Using self-assembled monolayers to inhibit passivation at the lithium electrode polymer electrolyte interface

This work investigates the use of surface chemistry to modify the lithium electrode/polymer electrolyte interface by placing a molecular layer, most likely in the form of a self-assembled monolayer (SAM), of H-(CH2)(32)-(CH2-CH2-O)(10)-H onto the surface of PEG, poly(ethylene oxide), electrolyte films. It is proposed that the PEG-like "head" of the molecule above preferentially orients itself to absorb onto the PEO electrolyte, leaving the hydrocarbon CH2 "tail" to self-assemble. SAM placement was confirmed using attenuated total reflection FTIR spectroscopy and wetting studies, and AC impedance measurements were used to investigate the passivating layer development. Extended time period studies of untreated polymer films in contact with lithium exhibited the rapid rise of an interfacial passivating layer whose resistance overtook that of the bulk electrolyte. Similar studies demonstrated that samples with SAMs actually had a small increase in ion conductivity and developed interfacial passivation much slower, supporting the assertion that SAMs could be used to deter the formation of a barrier to Li+ transport during cycling of lithium polymer batteries. (C) 1999 Published by Elsevier Science B.V. All rights reserved.