The underpotential deposition (UPD) of lithium on a polycrystalline gold electrode from poly(ethy1ene oxide) (PEO)/LiC104 electrolytes has been investigated both in ultrahigh vacuum (UHV) and in an inert gas atmosphere at ambient pressure. The results obtained at temperatures in the range 55-80 "C indicate, as found in earlier studies, that the exposure of a gasket-free, sandwich-type cell to UHV does not affect the overall electrochemical response of the interface observed in more conventional environments. In addition, the voltammetric curves for Li UPD were found to be similar to those reported in liquid nonaqueous solvent electrolytes, displaying in each case deposition and stripping peaks with areas corresponding to a charge equivalent to the full discharge of a single Li+ per surface site.

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The electrochemical insertion of lithium into the basal plane of highly ordered pyrolytic graphite (HOPG-(bp)) from a LiClOdPEO solid polymer electrolyte has been examined in ultrahigh vacuum (UHV) using a carefully designed electrochemical cell. On the basis of a comparison of the data obtained with those recorded for the same interfacial system in an inert gas at atmospheric pressure, it has been concluded that the electrochemical behavior observed in UHV is indeed characteristic of the Li/L,iClO4(PEO)/HOPG(bp) system and therefore not affected in any discernible way by the ultralow pressures. Coulometric analysis of cyclic voltammetry experiments showed that the charge associated with lithium intercalation is larger than that observed during subsequent deintercalation, particularly during the first few intercalation-deintercalation cycles. However, the total amount of impurities observed in Auger electron spectra of emersed HOPG(bp) surfaces following lithium intercalation was very low. This last finding is inconsistent with the presence of a film of any significant thickness on the surface, suggesting that the charge imbalance for this interface is due to kinetic hindrances during lithium deintercalation. IntroductionThe advent of solid polymeric materials displaying high ionic mobilities at relatively low temperatures has opened new prospects for improved electrical energy-storage and energygeneration devices.' Battery technology has been focused on solid organic polymers capable of transporting lithium ions but impervious to attack by metallic lithium.* Highly promising is poly(ethy1ene oxide) (PEO),3 a material that shows moderate Li+-ion conductivities at temperatures as low as 55 "C even in the absence of cosolvents or other additives.During in situ characterization of the short-term stability of ultrapurified PEO-based electrolytes toward metallic Li using

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