A thin-film spinel Li2Mn4O9 electrode was prepared by spin coating onto a Pt substrate, Spectroscopic ellipsometry, X-ray diffraction and current-sensing atomic force microscopy (CSAFM) were used to characterize interfacial processes and film formation at this electrode in the presence of 1.0 M LiPF6, EC:DMC (1:1 by volume) electrolyte. Prolonged exposure of the film to the electrolyte at ambient temperature resulted in spontaneous decomposition of the spinel to lambda-MnO2 without disruption of the original structure. The surface of the resulting lambda-MnO2 film exhibited no significant change in morphology, however a thin passive electrode surface layer was detected by the CSAFM probe. This electrode surface layer exhibited insulating properties and most likely contained Li2O, a by-product of Li2Mn4O9 decomposition.