MODIFICATION OF THE LITHIUM METAL-SURFACE BY NONIONIC POLYETHER SURFACTANTS - QUARTZ-CRYSTAL MICROBALANCE STUDIES

In order to stabilize and obtain highly ion-conductive surface films, lithium metal was modified using two kinds of nonionic polyether-type surfactants, viz., poly(ethylene glycol) dimethyl ether and dimethyl s ilicone/propylene oxide copolymer in propylene carbonate (PC) and ethy lene carbonate (EC) + dimethyl carbonate (DMC) solutions. Impedance sp ectroscopy indicated that the surface films formed in the presence of these surfactants were very stable, whereas that in the absence of sur factant was not. Scanning electron microscopy images showed that local ized deposition is suppressed for the modified systems compared with t he unmodified system. The mass change associated with the deposition-d issolution of lithium was investigated with cyclic voltammetry (CV) an d quartz crystal microbalance (QCM) methods. The CV/QCM analyses indic ated that the extent of inactive lithium is diminished markedly (by ca . 50% for PC and ca. 30% for EC + DMC solutions) and that the surface film was very stable and its thickness was apparently invariant due to the presence of the surfactants on the electrode surface. The QCM ana lysis also suggested that the surface-film may partly consist of the s urfactant molecule constituents, including ethylene oxide (EO) groups reacted with lithium metal. It appears that a modified surface film in cluding EO chains enhances the ionic conductivity, surface uniformity, and cyclability.