Effects of the electrolyte composition on the electrochemical intercalation of lithium ions into polyparaphenylene

The electrochemical intercalation of lithium ions into polyparaphenylene (PPP) was studied using the electrolyte composed of propylene carbonate as the solvent and LiClO4 as the salt. Such compositions lead to the partial degradation of the polymer during the first reductions. Consequently, the highest Li/C6H4 ratio obtained is lower than 0.2. Such a value is much lower than the expected one, equal to 0.5, found in compounds doped with alkali metal in the vapour phase and corresponding to fairly well defined structures. Then, the intercalation-deintercalation process, limited in this PC/LiClO4 electrolyte, can be originated by the coinsertion of solvent molecules which decompose between the polymer chains with consequently a structural disorganization of the polymer similar to the so-called graphite exfoliation. Addition of a co-solvent, ethylene carbonate to PC with LiClO4 salt results in the electrochemical intercalation of Li+ into PPP with formation of the richest binary compounds Li-0.5(C6H4). Similar results are obtained when PC is treated with a wetting agent like chloro-EC, acting also as a solvent. In both cases, the formation of a passivating layer on the electrode surface allows intercalation of lithium and prevents the coinsertion of the solvent molecules, limiting consequently the electrode degradation. (C) 1998 Elsevier Science Ltd. All rights reserved.