2009-EEPS Secondary batteries-lithium rechangeable systems Electrolytes Overview.pdfVIP

Electrolytes: OverviewFi of w re elK Xu, U.S. Army Research Laboratory, Adelphi, MD, USA Published by ElsevierIntroduction Electrochemistry is the science of controlled redox re- actions proceeding at various electrolyte–electrode inter- faces. In electrochemical power sources, these interfaces are responsible for the sustainable operation of the devices not only by preventing parasitic reactions between the electrolyte components and the strongly oxidative or re- ductive electrodes but also by exerting critical controls on the reaction kinetics, especially at high rates or low tem- peratures. Focusing on the cell chemistries based on lith- ium, this article describes how the selection of electrolyte components (salts, solvents, and additives) dictates the formation chemistry, and consequently affects the physi- cochemical properties of these interfaces on both negative and positive electrode materials. Approaches to tailor the surface chemistries are also summarized.Zn Zn2+ Site and direction of electron exchange (a) (b gure 1 The difference between a chemical and an electrochemica Daniell’s cell is used as an example eCu2tSO42 t ZnZn2tSO4 here chaotic collision between reactants is accompanied by disorder actants localizes the electron exchange sites to the two electrolyte ectrons through an external circuit.What Is Electrolyte? In electrochemistry, redox reactions are forced to pro- ceed in such a manner that there are no direct electron exchanges between the reactants amid their chaotic col- lisions (Figure 1(a)). Instead, the oxidant (positive elec- trode or cathode) and reductant (negative electrode or anode) are physically separated by an electronic barrier, so that, during the reaction, the interfaces between the barrier and the two electrodes are the only locations where electron exchanges can occur, thus producing a steady orientation flow of electrons via an external circuit (Figure 1(b)). An electrochemical cell is a reactor for such controlled redox re