Effect of Adsorption or Electro-Adsorption of Electrolyte Components at Interfaces on the Electrochemical Performances of Li-Ion Batteries and Supercapacitors

Abstract

Electrolytes of relevance for high performance Li-ion batteries have to fulfill a variety of criteria such as a high conductivity and phase stability in a wide range of temperatures, a large electrochemical window and high Li+ transference number across interfaces to reduce interfacial resistances and anion concentration polarization. Beyond that, electrolytes should wet separators and electrodes, which is particularly crucial if nano-structured electrodes are used. From this point of view, liquid electrolytes present some advantages over solid or gel electrolytes. Finally the electrolyte must be not too costly or environmentally problematic. Owing to their strong dipolar moment cyclic alkyl carbonates, sulfones and nitriles exhibit high surface tensions which make them unsuitable for wetting polyolefin separators. Another consequence of the use of highly polar solvents is that there are preferentially adsorbed at charged interface and hence, the composition of the adsorbed film will be different from the bulk when binary mixtures of solvent are used, generally composed of a polar and non-polar solvent. As an example, the oxidation potentials of binary solvent electrolytes at the LMNO/electrolyte interface do not depend in a simple manner on the composition of the mixture. Moreover, in order to improve the stability of interfaces, film forming additives, such as vinylene carbonate (VC) or fluoro-alkylcarbonate (FEC), are usually added in small quantities. These additives will compete with the solvent molecules and electrolyte ions, solvated or not, for adsorption at the electrode surface. Hence, it is not surprising to find that the reduction potential of such additives varies strongly with the composition of the electrolyte or even with the addition of a second additive. Given examples will concern the electrochemical performances of TiSnSb conversion active material in the presence of VC and FEC as additives in a mixtures of alkyl carbonates. Other examples will concern the performances of positive active carbon electrodes, designed for use in Li-ion supercapacitors, in order to enlighten the role of solvation on ion electroadsorption in small pores.