research could also lead to the development of much more effective and durable superconducting magnets. Contact i nformation: Pascal Tixador > pascal.tixador@g2Elab.grenoble-inp.fr magne tic energy storage essential for the oxygen electrode. This is not expected to happen in the near future.” These problems are not limited to car batteries. The mass storage of electricity aimed at supplying the power grid relies on huge sodium-sulfur batteries. But these operate at 300°C, raising important safety issues. In response, the RS2E is actively investigating the use of sodium-ion batteries, which operate at ambient temperature, as well as a promising technology called 05 A superconducting coil that can store energy in the form of a magnetic field. “redox flow” (see caption 7). In the latter, the electrodes are formed by two fluids in motion, separated by a membrane through which ions are moving. This method is inexpensive but delivers insufficient energy. Researchers at the RS2E are working on the formulation and properties of “inks” (liquids containing particles in suspension rather than free ions), which offer a higher concentration of ions and thus greater energy density. POW ER PRESERVAT ION Another problem with batteries, including lithium-ion ones, is that they lose capacity and power over time. “This is the result of electrochemical reactions at the interface between the electrodes 06 A test bench for electrochemical storage materials at the LRCS in Amiens. 07 The operating principle of a redox flow battery: the electrodes consist of fluids in motion (electrolytes) separated by an ion-permeable membrane. © CEA/C. Beurte y 05 06 © D. M orel © C. FRESILLON/CNRS Photothèque 07
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